biodegradable polyesters(r1)

Polymer International 47 (1998) 89È144

Featured Article

A Review of Biodegradable Polymers:Uses, Current Developments in theSynthesis and Characterization of

Biodegradable Polyesters, Blends ofBiodegradable Polymers and RecentAdvances in Biodegradation Studies

Wendy Amass,* Allan Amass & Brian Tighe

Speciality Polymer Research Group, CEAC, Aston University, Aston Triangle, Birmingham B4 7ET, UK

(Received 27 May 1998 ; accepted 10 July 1998)

Abstract : This review considers the uses of biodegradable polymers in terms oftheir relevance within current plastic waste management of packaging materials,biomedical applications and other uses ; research papers and patents are cata-logued. The chemical synthesis of polyesters and the microbial production ofpoly(hydroxyalkanoate)s, including recent publications in these areas, arecovered and methods of characterization and structural analysis are outlined.Current research into two- and three-component blends is reviewed as a methodof reducing overall costs and modifying both properties and biodegradation ratesof materials. Finally, there is a summary of degradation processes. Both abioticand biotic reactions are discussed, together with the development of biodegrada-tion test methods, particularly with respect to composting. Society of( 1998Chemical Industry

Polym. Int. 47, 89È144 (1998)

Key words : biodegradation ; biodegradable polyesters ; packaging ; synthesis ;characterization ; blends

1 INTRODUCTION

Within this review, biodegradation will be deÐned as byAlbertsson and Karlsson1 as an event which takes placethrough the action of enzymes and/or chemical decom-

* To whom all correspondence should be addressed.

position associated with living organisms (bacteria,fungi, etc.) or their secretion products. However, it isnecessary to consider abiotic reactions (e.g. photo-degradation, oxidation and hydrolysis) that may alsoalter the polymer before, during or instead of biodegra-dation because of environmental factors. Albertsson andKarlsson2 have also recognized that nature can be used

891998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

90 W . Amass, A. Amass, B. T ighe

as a model in the design of degradable polymericmaterials, because it can combine polymeric materialswith di†erent degradation times into a hierarchicalsystem that optimizes both energy and material proper-ties. Naturally occurring biopolymers are susceptible toenvironmental degradation factors and their breakdownmay be caused by a combination of these ; for example,wood is both oxidized and hydrolysed. Tailoring theproperties of polymers to a wide range of uses anddeveloping a predetermined service life for the materialshave become increasingly important and Albertssonand Karlsson suggest four di†erent strategies :

(1) the use of cheap, synthetic, bulk polymers withthe addition of a biodegradable or photo-oxidizable component ;

(2) chemical modiÐcation of the main polymer chainof synthetic polymers by the introduction ofhydrolysable or oxidizable groups ;

(3) the use of biodegradable polymers and theirderivatives, with poly(hydroxyalkanoate)s(PHAs) being the most studied ;

(4) tailor make new hydrolysable structures, e.g.polyesters, polyanhydrides and polycarbonates.

All four approaches are considered within this review.Current research interest in biodegradable polymers

is connected with well-deÐned areas of use. Biodegrad-able plastics o†er one solution to managing packagingwaste. However, biomedical applications of biodegrad-able and biocompatible polymers generate an enormousamount of research and interest. Uses in this Ðeld rangefrom wound dressings, drug delivery applications, surgi-cal implants and other medical devices. There are alsoagricultural uses, e.g. for controlled release of fertilizersand pesticides, applications in the automotive industryand as surfactants.

As well as the uses of biodegradable polymers, thisreview will include sections on the chemical synthesis ofpolyesters, the microbial production of poly(hydroxy-alkanoate)s (PHAs), blends of these biodegradablepolymers, methods of characterization and structuralanalysis and, Ðnally, recent research into the rates andmechanisms of the biodegradation of relevant polymericmaterials. The latest published papers which have bio-medical applications and relevant patents for the revieware included as appendices.

2 USES OF BIODEGRADABLE POLYMERS

2.1 Biodegradable plastics for packaging

Synthetic polymers have become technologically signiÐ-cant since the 1940s and packaging is one industry that

has been revolutionized by oil-based polymers such aspolyethylene (PE), polypropylene (PP), polystyrene (PS),poly(ethylene terephthalate) (PET) and poly(vinylchloride) (PVC). Despite the major advances in the syn-thesis, manufacture and processing of these materials,two major problems still confront the industry : the useof non-renewable, oil-based chemicals for the manufac-ture of commodity polymers and the ultimate fate of thewaste materials.

2.1.1 Plastic waste management. The need to confrontthe problem of recycling packaging material of all typesis recognized by governments and the European Com-munity Waste Management Legislation is the Ðrstattempt to unify the approach to the problem by all themember states. The targets from the EU directive areshown in Table 1.3

In the UK, overall recovery targets for all packagingwaste have been set for each year (1998È2001) with theobligation from 2001 onwards of over 50% recovery, ofwhich at least half must be met by recycling, and up to16% of each material, e.g. plastics, must be recycled. Itis estimated that 200 000 t of post-consumer plasticwaste were recovered in 1997 by incineration for energyor by recycling ; as the consumption of plastics for pack-aging in the UK is about 2 ] 106 t year~1, this rep-resents only 10% recovery.

In the UK, Valuplast is the organization responsiblefor implementing industryÏs obligations under the newrecycling legislation. The aim is to increase the annualrecycling capacity from the current 100 000 to 300 000 tby 2001, with upwards of 500 000 t year~1 of post-useplastics being recycled by 2005. Many companies haveregistered with Valpak, an organization that has under-taken to manage their recycling responsibilities. Theimplications for raw material producers and convertersas well as packagers and retailers is considerable and areview of the legislation seems inevitable.

“Plastics RecoveryÏ examining data from 1995 is themost recent report available on plastic consumptionand recovery in Western Europe.4 It is produced by theAssociation of Plastics Manufacturers in Europe(APME) and this is the seventh year that data frommanufacturers, trade associations, waste managementorganizations and government bodies have beenanalysed in this way. The UK is shown to be the fourth

TABLE 1. Targets for EC waste management

legislation

1 By 30/6/2001, between 50% and 65% by weight of

packaging waste will be recovered

2 Within this general target and within the same time

limits, between 25% and 45% by weight of packaging

materials in packaging waste will be recycled

3 Within 2 above, a minimum of 15% of each material

must be recycled, e.g. plastic packaging

POLYMER INTERNATIONAL VOL. 47, NO. 2, 1998

A review of biodegradable polymers 91

largest consumer of plastics in Western Europe, using3É3 ] 106 t in 1995 of the total 24É3 ] 106 t demand(UK demand in 1997 was 4] 106 t). The consumptionof plastics for packaging between 1993 and 1995remained at about 40% of the total. Despite the hugedemand for plastics in this and other industries, poly-mers account for only 0É6% by weight of total waste. Ithas been estimated that plastics make up about 8% ofmunicipal solid waste (MSW)4 and this represents theultimate fate of about 63% of all plastic waste. Remo-ving plastics from packaging would create problems,not solve them, as a German study has estimated thedramatic increase in terms of weight and volume ofusing non-plastic packaging and also the additionalenergy consumption required.5 However, total plasticwaste in Europe in 1995 was estimated at 16 ] 106 tand, because of its low density and hence large volume,it has considerable environmental impact. Table 2shows the Ðgures for di†erent disposal methods.

The three main strategies available for the manage-ment of plastic waste are incineration, recycling andlandÐll. Over 25% of the 16] 106 t of plastic waste wasrecovered in 1995 either as energy (D17%) or by recy-cling ([9%). Plastic waste disposal to landÐll represent-ed a 14% fall from the 13É3 ] 106 t lost in 1994. Reviewsof polymer waste management have been published byseveral authors.6,7 The use of degradable plastics is, asyet, a minor factor in this overall strategy, but develop-ment of photo- and biodegradable polymers has con-siderable implications for medical, food and agriculturalpackaging.8

Incineration has the advantage that the plastics havehigh caloriÐc value, but incineration plants have to bemodiÐed for efficient combustion of the polymers andregulation of the gas emissions to ensure that no toxicgases are released. Energy recovery through inciner-ation is an essential part of an overall waste manage-ment scheme.

“In-houseÏ recycling and interception of bulk waste atsupermarkets, etc. are strategies that will have to beextended to meet the new British packaging regulations,

TABLE 2. European plastic waste disposal (1995)

Plastic fate Amount (103 t) %

Landfill 11354 70·7

Incineration without 517 3·2

recovery

Waste exported from 166 1·0

Western Europe for

recovery

Energy recovery 2698 16·8

Feedstock recycling 99 0·6

Mechanical recycling 1222 7·6

Total 16056 100

which seek to double the amount of plastic packagingrecycled by the year 2001, although a quadrupleincrease is more in line with EU targets. There are alsoemerging technologies for plastic feedstock recycling,which can take mixed plastic waste and turn it backinto naphtha, monomers or other oil derivatives. Alarge-scale reÐnery-type operation is required to makethese pyrolysis and hydrogenation procedures viable,but it is anticipated that the necessary economic climatewill exist within the next 10 years.9

Some polymer manufacturers have their own recy-cling organizations, e.g. Petcore, the European PETcontainer recycling organization. PET container collec-tion for recycling in Europe has increased on average by40% year-on-year from 13 000 t in 1991 to 86 000 t in1997 although there has been a 10% annual growth inPET usage. However, the percentage increase in PETrecycling in the UK was only 21% in 1997 and expectedto be the same for 1998, because there is no fundingmechanism to cover the cost of material collection asexists in other European countries. The new packaginglegislation is bound to increase the UK totals, and recy-clers are known to have excess capacity. However, thedepressed price of virgin PET and all vinyl polymersmakes it difficult for collectors and recyclers to runschemes proÐtably at the moment.

The non-statutory requirements given to localauthorities by central government suggest a Ðvefoldincrease in materials collected from households in thenext 4 years. Today about 40% of local authorities havepackaging collection programmes, but it has been esti-mated that 80% of homes will have to participate insome form of kerbside collection by 2001. The two mainchallenges of plastic recycling are the separation andidentiÐcation of the vast range of plastic items and theproduction of recycled material that has reproducibleperformance and is economically viable. The slight dete-rioration in the performance characteristics of a speciÐcpolymer only becomes signiÐcant after 5È8 cycles, butthis closed loop recycling does not occur. Thus PETand PVC recyclates are used as Ðbres for thermal insu-lation (duvets, anoraks) and PS, PP and PE are reusedfor a range of office accessories, cassette cases, crates,pipes and refuse bags. Legislation prevents the use ofrecycled polymers in direct food contact packaging, andplastics with high technical speciÐcations cannot bemade from recyclates.

Plastic identiÐcation codes are one aid to separationthat has been introduced, and mechanical sorting basedon the speciÐc gravity of the di†erent polymers is welldeveloped, although this will not e†ectively separatePVC and PET. Infrared technology is used to “recog-nizeÏ PVC from PET bottles, and both UV and surfacedielectrics have been used to identify di†erent polymers.Recently, Pira International has introduced a full-scaleautomated plant for the identiÐcation and separation ofarticles from mixed waste, using Ñuorescent tracers in



various polymers which are then identiÐed using ultra-violet light. Efficiencies of 95% have been achieved atsort speeds of six articles per second.

However, recycling from MSW is even more techni-cally difficult and so far has proved uneconomic. Land-Ðll sites are the main destination of plastic waste andthe fate of even organic solids depends on the manage-ment of the site. The lack of air and water in the bulk ofthe material means that paper has been found intacteven after 7 years. However, within these sites, tem-peratures up to 70¡C can occur and anaerobic degrada-tion can produce methane and other degradationproducts. Some bulk plastics undergo photochemicaldegradation, but most are not degraded.

Composting MSW has been attempted by some localauthorities. This involves separating some of the landÐlland, after preliminary screening, mechanically shred-ding the solid waste. The moisture content is thenincreased by the addition of sewage sludge and the mixis slowly agitated and aerated. After 5 days and screen-ing, a large percentage of the material can be left tomature for use as compost. However, the plasticmaterial is unchanged.

In the 1970s, work was started in the US and else-where to produce photodegradable and biodegradableplastics for the packaging industry. The requirementswere :

(1) non-toxic materials with non-toxic degradationproducts that would not a†ect the drainagewater from landÐll sites ;

(2) polymers with suitable mechanical properties forspeciÐc uses ;

(3) economic viability ;(4) degradation control of the plastics via polymer

modiÐcation ;(5) processability.

The source of these materials was of two main types :natural materials known to biodegrade and syntheticbiodegradable polymers. Starch and cellulose are twonatural materials that have been extensively investi-gated for use as packaging material and they have theadded advantage of being renewable sources of poly-meric material. The processability of the starch has beena major problem and the cellulose has to be signiÐ-cantly modiÐed both to increase its biodegradabilityand to optimize its mechanical properties.

Poly(hydroxyalkanoate)s (PHAs), which are producedin plant cells and can be synthesized biochemically byfermentation, are another source of natural polymers.Poly(3-hydroxybutyrate) (PHB) and the copolymerpoly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)are produced commercially by Monsanto and sold as

PHBV copolymers were Ðrst manufactured byBiopol}.ICI in 1983 (see Section 3)10 and were originallyintended as biodegradable substitutes for oil-basedpolyoleÐns in plastic containers, Ðlms and bottles.11

Their potential use was also seen for plastic items thatcould not easily be separated from other materials, e.g.nappies and sanitary towels. In 1990 the manufacture ofblow-moulded bottles using Biopol for packagingshampoo was started in Germany by Wella AG, Darm-stadt.12 Hocking and Marchessault13 have reviewed theactual and potential uses of PHB and PHBV for e.g.motor oil containers, Ðlm formation and paper-coatingmaterials. The last two applications are based on biode-gradable latex produced from PHAs with medium-length b-side chains.14 The patents taken out over thelast 10 years, which are summarized in Appendix II.1,give some indication of the range of possible uses.

Synthetic biodegradable poly(a-ester)s, e.g. poly(lacticacid) (PLA), poly(glycolic acid) (PGA) and copolymersof these, have been manufactured for biomedical usessince the 1970s. These and other polyesters known tobiodegrade, e.g. poly(caprolactone) (PCL), have beeninvestigated as packaging materials. In order to opti-mize the properties of all these biodegradable materialsand reduce costs, di†erent research strategies have beenadopted. Block and statistical copolymers have beensynthesized and blending studies have been carried outto obtain compatible mixtures of components whichstill biodegrade. The use of plasticizers to increase theÑexibility of polymers and the use of Ðllers to reduce thecost are two advantages of the blending approach. Afurther bonus comes from components that are photo-or biodegradable and induce this behaviour in com-ponents that do not degrade as pure materials.

The main constraint on the use of biodegradablepolymers for bulk packaging is the di†erence in theprice of these polymers compared with that of bulk-produced, oil-based plastics. The current cost of

is approximately £8000 per tonne, comparedBiopol}with the current UK prices of commodity polymers ofbetween £500 per tonne (PVC and PP) and £600 pertonne (HDPE and high-impact PS). Current low oilprices, increased recycling capacity and improved tech-nologies for the separation of plastics and their reusemake the use of biodegradable polymers for most pack-aging requirements uneconomic.9

There are several problems besides cost associatedwith certain biodegradable polymers as packaging.PGA and PLA are highly susceptible to bulk hydro-lysis, so this limits the range of contents for the pack-aging, and rigorous storage conditions must be used.Microbial polyesters are relatively resistant to chemicalhydrolysis but are susceptible to bacteriological attack,which restricts their use as packing for foodstu†.However, PHBV has excellent gas barrier properties15and Scherzer16 has recently reported the developmentof barrier layers against oxygen transmission usingradiation-cured methacrylated gelatin. The layersshowed extremely low oxygen permeability, high resist-ance against boiling water and good adhesion charac-teristics. It is postulated that, using substrate foils of



biodegradable polymers, the methacrylated gelatinswould be suited as barrier adhesives in laminates forcompletely biodegradable food packaging.

Another recent paper by Suominen et al.17 reportstheir investigation into the migration of microbes infood-packaging paper and board coated with PE,mineral pigment or a biodegradable polymer. Confocallaser-scanning microscopy was used to examine thespatial distribution of microscopically observable bac-terial cells. The paper and paperboard studied were anefficient barrier against translocation of microbes and itwas concluded that there was limited access to freewater within the cellulose. However, there were rela-tively high concentrations of microbes residing betweenthe paperboard and its polymer coating, which on thecoating facing the food is a potential site for leakageinto the food. The need for high-hygienic-qualitysurface-sizing chemicals was emphasized and mineral-coating pigments were found to be a source of microbesand thus to be discouraged.

The relative merits of the di†erent methods of manag-ing plastic waste materials are eventually related to costand legislation. This paper will review the current situ-ation in the development of biodegradable blends ofmaterials for uses including packaging.

2.1.2 Use of renewable resources. Renewable sources ofpolymeric materials o†er an answer to maintaining sus-tainable development of economically and ecologicallyattractive technologies. The e†ect on the US economyof substituting production of corn-based polymer resinsfor petroleum-based polymers has been analysed byBeach et al.18 and the use of agricultural materials andbiomass has been reviewed by Mulhaupt.19 Mulhauptconcluded that although Germany is at the forefront ofgreen technology and a wide range of biodegradablepharmaceutical and novel surfactant materials can bemade from renewable materials, it is only as com-ponents of packaging and as natural Ðbre compositesthat these materials are currently viable in terms ofprice and performance.

2.2 Biomedical uses

The most widely researched biodegradable polymers arethe poly(a-ester)s (e.g. PLA, PGA) and the microbialpoly(hydroxyalkanoate)s (e.g. P3HB, P3HV). Thesehave been shown to be non-toxic and biocompatibleboth as polymers and as their degradation products,which in most cases occur naturally in the body. Theyhave a range of pharmaceutical and biomedical usesbased on these characteristics and their physicochemicalproperties.

The extremes of degradation rates between solublemacromolecules such as poly(vinyl alcohol) (PVA),which has a half-life of weight loss measured in hours,and the almost completely inert high-molecular-weight

homopolymers such as polystyrene are more correctlydeÐned as a measure of hydrolytic attack. Biodegrada-tion occurs through the action of or in association withliving organisms and its measurement is based on stan-dardized tests developed over the last 30 years. PLAand PGA, which are used as absorbable suture material,are hydrolysed in vitro and in vivo in days, while PHAsare broken down much more slowly. Holland20 produc-ed the hydrolysis data for these polymers using physio-logical conditions and on which Table 3 is based. Theuses of these polymers would be extended considerablyif a wider range of degradation proÐles were available.PHAs have the most potential, because although theirrate of abiotic hydrolysis is relatively slow, microbialhydrolysis is more rapid and can be manipulated byvariations in processing techniques, molecular weight ofthe polymer, copolymer composition and blending.21Although there have been considerable advances in con-trolled synthesis, the chemical synthesis techniques forthese polymers have not yet produced tailor-made bio-polymers with the Ðne structure (e.g. chirality, monomersequence, tacticity) necessary to match enzyme speci-Ðcity in degradation reactions in di†erent microbialenvironments.

2.2.1 Speciality packaging. Speciality packing is poten-tially the largest use of biodegradable polymers. Poly-meric materials and blends that have undergonetoxicological and cytocompatibility testing for in vivouse and are biodegradable have obvious beneÐts aspackaging materials for pharmaceutical products, drugsand wound dressings. The physiomechanical propertiesplus the gas and liquid barrier properties of thesematerials are obviously important, as is a knowledge ofthe conditions and rate of biodegradation. However, ifprocessable biodegradable pure materials or blendscould be manufactured, the economic imbalancebetween these materials and bulk-produced packagingmaterials would not be so great in this specialist pack-aging application.

2.2.2 Surgical Ðxation (sutures, clips, bone pins andplates). The development of the uses of PGA, PLA andPGLAs for surgical Ðxation has been summarized byHolland and Tighe.21 These materials have been usedincreasingly from the late 1960s as absorbable, syntheticsutures because they could be produced as strong Ðla-ments and were shown to degrade rapidly. The mostwidely used absorbable sutures are a multi-Dexon},Ðlament PGA material, a copolymer withVicryl},composition PLLA (8%)-co-PGA (92%) and PDS},poly(p-dioxanone).

The use of biodegradable implants for the Ðxation offractured bones and joints has been reviewed byHofmann22 and contrasted with the use of metal pinsand clips. About 40 di†erent biodegradable polymersand copolymers are currently being used as alternatives



TABLE 3. Comparative weight loss for different

biodegradable polymers, given as time for 10%weight loss at 37ÄC and pH 7·4

Polymer t10

(h)

Vicryl} absorbable suture 450 (18·8 days)

(PLLA (8%)-co-PGA

(92%))

Dexon} 550 (22·9 days)

(PGA suture material)

PDS} 1200 (50·0 days)

(poly(p-dioxanone) suture

material)

PHBV (20% HV) 4·7%=5500 (229·1 days)

(Mw

¼3 Ã105)

PHBV (12% HV) 5·6%=5500 (229·1 days)

(Mw

¼3·5 Ã105)

PHB (0% HV) 18%=2500 (104·2 days)

(Mw

¼8 Ã105)

to metal implants. The disadvantages of metal implantsare cited as the need for “stress protectionÏ, the sensi-tivity of patients to metals (particularly nickel) and theneed for a removal operation. The use of self-reinforcedPGA and polydioxanone, a polyester variation, asimplants was also assessed and the main difficultieswere listed as loss of sti†ness in the material, in a timeinterval which is not long enough to guarantee bonehealing, and the development of a sterile sinus over theimplantation site. It is recognized that designs andassembling principles of metal implants in orthopaedicsurgery cannot be directly transferred to biodegradablepolymers. However, Hofmann states that these poly-mers can be used to treat osteochondral fractures andother deÐned injuries and a standard set of possibleindicators for their use is given. Some speciÐc examples

of their use are ligating clips and bone pins producedfrom P(LLA/GA70/30), and from variousLactomer},polydioxanones and polyoxalates, and high-strengthbone implants of PGA, PLA and PGLA. Tunc and co-workers23,24 have published a series of papers on theuse of an “OrientrusionÏ process for realigning PLAmolecules to create a strong matrix with a Ðvefoldincrease in tensile strength over unprocessed PLA.

PHBV has piezoelectric properties similar to those ofnatural bone.25 Electric stimulation can be used torepair and strengthen bone and there is a potential usefor reinforced PHB composites, with bone-matchingmechanical properties, as fracture Ðxative plates. Theo-retically, growth and healing of the bone can be stimu-lated as the polymer slowly degrades, with no need of areplacement operation. The most recent developmentsthat have been reported on using biodegradablematerials for surgical implants are summarized inAppendix I.1.

2.2.3 Controlled drug delivery. This is the most impor-tant and versatile application of these polymers. Con-trolled drug delivery has applications not only inmedicine but also in veterinary and agrochemical Ðelds.Active ingredients from pesticides to contraceptives canbe delivered by sustained release with the ultimate bio-degradation of the carrier medium.

Drug delivery proÐles of the concentration of thedrug in plasma against time can be used to comparetraditional methods of drug delivery (oral, intravenous)and controlled release. Figure 1 is based on a Ðgurefrom the review of Holland and Tighe21 and illustratestypical drug delivery proÐles, showing the advantages ofcontrolled drug delivery.

The initial drug release systems involved incorpor-ation of the active substance into a polymer matrix,which was implanted into the patient in various ways,

Fig. 1. PlasmaÈdrug concentration proÐles for various methods of administration.



and a di†usion mechanism produced the maintaineddrug release followed by bulk hydrolysis of the polymer.Controlled drug release has become the main applica-tion of (D,L)LA/GA copolymers and the best results, forrelatively small drug molecules, have been obtainedusing PLGA copolymers with 20È50 mol% GA andmolecular weights of approximately 25 000. However,modern techniques allow the incorporation of drugswhich vary in nature and molecular size (from low-molecular-weight steroids to polypeptides (Mw \22 000)). A di†usion mechanism is too slow for therelease of large molecules, so an erosion release mecha-nism may be required for some polymer matrices. Cur-rently it is claimed that drug loadings of upwards of50%w/w and drug release rates from 170 mg day~1 to20 lg day~1 can be achieved.

Polylactones such as PCL and PVL are also used fordrug delivery. However, the most recent emphasis hasbeen to blend PCL with PLLA and PGA or to produceblock copolymers to control biodegradation and drugrelease characteristics. The slower hydrolysis rates ofmicrobially produced polyesters suggest that PHB,PHV and their PHBV copolymers can be used toextend the range of drug delivery systems and Poultonand Aktar26 have reviewed the potential of PHAs inthis Ðeld. The relatively high melting temperature andrapid crystallization rate of PHB homopolymer make itdifficult to entrap the drug. PHBV copolymers are alsosemicrystalline, but di†erent solid matrices are produc-ed by their slower crystallization rates. Release of low-molecular-weight drugs (drug loading [5%) from thesematerials is shown to occur by water penetration andpore formation and is largely independent of erosion.However, surface erosion is particularly viable for drugdelivery systems and with lower drug loading moree†ective entrapment occurs, and although it has beenshown that porosity is initially poor, surface erosiondoes occur followed by greater water penetration. OtherPHAs may provide drug delivery materials, as medium-chain PHAs have lower melting points and are morerubber-like than PHB. The various factors that controlthe rate of hydrolysis of PHAs have been extensivelystudied by Holland20 and Yasin et al.27 PHBV has beenused for drug delivery both as tablets and as micro-capsules, although the biodegradation mechanism hasbeen found to be more complicated than the randomchain ester hydrolysis of other polyesters.

Poly(orthoester)s, polyanhydrides and their blendsare other groups of biodegradable polymers used fordrug delivery and Thomas et al.28 have studied micro-sphere production based on a blend of a soft, rapidlydegrading polyanhydride, poly(carboxyphenoxyvalericacid) (PCPV), and a rigid, slowly degrading polymer,poly(carboxyphenoxyhexane) (PCPH). Incorporatingthe soft polymer increased the density of the micro-spheres produced, but its rapid degradation led to amicroporous structure from which small drug particles

could di†use rapidly, at rates that were independent ofthe blend composition. However, the rate of release ofmacromolecular particles of lysozyme was controlled byblend composition.

Park et al.29 have recently reported on a new class ofbiodegradable compounds containing fatty acid dimer-based polyanhydrides, e.g. poly(fatty acid dimerÈsebacicacid) (P(FADÈSA)). They were reported to have theright physicochemical and mechanical properties fordrug delivery devices and to undergo pure surfaceerosion. The erosion characteristics of a polymer arevery important and Park et al. investigated the degrada-tion mechanism and the release proÐle and mechanismat various pH values. Disc-shaped devices, incorpor-ating three model compounds with di†erent hydrophi-licity (mannitol, inulin and stearic acid), were fabricatedand the release was studied in the range pH 1È9. Allthree model compounds were released faster in alkalinesolution (release rate decreased pH 9 [ pH 7É4 [ pH1È5). This was thought to be dependent on base-catalysed polyanhydride erosion, but it was found thaterosion was not the sole controlling mechanism fordrug release. In acidic solution the erosion of the poly-anhydride was negligible but the highly water-solublecompounds were released rapidly. The release rate at allpH values mirrored hydrophilicity, decreasing from thevery water-soluble mannitol to inulin to the lipophilicstearic acid. Pure erosion release proÐles matched mostclosely the behaviour of the stearic acid. Molecularweight was also found to be a factor in the drug release.100% of the low-molecular-weight mannitol was rel-eased within 6 weeks, but, after an initial fast release,only 60%È70% of the high-molecular-weight inulin wasproduced over this period and this was attributed tohindered di†usion through water-Ðlled pores in the disc.No overall kinetic model was devised to Ðt the observedbehaviour at all pH values and it was concluded thatboth bulk and surface erosion occurred but the balancedepended on the pH of the medium and the nature ofthe incorporated drug.

Chiellini et al.30,31 have reviewed the synthesis andsemi-synthesis of multifunctional polymers and evalu-ated both their biodegradation and bioerosion. Theyhave also assessed their potential for controlled drugrelease in pharmaceutical and agricultural areas and inparticular focused on hydrophilic and/or water-solublepolymeric materials and their blends.

The most common methods of sustained drugdelivery are as injectable micro- or nanospheres or assubcutaneous implant systems,32,33 which involve somedi†usion or erosion matrix. However, some novelmethods have been devised recently. Giordano et al.34have manufactured and characterized non-porous, Ñex-ible, thin Ðlms made of PLLA and P(D,L)LGA (75 : 25and 50 : 50) to which foetal human retinal pigment epi-thelium (RPE) cells were found to adhere when culturedin vitro. Films of controlled thickness and reproducible



surface morphology were produced. This technique hasthe potential to provide a means of transplanting allo-genic RPE cells as a therapy for several ocular diseasesrelated to RPE dysfunction.

Jeong et al.35 have investigated the use of ther-mosensitive biodegradable hydrogels consisting ofpoly(ethylene oxide) and PLLA, aqueous solutions ofwhich exhibit temperature-dependent reversible gelÈsoltransitions. The hydrogel can be loaded with bioactivemolecules in water at approximately 45¡C; a sol isformed, so subcutaneous injection can be carried out.Rapid cooling to body temperature occurs and theloaded copolymer forms a gel that can act as a sus-tained release matrix for the drug.

Akamatsu et al.36 have synthesized a polymeric pro-drug of prostaglandin E-1 (PGE(1)) using galaclosylatedpoly(L-glutamic acid) (Gal-PLGA) as a biodegradableand targetable carrier of liver parenchymal cells as poly-meric conjugates. After intravenous injection into mice,conjugates rapidly accumulated in the liver with up to65% of the dose, suggesting e†ective targeting.

Microspheres. The majority of recent papers on drugdelivery involve the use of microspheres fabricatedusing single or multiple emulsion techniques followedby solvent evaporation, although there are several otherstandard procedures. Okada and Toguchi37 havereviewed the preparation, characterization and biodeg-radation of PLA and PLGA microspheres, and sus-tained release depots and target drug delivery are alsodiscussed. Recent developments in the production ofmicrospheres of polyesters and their blends are sum-marized in Appendix I.2.

Nanoparticles. In certain circumstances the use ofmicroparticles for drug delivery is either inappropriateor can be extended by reducing the particle size. Theminimum microparticle size is about 10lm, but nano-particles with size under 100 nm can be fabricated.Research areas cover novel properties that have beendeveloped for nanoparticles, increased efficiency of drugdelivery, improved release proÐles and drug targeting.Appendices I.3 and I.4 summarize the latest reportedwork using nanoparticles : the former highlights thepreparation and characterization procedures and thelatter deals with novel applications.

In vivo and in vitro studies of e†ectiveness of drugrelease. Monitoring sustained drug release can becarried out using a wide range of di†erent techniquesdepending on the nature of the biodegradable materialand the entrapped drug. The ideal drug delivery proÐlewas illustrated in Fig. 1 and the purpose of in vitro andin vivo studies is to provide information about theseproÐles, including delivery dosage and release time.Recent developments are summarized in Appendix I.5.

Biodegradation, biocompatibility and cytology studies.Although various biodegradation studies using stan-dard conditions have been set up to look at the rate andmechanism of the breakdown of individual polymers for

non-medical use, the degradation of drug deliverysystems represent a much more rigorous area of study.In Appendix I.6 the papers published in this area during1994È1997 are reviewed. Some of the in vivo and in vitrostudies covered in Appendix I.5 also relate to degrada-tion of the polymeric carrier.

2.2.4 Other biomedical uses. Advances in tissue cultureand tissue engineering have generated research intonovel methods of producing biodegradable networksthat are e†ective for a variety of applications both ashard and soft sca†olds. These latter networks have usesas wound dressings, as tubular conformations for intes-tine or vascular grafts and as skin substitutes. Advancesin the use of biodegradable polymers for tissue engin-eering have been reviewed by Vacanti et al.38 Althoughthe use of biodegradable polymers for drug delivery hasalready been dealt with, there has been work carried outon the development of pastes that can be applied aftersurgery to inhibit tumour growth by the slow release oftaxol. The latest papers and patents reporting work inthese areas are summarized in Appendices I.7 and II.2.

2.3 Other uses

2.3.1 Detergent applications. The use of polymeric car-boxylic acids (poly(acrylic acid) and its copolymers) indetergent powder formulas is reviewed by Paik et al.39They were initially introduced in the 1980s in com-bination with zeolites as partial replacements for poly-phosphates. Wastewater treatment plants are unable toremove phosphates and, as these salts promote plantgrowth in rivers, this can lead to eutrophication andeventually environmental imbalance. The non-biodegradability of the original polymers eliminates thepossibility of eutrophication, but acid accumulationeventually results from their use. Although the acidsused are non-toxic, questions about their fate and long-term environmental impact remain. The review tracesthe history of the biodegradable polymer options thathave been considered and the current state of theresearch. Freeman et al.40 have identiÐed poly(asparticacid) as a biodegradable, synthetic polycarboxylate withacceptable detergent properties. Semi-continuous acti-vated sludge (SCAS) removal tests and modiÐed Sturm

production tests show that poly(a,b-D,L-aspartate)sCO2prepared by acid-catalysed thermal polymerization arerapidly and completely removed by municipal treat-ment plant micro-organisms. However, polyaspartatesprepared by other methods were only partially bio-degraded. Structural analysis shows that a linear poly-amide backbone is needed for total degradation.41Talaba et al.42 have reported the modiÐcation of O-(2-sulphoethyl)cellulose (2-SEC) to produce alkylatedSECs as cellulose-based biodegradable surfactants.



2.3.2 Agricultural and veterinary uses. Although theapplication of controlled drug release with respect toagricultural use has already been mentioned, it is worthemphasizing the fact that the bacterium Alcaligeneseutrophus, which can be used both to synthesize anddegrade PHB and PHBV copolymers, was Ðrst isolatedfrom soil. Holmes, in his original paper on the indus-trial production and potential uses of PHBV, recog-nized its potential for the release of insecticide intosoil.10 The PHBV/insecticide pellet would be sown withthe seed and the pest activity would be directly con-nected with the bacterial activity, thus degrading thepolymer and releasing the chemical at the appropriatetime.

Veterinary controlled drug release also has speciÐcapplications. PHB and PHBV are broken down rela-tively slowly in vivo unless speciÐc PHB depolymerasesare present. The rumen in cattle has been found toproduce good degradation conditions and so controlledrelease of medicine at preset intervals is achieved usinga bolus of PHBV that remains in the rumen.43 AppendixII.2 contains a summary of the patents taken outrecently for agricultural uses.

2.3.3 Miscellaneous uses. Several alternative uses of bio-degradable polymers have been postulated, e.g. thesorption of oil-based aromatic compounds from low-carbon sand by microbial polyesters, which has beeninvestigated by Dohse and Lion,44 and the applicationof these polymers in the Ðeld of oil pollution has beenrecognized.

A 1994 patient cited the use of poly(caprolactone) Ðl-aments blended with other biodegradable polymers as a

biodegradable carrier for denitrifying bacteria in waterpuriÐcation. The recent patents identifying potentialuses of these biodegradable polyesters are summarizedin Appendices II.1 and II.2.

3 REVIEW OF BIODEGRADABLE POLYMERS

Many authors have reviewed biodegradable polymers,but, to give an overview of the range of materials avail-able, Holland and Tighe21 provide a concise summaryon which Table 4 is based. Synthetic and microbialpolyesters form the greatest number of reported types ofbiodegradable polymers and are the main focus for thisreview, so an outline of their preparation and propertiesfollows. The processing of starch and cellulose is alsocovered because of the extensive use of these naturalpolymers in biodegradable blends.

3.1 Chemical synthesis of polyesters

Polyesters can be synthesized by polycondensation ofthe hydroxy acid or by ring-opening polymerization ofthe anhydrosulphite, the anhydrocarboxylate or thelactone/dilactone. The disadvantage of the stepwisepolycondensation preparation is that water has to beremoved continuously, requiring extreme conditions,and the polymerization requires long reaction times andproduces chains with widely varying length. Ring-opening polymerization of the cyclic lactone/dilactoneis the preferred method of producing high-molecular-weight polyesters of this type. However, the anhy-drosulphites and anhydrocarboxylates can be synthe-sized from a-hydroxy acids and then the heterocycles

TABLE 4. Biodegradable polymers

Type Comments Examples

Polyesters Formed by condensation, ring-opening Poly(a-ester)s, polylactones

polymerization or bacterial synthesis and poly(b/c-ester)s

Polyamides Only structurally modified synthetic Hydroxylated nylon

polyamides are biodegradable

Polyurethanes Only structurally modified synthetic Hydrophilic ether urethanes

polyurethanes are biodegradable

Polyureas Virtually ‘non’-degradable Urea formaldehyde

Polyethers Dissolve if carbon chain is short ; General formula w(wOw(CH2)xw)

n.

but also found to degrade Poly(ethylene oxide) (PEO), x ¼2

Polyanhydrides Degradation thought to be mainly by Poly(bis(p-carboxyphenoxy)alkane

surface erosion anhydride) or PCPX

Poly(orthoester)s Degradation thought to be mainly by Poly(3,9-bis(ethylidene-2,4,8,10-

surface erosion tetraoxaspiroÍ5.5Ë)undecane-co-

hexane diol) or DETOSHU-HD

Polypeptides and proteins Naturally occurring polyamides w(wC(R)HwCOwNHw)n

with different

containing amino acid units R groups and chain lengths e.g.

natural proteins collagen, gelatin

Polysaccharides Basic sugar units joined by Naturally occurring starches and

glycoside linkages ; hydrolysed different forms of cellulose

abiotically and by enzymes



can be polymerized thermally or using nucleophilic- orbase-catalysed reactions. The anhydrosulphites aremore susceptible to ring-opening polymerization thanthe anhydrocarboxylates.

Table 5 summarizes the range of monomers used toproduce polyesters, and polymerizations can be carriedout either in the bulk or in solution. Ring-opening poly-merization (ROP) of the dimeric cyclic glycolide orlactide produces the poly(a-ester) poly(glycolic acid)(PGA) or poly(lactic acid) (PLA) respectively. The con-ditions are sealed, dry containers under reduced pres-sure and at high temperature. A wide range of initiatorscan be used such as tin-, zinc- or aluminium-containingorgano-compounds, e.g. tetraphenyl tin, stannous octa-noate, aluminium iso-propoxide. Also anionic poly-merization catalysts can be used, e.g. potassiummethoxide, potassium tert-butoxide, butyl lithium andmechanisms for the polymerizations have been pro-posed.

The properties of PGA are related to its high molecu-lar weight, highly crystalline structure and the fact thatit is the most hydrophilic of the polyesters. It has a veryhigh melting point (224È226¡C) and much higher Tg(36¡C) than poly(b-ester)s. Its use as a degradable suturematerial and its bulk, mainly abiotic, hydrolytic degra-dation are covered in other sections.

The conformations of both lactic acidand dilactide monomers(HOwC(CH3)HwCOOH)

a†ect the resultant tacticity of the polymer and this hasbeen the basis of many mechanistic studies. There is stillsome confusion about the naming of both the mono-mers and the polymers, as both the D,L and R,S systemsappear in the literature. Some attempt to clarify the

nomenclature seems to be required ; the examples thatfollow are based on published work in which the tac-ticity is determined by application of Bernouillian sta-tistics to monomer sequences found from the 1H and13C NMR spectra of the polymers. Naturally occurringlactic acid is LLA and the pure enantiomeric polymers(PLLA and PDLA) are semicrystalline, but P(DL)LA ismore amorphous, less mechanically strong and has alower melting point (P(DL)LA, Tm\ 130È135¡C;PLLA, P(DL)LA is more processable, asTm\ 180¡C).PLLA has a relatively high (58¡C). Both PGA andTgPLA undergo in vitro and in vivo degradation at similarrates. The hydrolytic scission of PLA is at a slightlyslower rate than for the more hydrophilic PGA. Di†er-ent copolymers of GA and LA are manufactured for avariety of biomedical applications. They are generallymore amorphous than the pure polymers and are moreresistant to hydrolysis if they contain high percentagesof one polymer.

Stereo structure of lactic acid (LA)/lactide (L)

monomers and the poly(lactic acid) or polylactide(PLA or PL) product

lactic acid (or R([) lactic acid) produces iso-I D([)tactic P(DLA).*

* Unless the mechanism for polymerization involves race-mization46 of the asymmetric centre or transesteriÐcation48,49occurs.

TABLE 5. Biodegradable polyesters

Type Monomers Examples of polymers

Poly(a-ester)s a-Hydroxy acid Poly(glycolic acid) (PGA) (R ¼H). Poly(lactic

e.g. HOwC(R)HwCOOH. acid) or polylactide (PLA or PL) (R ¼CH3).

Dimeric cyclic lactide or Copolymers poly(lactic acid-co-glycolic acid)

glycolide undergoes ROP (PLGA)

Poly(b-ester)s or poly(3-hydroxy b-Hydroxy acid, Poly(3-hydroxybutanoate) (P3HB) (R ¼CH3).

alkanoate)s (PHAs) e.g. HOwC(R)HwCH2wCOOH, Poly(3-hydroxyvalerate) (PHV) (R ¼C

2H

5).

or cyclic lactone Copolymers poly(hydroxybutyrate-co-

hydroxyvalerate) mol% HV defined

(PHBV (8% HV))

Poly(c-ester)s HOwC(R)Hw(CH2)2wCOOH Poly(4-hydroxybutanoate) (P4HB) (R ¼H)

or cyclic lactone

Other polyesters or polylactonesPoly(e-caprolactone), n ¼5.

Poly(valerolactone), n ¼4

cyclic lactone



lactic acid (or S(]) lactic acid) produces iso-I L(])tactic P(LLA).*

mixture of D/L-lactic acid producesI Racemicatactic P((D,L)LA).¤

or L,L-dilactide produces isotacticI D,D-dilactideP(DLA) or P(LLA).*45,46

mixture of D,D/L,L-dilactide producesI Racemicpredominantly isotactic P((D,L)LA).47

of D,L-dilactide produces syn-I Diastereoisomerdiotactic P((D,L)LA).45

3(or b)-Hydroxyalkanoic acids with b-alkyl sidechains also exhibit ste-(HOwC(R)HwCH2wCOOH)reoisomerism. The simplest example is 3-hydroxy-butyric acid and again it is important to(R\ CH3w)summarize the notation. Poly(b-ester)s (P3HAs) andpoly(c-ester)s (P4HAs) can be synthesized from the ap-propriate cyclic lactone or produced microbially, andrecent research in both these areas will be consideredseparately.

Stereo structure of 3-hydroxybutyric acid(3-HBA)/3(b)-butyrolactone (3-BL) andpoly(3-hydroxybutyrate) (P3-HB)

naturally occurring 3HAs have R (or D) con-I AllÐguration and microbial synthesis produces iso-tactic P(R)3-HAs.

(R-3-BL) also produces iso-I (R)-b-butyrolactonetactic P(R)3-HB.

mixture ((R,S) or (D,L) 3-BL) producesI RacemicP(3-HB) in which the tacticity depends on thecatalyst used : atactic, isotactic and syndiotactichave been produced.50h52

Poly(e-caprolactone) is synthesized by the ring-opening polymerization of the cyclic e-caprolactonemonomer using similar catalysts to those already men-tioned. The semicrystalline polymer (\60% crystalline)has a low melting temperature even for(Tm\ 60¡C)relatively high-molecular-weight samples (Mw \ 80 000).The glass transition occurs well below ambient tem-peratures ([60¡C) and the polymer has relatively lowtensile strength (23 MPa) but extremely high elongationat break ([700%). The biodegradation of PCL in bothsoil burial and activated sludge tests was found to berelatively fast with rapid weight loss, indicating a bulkrandom chain scission mechanism, but abiotic hydro-lysis occurs more slowly. Low-molecular-weight PCL isused as a processing elastomer for polyurethanes andfor fabric coatings. The high-molecular-weight materialis used as a thermoplastic processing aid for adhesivesand a range of polymers.

¤ Unless the catalyst is stereo-selective, producing isotacticpolymer from one entantiomer.

3.1.1 Recent developments in chemical synthesis ofPHAs. Distannoxane complexes have been used tocatalyse the ring-opening polymerization of opticallyactive 3-butyrolactone (b-BL) to produce high-molecular-weight P3HB. Hori et al.53 report that when1-ethoxy-3-chlorotetrabutylstannoxane was used as acatalyst (catalyst/monomer ratio 1 : 8000, at 100¡Cfor 4 h), a 99% yield of P(R)3HB was obtained. Theprocess was extended to produce stereo-copolymers, bycopolymerization of (R)b-BL and racemic-(b-BL), andstatistical copolymers, from (R)-(b-BL) with e-caprolactone, b-methyl-d-valerolactone and L-lactide.These novel, optically active, biodegradable copolyes-ters were found to have high molecular weights andthose with over 80 mol% (R)3HB have almost the samebiodegradability as PHBV (11% HV) produced micro-bially. The reaction mechanism for (R)-(b-BL) catalysedby distannoxane complexes was shown to proceed bybond breaking between the carbonyl carbon and theoxygen atom of the lactone ring (acyl cleavage), withretention of the conÐguration, and little or noracemization. 1-Ethoxy-3-chlorotetrabutylstannoxanehas also been reported54 as the polymerization catalyst,under similar reaction conditions (100¡C), for (R)-(b-BL)with other four-membered lactones, producing a seriesof P3HAs in good yields. Hori et al.55 have also synthe-sized a copolymer of (R)-(b-BL) with cyclic carbonatesto produce biodegradable poly(ester carbonate)s.

The usefulness of dimethyl, diethyl, dibutyl anddioctyl tin oxide (DMTO, DETO, DBTO and DOTO)for the preparation of syndiotactic poly(b-D,L-butyrolac-tone) was studied by Kricheldorf and Eggerstedt,56mostly in the bulk, but toluene or chlorobenzene wasused in a few experiments. The reaction temperaturehad to be above 50¡C to activate the catalysts, and tem-peratures between 50 and 100¡C were used. DMTOproved to be the least reactive initiator, whereas DBTOproduced high yields and the highest-molecular-weightpolymers and The(Mn[ 80 000 Mw [ 130 000).highest-molecular-weight polymers were obtained atlow monomer/initiator ratios (M : I \ 50È400). Unex-pectedly, high M : I ratios ([600) produced no conver-sion for any conditions used. The highest percentage ofsyndiotactic diads (72%) was found using DBTO at50¡C or with DETO at 100¡C. Flexible, elastic, trans-parent Ðlms were cast from the 63% and 70% syn-diotactic PBL samples, which may have an applicationas packaging materials.

Hirt et al.57 have described the synthesis ofmicrophase-segregated block copoly(ester-urethane)sfrom telechelic hydroxy-terminated poly(PHB-diol)(hard segments) and hydroxy-terminated PeCL or tele-chelic hydroxy-terminated poly(adipic acid)-alt-(1,4-butanediol ; diethylene glycol ; ethylene glycol) (Diorez})(soft segments), with either 2,2,4-triethyl-hexamethylenediisocyanate (TMDI) or methyl (S)-2,6-diisocyanatohexanoate (LDI). High-molecular-weight



copolymers were obtained with or without a catalystand the mechanical properties of the copolymer wereinvestigated. The Young modulus was found to bedirectly dependent on the mol% of crystallizable PHB-diol in the copolymer, increasing with increase in theamount of PHB-diol, which produced a correspondingdecrease in the percentage elongation at break.However, the amount of “soft segmentÏ or diisocyanatehad only a minor inÑuence. The chain length of thenon-crystallizable (soft) segments indirectly incluencedthe morphology and mechanical properties of thepolymer by altering the phase segregation behaviour.

Reeve et al.58 have prepared P(R)(3HB)ÈPeCL andP(R)(3HB)ÈPL diblock copolymers from low-molecular-weight (average DP\ 26) poly(hydroxy-butyrate) stereoisomers. These have been produced bydegradative, acid methanolysis of high-molecular-weight PHB. Macroinitiators were then produced bythe reaction of the puriÐed, dry hydroxy-terminatedunits with aluminium triethyl to form PHB-O-AlEt2species, which were used to catalyse the ring-openingpolymerization of e-caprolactone or lactide. The AÈB-type copolymers produced were characterized by VPO,GPC and 1H NMR. DSC and X-ray di†raction studiesshowed that with short PCL (DP 12) and L-PL (DP 13)chain segments, PCL and PLA crystalline phases didnot form. However, at longer chain length (PCL (DP 38and 51) and L-PL (DP 23)), both segments producedsuperimposed X-ray patterns. PHBÈL-PL diblock(PHB (DP 26) and L-PL (DP 23)), when melted andrapidly quenched, produced a kinetically frozen solidstate mrophology with miscible chain segments and anew (20¡C), but this undergoes phase separation onT gannealing at 55¡C for 24 h.

TransesteriÐcation has been carried out betweenseveral thermally stable polyesters.59,60 Attempts byVert61 using PHB and PLA both in the melt in thepresence of ethylene glycol and in solution usingethanol or dichloromethane with several di†erent cata-lysts have been unsuccessful. However, Hirt et al.62report the preparation of telechelic OH-terminatedP(R)-3-HB and P((R)-3-HB-co-(R)-3-HV) on a semi-preparative scale using transesteriÐcation from the high-molecular-weight polymers. The oligomers havewell-deÐned reactive end groups and are well suited forthe production of high-molecular-weight block copoly-mers by chain extension.

3.2 Microbial synthesis of poly(b-ester)s andpoly(g-ester)s

Poly(3-hydroxyalkanoate)s (PHAs) are biosynthesizedas storage material in plants, from di†erent organic sub-strates, using a wide range of bacteria. It has beenshown that poly(3-hydroxybutyrate) P(3HB) is accumu-lated in the cells of many bacteria under unbalancedgrowth conditions when exposed to an excess of

carbon.63 In the 1970s ICI developed a large-scale fer-mentation process, based on a two-stage batch reactor,to produce P(R3HB) using sugar-based feedstock andthe bacterium Alcaligenes eutrophus.10 The oil crisis wasthe initial incentive for this investment, but as oil pricesstabilized, the high cost of extraction and lack of pro-cessability of this plastic compared with polypropylene(PP) made its production uneconomic.

The properties of PHB, like those of PP, are of athermoplastic material ; Table 6 compares these proper-ties based on the review of Hocking andMarchessault.13 PHB is a sti†er, more brittle polymerthan PP, but hot rolling can reduce the size of thespherulites in its structure and produce a more ductilematerial. However, a nucleating agent is often needed,as the rate of crystallization of all PHAs is very slow,which was recognized as a major disadvantage.Although PHB is insoluble in water, it has poor chemi-cal resistance and undergoes isothermal degradationfrom 170 to 200¡C. A drop in molecular weight toabout half its original value was found when PHB washeld at 190¡C for 1 h. Packaging Ðlms, Ðbres and con-tainers were produced in the ICI evaluation usingmoulding and extruding processes and the Ðlm wasfound to have excellent gas barrier properties.

Further advances in the ICI fermentation processusing di†erent feedstocks (glucose/propionic acid)resulted in the production of statistical copolymers of3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV). Theproperties varied with the percentage of HV, thecopolymers becoming less brittle with more PHV. Asthe mol% of PHV increased, the Young modulus(sti†ness) decreased and the impact strength (toughness)increased. The copolymers had only slightly reducedpercentage crystallinity, but lower melting points andsimilar thermal stability. The copolymers showed evenlower crystallization rates with PHB. The biodegrad-ability of these polymers, using bacteria in a variety ofmedia, was an important consideration when in 1983ICI, as Marlborough Biopolymers Ltd, started the Ðrstcommercial production of a range of PHBVBiopol},

TABLE 6. Comparison of properties of PHB and PP

Property PHB PP

Molecular weight (g molÉ1) 5 Ã105 2 Ã105

Crystallinity (%) 80 70

Crystalline melting 175 176

temperature (¡C)

Glass transition 4–0 É10

temperature (¡C)

Density (g cmÉ3) 1·25 0·91

Tensile strength (MPa) 40 38

Extension at break (%) 6 400

Ultraviolet resistance Good Poor

Solvent resistance Poor Good



polymers. The advantage of a renewable feedstock wasone of the attractions of the process, but the artiÐciallyhigh price of fermentation sugars within the EC made itimpossible for the implementation of commodity-scaleproduction, although a small “high-added-valueÏbusiness was envisaged.

Biopol is now manufactured by Monsanto and atabout £8 per kilogram is about 10 times the price of PE.Biosynthesis of a range of homo- and copolymers ofPHAs is now possible using a wide range of microbesand feedstocks in the fermentation broth. Recently,Monsanto and other workers have been investigatingthe production of PHAs from transgenic plant cells. Asuspension culture of transgenic Arabidopsis thalianaplant cells has been shown to produce PHB with chemi-cal structure identical to that produced by bacteria.64Genetic engineering of the DNA of both arabidopsisand oilseed rape, to introduce the genes known toproduce PHB in microbes such as Alcaligeneseutrophus, has been investigated by a Monsanto/Durham University research collaboration.65 Pro-duction of high-molecular-weight PHAs occurs in boththe leaves and seeds of these plants. The aim is toobtain both the oil and polymer as cash crops from theoilseed rape. It has been estimated that 20% of the seedweight as PHA is needed for commercial viability andto achieve this workers have increased the productionof the naturally occurring enzymes in the plant used tomake the polymer. A method of turning o† the pro-duction of the PHAs in the leaves has also been devisedto concentrate the production in the seeds.

PHBV statistical copolymers with HV compositionup to 90 mol% have been synthesized microbially andcharacterized by several workers.63,66,67 Using Alcali-genes eutrophus in nitrogen-free conditions, with glucoseand propionic acid as cosubstrates, PHBV with up to47 mol% HV was produced.13 A recent paper by Chunget al.67 has suggested a method of overcoming theproblem of the inhibited growth of Alcaligeneseutrophus, at concentrations of propionic acid above0É5 g l~1 in the fermentation broth, by pH regulation.However, Doi et al.68 have reported PHBV with up to90 mol% HV using Alcaligenes eutrophus and feedstocksof di†erent ratios of pentanoic acid and butyric acid.

Although in 1988 ICI was manufacturing PHBV(0%È25% HV) of di†erent grades, from technical andstandard (95%È96% purity) to specially puriÐed gradesfor medical uses ([99É5% purity), the economic via-bility of the polymers was never really established.Hocking and Marchessault13 have comprehensivelyreviewed the potential uses and the three main com-ponents of the cost of PHBV production (carbonsource, fermentation processing/extraction and capital-related charges). As yet there has not been the necessaryeconomic breakthrough for massively increased pro-duction of PHBV. The large-scale manufacture ofBiopol by Monsanto now concentrates on a more

limited range of PHBVs, with PHBV (8 mol% HV) asthe major product. However, recent research pub-lications indicate that there is still considerable interestin novel and economic methods for both the microbialand the chemical synthesis of PHAs.

3.2.1 Recent developments in microbial synthesis ofPHAs. Two papers identify novel routes to the pro-duction of PHB and PHBV copolymers. Eschenlauer etal.69 have produced a working model for the synthesisof PHBV from a single carbon source (glucose) viaacetyl- and propionyl-coenzyme A, by expressing thePHA biosynthesis genes from Alcaligenes eutrophus inEscherichia coli strain K-12 under novel growth condi-tions. Maness and Weaver70 reported a process tophotoconvert low-grade organic material waste biomassinto PHB and PHBV. Unique strains of photosyntheticbacteria were contained in thermally gasiÐed drybiomass and under unbalanced culture conditions theseproduced up to 28% new cell mass of granular, high-molecular-weight PHAs. Extraction showed that thedominant monomeric unit was 3-HB, but the use of agreen alga as coculture with the photosynthetic bacte-rium, in lightÈdark cycles, produced PHBV (30% HV)as 18% new cell mass.

Pure P3HV production was originally reported bySteinbu� chel et al. in 199371 using the bacteriumChromobacterium violaceum and valeric acid as the solecarbon source in limited nitrogen. More recently, Stein-bu� chel and Schmack72 have improved this process byusing complex nutrients as supplements. A yield of 40 gof dry cell matter per litre of fermentation broth wasobtained and 70%w/w of the cells were found to beP3HV.

Gross et al.73 have carried out extensive research onthe biosynthesis and characterization of PHAs usingPseudomonas oleovorans and sodium salts of n-alkanoicacids as the carbon source. PHAs containing at leasttwo major ([5 mol%) monomer units were obtainedand, depending on the carbon source, n-alkyl- pendantgroups from methyl- to nonyl- were found. Maximumcellular yield and polymer content (percentage cellulardry weight) obtained were 1É5 g l~1 and 49% respec-tively using nonoate as the carbon source. GPC dataindicated that the polymers had between 160 000Mwand 360 000, whilst DSC and X-ray di†raction measure-ments showed that the polymers with longer side chains(ºn-pentyl) were crystaline polymers from 45 to(Tm61¡C), with participation of both main and side chainsin a layered packing order. Those with n-propyl- andn-butyl- side chains showed little tendency to crystallize.A more recent paper from the same group by Peres andLenz74 has reported the use of this microbial synthesisof optically pure copolymers. A copolymer containing85 mol% b-hydroxyoctanoate units was converted into97% optically pure (S)-b-pentyl-b-propiolactone (S-PPL) and then polymerized by a ring-opening process,



using aluminoxane and zinc alkyl catalysts, into stereo-regular poly(b-hydroxyoctanoate). The diad stereoche-mical sequence in the polymer was determined using50É3 MHz 13C NMR spectroscopy and the polymerssynthesized were also characterized by comparing theirDSC proÐles with those of racemic and optically activeforms of poly(b-hydroxyoctanoate).

De Konig75 has recognized two major problems withthese PHAs : their tendency to soften and lose theircoherence at temperatures as low as 40¡C and their lowcrystallization rates limiting the practicability of manyprocessing techniques. However, crosslinking was pro-posed as a solution and successfully implemented usinga carbon source that introduced unsaturated bonds intothe polymer. Crosslinking was achieved using electronbeam irradiation and produced a true rubber with onlychemical crosslinks. The properties were constant from

(30¡C) to the decomposition temperature (170¡C) andTgthe degree of crosslinking was controlled by the fractionof unsaturated monomer units used and the radiationdose. The enzymatic degradation rate of these cross-linked PHAs was similar to that for the unmodiÐedpolymers and characterized by surface erosion. Theapplication of these PHAs as latex to substrates such aspaper and the use of heat, ultraviolet light or an elec-tron beam source to establish crosslinking have beenproposed.

Inoue et al.76 have recently published a paper on anextension of Satoh and co-workersÏ77 research into theuse of activated sludge from wastewater treatmentworks, developed for anaerobicÈaerobic biologicalphosphate removal, for PHA synthesis. Activated sludgeis a valuable biomass source, but currently it is exces-sively produced and the majority is disposd of as land-Ðll without reuse. Under optimum conditions it hasbeen shown that it is possible to accumulate PHAs ofmore than 30% dried weight of sludge. Satoh et al.78have investigated an anaerobic take-up mechanism ofacetate, propionate and lactate by the sludge developedin the phosphate removal system. The latest research76examines the microstructure of the PHAs producedfrom this sludge, using propionate as the carbon source,with 13C NMR spectroscopy and identiÐes four types ofmonomer unit : 3HB, 3HV, 3H2MB (3-hydroxy-2-methylbutyrate) and 3H2MV (3-hydroxy-2-methyl-valerate). It is suggested that these are essentially sta-tistical copolymers, although some diad sequencescould be allotted. It was concluded that there was noclear evidence for true four-monomer-componentcopolymers.

Neither of the two common bacteria used to producePHAs (Alcaligenes eutrophus for PHB and PHBV pro-duction and Pseudomonas oleovorans for longer-side-chain PHAs) can be used to produce copolymers ofhydroxybutyrate (HB) and other monomers with highercarbon number than valerate. However, Caballero etal.79 have biosynthesized and characterized PHB-co-

HC copolymers (2È4 mol% hydroxycaproate (HC)) andPHB-co-HC-co-HO terpolymers (hydroxyoctanoate(HO) units). The polymers were produced in good yieldusing Comamonas testosteroni, bacillus cereus and anunknown third organism when grown on caproate oroctanoate. X-ray analysis and quantitative analysis ofthe melting point depression showed that the minorcopolymer was rejected by the PHB crystallites. Thereduced melting point but high degree of crystallinityshown by these materials leads the authors to suggesttheir use as melt-processable thermoplastics.

Doi80 and co-workers have carried out extensiveresearch on the synthesis and characterization of micro-bial polyesters. Most recently they have reported workon the microbial synthesis of copolymers of [R]-3-hydroxybutyrate (3HB) and 4-hydroxybutyrate (4HB)from various substrates by Alcaligenes eutrophus, Alcali-genes lotus and Comamonas acidvorans.81 The P3HB-co-4HB copolymer composition varied from 0 to100 mol% 4HB depending on the micro-organism andthe carbon source mixture. The thermal and physicalproperties of the copolymers over the whole composi-tion range were investigated and the results based onthe table from the paper are summarized in Table 7.The copolymers exhibit a wide range of materialproperties, ranging from crystalline plastics to elasticrubbers, depending on composition. The crystallinity,shown by X-ray di†raction, decreased from 60% to40% as the 4HB content increased from 0 to 27 mol%and the samples all had P3HB crysal lattices. However,the copolymers with [64 mol% 4HB showed increasedcrystallinity (from 15% to 34%) with increasing mol%4HB and had P4HB crystal structure. The copolymerswith high 4HB content ([64 mol% 4HB) exhibit thecharacteristics of thermoplastic elastomers, with tensilestrength increasing from 17 to 104 MPa as the mol% of4HB increases, whereas for low mol% 4HB the tensilestrength decreases from that of pure P3HB (43 MPa) to24È26 MPa for 10È16% 4HB in the copolymer. Theincreasing Ñexibility of these materials is reÑected in theincrease in elongation at break over the same 4HBrange (0%È16%) from the value for the very brittleP3HB (5%) to 444% (16% 4HB copolymer). Theremarkable biodegradation characteristics of thecopolymers in di†erent environments were also reportedin this paper and will be reviewed later.

3.3 Natural biodegradable polymers

Abundant naturally occurring, biodegradable polymerscan be processed into useful plastic materials and usedto supplement blends of synthetic and microbial poly-mers. The polysaccharides starch and cellulose havebeen used most extensively. Cellulose is a constituent ofwoody plant material and is described as a complexthree-dimensional, biopolymeric composite. It consistsof repeating glucopyranosyl units. There are two groups



TABLE 7. Physical and thermal properties of P(3HB-co-4HB) copolymers at 23ÄC

Property 4HB fraction (mol%)

0 3 7 10 16 27 64 78 82 90 100

Melting temperature 178 172 130 50 49 52 50 53

Tm

(¡C)

Glass transition 4 É2 É7 É35 É37 É39 É42 É48

temperature Tg

(¡C)

Crystallinity (%) 60 55 50 45 45 40 15 17 18 28 34

Density (g cmÉ3) 1·25 1·23 1·23 1·23

Water uptake (wt%) 0·32 34 0·20 0·14 0·45

Stress at yield 4 28 19 14

(MPa)

Elongation at 28 5 7 17

yield (%)

Tensile strength 43 45 24 26 17 42 58 65 104

(MPa)

Elongation at 5 242 444 591 1 120 1 320 1 080 1 000

break (%)

of cellulose-based materials used on an industrialscale.82

cellulose, which is suitable only forI RegeneratedÐbre and Ðlm production and cannot be moulded.

modiÐed cellulose, e.g. cellulose estersI Chemically(CEs) (cellulose acetate (CA), cellulose acetatebutyrate (CAB)). These only biodegrade undercertain circumstances as the three hydroxylgroups on the glucopyranosyl rings are replacedby the more hydrophobic ester groups, and it isimportant to know the degree of substitution (DS)of a CE. CEs are thermoplastics, but as hydrophi-licity retards the rate of thermoplastic processing,these compounds are insoluble in water. A studyof ether-substituted cellulose showed that biodeg-radation is associated with C-2 hydroxyl groupson contiguous repeating glucopyranosyl rings,with preferably a Ðve- or six-segment run of C-2unsubstitution.83

Research has been carried out on bacterial cellulose(BC) and water-soluble polymer (WSP) based on cellu-lose. Tajima et al.84 report the production of bacterialcellulose from Acetobacter xylinum which has bothgood mechanical strength and biodegradability. Thishas been achieved by the incorporation of WSPs, e.g.carboxymethyl cellulose (CMC) and methyl cellulose(MC), in the incubation medium of the bacterium. Therapid biodegradation of the BC (CMC) product wasshown both enzymatically, using cellulase, and in soil.

The biodegradation and non-toxicity of CMC havebeen tested by other workers,85 as it can be found inwastewater treatment works. The continuous-Ñow acti-vated sludge (CAS) test simulates sewage treatment andCMC (DS 0É7) added to raw sewage was partly

degraded by micro-organisms in the bioreactor. Aquatictoxicity tests on the effluent and CMC intermediatesproduced by a pure culture were negative.

Citric esters have recently been introduced as biode-gradable plasticizers by Gross and co-workers. Theyhave reported86 that cellulose acetate formed miscibleblends (based on with both triethyl citrate and acetylTg)triethyl citrate. The e†ect of an increase in the amountof plasticizer was to reduce the tensile modulus, increasethe elongation at break and increase the rate of biodeg-radation.

Starch is a homopolymer of glucopyranose units butconsists of two a-types of polymer :

linked essentially linear amylose ;I a-(1,4)and a-(1,6) highly branched amylopectin.I a-(1,4)

The botanical and genetic background of the starcha†ects the branch chain lengths, chemical structure andfunctional properties. It is an enormous source ofbiomass and in most applications semicrystalline, nativegranule starch is either destroyed or reorganized. Wateris frequently used as a plasticizer ; the percentage ofwater has a great e†ect on the physical properties of thestarch and therefore its uses. Recently, other plasticizers,low-molecular-weight alcohols, have been used for theproduction of thermoplastic starches.87

There are three ways starch can be used as a biode-gradable material.

1. High-amylose starch is used directly as a biode-gradable plastic and can be fabricated as strongÐlm.

2. Small-granule starch can be used as a Ðller forPE Ðlm (6%È15%) with its original structure andgranular form. If antioxidants are present in the



mixture, peroxides are produced and this cleavesthe polymer chains, degrading the starch.

3. High-starch-containing mixtures (40%È60%) usea destructurized form of the polymer. Starch pro-cessing can involve di†erent procedures, e.g. heatprocessing, pre-gelatinizing (this produces cold-water-soluble starches which can be dispersedwithout heat treatment) and chemical modiÐ-cation (to produce hydrophobic starch deriv-atives or dialdehyde starch, which can formcrosslinks between starch and components withfunctional groups).88

4 BIODEGRADABLE POLYMER BLENDS

The blending of biodegradable polymers is a method ofreducing the overall cost of the material and o†ers amethod of modifying both properties and degradationrates. Miscibility is usually characterized by a singleglass transition in the blend, which varies with composi-tion and is measured by DSC or DMTA. The advan-tages of producing miscible blends include single-phasemorphology in the melt and reproducible mechanicalproperties. However, forming a miscible blend, particu-larly with a non-biodegradable polymer, can reduce oreven inhibit the degradation of the biodegradable com-ponent ; e.g. PHB and poly(vinyl acetate) (PVAc) formmiscible blends,89 but even those containing a highwt% of PHB undergo only slight enzymatic degrada-tion.90 Immiscible blends have the disadvantage ofhaving properties that are dependent on the blend mor-phology produced by processing and these are often notreproducible. However, some can show higher biodeg-radation rates than the unblended biodegradablepolymer(s) ; e.g. PHB/PHBV (76 mol% HV) and PHB/poly(1,4-ethylene adipate) (PEA) blends are both immis-cible, but the rate of enzymatic degradation of blendÐlms was higher than for each single-component Ðlm.91

4.1 PHA-based blends

PHA blends have been reviewed by Verhoogt et al.92and Scandola90 and some of their conclusions areincluded along with recent research by other workers.

4.1.1 PHA/synthetic non-biodegradable polymer blends.Hocking and Marchessault13 identify a range of syn-thetic polymers which have been found to be misciblewith PHB, including poly(vinyl acetate) (PVAc)89 andpoly(vinyl chloride) (PVC).93 A recent study of PHBV/PVC blends94 has produced DSC and DMTA resultswhich show that PHBV (8 mol% HV) and PVC haveseparate values for all compositions. However,TgPHBV (18 mol% HV)/PVC blends were miscible with asingle and a melting point depression for the wholeTg

composition range. FTIR studies revealed shifts in theabsorbance for the CwOwC stretching vibration andthe CHwCl deformation, indicating speciÐc intermolec-ular attractions between the two components. Anincrease in the PVC content of the blends increased thetensile strength and Young modulus of the blends butdecreased the percentage elongation at break.

PHB/poly(methyl methacrylate) (PMMA) blendshave been studied by Lotti et al.,95 who have reportedcompatible blends of PHB/PMMA up to 20 wt% PHBwhen prepared by melt compounding followed by quen-ching to room temperature. Single-phase amorphousglasses with composition-dependent single areTg sformed. Above 20 wt% PHB (the solubility limit forPHB in PMMA), partially crystalline PHB coexistswith the constant-composition PHB/PMMA (20 : 80)mixture. Crystallization of PHB is retarded in theseblends by increasing amounts of PMMA both from themelt and when heated from the rubbery state. Thegroup found poly(cyclohexyl methacrylate) (PCHMA)to be immiscible with PHB.

More recent work by Scandola and co-workers90,96conÐrms these Ðndings and also investigates the misci-bility of other blends, including poly(epichlorohydrin)(PEC) which is a low-temperature rubber (Tg \[21¡C),and PHB. The mixtures had to be solvent cast initiallybut were then melt processed, quenched in liquid nitro-gen and stored at room temperature for at least 3 weeksbefore testing. The blends were shown by DSC andDMTA measurements to be miscible, with andTm *Hmboth increasing linearly with increasing wt% PHB. Thesingle glass transition temperature of the blends isbelow room temperature for all compositions, so PHBpartially crystallizes out at room temperature, and theincrease in with increasing wt% PHB can be*Hmrelated to a constant fraction of this PHB crystallizing(58%). A space-Ðlling spherulitic morphology developsfrom the melt, with the radial growth rate decreasingwith PEC content.

Biodegradation studies, using wastewater activatedsludge exposure for up to 1 year, were carried out onsome of the miscible blends (PHB/PEC90,96 and PHB/PVAc91) and showed a relationship between the ofTgthe blend and its biodegradation rate. Kumagai andDoi91 report that PVAc and PHB(Tg \ 38¡C) (TgD

form miscible, non-biodegradable blends (i.e. even0¡C)the biodegradation of the PHB is inhibited), but Scan-dola found that PEC (80 wt% PHB)(Tg [ 21¡C)/PHBblends showed progressive weight loss and surfaceerosion. SEM indicated that localized erosion spreadover the sample surface with increased exposure time.He concluded that PHB/PEC blends, which havespherulitic morphology and lower values than pureTgPHB, have sufficient mobility in the interlamellaramorphous mixed phase to allow degradation to be ini-tiated, but in blends with higher values insufficientTgmobility inhibits biodegradation.



4.1.2 PHA/synthetic biodegradable polymer blends.Several papers have been published on PHA blendswith other biodegradable, synthetic polyesters.PHB/PEO blends are both miscible and show rapidrates of enzymatic degradation, but this is mainlycaused by elution of the PEO into the aqueous bu†erproducing large holes in the surface of the blend, accel-erating the enzymatic hydrolysis of the PHB.97

The e†ect of stereochemistry on the degradation andmorphology of poly(lactic acid) has been mentionedpreviously : isotactic PLLA is a semicrystalline formwhich undergoes rapid non-enzymatic hydrolysis,although there have been recent reports98 of someenzyme hydrolysis ; atactic P(D,L)LA is amorphous andagain is mainly hydrolysed abiotically. Two reports onthe blending of PLLA with PHB99 and PHBV100 haveproduced interesting results. Blumm and Owen99 inves-tigated the spherulitic structure, growth rates andmelting behaviour of PHB/PLLA 159 400 and(Mn1759) blends using polarized light microscopy. Resultsindicated complete miscibility in the melt, over thewhole composition range, for the PHB/low-molecular-weight PLLA blends, but with the high-molecular-weight PLLA, biphasic separation occurred in the meltand two types of spherulite formed on cooling. Someblends showed interpenetration of spherulitic growth.Iannace et al.100 have concluded that PHBV/PLLAblends show partial miscibility based on calculationsand their mechanical properties.

Koyama and Doi101 have investigated the morphol-ogy and biodegradability of binary blends of P(R)-3HB

300 000) and P(R,S)LA 9000 and 21 000).(Mn (MnSolvent-cast Ðlms were prepared and aged for at least 3weeks at room temperature. Miscibility in the melt andin the amorphous state was demonstrated by single T gvalues between that of pure PHB (4¡C) and that of purePLA (44¡C) from DSC results for the PHB/P(R,S)LA(9000). Polarized optical microscopy was used to followthe spherulitic growth rate, which decreased as the PLAcontent increased ; the spherulites were volume Ðlled,indicating inclusion of the amorphous PLA within theirstructure. Biodegradation studies were based on bioticand abiotic conditions. Enzymatic hydrolysis usingextracellular PHB depolymerase (Alcaligenes faecalis)was carried out at 37¡C in phosphate bu†er at pH 7É4for 19 h. The abiotic hydrolysis was carried out for 150days in bu†er (pH 7É4) at 37¡C.

The pure PLA showed no weight loss over 19 h in theenzyme medium, but all the other Ðlms showed massloss (erosion) proÐles which increase with time. The rateof mass loss increased with the wt% PHB in the blend.The non-enzymatic hydrolysis produced no weight lossin the pure PHB over 150 days, but pure P(R,S)LAweighed only 17% of the initial mass. The blendsshowed no initial mass loss, but after an inductionperiod of 14È28 days they started to lose mass. Themass losses from the blends containing 25% PLA (23%

loss) and 50% PLA (48% loss) are almost consistentwith the initial mass of the PLA component, so it maybe that only this component is being hydrolysed.

Molecular weight changes during the abiotic hydro-lysis were recorded. The rate of decrease for PHBhomopolymer was slower than for pure PLA. In theblends the values decreased for both componentsMnmore rapidly. The mechanism of the hydrolysis is dis-cussed more fully later, but the acceleration of the chainscission of the PHB was attributed to the catalytic e†ectof PLA oligomers in the matrix.

The immiscibility of binary blends of PHB with PeCLhas been established by Kumagai and Doi.91 However,this group has reported more recently on the miscibility,morphology and biodegradability of binary blends ofP(R)-3HB 300 000) and copolymers of PeCL-co-LA(Mn

1500È40 000).102 The copolymers were synthesized(Mnusing aluminium tri-iso-propoxide from mixtures ofe-CL and (R,S) lactide monomers. The statisticalcopolymers were analysed using 13C NMR, and enzy-matic hydrolysis was carried out in the presence of alipase (Rhizopus delemar). It was found that enzymatichydrolysis of the copolymers was faster than for PCLhomopolymer. DSC measurements showed that misci-ble blends of PHB/PeCL-co-LA were prepared usingamorphous PeCL-co-LA ranging from 30 to 100 mol%LA. The spherulites were volume Ðlled in the miscibleblends and the rate of spherulitic growth decreased withincreasing wt% of copolymer. Enzymatic hydrolysisusing extracellular PHB depolymerase (Alcaligenesfaecalis) was carried out at 37¡C in phosphate bu†er atpH 7É4 for 19 h and, as with the studies using PHB/PLAblends, the rate of hydrolysis increased with increase inthe PHB component.

Avella et al.103 have carried out blend studies onPHB/poly[(D,L)lactide-co-poly(ethylene glycol)] (PELA).DSC measurements showed some shift in the of theT mPHB and a linear decrease in as the wt% of PHB*Hmdecreased. values for the blends were found in theT grange 10È30¡C (PEG, and PHB spherulitesT g\ 30¡C)were found in blends with º60 wt% PHB.

4.1.3 PHA/natural polymer blends. Blends of PHB andPHBV with cellulose that has been chemically treatedto produce cellulose esters (CEs) are common. The CEshave thermoplastic properties, e.g. cellulose acetate (CA)and cellulose acetate butyrate (CAB), but are non-biodegradable. Scandola and co-workers90,104 reportedthe formation of miscible blends of P(3HB) with CEs bymelt compounding. Blends with ¹50 wt% PHB witheither CAB or CAP produced transparent, stable,homogeneous, amorphous glasses, while blends withhigher PHB content were partially crystalline andbecame opaque upon room temperature storage.104 Thedata on the degree of substitution of the CEs and their

in Table 8 are taken from ScandolaÏs paper.90 ThisTg spaper also presents graphical evidence of the linear



TABLE 8. Composition and glass transition temperatures of polymers

used in work by Scandola90

Polymer Degree of substitution Tg

(¡C)

Bu- Pr- Ac- Et-

PHB 2–4

Cellulose tributyrate (CTB) 3·00 81

Cellulose acetate butyrate (CAB-1) 2·58 0·36 103




Cellulose acetate propionate (CAP) 2·39 0·37 146

Ethyl cellulose (EtC) 2·3 144

variation in glass transition, temperature, measured byDMTA, with composition of the blends of highly substi-tuted CAP, CAB-1 and CTB with PHB (for 0È50 wt%PHB). This provides evidence, based on WoodÏs equa-tion,105 that the blends are miscible in the melt in theseproportions, and as the for all these blends areTg shigher than room temperature, the blends stay as stable,homogenous glasses. When the PHB content increasesabove 50 wt%, the decrease in to below room tem-Tgperature causes the PHB to partially crystallize out ofthe rubbery blend. Partially crystalline PHB/CE blendshave spherulite morphology, but the crystallization rateof the PHB is reduced with increasing wt% CE becauseof the intimately mixed macromolecules. Scandola90also reports the non-biodegradability of PHB/CEblends even with high wt% PHB (no weight loss orsurface erosion after 1 year in activated sludge for PHB/CAB-1 (80 : 20) blends ; cf. PHB/PVAc blends reportedby Kumagai and Doi97). The amorphous phase com-position of partially crystalline 80 : 20 PHB/CAB-1 is58 : 42 and this Ðlls the interlamellar space between thepure PHB spherulites. The non-biodegradability of thePHB is explained as before by insufficient mobility inthe amorphous phase, which prevents the known prefer-ential hydrolysis that occurs with PHB degradation,and so subsequent enzyme attack on the lamellar PHBis prevented.

Unlike the cellulose esters, ethyl cellulose (EtC), witha similar to that of CAP, forms only immiscibleTgblends with PHB when they are melt blended at 195¡Caccording to Scandola.90 The PHB in low wt% PHBblends of PHB/EtC crystallizes out quantitatively onquenching and DSC curves on the second scan showthe characteristic transitions of the two pure com-ponents. However, Zhang et al.106 report thatPHB/EtC blends appeared by solvent casting (60, 40and 20 wt% PHB) and annealing (86, 60, 40 and 20 wt%PHB) showed composition-dependent glass transitions :

decreased with increasing wt% PHB. However, meltTgquenching or DSC cooling runs resulted in detection ofonly the of pure PHB (5È9¡C). Changes in the hydro-Tg

gen bonding of the hydroxyl groups in the EtC weredetected by changes in their FTIR absorption, as inter-molecular hydrogen bonding between EtC and PHBcarbonyl groups changes to intramolecular EtC hydro-gen bonds.

Ceccoruli et al.107 have also looked at the e†ect ofthe plasticizer di-n-butyl phthalate (DBP) on CAB/PHBblends. The DBP is miscible in all proportions withboth CAB and PHB, showing variation in with com-Tgposition. Addition of a Ðxed amount of DBP plasticizerto CAB/PHB blends (0È100 wt% PHB) caused a signiÐ-cant decrease in the of the binary polymer blends.TgThe depression increases with the DBP content inTgthe ternary blends. In the two-component blends ofPHB/CAB (\50 wt% PHB) the value of the single Tgdecreases linearly with increasing wt% PHB (pureCAB-1, 50 : 50 CAB/PHB,Tg\ 115¡C; TgD 50¡C).However, there is also a characteristic relaxation in theDSC trace at lower temperatures which was associatedwith mobilization of the component (PHB).lower-TgConcomitant with the expected “plasticizingÏ e†ects ofDBP on it also induces a decrease in the character-Tg ,istic temperature for these additional low-temperaturetransitions.

Yasin and co-workers108 have studied two- andthree-component blends of Biopol with CEs or PCL orPLA and biodegradable plasticizers. Three-componentphase diagrams were produced based on the opticalproperties of the blends and their glass transition char-acteristics. Compatibility was achieved over a limitedcomposition range (30 wt% CAB, 40 wt% poly(esterglutarate) (P7092)) for PHBV/CAB/P7092 blends basedon the optical clarity of these melt-pressed Ðlms andDSC measurements. However, for blends with low wt%CAB and high wt% PHBV, translucent and opaqueÐlms were formed and it was concluded that the blendswere partially or totally immiscible. Increased crys-tallinity was found in Ðlms that were melt pressed com-pared with solvent-cast Ðlms and this a†ected theirmechanical properties (melt-pressed Ðlms have greatertensile strength). High wt% CAB increased the tensile



strength but decreased the Ñexibility of the Ðlms. PCL/CAB/poly(ester glutarate) blends showed miscibility forall ratios of plasticizer with very high wt% CAB.However, there was little miscibility for blends with lessthan 50 wt% CAB. Blends of PHBV/PCL/poly(esterglutarate) showed no compatibility.61

Verhoogt et al.109 have reported on PHBV (12% HV)blends with thermoplastic starch. Ramsay et al.110 hadpreviously established that blends of granular starchand PHBV (19% HV) had increased biodegradationrates but inferior mechanical properties compared withpure PHBV and formed a composite rather than a trueblend. The Verhoogt group used thermoplastic starchplasticized with water and glycerol and processed in dif-ferent ways. The possible advantages of gelatinizedstarch were that the starch can be deformed and distrib-uted during blending and, by using glycerol/waterinstead of just water for gelatinization, reprocessing ofthe thermoplastic at elevated temperatures is possible.True blends were obtained, but they were brittle despitethe Ñexibility of the starch. SEM studies showed irregu-lar particles thought to be caused by viscosity di†er-ences.

4.1.4 Polymer blends of PHAs. The improved biodegra-dation of PHAs blended with other biodegradable poly-mers, particularly other PHAs, compared with that ofpure PHAs has been established by several workers.Pearce and Marchessault111 have studied the meltingand crystallization of PHB/PHV and PHBV blends tocompare phase separation with that in PHBV copoly-mers, which form isodimorphous crystals. They showedthat in the melt, PHB/PHV blends are phase separated,which has been conÐrmed by Gassner and Owen.112The mechanical behaviour of random copolymers ofPHBV was compared with that of blends of almost thesame composition and found to be markedly di†erent.

Blends of synthetic atactic PHB (a-PHB as 10È50 wt%) and bacterial isotactic PHBV (10 mol% HV)were prepared as solvent-cast Ðlms by Scandola etal.113a The blends were found to be miscible in the meltwith decreasing crystallinity as the a-PHB contentincreased ; this also increased the elongation at break ofthe 50 : 50 blend by a factor of 30 over pure PHBV.Abiotic and biotic hydrolysis studies were carried out.Enzyme hydrolysis was accelerated in the blends com-pared with PHBV. The pure a-PHB did not biodegrade.HPLC identiÐed 3-HBA monomer and dimer as degra-dation products of the blends and this was thought tobe a factor in their subsequent enzyme degradation.However, more recent work113b has indicated that thenon-biodegradability of a-PHB is related to its amorp-hous character and the PHB depolymerase can onlydegrade PHB when a crystalline binding site is present.

Satoh et al.114 have studied the hydrolysis (in phos-phate bu†er at pH 7É4 and 55¡C) of PHB/PHBV(22 mol% HV) blends. DSC studies on the original

polymer indicated that PHB was the main componentof the crystalline phase, and the crystallinity decreaseswith increasing wt% PHBV in the blends. There wascorrelation between an accelerated degradation rate anda decrease in the degree of crystallinity in the blends.

MarchessaultÏs group have also investigated themelting behaviour of blends of PHB with di†erent tac-ticity and using solvent casting, melt crystallization andrapid co-precipitation procedures.115,116 Solvent-castÐlms of isotactic PHB and atactic PHB were found notto be fully miscible prior to Ðrst melt for DSC scan ;both annealing and recrystallization behaviour wereidentiÐed. The work on bacterial and partially isotacticPHB blends showed that co-precipitation producedcompatible blends, whereas atactic PHB was not com-patible with bacterial PHB. However, blends withatactic PHB[ 60 wt% exhibited a signiÐcant drop inthe melting point of the bacterial PHB.

Abe et al.117 related stereostructure to melting behav-iour by studying a range of blends of microbial stereore-gular P(R)-3HB and synthetic stereo-copolymers (P(R,S)HB), both atactic and syndiotactic forms. The of theTgblends showed little variation (5 ^ 2¡C) but the meltingtemperature decreased from that of P(R)HB (191¡C)with increasing wt% P(R,S)HB (174¡C for 75 wt% P(R,S)HB), suggesting that the polymers are miscible in themelt and in the amorphous state and the degree of crys-tallinity decreases with increasing wt% P(R,S)HB. Enzy-matic degradation of the blends in phosphate bu†er atpH 7É4 and 37¡C was carried out on the homopolymersand the blends. Surface erosion occurred on the bac-terial PHB, but the synthetic polymer showed littlesurface hydrolysis. Blending of the two accelerated thehydrolysis, with the 50 : 50 wt% P(R)-3HB/(P(R,S)HB)showing the fastest erosion rate (similar to the resultswith a-PHB/PHBV blends investigated by Scandolaand co-workers113). HPLC analysis of the degradationproducts suggested that synthetic PHB (atactic andsyndiotactic) is hydrolysed by PHB depolymerase in thepresence of P[(R)3HB] or its degradation products.

Abe et al.118 have also reported the use of the blockcopolymer of atactic P(R,S)3HB and poly(6-hydroxy-hexanoate) (PHH) to compatibilize blends of P(R)3HB/PHH. The AB block copolymer was preparedsynthetically and the addition of small amounts toblends of the two homopolymers reduced the size of thePHH dispersed domains and improved their mechani-cal properties. Enzymatic hydrolysis results showed thatPHH displayed least surface erosion, then pure PHB,and the ternary blend degraded more slowly than thebinary (75 : 25) PHB/PHH blend.

4.2 Blends based on starch and cellulose derivatives

There has been some recent research on blends usingother biodegradable polymeric materials, and thoseinvolving starch and cellulose are relevant to this



review. Pranamuda et al.119 report on the feasibility ofpoly(e-caprolactone) (PCL)/tropical starch blends,similar in type to PCL/corn starch blends, as biode-gradable thermoplastics, but all these blends have lowtensile properties compared with PE, although contin-uous phase dispersion of the starch in the PCL wasobserved in the Ðlms using SEM. The miscibility ofblends of PCL/cellulose butyrate (DSº 2É0) wasobserved using DSC measurements by Nishio et al.,120but the miscibility alters with the nature of the estergroup and with the degree of substitution, and evenbutyl esters with DS \ 1É5 showed poor miscibility withPCL. The fabrication of PLA/native starch blends withlow-molecular-weight PEG as a plasticizer has beenreported,121 with DSC studies and thermogravimetricanalysis carried out.

Mayer et al.122 report that a corn starch (St)/celluloseacetate (CA, DS\ 2É5)/propylene glycol (PG) blendwith St/CA/PG wt% ratio 57 : 25 : 19 has mechanicalproperties similar to those of polystyrene, and these rep-resent acceptable properties for injection-mouldingapplications. Increasing the wt% starch to PG reducesthe tensile strength. The biodegradation of the starchand PG was observed in both soil and compostingstudies, but extended incubations were required todetect any weight loss from the CA in the blends.However, it was reported that in simulated municipalcompost there was 90% weight loss from the pure CAafter 90 days.

The properties of the blends of starch octanoates(OCST1É8 and OCST2É7, DS \ 1É8 and 2É7 respectively)and starch dodecanoate (DODST2É7, DS \ 2É7) withLDPE have been investigated.123 The ester groups actas an internal plasticizer with increasing e†ect as thenumber and size of the grafted fatty acyl chainsincrease. The DODST2É7/LDPE blends showed gener-ally better thermal stability and higher elongation butlower tensile strength and water absorption comparedwith the OCST/LDPE blends. However, the degrada-tion rate of all blends was very low. Another group hasreported di†erent methods of modifying starch beforeblending with LDPE in order to maintain its thermo-plastic properties but increase its biodegradability. Parkand co-workers used Dupont acetic anhy-Surlyn},124dride and three types of ionomers125 to esterify thestarch, with the aim of using the blends for packagingÐlm. Ionomer-treated starches were reported to be morecompatible with LDPE and to give better mechanicalproperties than other blends.

The production of biodegradable Ðlms for food pack-aging was the aim of a study by Arvanitoyannins etal.126 The mechanical and gas/water permeabilities ofextruded blends of LDPE/wheat starch/ethylene acrylicacid (EAA) or PCL were investigated after conditioningat various relative humidities. Both starch and PCLwere found to have adverse e†ects on the mechanicalproperties of LDPE (particularly [30% starch) and

both gas and water transmission rates increased withincreasing amounts of starch/PCL in the blends. EAAacted as a compatibilizer and increased the percentageelongation at break of the blends.

The production of high-strength and high-Young-modulus PE/starch composite Ðlms has been reportedby Nakashima and Matsuo127 using ultrahigh-molecular-weight (6] 106) PE and corn starch pro-cessed by gelation crystallization from dilute solutions.PE/starch compositions of 3 : 1, 1 : 1, 1 : 3 and 1 : 5 weretested and found to have high draw ratios at 135¡C(1 : 5 composition reached 80) and good Young moduli(1 : 5 was 10 GPa up to 1 : 1 with 105 GPa, cf. 70 GPafor aluminium). The starch particles in most compositesremained within the composite Ðlms even at high drawratios. Enzymatic degradation with di†erent fungalspecies on agar nutrients was investigated, and althoughthe biodegradation of the starch and a decrease in themechanical properties of the Ðlms were observed, nodisruption of the PE Ðbrous texture was discernible bySEM.

5 CHARACTERIZATION AND STRUCTURAL

ANALYSIS

In this section a range of the current methods used tocharacterize the properties and determine the structureof biodegradable polymers will be identiÐed. Spectro-scopic studies are used to characterize the compositionof polymers, e.g. Fourier transform infrared (FTIR),Raman, electron spin resonance (ESR), 1H and 13Cnuclear magnetic resonance (NMR) and ultraviolet/visible light spectroscopy. Molecular weight character-istics can be determined by an extensive range ofmethods, e.g. colligative properties, as in vapour pres-sure osmometry (VPO), end group analysis, e.g. quanti-tative 31P NMR,128 as well as viscometry, lightscattering, small-angle X-ray and neutron scattering(SAXS and SANS) and gel permeation chromatography(GPC) or size exclusion chromatography (SEC).

Erlandsson et al.129 have recently published a paperthat indicates that SEC of the degradation products ofsome polymers may not be very accurate. Thermallydegraded and UV-aged PP containing recycled materialwas found to have a changing relationship between itsmolecular weight and retention volume. The MarkÈHouwinkÈSakaruda (MHS) equation o†ers a conve-nient means of determining the molecular weight ofpolymers soluble in organic solvents. For linear poly-mers the MHS parameters are constant over a widerange of molecular weights, but for highly branchedpolymers there are deviations because the hydrody-namic volume is not directly related to molecularweight. The observed deviation in the degraded polyole-Ðns was related to the e†ects of chain branching andincreased polydispersity with prolonged degradation.



Structural analysis mainly involves measurement ofeither bulk or surface properties of polymers in the solidphase. However, studies of crystallization from the meltand from solution, thermal testing and methods ofanalysis for degradation products in the liquid or gasphase are also important.

5.1 X-ray diffraction

Single-crystal X-ray goniophotometry has limited use inthe study of semicrstalline polymers, whereas stretchedÐbre, thin Ðlm and powder X-ray di†raction do haveapplications. Gazzano et al.130 report a recent study ofbacterial P(3HB) Ðlms which were crystallized from themelt at di†erent temperatures up to 120¡C. The di†rac-tion patterns at increasing crystallization temperatures

were seen to di†er more and more from that of the(Tc)original powder, and the quenching range also a†ectedthe relative intensity and peak width of the reÑections.The di†erences were correlated with changes in averagespherulite size (0É1È2 mm) in Ðlms of limited thickness.It was found that when the spherulites were large, thelamellar crystals were strongly oriented parallel to theÐlm plane.

X-ray di†raction studies for examining the crystalstructure of solids have been extended by the develop-ment of small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) or di†raction (WAXD).A typical WAXS curve shows the intensity of X-rayscattering as a function of the di†raction angle and fea-tures relatively sharp peaks for crystalline scatteringand broader “humpsÏ for the scattering from amorphousareas, allowing the degree of crystallinity to be assessed.SAXS is used to determine lamellar thickness, andbetter results are obtained from solution-grown crystalsthan from melt-crystallized polymers. These techniquesare used with computer modelling to Ðnd out aboutcrystal phases, lamellar thickness, degree of crys-tallization, as well as rates of crystallization, meltingand spherulitic growth.

Orts et al.131,132 have used SAXS and WAXD andScandola et al.133 have used WAXS to study the iso-dimorphic systems of PHBV random copolymers. Morerecently, Gross and co-workers134 have used bothX-ray di†raction and WAXS to study the crystallizationbehaviour of predominantly syndiotactic PHB.

X-ray di†raction studies are often used in conjunctionwith other characterization techniques, and Jerome andco-workers135 have employed SAXS, WAXS, TEM,AFM and PCS to study a stable non-aqueous colloidaldispersion formed by poly(caprolactone-co-glycolide)(P(eCL-co-G)) block copolymers in toluene. The poly-merization produced “livingÏ polymers, so there wascontrol on both molecular weight and composition. Amicelle model was proposed consisting of PG core anda PCL corona, with both constituents semicrystalline.SAXS observations indicated that the core is two con-

centric spheres, with the internal sphere containing crys-talline PG.

5.2 NMR spectroscopy

5.2.1 Solid state CPMAS 13C NMR. The advances insolid state 13C NMR allow analysis of speciÐc regionswithin the solid structure. The monomeric ratio forPHBV copolymers can be determined in the amorphousand crystalline regions, and Orts and co-workers132,136report di†erent ratios of HV in the crystalline regions ofPHBV related to the overall crystalline disorder. TheHV incorporation into the PHB-type crystalline phaseswas shown to be over the composition range.

5.2.2 High-resolution 13C NMR spectroscopy. This tech-nique can be used to determine the tacticity of a widerange of polymers and is particularly useful for analys-ing P(3HB). Hocking and co-workers137,138 haveanalysed a range of racemic synthetic polymers with dif-ferent tacticity and interpreted the results based onseveral statistical models. Average isotactic and syn-diotactic block lengths were calculated.

5.3 Microscopy techniques

A range of di†erent microscopy-based techniques havebeen used to study the surface structure and erosioncharacteristics during biodegradation.

5.3.1 Optical microscopy. Isothermal crystallizationkinetics in miscible blends can be examined usingoptical microscopy. Pearce et al.139 have used the tech-nique to study isotactic/atactic PHB blends and seen areduction in the rate of spherulite growth in the iso-tactic polymer.

5.3.2 T ransmission and scanning electron microscopy(T EM and SEM). Lauzier et al.140 have examined thestructure of PHB granules isolated from bacterial callsusing SEM and identiÐed the outer lamellar crystallinecoating and non-crystalline core. Miyata andMasuko141 reported the use of TEM to examine themorphology and structural features of PLLA grownfrom acetonitrile solution by an isothermal crys-tallization technique. Di†raction mode WAXS di†racto-metry and AFM were also used and the packing of themolecular chains within the crystals was investigatedalong with the orthorhombic unit cell parameters.

5.3.3 Atomic force microscopy (AFM). The surfaceerosion of biodegradable polymers in aqueous solutionshas been followed in situ using AFM by Shakesheef etal.,142 showing the versatility of the technique and itsability to achieve resolutions comparable with vacuum-based SEM. This group143 has also achieved sponta-neous acquisition of atomic force microscopy andsurface plasmon resonance data with a combinedAFM/SPR instrument which can be used to study thee†ect of morphology on erosion. This has particular



applications for the degradation of immiscible polymerblends. A study of the degradation of thin Ðlms ofpoly(sebacic anhydride) (PSA) and P(D,L)LA enableddynamic changes in surface morphology during degra-dation to be related to SPR-recorded kinetics of Ðlmerosion. Three stages in the Ðlm erosion at pH 11 weredemonstrated : initial rapid loss of PSA, slow loss ofPLA in the Ðnal stage and an extended transition in theintermediate stage.

5.3.4 Scanning probe microscopy. The use and design ofthese microscopes, for investigating drug deliverysystems, have been reviewed by Shakesheef et al.144These instruments should provide information aboutthe molecular structure of polymeric surfaces, the con-formation of target molecules, the inÑuence of morphol-ogy on biodegradation, the adsorption of proteins ontosynthetic surfaces and the structure and interactions ofcolloidal particles.

5.3.5 Confocal laser-scanning reÑection microscopy(CL SM). Semler et al.145 have used CLSM to visualizeand quantify the microtopography of the surface of theporous matrix of PLA-co-GA copolymers after degra-dation. The surface matrix was reported to change sig-niÐcantly during degradation, with increased localclustering of textured regions.

5.4 X-ray photoelectron spectroscopy (XPS)

Shakesheef et al.146 have shown that XPS provides amethod of tailoring the surface properties of micro-particle drug delivery systems so that the polymerchemistry and fabrication parameters can be related tothe Ðnal surface chemistry of the microparticles. SigniÐ-cant variations in the surface chemistry of micro-particles containing the polymers PLA and PLGA andthe block copolymers PLA-co-PEG and PLGA-co-PEG were identiÐed. These variations were related tothe mechanism for the microparticle/water interface sta-bilization, which is controlled by the interfacial surfacetensions of the polymers within the aqueous environ-ment. Di†erences in the adsorption of a surfactant, e.g.poly(vinyl alcohol) (PVA), were connected to the com-position of the polymer, with PEG copolymers adsorb-ing less PVA because the PEG segments stabilize thepolymer/water interface.

Leadley et al.147 have also reported the use of XPSanalysis to correlate the experimental and theoreticalchemical composition of the surface of drug deliverysystems using biodegradable poly(b-malic acid) and itsester derivatives.

5.5 Differential scanning calorimetry (DSC)

DSC is a standard technique for studying thermalbehaviour in semicrystalline samples of pure polymers,copolymers and blends to determine enthalpy changes

during melting. The process can be used to identifyglass transition and melting temperatures and(Tg Tm) ;melting endotherms can be used to calculate enthalpychanges and show annealing and recrystallization(*Hm)characteristics. The degree of crystallinity of semicrys-talline polymers can be assessed by comparison with

for completely crystalline samples. DSC is fre-*Hmquently used to determine the miscibility of polymercomponents when blended, i.e. single-phase morphologyin the melt, as this is characterized by a single glasstransition of the blend, which varies with compositionand can be measured by DSC or DMTA.

A recent study by Kemnitzer et al.134 looked at crys-tallization in predominantly syndiotactic, syntheticP(3HB) with di†erent degrees of syndiotacticity. Aftermelting and annealing, at least one and often two dis-tinct melting transitions were seen over a broad tem-perature range (often at about 40¡C). This wasinterpreted as evidence of di†erent crystalline regionswith di†erent thermodynamic stabilities. The annealingtemperature was related to the crystallization(Tc)behaviour, with maximum crystallization at Tc \ 30¡Cand partial or complete exclusion of the syndioregularchain segments occurring at From theTc º 45¡C.dependence of the higher melting transition on theTcequilibrium melting temperatures for the di†erentsamples were obtained. Avrami analysis of syn-PHBresults showed similarities to those obtained for micro-bial PHB and so a two-dimensional crystal growthmechanism was proposed.

5.6 Dynamic mechanical testing and analysis(DMTA)

Dynamic mechanical testing involves a torsion pendu-lum in which polymer samples undergo sinusoidal oscil-lations at a Ðxed temperature and known testingfrequency. The transition behaviour of polymers can bestudied as a function of testing temperature, and infor-mation about changes in tan d (where d is the phaseangle or lag for any viscoelastic material) and the shearmodulus gives clear evidence of values (referred(G1) Tgto as the a-transition). Variations in tan d at tem-peratures below the glass/rubber transition of a polymerrepresent other secondary transitions, which can be dif-Ðcult to assign but may be related to phenomena suchas side chain rotation and co-operative motion of seg-ments of the main chain.

ScandolaÏs paper90 presents a clear example ofgraphical evidence of the linear variation in glass tran-sition temperature, measured by DMTA, with composi-tion of the blends of highly substituted CAP, CAB-1and CTB and PHB (for 0È50 wt% PHB). This providesevidence, based on WoodÏs equation,105 that the blendsare miscible in the melt in these proportions, and as the

for all these blends are higher than room tem-Tg sperature, the blends stay as stable, homogenous glasses.



In a previous paper, Celli and Scandola148 relate DSCand DMTA results to the aging of PLLA.

5.7 Mechanical testing

Standard tests for measuring tensile modulus and per-centage elongation at yield and break are used to char-acterize polymers and to study bulk property changesduring degradation.

Hiljanenvainio et al.149 have synthesized copolymersof e-CL/L-lactide and e-CL/D,L-lactide with di†erentcompositions (80È40 wt%) of e-CL in the monomerfeed. The copolymers di†ered widely in their physicalcharacteristics from weak elastomers to tougherthermoplastics. Their properties were compared withthose of the homopolymers and the tensile modulus andtensile strength were higher for the homopolymers thanfor the copolymers. However, the maximum strain,which is very low for the pure PLA polymers and highfor PCL, also showed large elongation for the copoly-mers.

Merloz et al.150 have used static and dynamic mecha-nical testing in vitro for assessing the potential of PLLAfor surgical implants. Traction and Ñexion measure-ments were carried out on bars of PLLA in di†erentenvironments. StressÈstrain results showed that the barswere fragile but una†ected by sterilization procedures.Bars placed in a physiological environment at 37¡Cbecame ductile, and after 1 month at this temperaturethe Young modulus and maximal strain before breakingwere reduced by 50% of their original value.

5.8 High-performance liquid chromatography(HPLC)/gas chromatography (GC)

HPLC is used to isolate and analyse mixtures ofpolymer degradation products. The method can be usedto give biodegradation product proÐles and yield con-centrations for di†erent products, which are required toassess the toxicological nature of the polymers and theirdegradation products. Marcato et al.151 have used thetechnique to identify some of the hydrolytic decompo-sition products of PaHAs. The hydrolysis of PLLA,P(D,L)LA, PGA and PLGA was carried out in astirred, aqueous suspension for 12 h. Peak identiÐcationwas carried out by comparison with pure standards andthe optimum shape and separation of the peaks wereachieved by preliminary studies on the composition andpH of the mobile phase (ratio (v/v) of acetonitrile/phosphate bu†er was 75 : 25 and pH 5É8). PLLA wasfound to be quite stable, PGA degraded easily to GA,whereas P(D,L)LA and PLGA exhibited intermediatebehaviour, producing not only the monomers but alsodimer, trimers and higher oligomers.

Microbially produced PHBV with di†erent compo-sitions was analysed by Nedea et al.152 using NMRspectroscopy and, after partial methanolysis or ammon-

olysis, HPLC and fast atom bombardment mass spec-trometry (FAB-MS). Normalization of the oligomerpeaks provided an estimation of both the copolymercomposition and the monomeric sequence distribution.It was concluded that statistical copolymers had beenproduced.

Wang et al.153 have investigated the in vitro break-down products associated with the biological hydrolysisof a biomedical polyurethane. UltraÐltration, freezedrying and solid extraction were used to remove proteincontamination, and more than 20 degradation productswere separated using gradient HPLC, optimized using aphotodiode detector, and simultaneously identiÐedusing a tandem mass spectrometer (MS).

A qualitative method for determining the mechanismof hydrolytic scission in biodegradable polymers hasbeen developed by Shih.154 The molar fraction of themonomer was determined by 1H NMR or HPLC andthe degree of polymer degradation was also measuredusing proton NMR. Experimental data from di†erenthydrolysis reactions were Ðtted to theoretical curvesand the hydrolysis mechanism was then proposed.

Easily hydrolysable polyesters produce few degrada-tion products, mainly monomer, dimer, etc., but thesecan be obtained by head space GCÈMS and identiÐed.For the degradation of PE, which cannot be carried outby hydrolysis, hundreds of products may be obtainedaccording to the reaction conditions and the use ofadditives, e.g. pro-oxidants and starch. The same ana-lytical technique can be used to Ðngerprint the polymerand the degradation conditions.155

In a quantitative study of intact ester linkages inP(D,L)LA and PLGA, Shih et al.156 have converted theester linkages into amides using n-butylamine, inambient conditions, and then used gas chromatography(GC) to analyse the product mix. The amidation of bothtypes of ester bond was found to be quantitative andwas complete after 3 h for the glycolate and 8 h for thelactate. The values were within 5% of those obtained by1H NMR.

5.9 Other methods of structural analysis

The use of several novel methods of analysis has beendescribed in the literature. Holland and Tighe21 andYasin and co-workers108 report the use of goniophoto-metry to investigate the surfaces of Ðlms and have foundthe technique very sensitive to changes in surfacerugosity. Changes in the relative gloss factor were usedto characterize small changes in the roughness of thesurface of PHBV (20% HV) Ðlms over a period of 72 h.

Foster and Tighe157 used enzymatic assay of HBAmonomer formation as a measure of the degradation ofP(3HB). This technique has applications for forms ofthe polymer e.g. Ðbres and microcapsules, which cannotbe monitored by standard surface or gravimetricmethods. The potential use was demonstrated in the



enzymatic hydrolysis of gel-spun Ðbres of PHB. HBAdehydrogenase is used to hydrolyse the polymer in thepresence of nicotinamide adenine dinucleotide (NAD).This is reduced to NADH and the conversion is associ-ated with an increase in the absorption of light at340 nm, which also serves to indicate the concentrationof HBA monomer in the sample.

6 BIODEGRADATION CONDITIONS AND

TEST PROCEDURES

The deÐnition of biodegradation given by Albertssonand Karlsson1 was quoted at the beginning of thisreview. These biotic changes must be considered along-side environmental factors which may produce chemicaland other abiotic reactions, e.g. oxidation, photo-degradation or hydrolysis.

6.1 Degradation processes

6.1.1 Oxidation. The initial step in the oxidation ofpolymer Ðlms may occur without any biodegradativeattack, although both microbes and fungi may acceler-ate the process. The rate of initial oxidation of polymersis likely to decrease when air is limited, e.g. in the bodyof landÐll sites (1È2 m below the surface). However,certain micro-organisms can utilize oxygen in chemi-cally bound form (e.g. nitrate, sulphate, carbonate) tobiodegrade the polymers in anaerobic conditions. Amechanism for the abiotic oxidation of polyethylene(PE) has been proposed using iron ions in anaerobicconditions.158 A recent paper has also reported that thetoxicity of oxygen free radicals is enhanced by iron. Itsuggests that the human body can therefore producevery toxic hydroxyl radicals, which may be one of themain causes of degradation in implanted polymericdevices.159

The mechanism for the biodegradation of PE waspresented in 1987,160 involving initial abiotic oxidation.Hydroperoxides are introduced into the polymer chainwith a gradual increase in “in-chainÏ keto groups, fol-lowed by a decrease as short-chain carboxylic acids arereleased ; the Ðnal stage involves microbial biodegrada-tion by a b-oxidation mechanism. To elucidate theinitial mechanism further, LDPE Ðlms containing bio-degradable starch and a pro-oxidation formulation wereinoculated with bacteria or fungi and left in an aqueousmedium at ambient temperatures for 1 year.161 Twointeractive mechanisms occurred : the water containingtrace elements triggered autoxidation of the pro-oxidantand, with the synergistic biodegradation of the starch,this initiated the autoxidation of the LDPE, which wasmonitored by chemiluminescence, DSC and CSLM.There are several reasons why the initial degradation ofPE has to be abiotic. The biotic degradation of long-

chain alkanes by the b-oxidation mechanism, which issimilar to that of fatty acids and alkanes in man andanimals, cannot occur enzymatically beyond n-C44because of steric hindrance. The initial step of CwCbond splitting cannot be carried out by any naturalenzyme in either the side or main chain.1

LDPE, polystyrene (PS), poly(vinyl chloride) (PVC)and urea formaldehyde resin (UF) that had been buriedin bioactive soil for over 32 years have been examinedfor biotic degradation.162 Only the LDPE showed anysigns of this, as the surface in contact with the soil hadwhitened and was severely degraded. The inside of theLDPE was still transparent and di†ered in its FTIRspectrum from the surface. The oxidation products inthe clear part were carboxylic acids and ketones,whereas the whitened surface showed some carbonylabsorbance but increased CxC (double-bond) absorp-tion. The usual oxidation mechanism was proposed forthe clear regions and a more complex mechanism pos-tulated for the whitened surface. Under biotic condi-tions the alkoxyl radical scission may proceed througha c-scission mechanism, a form vinyl groups and vola-tile products, in addition to the b-scission mechanism.This group has published more recent results onattempts to accelerate the biodegradation of LDPE.163UV irradiation was used and several oxidizing agents(biodegradation-inducing agents) were added to theLDPE, which was left in bioactive soil for 12 months. Itwas found that UV radiation accelerated the biodegra-dation very e†ectively and that addition of vegetable oil,metal carboxylates (iron stearate) and small amounts ofdicumyl peroxide (DCP) also promoted the degradationof LDPE.

Weiland et al.164 have investigated the biodegrad-ability of thermally oxidized PE using a wide range ofbiotic conditions, including di†erent bacteria on solidagar, various composting units (varying in temperature,moisture content or composting medium) and di†erentliquid media. There was some evidence of oxygenuptake in the respirometric method using liquid suspen-sions of compost micro-organisms and very small con-centrations of substrate. Day et al165 have carried outabiotic and biotic degradation testing on PE-basedÐlms. The results conÐrmed that they were susceptibleto abiotic thermal oxidative degradation at 60¡C, andalthough this temperature was achieved in thelaboratory-scale, aerobic composting test, no degrada-tion was obtained. It was concluded that the test timewas insufficient compared with the induction periodnecessary for the desired thermal oxidation process.

LDPE and HDPE Ðlms have been Ðlled with up to20% by weight of starch and tested for biodegradationunder a wide range of microbial conditions.166 There isevidence of the removal of starch from the di†erentÐlms by massive colonization of various organisms, andthis is accompanied by a small but signiÐcant drop inthe average molecular weight and decrement in mecha-



nical strength of the Ðlms. The composting trial, whichexperienced prolonged severe temperature conditions,produced small but detectable spectroscopic evidence ofoxidation of the PE matrix. The hydrophobicity ofpolyoleÐns is considered to be an obstacle to microbialattack. The incorporation of surfactant onto PE Ðlmshas been shown in a di†erent study to increase the rateof biodegradation.167

6.1.2 E†ect of light and other forms of EMR. Polymersare susceptible to electromagnetic radiation, with bothc-irradiation and light having an adverse e†ect onpolymer properties ; e.g. poly(a-ester)s show reducedmolecular weight. Photo-oxidation has a profounde†ect on the biodegradability of certain polymers. PEwithout photo-oxidation has been shown to degrade byonly 0É2% over a 10 year period, but with 42 days prioroxidation a 2% weight loss was obtained after soil incu-bation for a similar time period.1 Research has recentlybeen presented on the use of a photodegradable coatingon polymeric materials.168 Three polymeric materialswere prepared : starch/PE blend, photoactivator/starch/PE blend and photo/activator/starch/PE coated ingelatin. The results suggested that a more powerfulphoto-oxidizer would introduce control into the initialdegradation. The gelatin coating (which containediron(III) salts) extended the induction period, but after ithad degraded, there was accelerated photo-oxidation.

As c-radiation is a means of sterilization, it is animportant consideration particularly in biomedical usesof these polymers. However, Collett et al.169 havereported a method of minimizing the e†ects by carefulreduction of the oxygen pressure during c-irradiation.Mitomo et al.170 have reported more recently on thee†ects of c-radiation on PHB and PHBV in air or avacuum. They found that polymer scission and cross-linking occurred. The samples irradiated in air showedgreater relative decrease in and number-averageTm , Tgmolecular weight, with less crosslinking. It was foundpossible to radiation graft both PHB and PHBV withMMA and hydroxyethyl methacrylate (HEMA).

6.1.3 Hydrolysis. Chemical and microbial hydrolysismechanisms are the most important biodegradationreactions involving polyesters. ShihÏs graphical methodfor the determination of the mode of hydrolysis of bio-degradable polymers has already been mentioned154and is just one mathematical treatment used to helpdetermine the mechanism of hydrolysis. The methodrequires the determination of the mole fraction of themonomer by 1H NMR or HPLC and the degree of(m1)polymerization (a) by 1H NMR. Kuhn predicted that ifcompletely random scission of backbone bonds occurs,

and if exclusive chain-end unzipping occurs,m1\ a2,If the data fall between the two curves, chain-m1\ a.

end scission is faster than random scission, and for data

that fall below both curves, the chain-end bonds aremore stable than the internal bonds. Shih showed thatacid-catalysed hydrolysis of poly(orthoester)s and basehydrolysis of P(D,L)LA occur by random scission,whereas the acid-catalysed hydrolysis of PLA involvesfaster chain-end scission.

Two controlling mechanisms of polyester breakdownare recognized either at the surface (homogeneous) orwithin the bulk (heterogeneous). The relative contribu-tions of each depend on the following factors :

(1) the nature of the polymer or copolymer(including their hydrophobicity/hydrophilicity,molecular weight and glass transitiontemperature) and the nature of additives, e.g.other polymers, plasticizers, Ðllers ;

(2) the degree of crystallinity of the polymer and themorphology/miscibility of any other com-ponents ;

(3) degradation products of degradable polymers ;(4) the degradation environment.

Hydrolysis of poly(a-ester)s. In vitro systems, in whichabiotic hydrolytic processes are studied, are usuallybased on physiological conditions, i.e. iso-osmolarphosphate bu†er at pH 7É4 and 37¡C, whereas micro-bial systems are more varied both in the conditions andthe nature of the inoculum. Data for the rate of hydro-lysis of di†erent biodegradable polyesters under physio-logical conditions were given in Table 3. Poly(a-ester)s(PLA, PGA, etc.) hydrolyse much more rapidly thanPHAs and by an almost abiotic bulk hydrolysis mecha-nism because of their more hydrophilic character. Therate of abiotic hydrolysis is mainly controlled by theirmolecular weight, although it has been reported171 thatthe rate of any enzyme hydrolyse of poly(a-ester)s isprincipally determined by crystallinity.

A two-stage hydrolysis mechanism for poly(a-ester)swas established in the early 1980s by Chu172 using invitro studies ; di†usion of water into the amorphousregions of the polymer occurs, producing randomhydrolytic scission at the susceptible ester linkage.When most of the amorphous regions have beeneroded, the second stage commences, as hydrolysis ofthe crystalline regions, so during the Ðrst stage there is acharacteristic increase in the percentage crystallinity ofthe polymer. The rate of hydrolysis of PLLA has beenshown to be modiÐed by using blends of monodispersePLLA 7600 and 82 500). In physiological condi-(Mwtions for 28 days the rate of release of lactic acid wasshown to increase with the percentage of low-molecular-weight PLLA in the blend.173 The rate ofhydrolysis of blends of amorphous P(D,L)LA (a-PLA)and isotactic crystalline PDLA or PLLA (c-PLA) assolvent-cast Ðlms was studied in the same in vitro condi-tions. The mass of blend Ðlms remaining after 20months was greater for Ðlms with a higher percentage ofc-PLA initially. The tensile properties remained



unchanged in the initial period of hydrolysis but thendecreased rapidly, although the tensile strength wasretained longer in blends with higher amounts of a-PLA.174

A recent investigation175 looked at the e†ect ofdimensions on the degradation rate of P(D,L)LAmaterial. Compression-moulded plates, millimetricbeads, microspheres and cast Ðlms were fabricated fromthe same batch of polymer and allowed to age in iso-osmolar phosphate bu†er at pH 7É4 and 37¡C. It hadpreviously been recognized that large PLA devicesdegrade faster than smaller ones and this was related tothe di†usion mechanism, in which the outer layer ofmaterial degrades less rapidly than the bulk internalpolymer. In this study it was found that the plates andbeads degraded more rapidly, with bulk disintegration,compared with the slower, surface only, hydrolysis ofthe Ðlms or microspheres. Park reported176 that the invitro degradation behaviour of a wide range of PLGAcopolymers was investigated for up to 53 days. It wasfound that amorphous PLGA showed transient multi-ple crystallization behaviour of D- or L-lactic acid oligo-mers during degradation, indicating preferentialhydrolytic scission of D- or L-lactic acid to GA linkagesor of GA to GA linkages. DSC reveals two distinct Tg swhen the crystallization occurs as these transientdomains are produced.

The degradation rate of solution-cast Ðlms of PLAwas studied at 37¡C in NaOH solution to give acceler-ated hydrolysis.177 Four di†erent PLLA Ðlms with dif-ferent morphologies and molecular weights (3É0

were tested and SEM] 105\ Mw \ 3É0 ] 106)revealed swelling particularly of the spherulites, whichare then eroded from the centre. The highly amorphousÐlms became more crystalline as degradation proceededand the reduction in transparency measured at 570 nm,using a spectrophotometer was ascribed to increasedspherulite density.

The hydrolytic degradation of PEG/PLLA blockcopolymers has shown that ini-(1000 \Mw \ 6000)tially there is preferential scission of the ester linkagebetween the PEG and PLLA blocks.178 GPC analysisof the molecular weight characteristics of the copoly-mers showed the formation of bimodal material andthere is NMR and FTIR evidence, in the Ðrst 200 h, ofthe formation of wOH-ended PEG blocks andwCOOH-ended PLA.

The factors that control the enzyme hydrolysis ofpoly(a-ester)s have only recently been investigated.McCarthy and co-workers171 have investigated thee†ects of the stereoisomeric content of PLA on the rateof degradation of solvent-cast Ðlms incubated with pro-teinase K. Surface changes in the Ðlm and weight losswere investigated and changes were found in both. Themain controlling factor was the degree of crystallinity inthe samples, although high mol% LLA was also impor-tant. It was concluded that initial degradation occurred

at the surface and that the rate of degradation increasedwith high LLA content (92% optimum) and a high per-centage of amorphous domains. There was little changein molecular weight over the period of the investigation.Torres et al.179 have used a Ðlamentous fungus(Fusarium moniforme) and a bacterium (Pseudomonasputida) to study the enzymatic degradation of lactideoligomers and dimers and lactic acid monomers withdi†erent stereostructure in liquid cultures using HPLC.The micro-organisms totally used the oligomers regard-less of enantiomeric composition, although the L-dimerwas consumed rapidly. The fungus was faster actingthan the bacterium, and the racemic oligomers wereslowly assimilated but the higher L-oligomers were bio-stable ; the crystallinity of these latter species wasthought to be the cause.

Hydrolysis of water-soluble polyethers. The microbialaspects of the degradation of PEO have been studied byOtal et al.180 using a model, integrated, wet airoxidation/aerobic wastewater treatment. The partialwet air oxidation was found capable of converting theoriginal polymers 10 000) into lower-molecular-(Mnweight compounds (oligomers and short-chain acids),and subsequent treatment with wastewater achieved an80% total organic carbon (TOC) removal rate com-pared with almost no removal of the polymer using 0É5day residence time with just the biological treatment.After a 4 day residence time, 60%È70% TOC removalwas recorded with no pretreatment and [90% TOCremoval was achieved with the integrated wet air oxida-tion step. Kawai181 has also commented on the micro-bial degradation of water-soluble polymers (e.g. PEO)as a combination of oxidation and hydrolysis. In boththese studies the e†ect of increased molecular weight ofthe PEO on the reduction in the rate of hydrolysis wasnoted. Penco et al.182 have studied the degradation ofmultiblock PLGA and PEG copolymers using physio-logical conditions and found that the solubility of theblock copolymers and the degradation rate increasedwith increasing length of the PEG segment ; this is pre-sumably connected with the subsequent ease of hydro-lysis of the PLGA segments.

Hydrolysis of poly(e-caprolactone). The biodegrada-tion of poly(e-caprolactone) (PCL) using soil burial,activated sludge tests183 and composting184 is reportedto produce loss of mechanical properties and rapidweight loss.183 A bulk random chain scission mecha-nism has been proposed, but other workers185 haveproduced evidence that mixed micro-organisms fromindustrial compost of household refuse initiate the deg-radation by chain-end scission. Abiotic hydrolysisoccurs more slowly because of the hydrophobicity ofthe polymer and in this way it is similar to PHB.Koenig and Huang186 have investigated the use ofcrosslinked PCL and solution-cast PHBV (12% HV) asbiodegradable hydrophobic coating materials forhydrophilic substrates (Ðlter paper, (a cornMater-Bi}



starch/vegetable seed oil thermoplastic) and a PCL/starch blend). The PCL was a better water barrier thanthe PHBV for the paper at 23¡C but acted as a semiper-meable membrane with the other substrates andincreased the water absorption over untreated samples ;also, the low melting point of PCL (60¡C) was an addi-tional problem.

Hydrolysis of cellulose derivatives and starch. Thee†ect of the esteriÐcation of cellulose on its biodegrada-tion has been mentioned and the production of a biode-gradable form of cellulose has been approached bylooking at the e†ect of substituent distribution in cellu-lose esters.187 A range of acyl substituents wereattached to the cellulose and DS between 0É1 and 3 wasassayed. It was found that the rate of cellulolyticenzyme degradation decreased with increased DS andwith increase in the number of carbons in the sidechain. 75%È80% of the maximum rate of biodegrada-tion was reached within approximately 7 days of incu-bation. The maximum degree of acylation which (a)could be tolerated before the polymer became unde-gradable or (b) resulted in more than 10% weight losswas related to ester type and varied from DS 0É5 to 1É0.

Gardner and co-workers188,189 and Gross et al.190have investigated the biodegradation of CA Ðlms inbench-scale composting medium (at 53¡C and 60%moisture content). Based on the disintegration of theÐlms and weight loss, they concluded that CAs withDS \ 2É20 decomposed in compost at similar rates toPHBV and PCL Ðlms and were found to totally disap-pear after 14 days. There was NMR and GPC evidenceof the preferential removal of low-molecular-weightfractions of CAs, leaving the more highly substitutedmaterial.188 Little weight loss was observed usingabiotic conditions with similar temperature and mois-ture conditions. The nature of the composting formula-tion was investigated and found to have little e†ect inÐve out of the seven types tested. However, moisturecontent had a signiÐcant e†ect and a reduction from60% to 40% resulted in changes in the completepolymer degradation time of CA (DS 1É7) from 6 to 30days. CA (DS 2É06) and triethylcitrate (TEC) were ther-mally compounded189 and fabricated as bothcompression-moulded Ðlms and injection-moulded bars.The former degraded rapidly using the compost and10%È12% weight loss was recorded for the bars. Misci-ble blends of cellulose acetate propionate (CAP) andboth poly(ethylene glutarate) (PEG) and poly(tetra-methylene glutarate) (PTG) were prepared and com-posted. The nature of the polyester was found to haveno e†ect on the rate of degradation, but the ratio ofCAP to polyester and the DS of CAP both a†ected thedegradation. When the DS of the CAP was high, almostall the weight loss was from the polyester, but for blendswith DS of CAP \2É0 both components degraded. Therate of degradation increased as the wt% of polyester inthe blend increased.

Mino et al.191 have studied the biodegradation ofstarch under anaerobic, anoxic and aerobic conditionsas a model organic substrate using an analyticalmethod based on starchÈiodine complex formation. Thetests show the range of bacterial media that will degradethe polysaccharide from pure a-amylase enzymatic deg-radation to hydrolysis by the complex mixture of inocu-lum found in activated sludge. This was found to followsurface-limited adsorption kinetics as it was indepen-dent of the biomass concentration. An in vitro study192of the biodegradation of starch-based materials wascarried out using excess Bacillus licheniformis a-amylaseand Aspergillus niger gluco-amylase at 37 and 80¡C andthere was preliminary evidence of biodegradation fromweight loss, but the complexity of the materials testeddid not allow the extent of biodegradation to be accu-rately assessed.

Studies of starch/PCL systems have shown thatblends using high-amylose corn starch are the strongest,as the small size of the starch granules produces gooddispersion in the PCL matrix, but, like PHBV(11É6 mol% HV)/starch blends, they are phase separat-ed.193 The biodegradation of starch/PCL systems hasbeen studied194 in a variety of biotic and abiotic condi-tions. The rate of biodegradation of the blend incompost was found to increase in the presence of starch.

Hydrolysis of poly(3-hydroxyalkanoate)s. PHAsundergo quite rapid enzymatic hydrolysis in soil,sewage sludge and seawater especially in the presence ofextracellular P(3HB) depolymerases, but the polymersare hydrophobic and abiotic hydrolysis is relativelyslow. It has been shown195 that surface area and thick-ness of Ðlm have little e†ect on the rate of biodegrada-tion of PHB or PHBV. The major factor controlling themicrobial hydrolysis rate of the PHB or PHBV is thedegree of crystallinity.

The enzymatic degradation of PHB, both intra- andextracellular, has been extensively investigated andHocking and Marchessault13 and Doi80 have reviewedthe role of extracellular P(3HB) depolymerases, whichwere found to degrade PHB by di†erent mechanismsaccording to their bacterial source. Saito and co-workers196 showed that Pseudomonas lemoignei depoly-merase could be separated into four separate enzymes :the A pair and the B pair. The B pair produced a higherpercentage of trimer during degradation and the Ðnalproduct ratio of monomer to dimer was higher than forthe A enzymes (0É75 compared with 0É2 for the A pair).All the enzymes speciÐcally hydrolyse the polymer andoligoesters of D([)3-hydroxybutyrate but show noactivity towards the dimer and have an affinity for thehydroxyl rather than the carboxyl end of the oligomers.Cleavage occurs at every second or third ester bondfrom the hydroxyl end by these exoenzymes. However,Pseudomonas lemoignei also produces intracellularD([)3HB dimer hydrolase and this can absorb thedimer, hydrolyse it and then oxidize it into acetoacetate,



which leads to its ultimate biodegradation via acetyl-CoA and the citric acid cycle.

A strain of Alcalignes faecalis isolated from activesewage sludge has been found to excrete an e†ectiveP(3HB) depolymerase (optimum pH 7É5) and an extra-cellular D([)3HB dimer hydrolase when grown in amedium with PHB as its only carbon source. Themechanism of the depolymerase enzymeÏs activity hasbeen described as an endo-type hydrolase, with thesecond ester linkage from the wOH end being particu-larly susceptible to hydrolysis. It has been shown197that as well as its catalytic site the enzyme has a hydro-phobic site, which is not essential for the hydrolysis ofwater-soluble oligomers but is necessary for the hydro-lysis of hydrophobic substrates. The extracellularD([)3HB dimer hydrolase excreted was found to becapable of hydrolysing the DD dimer and has someactivity towards the LD and DL dimers, but not the LL

dimer.The rate of enzymatic hydrolysis of copolymers of

PHAs was studied by Doi et al.198 using the puriÐeddepolymerase from A. faecalis in a phosphate bu†er (pH7É4) at 37¡C. The rate of erosion was measured byweight loss and found to be strongly dependent on thecopolymer composition, with copolymers containing C6to in the basic 3HA unit showing no hydrolysis.C10However, the copolymers P(3HB-co-4HB) 10È17 mol%4HB) showed accelerated hydrolysis under these condi-tions. There was no correlation between the crystallinityor molecular weight of the microbial Ðlms and the rateof hydrolysis. This latter point supports the endo-typehydrolase mechanism, as the value of the ÐlmsMnremains almost unchanged during the degradation, indi-cating that P(3HB) depolymerase hydrolyses only thesurface layer of polymer chains and polymer erosionoccurs by surface dissolution. There is SEM evidence ofsurface erosion and no bulk hydrolysis is thought tooccur.

Albertsson and co-workers199 have established aseries of biotic and abiotic test conditions to investigatethe biodegradation of P(3HB-co-6% 3HV) copolymerÐlms. Microbial degradation was investigated, with theÐlms as the main carbon source, (a) in a stirred,complex aqueous, mineral medium inoculated withAspergillus fumigatus at pH 5É5 and 25¡C, (b) in a sterilecontrol of this mineral medium and (c) in a compostingfacility, which could be turned and consisted of typicalgarden waste, with a maximum internal recorded tem-perature of 64¡C and approximately 60 wt% moisturecontent.

Microbial degradation produced SEM evidence ofrapid surface degradation, also observed by Doi etal.198 and explained by Nobes et al.200 as a splinteringphenomenon. This was also seen in single-crystal degra-dation by a chain-end mechanism, causing no decreasein average molecular weight. Hocking et al.201 havenow proposed that A. fumigatus uses both endo- and

exoenzymes in the degradation of PHB, explaining thefast rate of degradation. Doi has shown that PHBdegrades faster enzymatically than PHBV and Albertss-on has produced ATRÈFTIR spectral evidence of struc-tural changes in the surface layer of polymer duringmicrobial degradation, which indicated that the PHBunits were more easily degraded than the PHV and thisincreased the abundance of the latter in the surfacelayer.

The abiotic rate of hydrolysis of microbial polyestersin physiological conditions is slower than the enzymaticrate under similar conditions by a factor of two orthree. Using phosphate bu†er (pH 7É4) and tem-peratures ranging from 37 to 70¡C, Doi80 found thatP(3HB), PHBV (68% HV), P(3HB-co-9% 4HB) andP(3HB-co-16% 4HB) showed no weight loss after 180days at 37¡C, although after a characteristic inductionperiod, molecular weight loss was observed. The induc-tion period varied from 80 days for PHB and PHBV toonly 20 days for the P(3HB-co-4HB) copolymers and itwas postulated that this may be the time for the waterto permeate the polymer matrix. The molecular weightloss for the P(3HB-co-4HB) copolymers was alsogreater than for the others and increased with mol%4HB.

Accelerated hydrolysis at elevated temperatures wasinvestigated by Doi80 (55 and 70¡C) and Albertsson andco-workers199 (at 60¡C for 347 days) and found toinvolve a homogeneous process in two stages. Therewas no induction period above 60¡C and the initialrandom scission of the ester groups occurs throughoutthe polymer (amorphous and crystalline regions), whichleads to a decrease in molecular weight but little changein the polydispersity and almost no bulk mass loss.Albertsson used DSC changes to investigate further themultiple melting behaviour of PHBV during initialrandom scission hydrolysis in sterile water. Whenmolecular weight values have decreased to about13 00080 and 8000È9000 (after 200 days),199 the secondstage occurs involving weight loss ; the polydispersityrapidly increases to a value of 4 and then remains con-stant.

The kinetics during the second stage indicated achange to non-random chain scission and, based ondata of Knowles and Hastings202 on the e†ect of pH onhydrolysis, it was found that after 200 days the pH hadfallen from 7 to 3 because of hydroxy acid production.1H NMR evidence showed that the mol% of HV in thepolymer had decreased from its initial value and it wasinferred from this that the Ðnal stage of the abiotichydrolysis favours hydrolysis of HV units. Doi foundthat P(3HB-co-4HB) copolymers again show morerapid hydrolysis in abiotic conditions compared withP3HB and PHBV copolymers.

In AlbertssonÏs study there were irregular changes inthe average molecular weight of the Ðlms in compostbut there was no general trend. The results were in



general agreement with those of Margaert et al.,203 whoobserved no decrease in the average molecular weight ina compost over 150 days at temperatures ranging from7 to 32¡C and concluded that the degradation wasmicrobial. A study at slightly higher temperatures (28È40¡C) has produced both abiotic molecular weight lossand biotic mass loss. The variations in the nature of thecompost, its water content and temperature make it dif-Ðcult to generalize about the mechanism involved. Thepolymer Ðlm in the study by Albertsson became brittlein sterile water and in compost, indicating some abiotichydrolysis, and Gilmore et al.204 noted the sameincreased brittleness during composting studies onPHBV.

There have been several studies on the biodegrada-tion of PHB with di†erent stereoisomer compositionand di†erent tacticity.205,206 The factors that controlthe rate of biodegradation of chemically synthesizedPHB stereoisomers are the nature of the micro-organism, the morphology of the polymer, the tacticityof the polymer and its stereostructure.

6.2 Biodegradation testing

6.2.1 T est methods. Testing chemicals for their biode-gradability has been carried out for over 30 years,although the development of scientiÐcally acceptablemethodologies in the Ðeld of polymers and plastics isstill inadequate. Current standard test procedures eitherdescribe biodegradability inadequately or fail to providemeaningful practical data which can be used to makeenvironmental assessments.

DeÐnitions of biodegradability which specify 100%biodegradability are impracticable, but standardsauthorities have produced deÐnitions, e.g. ISO472 :1988, ASTM (D20.96 proposal) and DIN103.2,which are all quoted by Seal.207 These deÐnitions, alongwith that given at the start of this review, are pragmaticas they concentrate on the biodegradation of the plasticmaterial without assessment of its ultimate fate andwith no attempt at classiÐcation in terms of rate of bio-degradation.

A tiered approach is needed to assess chemicals fortheir e†ect on and fate in the environment and this hasapplications whether they are biodegradation or not.Tier testing for biodegradable polymers can be rep-resented as follows.

1. The Ðrst tier is short-term screening under strin-gent conditions in a controlled laboratoryenvironment. Chemicals which biodegrade underthese conditions, with limited time for acclima-tization, are described as readily biodegradable.

2. The second tier involves prolonged exposure to anatural inoculum of micro-organisms. Favour-able test conditions, including an acclimatizationperiod, are included, so the classiÐcation inher-

ently biodegradable cannot be assumed to rep-resent rapid and reliable biodegradation in anatural environment.

3. The third tier is a range of protocols designed tosimulate repeated exposure of the material to adisposal route, e.g. sewage treatment, composting,The tests have to be long-term and requirecareful validation and justiÐcation, so they areexpensive.

The degradation tests must incorporate realisticenvironmental disposal conditions which are either ter-restrial (landÐll, soil, composting) or aqueous(groundwater, marine, waterways, sewage or sediment).The development of some tests is quite advanced, butthe standardization of other tests, e.g. composting, withvariation in both composition and conditions, is verydifficult. Five screening test protocols have beenaccepted by the EC and EPA and are described in theOECD guidelines, but two involve measurement of dis-solved organic carbon (DOC), which cannot be used forbiodegradable. The three appropriate tests for biode-gradable polymers are the modiÐed Sturm test, themodiÐed MITI test are the closed bottle test (OECD301B, C, D). They are all aquatic, aerobic tests and thetest substance provides the sole carbon source and isexposed to a low level of inoculum for 28 days withstirring. The evolved or consumed is deter-CO2 O2mined and calculations based on the total carboncontent of the polymer are used to Ðnd the percentagedegradation. The criterion for a polymer to be readilybiodegradable is to achieve 60% of the total theoreticaloxygen demand (TOD) within 28 days and this shouldbe reached within 10 days of the biodegradation reach-ing 10% TOD.

The next tier of testing involves inherent biodegrad-ability and there are four test protocols recommended,which do not have pass/fail criteria, but at least 20%biodegradation within the 28 days suggested exposuretime is recommended by the OECD. They are the modi-Ðed SCAS (semi-continuous activated sludge) test, themodiÐed ZahnÈWellens test, the modiÐed MITI testand inherent biodegradability in the soil (OECD 302A,B, C and 304A). They allow for greater Ñexibility inexposure time, acclimatization procedures and micro-bial selection. Seal207 has comprehensively reviewedthese di†erent test methods, the test parameters and thecriteria for the pass level. He has also summarized thestandards for resistance testing of plastics and the teststrains of fungi and bacteria used in these tests. Thedegree of sophistication in measuring the degradationvaries from visual examination of surface growth offungi or bacteria and material weight loss to testing ofmechanical properties.

Itavaara and Vikman208 have also given an overviewof methods of biodegradability testing used at the Tech-



nical Research Centre of Finland (VTT) for testing solidpolymers and packaging materials. Aquatic screeningtests include an automated, aerobic Sturm test (OECD301B; ASTM D5209), the VTT head space test and ananaerobic test (ASTM D5210). Also reviewed are di†er-ent composting systems and, in a later paper,209improvements in the VTT head space test for insolublepolymers are outlined, including the method of deter-mining the distribution between the gas and liquidCO2phases.

A screening and long-term test procedure has beenproposed by Puechner et al.210 Aerobic and anaerobicaquatic and soil screening was carried out using carbonbalances to evaluate the biodegradation of PHB,PHBV, PCL and other manufactured biodegradablepolymers. However, polymers that were not fullydegraded were then tested in an aquarium system for 1year, monitoring biodegradation by a variety ofmethods, including DOC release into the water, massloss and SEM.

Margaert et al.211 have looked at the biodegradationof PHB and PHBV (10 and 20 mol% HV) copolymersin natural waters. After 358 days in a freshwater canalthe mass loss was 34% for PHB, 77% for PHBV (10%HV) and the other copolymer completely disintegrated.In seawater after 270 days the PHB lost 31% and thecopolymers 49%È52% of their initial mass. Tem-perature had an e†ect, as mass loss was greater in thesummer. Degradation reduced the tensile properties ofthe copolymer samples, but no relevant changes in themolecular weight were observed indicating surfaceerosion. Over 90 micro-organisms were isolated andidentiÐed from the polymer surfaces. Mayer et al.212have also investigated the biodegradation of polymericÐlms in marine and soil environments. They have devel-oped standardized accelerated test methods and quanti-Ðed the degradation using weight loss/surface areameasurements. A range of unblended polymers weretested, but the increased complexity of testing blendswas recognized, in which leaching of plasticizers andother factors can a†ect the results.

6.2.2 Recent developments in composting test methods.The biodegradation of polymeric materials and theimplications of this factor in the management of solidwaste have been discussed. The standardization of com-posting tests and the correlation between biodegrada-tion behaviour in miniaturized, laboratory-basedcomposting facilities and that observed in actual com-posting conditions are problems that have to beaddressed.

Pettigrew et al.213 have compared a tiered testingstrategy for assessing the compostability of PCL(OECD 301B and ASTM D5338 test methods) with rea-listic composting of 14C-PCL. These tests conÐrmedthat screening-level biodegradation tests can provide

information about the inherent biodegradability ofpolymers, but the rate of degradation and the ultimatefate of polymers in environmental conditions must bedetermined using realistic tests.

Urstadt et al.214 have shown that calculating thedegree of biodegradation of PHB in a mixed culturecompost could not be done accurately using CO2release alone. However, establishing carbon balanceshas the potential to be more accurate, with di†erentanalytical methods being adopted to determine thecarbon fraction in particular test conditions. Quantitat-ive determination of biomass was assessed theoreticallyand practically from the protein content and by selec-tive oxidation with hypochlorite. Limitations in themethods were recognized ; but reasonable assays wereachieved within acceptable standard deviation range.Protein assay was the basis of another study of thecarbon content of biomass by Spitzer et al.215 TheLowry method was used and this showed that the ratioof protein to carbon content is not constant butdepends on the composition of the microbial popu-lation, the growth phase and the substrate supply.Correlation of the absorbance with carbon contentobtained during the Lowry test was used to estimatebiomass/carbon content dependence during biodegrada-tion by a mixed population of micro-organisms, andcalibration curves were obtained.

The problem of quantifying biodegradation has alsobeen approached in other ways. David et al.216 investi-gated a manometric method of measuring oxygen con-sumption during biodegradation using various inocularanging from a single bacterium to complex mixtures ofmicro-organisms in compost and sewage sludge. Paggaet al.217 in Germany and Gross and co-workers218 inthe US have produced laboratory-controlled com-posting facilities which have the potential to becomestandard test facilities. Gross and co-workers havesimulated municipal solid waste compost at a constanttemperature of 55¡C and 54% moisture content. It wasproposed that the e†ect of acclimatization time andexposure time should be studied, as well as the rate ofbiodegradation of PHBV (20 mol% HV) and PHBsamples at various points in the composting cycle. SomePHBV samples were exposed for 18 days continuouslyand weight loss was recorded every 3 days. Othersamples were exposed to the compost for only 3 daysbut at di†erent times in the composting cycle. The cyclewas found to have three stages, with the maximum rateof degradation occurring between the 10th and 15thday. The continuously exposed samples degraded mostand PHBV samples degraded more than pure PHB.Very recently, Itavaara et al.219 have proposed amethod of using steel frames to test the biodegradationof polymer packaging materials in windrow compost.The polylactide- and Biopol-coated cardboard samplescompletely degraded and there was no toxicity intro-duced into the compost.



6.2.3 Biocompatibility and toxicity screening for biode-gradable polymers. The biocompatibility of any macro-molecules that are used as implants or as drug deliverysystems involves cytotoxicity testing using appropriateextraction protocols, which help to establish the pres-ence of potential “leachablesÏ that are capable of indu-cing a measurable degree of systematic toxicity,localized tissue irritation sensitization or other bio-logical response. Tissue culture testing is a rapid, eco-nomical, in vitro approach to biocompatibility testing,but because of its sensitivity it should always be used inconjunction with in vivo studies. Holland and Tighe21have compared two approaches to cytocompatibilitytesting : the semiquantitative agar overlay test and thequantitative cell growth inhibition test. Chaput et al.220have used in vitro direct contact and agar overlay cellcultures of mouse Ðbroplasts to test PHBV Ðlms andextracts in di†erent media. Direct contact cultures withthe Ðlms produced mild cell reactions and lyses, andagar overlay cultures did not give any signiÐcant celldeath or changes. Recently, Dang et al.221,222 havecarried out toxicity screening tests on biodegradablepolymers (CA, Biopol, PCL and others) using standardin vitro animal cell culture tests. Phase contrast lightmicroscopy enhanced by neutral red staining was usedto provide qualitative evaluation of cell morphologyand lysis. The method was shown to be more sensitiveand accurate than other comparable tests.

Saad et al.223 have studied the biocompatibility ofmultiblock copolyesters. Cell adhesion, cell growth andcell activities of macrophages and Ðbroblasts culturedon the newly developed degradable copolymers werestudied to establish whether the latter were suitable forbiomedical applications. Their biocompatibility wasestablished by subcutaneous implantation of thepolymer foil into rats and their tissue compatibility andbiodegradability were also demonstrated. It is beyondthe scope of this review to give more than an indicationof the range of toxicity and biocompatibility studiesthat need to be carried out on polymers for biomedicalapplications.

7 GENERAL SUMMARY OF FINDINGS

The thorough literature search that has been carriedout has made it possible to draw a number of conclu-sions.

1. The use of plastics in packaging and the wasteproduced have had considerable environmentalimpact because of the low density and hencelarge amount of most commodity polymers.However, the removal of plastics from pack-aging is not a viable option, as there would be adramatic increase in terms of weight and volumeusing non-plastic packaging and the additionalenergy consumption would be prohibitive. Recy-

cling of materials and energy recovery are obvi-ously primary strategies for reducing plasticwaste, but the use of biodegradable polymersalso has some advantages.

2. Renewable sources of polymeric materials andsynthetic biodegradable polymers may provideecologically attractive technologies, but even incountries that are at the forefront of green tech-nology it is only as components of packagingand as natural Ðbre composites that thesematerials are currently viable in terms of priceand performance. The main constraint on theuse of biodegradable polymers for bulk pack-aging is the di†erence in the price of these poly-mers compared with that of bulk-produced,oil-based plastics. The current cost of isBiopol}approximately £8000 per tonne, compared withthe current UK prices of commodity polymersof between £500 per tonne (PVC and PP) and£600 per tonne (HDPE and high-impact PS).Current low oil prices, increased recyclingcapacity and improved technologies for theseparation of plastics and their reuse make theuse of biodegradable polymers for most pack-aging requirements uneconomic.

3. Polymeric materials and blends that are bio-compatible and biodegradable have obviousbeneÐts as packaging materials for phar-maceutical products, drugs and would dressings.The physicomechanical properties plus the gasand liquid barrier properties of these materialsare obviously important, as is a knowledge ofthe conditions and rate of biodegradation. Ifprocessable biodegradable pure materials orblends can be manufactured, the economicimbalance may be less for specialist packagingapplications.

4. The production of biodegradable polymers withthe necessary controlled structure (e.g. monomersequence, tacticity, chirality) to match enzymespeciÐcity in degradation reactions in di†erentmicrobial environments is not possible with cur-rently available techniques for chemical synthe-sis. However, some progress has been made inthe synthesis of block copolymers with con-trolled molecular weight and tacticity. There istherefore potential to make materials withimproved physicomechanical properties andvaried degradation proÐles.

5. The degradation of poly(a-ester)s is mainly byabiotic hydrolysis, but although the use of poly-mers with di†erent molecular weight and tac-ticity has varied the biodegradation rates ofthese materials, the range is still quite limited.PHAs have the most potential, as although theirrate of abiotic hydrolysis is relatively slow,microbial hydrolysis is more rapid and can be



manipulated by variations in processing tech-niques, molecular weight of the polymer, copoly-mer composition and blending.

6. Pharmaceutical uses of biodegradable polymersare well established, e.g. resorbable sutures, hardand soft tissue implants and controlled drugdelivery systems, mainly because the uniqueproperties of the materials override the costsinvolved. Drug release systems incorporate theactive substance into a polymer matrix which isimplanted into the patient. A di†usion mecha-nism produces maintained drug release followedby bulk hydrolysis of the polymer, which is idealfor the delivery of relatively small drug mol-ecules. However, modern techniques require theincorporation of drugs which vary in nature andmolecular size and a di†usion mechanism is tooslow for the release of large molecules, so anerosion release mechanism must be used. Therange of degradation proÐles available is limitedin terms of both degradation time and character.The balance between abiotic and biotic degrada-tion, both in vivo and in vitro, is not fully under-stood, even for the most studied biodegradablepolymers, namely poly(a-ester)s andpoly(hydroxyalkanoate)s. More pure or blendedmaterials with surface erosion characteristics arerequired for drug delivery.

7. The blending of biodegradable polymers is amethod of reducing the overall cost of thematerial and o†ers a method of modifying boththe properties and the degradation rates of thematerials. Miscibility is usually characterized bya single glass transition in the blend, whichvaries with composition and can be measuredby mechanical or thermal testing. The advan-tages of producing miscible blends includesingle-phase morphology in the melt and repro-ducible mechanical properties. However,forming a miscible blend, particularly with anon-biodegradable polymer, can reduce or eveninhibit the degradation of the biodegradablecomponent. Immiscible blends have the dis-advantage of having properties that are depen-dent on the blend morphology produced byprocessing and these are often not reproducible.However, some can show higher biodegradation

rates than the unblended biodegradablehomopolymer(s).

8. The development of two- and three-componentblends represents an area of considerable inter-est. The use of natural polymers and biodegrad-able polyesters has been studied by severalgroups, but there is still considerable potential,especially if compatible blends can be madeusing materials with relatively low glass tran-sition temperatures. The use of low-molecular-weight plasticizers and photoactive materials intertiary blends of biodegradable and non-biodegradable, synthetic and natural polymers isanother potential Ðeld of study.

9. Pragmatic deÐnitions of biodegradation concen-trate on the degradation of the plastic materialwithout assessment of its ultimate fate and withno attempt at classiÐcation in terms of rate.Standards authorities have produced deÐnitions,e.g. ISO 472 :1988, ASTM (D20.96 proposal) andDIN103.2. Tier testing for biodegradable poly-mers can represent a method of classifying poly-mers, and readily biodegradable, inherentlybiodegradable and a third tier have been identi-Ðed.

10. Many current standard test procedures for bio-degradable polymers either describe biodegrad-ability inadequately or fail to providemeaningful practical data which can be used tomake environmental assessments. The degrada-tion tests must incorporate realistic environ-mental disposal conditions which are eitherterrestrial (landÐll, soil, composting) or aqueous(groundwater, marine, waterways, sewage orsediment). The development of some tests isquite advanced, but the standardization of othertests, e.g. composting, with variation in bothcomposition and conditions, is very difficult.The development of a laboratory-scale com-posting facility would provide valuable informa-tion about the optimum conditions needed forlarge-scale composting, and there is a need toimprove the knowledge base of abiotic hydro-lytic degradation rates and mechanisms of di†er-ent biodegradable polymers, copolymers andblends.



APPENDIX I RELEVANT RESEARCH PAPERS

I .1 Summary of current research papers on use of biodegradable polymers as surgical implants (1994–1997)

Aim Materials Points Ref.

The design and fabrication of Porous films of P((D,L) 1. There is ingrowth of fibrovascular 224biodegradable polymer devices LGA) formed into tubes tissue after transplant of device.to engineer tubular tissue and as capable of resisting 2. Using intestinal epithelial cells, 1994a delivery vehicle for transplanting cells. compression forces in attachment was found and an

vitro and in vivo. organized epithelial layer.

PHA polymers as culture PHBV (HV ¼7, 14 and 1. Proliferation rates were slow 225surfaces for spinal ligament 22 mol%) surfaces for PHBVfibroblasts (HSF) from scoliotic tested with PS and type I 2. Surface chemistry and wettability 1995patients. collagen. The surfaces PHBV had no effect.

were used as culture 3. Preferential growth on PS andsubstrates for HSF. collagen surfaces.

A novel way to make porous Macroporous sponges 1. Polymer sponges with large pores 226P(D,L)LGA sponges without made by exposing P(D, and porosity were produced.organic solvents. Applications : L)LGA discs to high- 2. The porosity could be controlled 1996tissue engineering to transplant pressure CO

2for 72 h at by a preform technique.

cells or growth factors and as room temperature, then fast 3. Fibre-reinforced foams produced.tissue regeneration templates. pressure reduction to

atmospheric level.

Biodegradable and macroporous Microcellular foams 1. Thermally induced phase 227PL foam implants for were produced by freeze separation studied w.r.t. severalcell transplantation. drying PL solutions in variables, e.g. ÍpolymerË. 1996

1. The preparation of supports 1,4-dioxane. 2. Tubular macropores with porousby solid–liquid phase substructure observed andseparation. variations in morphology with

conditions followed.

2. The preparation of polylactide Amorphous P(D,L)L and 1. The possibility of gels discussed. 228macroporous foams by semicrystalline P(L)L in 2. Freeze drying can produceliquid–liquid phase separation 87 : 13 dioxane : water flexible tough foams with 1996as a porogen technique. mixture producing isotropic morphology.

liquid/liquid phase 3. Potential for use established.separations.

A novel method for fabrication Stiff porous materials of 1. For a meniscal prosthesis the 229of biodegradable scaffolds with high-Mwt 50/50 P(L compression modulus has to behigh compression moduli LA-cc-eCL). Porous larger than 150 kPA to protect the 1997suitable for fibrocartilage microspheres agglutinated articular cartilage ; theregeneration in meniscal with NaCl crystals compression modulus of thisimplants and prothesis. and mixed with solid product could be varied over theExcellent adhesion properties of solvent to give range 40–1100 kPa.polymer known to enhance homogeneous solvent 2. The density could be varied fromhealing of meniscal lesions. distribution 0·07 g mlÉ1 to 0·5 g dlÉ1.

Soft tissue and cancellous bone Novel fibre-reinforced 1. In vitro mechanical performance 230reaction to the implantation of material based on test found to be superior to formernovel biodegradable pins and lactide. implants after 1, 6, 12 months. 1997plates in rabbits 2. In vivo test in a neutral environment

showed biocompatibility.

The influence of manufacturing Microcapsules prepared 1. 85/15 and 50/50 PLGA prepared by 231procedure on the degradation of by non-solvent-induced non-polar method degrade faster.PLGA implants. phase separation of 2. SEM shows water uptake faster 1997

1. 85/15 PLGA. solvent/non-solvent. in more porous matrix of non-polar2. 50/50 PLGA. 1. Methylene chloride/hexane product.Implants formed by compression (non-polar). 3. Average degradation times :of microcapsules and in 2. Acetone/phosphate I 85/15 PLGA Á26 weeks ;vitro studies at pH 4·5, 7·4, 9·4 buffer (polar). I 50/50 PLGA Á6–8 weeks.monitored by varied techniques.



I .2 Summary of research papers on production of microspheres for sustained drug delivery systems usingbiodegradable polymers (1994–1997)

Aim Material Points Ref.

Novel encapsulation technique PLGA and PLA 1. Particles with high trap ratio of 232by in-water drying method to microspheres drug and small initial burst wereproduce a sustained drug containing leuprorelin, obtained. 1994delivery system and a potent LHRH agonist, 2. The systems effectively reducedpersistently suppress were formed. required dose compared with dailysteroidogenesis for 1–3 months injection system, with moreafter a single injection. continuous receptor hits on the

target organs.

Microencapsulation of protein The ternary blend was 1. A mean particle size of 10–42 lm 233bovine serum albumin (BSA) based on P(e)CL was achieved altering the internalusing novel ternary blends. (high and low Mwt) phase volume of the primary 1995Inclusion of low-Mwt PCL- and poloxamer 181 emulsionenhanced phase mixing by a (PO-181), a triblock 2. Protein entrapment (11%w/w)reduction in the Mwt of the copolymer (PEO–PPO–PEO). was possible with protein-to-components. WOW multiple Various factors polymer ratio of 1 : 4.emulsion method was used to were varied to see their 3. Native-PAGE analysis of theencapsulate BSA. The PO- effect on the entrapped BSA indicated181 was found to retard rate of microsphere properties. maintenance of bulk structuralcrystallization and improve the integrity.sphericity and regularity of theparticles.

The production of peptide P(D,L)LA-co-GA 1. % yield of product decreased with 234containing microspheres from (Mwt Ä10 000) from increased water-soluble fractionlow-Mwt and hydrophilic copolymer. different suppliers was polymer.Initial analysis for synthesized in different 2. 75 : 25 PLGA showed decreased 1996Mwt, T

g, bulk density and ways. T

gincreased peptide incorporation with Mwt

carboxylic acid content was with Mwt of PLGA. but the 50 : 50 PLGA did not.carried out. The effect of these Bulk density and 3. Higher-Mwt microspheres showon the microspheres was wCOOH content of lower initial peptide release.investigated. microspheres decreased 4. Peptide incorporation can be

with increased Mwt. controlled by preparationmodification withlow-Mwt PLGA.

The effect of the processing P(D,L)LGA was 1. Only microsphere produced by 235technique on the Mwt of the investigated by GPC sonication (21% decrease) showedpolymer in biodegradable before and after any significant Mwt change. 1996multiphase microspheres. processing. Two 2. Potentiometric dispersionProduced by agitation, surfactants were produced the narrowest MWD.potentiometric dispersion or used at different levels 3. A decrease in the internalsonication. in the continuous phase. diameter of the infusion tube

used in the potentiometricdispersion technique decreasedproduct mean particle size.

4. Increased surfactant level gaveincreased mean particle size ofproduct containing BSA inpotentiometric dispersionmethod, attributed to increasedconductivity in the continuousphase with increased surfactantlevel.

Microparticle preparation by a salting-out Size, size distribution 1. Water-insoluble and low soluble 236process, which is mainly and morphology are compounds are wellsuited to the production of important parameters. encapsulated by this method. 1997nano- or microparticles for In vitro properties of the 2. The encapsulation efficiency forparental drug delivery systems. drug must also be water-soluble compounds such asCareful tailoring of the process considered. BSA is small.parameters is needed, based onthe nature of the polymer andthe drug.




The preparation of P(D,L)LA–PEG of 1. The size of microspheres increased 237biodegradable polymer known Mwt with increased Mwt of copolymer.microspheres encapsulating synthesized using 2. PLA–PEG (10%) produced micro- 1997protein with micron sizes. stannous octoate by spheres of improved size controlSolvent evaporation composite cationic ROP. PVA 3. ÍPVAË can affect the size ofemulsion technique is used to aqueous solutions were microspheres.produce particles with narrow used as dispersion 4. DSC data show proteinsize range. medium for microsphere efficiently encapsulated, with

preparation. low crystallinity, improvingability to trap protein.

5. The amount of protein carriedwas related to the nature of theprotein.

Polymer alloys as a new PLA–PLGA–methylene 1. The critical ÍpolymerË that 238preparation method for chloride ternary systems produces phase separation isbiodegradable microspheres. were studied. (i) independent of PLA : PLGA ratio, 1997This multi-reservoir technique Solubility parameters (ii) dependent on solvent, Mwtis based on phase separation of governed the powder and L : G ratio in PLGA.the two polymers when their distribution in the two 2. Particles of unique ‘polymerconcentrations exceed a critical phases. Cisplatin is all alloy’ with cisplatin distributedvalue. Amino acid powders were found in the PLGA-rich in internal PLGA-rich phase.added to the biphasal solutions. phase. 3. Encapsulation efficiency

approximately 100% at 16% loading.4. In vitro dissolution study showed

release of cisplatin over 45 dayswith no initial burst.

A novel method of PLGA and P(D,L)LA 1. Steady release of GM-CSF over 239encapsulation of a drug from (low-Mwt) microspheres 1 week; no significant proteinmicrospheres using silicone oil-based were prepared ‘burst’.phase separation process. to encapsulate granulocyte– 2. GPC analysis of degradation 1997Several different polymer macrophage colony kinetics shows low-Mwt PLAblends were used and the stimulating factor (GM-CSF). enhanced degradation of PLGA andencapsulation was affected release kinetics.characterized both in vivo and 3. GM-CSF release biologicallyin vitro . active and physically intact.

4. Formulation parameters affectencapsulation and controlledrelease of drug in PLGA/PLA.

5. There was an intense localtissue reaction in mice to oneGM-CSF formulation.

A review that covers PLA, PLGA and copoly 1. Degradation of these polymers 240(i) the techniques for the (lactic/glycolic is altered by copolymer ratio and

preparation of injectable acid) are reviewed as the the Mwt. 1997monolithic microcapsules for basis for the release of 2. The amount of releasedprolonged drug release, water-soluble drugs over microencapsulated drug correlates

(ii) a discussion of phase different times. almost linearly with polymerseparation, solvent evapor- degradation, indicating thatation and spray-drying procedures. controlled release formulations

(iii) manufacturing techniques can be designed using suitablefor efficient entrapment of PaHAs with differenthighly water-soluble drugs. degradation rates.

Production of biodegradable Poly ethylene glycol– 1. PLG particles modified by post- 241microparticles for drug dextran (PEG–DEX) adsorbed PEG–DEX conjugatesdelivery and vaccine conjugates were used as flocculated in 0·01 M salt 1997formulation using an emulsion/ combined stabilizer and solutions, but if PEG–DEX usedevaporation technique on surface modifier to as surfactant, PLG particlesmodified PLG. Surface produce resorbable stable in Ä0·5 M NaCl.attachment of the hydrophilic P(D,L)LG microparticles. 2. Immunological detection used tospecies of PEG was used to show surface-exposed dextran.anchor segments, as PEG or 3. The findings support the modeldextran polymers alone did of PEG in the conjugate actingnot produce microparticles. as an anchor with steric stabilityZeta potential measurements provided by DEX.and X-ray photoelectronspectroscopy confirmed PLGsurface modification.




Microcapsule fabrication using Biodegradable polymer 1. 80 : 20 PEAD : PeCL produced 242–243blends, including PHBV, with blends of smooth particles by bothBSA; surrogate protein-loaded (i) 80 : 20 poly(ethylene methods and 80 : 20 PEAD : PHBVagarose as the active adipate) (PEAD) : PeCL particles were smooth usingingredient. Factors affecting its by both techniques, DET.release were identified. Single (ii) 80 : 20 PEAD : PHBV 2. 40 : 40 : 20 PEAD : PHBV : PeCL(SET) and double (DET) (10·8% HV) by DET, produced a mixture of smooth-emulsion techniques with (iii) 40 : 40 : 20 PEAD : PHBV surfaced microporous andsolvent evaporation were used to (10·8% HV) : PeCL macroporous microsphereproduce spherical reservoir-type by SET. fractions using SET.microcapsules. A range of BSA 3. BSA incorporation has no

loadings were used. significant effect on particle sizeAll the polymers distribution.generated high yield of 4. Encapsulation efficiencies weremicrospheres (¿75%) low (Ä14·5% overall)

5. BSA release increasedsignificantly with theoretical %loading but could not beconfirmed.

6. Significant BSA release couldbe monitored up to 26 days.

I .3 Summary of research papers on production of nanospheres for sustained drug delivery systems usingbiodegradable polymers (1994–1997)


To form and stabilize a Nanoparticles were 1. Experimental model proposed 244polymeric colloidal suspension made of P(e)CL. Effect 2. Account taken of flocculationof nanoparticles by transfer of ÍP(e)CLË, L1/L2 concentration and L1/L2 ratio. 1995from a good solvent (L1) to a ratio and dielectric of 3. Model allows the optimumnon-solvent (L2). Photon final mixture on conditions for nanoparticlecorrelation spectroscopy morphology tested. formation to be determined.(PCS) is used to confirm theproduct.

The colloidal properties of A range of benzyl 1. PLGA/fully benzylated 245surfactant-free nanospheres esters of poly(b-malic PMLABe(100) system less stable 1995from biodegradable polyesters acid) (PMLA-BeH) and to electrolyte addition andinvestigated. PLGA. Differences coagulated near pH corresponding

in colloidal to K h of carboxy group.behaviour of systems 2. PLGA/partly benzylatedrelating to copolymer PMLABeH systems did notcomposition. coagulate at relatively high

ÍelectrolyteË and low pH ofsuspending buffer.

3. Difference in the constructionof nanospheres involvingsteric component postulated.

The use of polyester nanospheres P(e)CL nanospheres 1. PH and temperature have most 246in the area of ophthalmy for were made in sterile, effect on the degradation ofoptimizing the bioavailability of isotonic solutions, with the PCL.drugs depends on their stability a preservative, and 2. Temperature of 25¡C and pH 7 1996during storage in sterile aqueous stored using different produce negligible variations inmedia, i.e. for 6 months at 25 conditions. Physical stability judged M wt over 6 months.and 40¡C. by visual appearance 3. At 40¡C and in pH 7 buffer there

and mean particle size. was 50% loss of initial Mwt over6 months and 60% in unbufferedsolution.

4. Mean particle size of all nanospheresunchanged over 6 months.




Mechanistic study of the P(D,L)LA was the 1. Nanoparticle formation thought 247formulation of polymer nanoparticles model polymer. It was to be caused only by diffusion.by the emulsification– evaluated with different 2. ‘Diffusion stranding’ 1997diffusion technique. preparation conditions mechanism suggested forResults show each emulsion and by turbidity spontaneous emulsification, withdroplet forms several nanospheres measurements. Results local supersaturation nearand interfacial cannot be entirely interface, phase transformationphenomena during solvent explained by convection and polymer aggregationdiffusion determine size of effects caused by producing nanospheres.colloidal particles. interfacial turbulence. 3. Turbidity measurements support

this interpretation.

Physicochemical characterization The choice of stabilizer 1. Characteristic signal of 248of lipid nanoparticles, for the colloidal lipid supercooled melts obtained byprepared by melt emulsification suspension restricted high-resolution LH-NMR. 1997and evaluation of their because of lipid’s tendency 2. Viscous carriers not able todrug-loading capacity and to recrystallize. immobilize the incorporated drugsustained release potential. Drug payload of as well as a solid matrix, soSevere stability problems such crystalline particles is sustained drug release over daysas high gelatin tendency, generally low, so or weeks difficult.excessive particle growth and amorphous, supercooled 3. No success as yet in combiningdrug expulsion occur. cooled melts made. advantages of both systems.

The formulation by emulsification/ BSA-loaded PLGA 1. Faster in vitro release rate of 249solvent technique and model systems were BSA in lower-Mwt PLGA over 1997characterization of biodegradable investigated. Then 7 weeks.nanoparticles for Pharmacia and Upjohn 2. Crosslinking on particle surfaceintravascular local drug drugs (U-86983, U- reduced release rate.delivery. Different emulsion 61431, U-74389G) and 3. Nanoparticle uptake by arterialsystems were used and drug dexamethasone, which wall evaluated using ex vivoloading in particles from 10% to are related to the model. 26% of infused particles30% was found. group’s interest in the initially retained.Typical particle size 60–200 nm prevention of post-angioplasty 4. Higher arterial uptake ifand 85% in range 70–165 nm. restenosis. particles have smaller mean size

and lower drug loading.5. Irradiation to sterilize the

drug-loaded particles showed noadverse effect on properties.

The long-term stability of Cyclosporin-A (CyA)- 1. % CyA in the particles stored 250cyclosporin-loaded freeze-dried loaded PpCL at 8 and 25¡C for at least 3samples of biodegradable nanoparticles obtained months was constant and no 1997polymer nanoparticles stored at by a modified nanoprecipitation change in particle size.8 and 25¡C. The simultaneous method. The 2. After 4 months the physicaleffect of technical factors effect of variables on the stability was affected.temperature of aqueous phase, stability of 100–220 nm Reconstituted freeze-driedneedle gauge) and formulation particles measured. material was observed by lightvariables (volume of acetone, The cryoprotectors used microscopy and showedamount of polymer and surfactant) were mannitol, sorbitol, considerable increase in meanwas measured. Additional glucose and threalose. size and size distribution andexperiments looked at effect there was a mean 1% increase inof freezing temperature (É70 the trapped drug.and É196¡C) and of 5%, 10% and 3. Addition of ¿10% glucose/15% cryoprotector on 100 nm threalose produced adequateparticles. reconstitution of the freeze-

dried product withconservation of encapsulatedCyA.



I .4 Summary of research papers on novel uses of nanospheres for sustained drug delivery systems usingbiodegradable polymers (1994–1997)


The elimination of Monodisperse, biodegradable, 1. Particles had dramatically 251conventional drug carriers long-circulating increased blood circulationfrom reticuloendothelial polymeric times and reduced liver 1994systems within minutes of their nanospheres were made accumulation in miceintravenous injection prevents from amphiphilic 2. 45% by weight of the drug wastheir use for site-specific drug copolymers composed of trapped in the dense coredelivery or medical imaging two biocompatible produced in a one-step process,This trial uses biodegradable blocks. easily freeze dried and then redispersedpolymeric nanospheres. without additives in

aqueous solutions.

Biodegradable nanoparticle Nanoparticles of P(D,L)LA 1. Increase in polysac. content of graft 252DNA carrier which has sites and poly(L-lysine) copolymer increased thefor both polynucleotide (PL) graft polysaccharide polynucleotide adsorption 1997adsorption and targeting copolymers capacity.ligand on the surface. formed. Graft 2. Suggested that adsorptionProduced by either a solvent copolymers (with conformation of PL moietyevaporation or a diafiltration high % polysac.) had different in homo- and graftmethod, the particle size for minimum particle size copolymer.latter method controlled by of 60 nm but not from 3. Graft copolymer nanoparticlesvarying the initial Ígraft PL. Polysac. moieties are resistant to self-aggregationcopolymerË. on particle surface and non-specific protein serum

found, by aggregation adsorption.assay, to interact with a 4. Potential use of graft copolymerspecific lectin. as good DNA carrier in vivo.

Micro- and nanoparticles PLGA and PMMA 1. Anti-CBL titres in sera determined 253prepared from biodegradable nanoparticles were by enzyme immunoassayand biocompatible copolymers loaded with either CBL (ELISA). 1997successfully used as immunopotentiating only or clenbuterol– 2. CBL–Tfn-loaded PLGA particlesantigen delivery transferrin conjugate with Quil A had obvioussystems. The study was used to (CBL–Tfn) and administered advantages immunologicallyimprove polyclonal antibody subcutaneously over the positive control.production to clenbuterol to mice. PLGA 3. The combined adjuvanticity of(CBL), a modern hapten. particles given with or Quil A and the PLGA nanoparticles

without the saponin gave increased positiveadjuvant Quil A. response and higher antibodyThe current methods of titles in all four mice tested (cf.raising antibodies to positive control).CBL were the positive 4. Sustained immunogen releasecontrol. from nanoparticles, so

reduction in immunizingfrequency over 15 week study

I .5 Summary of research papers on sustained drug delivery profiles using biodegradable polymers (1993–1997)


The effect of incorporating Controlled release 1. Matrix stability of up to 39 254gelatin, an interactive polymer formulations of fenthion weeks enhanced to 55 weeks(below its isoelectric point), (10% and 20%) were with gelatin incorporation. 1993on the matrix stability and prepared with sodium 2. Variation in average release raterelease of fenthion from crosslinked carboxymethyl cellulose 5·15–11·91 mg weekÉ1

matrices (with Cu2½ and gelatin. 3. Variation in apparent diffusionions) of carboxymethyl coefficient 4·00 Ã10É9 andcellulose. Formulations 2·01 Ã10É8 cm2 sÉ1.analysed to monitor physical 4. Fenthion exhibits a Fickianintegrity of matrices and release pattern, but the mixturerelease profile of fenthion in containing gelatin shows morewater. non-Fickian release character.

5. Postulated use in mosquitocontrol programmes.




Biodegradable microspheres Purified tetanus toroid, 1. Protein release patterns related 255prepared by either solvent extraction a high-Mwt M8 protein, to polymer type and Mwt. Rateor solvent evaporation of entrapped in PLLA and was lower for PLLA than PLGA. 1994WOW multiple emulsion system. P(D,D)LGA microspheres. 2. Low-Mwt (3000) PLGA producedInvestigation of physical All types of burst then increased release rate.integrity and antigenicity of the particles (independent of Mwt of 3. Entrapped tetanus toroid significanceprotein treated under different polymer) displayed high more immunogenic than fluidprocessing conditions. Partial protein-loading toroid in mice, but antibodyloss of antigenicity associated efficiencies (¿80%), but response not significantly different inwith lyophilization process and microparticle size magnitude and duration fromaffected by the nature of related to Mwt of similar dose of aluminiumorganic solvent. polymer. phosphate-adsorbed toroid.

An in vitro study of release of a Microspheres of PLLA 1. The release rate of dye was 256dye using spectrofluorophotometry. and PGA used to deliver controllable by changing theCultured bovine substances to retinal Mwt and monomer composition 1995RPE cells were treated with the pigment epithelial of the copolymers in vitro .microspheres to study phagocytosis (RPE) cells directly. 2. Phagocytosis by RPE cells andof the particles by the Fluorescent dye intracellular dye releaseRPE cells using fluorescent (rhodamine (6GX) was occurred during incubationand transmission electron the drug marker 3. In vivo studies showedmicroscopy, and involved. degradation of microspheres inpost-phagocytosis intracellular the RPE cytoplasm, butrelease of dye was evaluated. fragments seen up to 4 weeks.An in vivo study in rabbits was 4. Retinal architecture overlyingalso carried out. delivery site well preserved.

The biodegradability and Microspheres were 1. PLA2000 microspheres undergo 257antigen release characteristics fabricated by blending controlled degradation andof polyester blend microspheres high-Mwt (61 000) produce continuous release 1995using BSA. The kinetics of P(D,L)LA and low-Mwt profiles of the BSA.degradation were derived from (2000) PLA 2. Immunization of rabbits bythe rate of formation of H½ ions (PLA2000). The model subcutaneous injection of BSA-loadedduring ester linkage hydrolysis protein antigen BSA particles enhancedmeasured by pH changes in was used. antigenicity of BSA andmicrosphere suspensions and increased significantly thethe water uptake. duration of humoral immune.

The influence of microcapsule PLA/PGA blends were 1. Microparticles used all had 258formulation on the continuous used with model continuous release profiles.release of proteins from biodegradable proteins BSA, 2. In vivo study showed continuous 1995polymer blends. In transferrin and trypsin release for more than 142 days.vitro study of biodegradation to make microparticles. Adjuvanticity superior toand protein permeability. In Control experiments Al(OH)

3, comparable with

vivo study in mice of potential and blank checks Freund’s incomplete adjuvant.vaccine adjuvant. carried out. 3. Immunization of animals with

BSA-loaded microcapsules moreeffective than one prime and twobooster injections of BSA/saline.

Properties and drug release Solvent evaporation 1. Drug release behaviour, encap- 259characteristics of PHB/PHBV technique used to sulation efficiency and loadingmicrocapsules. Some were encapsulate model drug loading influenced by polymer type. 1995made incorporating a polyphosphate 2,7-dichlorofluorescein 2. DSC showed crystallinity ofCa2½ complex into in PHBV or PHB polymer decreased with HVthe membrane. The microcapsules. incorporation. Hence increasedmorphology changed with the segment mobility and increasedtype of polymer, the drug loading and encapsulationintroduction and the complex efficiencyincorporation. 3. DSC showed the complex became

an integral part of the membrane.

Biodegradable PCL has been PeCL microspheres 1. The immune response compared 260considered as unsuitable for loaded with BSA were in size and time kinetics with three 1996protein delivery because of its studied. Slow rate of conventional injections.poor permeability to degradation of the PCL 2. As PCL degrades slowly comparedmacroparticles. An in vivo was monitored and so with PLA or PGA, low pH canstudy was carried out in rats no increased acidity. adversely affect the vaccineusing a single injection of BSA-loaded antigenicity. Thus PCL hasPCL microspheres. potential as a vaccine carrier.




Microspheres of PHBV were Spherical microporous 1. BSA loss produced loading 261/262fabricated using single (SET) reservoir-type PHBV efficiency Ä15% with SET andand double (DET) emulsion (10·8% HV) (Mwt 330 000)/20% 12% loading efficiency with 1996techniques with solvent evaporation. PCL microspheres DET.¿75% yield of microcapsules, prepared containing 2. BSA release could be monitored% incorporation BSA had no BSA-loaded agarose. for up to 24 days for botheffect on particle size. Size BSA loss from methods.distribution (SET, 6–50 lm; (i) partitioning into the 3. Total cumulative release of BSADET, 21–200 lm) for 20% BSA-loaded continuous aqueous as % of trapped drug showedmicrospheres obtained. phase, release only marginally altered

(ii) micropores during by theoretical % loading andprecipitation concomitant was influenced as much bywith solvent micropore numbers andevaporation. diameter.

The effects of drug properties, PLA and PLA/PEG 1. PLA/PEG copolymers are more 263particle size, drug loading and microspheres were hydrophilic than PLAmethod of encapsulation were prepared with drug homopolymers and have lower 1997examined. The release rate of loading by coarcervation T

gvalues.

the hydrophobic drug technique using solvent 2. PLA/PEG microparticles(lidocaine) and the hydrophilic evaporation and rougher than those of PLA.drug (propranolol hydrochloride) emulsion methods 3. Drug release from PLA/PEGwere examined. affected by copolymer properties.

4. In PLA/PEG release rate of bothhydrophilic and hydrophobic drugsfaster than in PLA.

5. The emulsion method producedporous particles with adequatedrug permeability.

Antibody responses in rabbits Micropheres of linear 1. Similar antibody response from 264tested after single copolymers of P(D,L)LGA single injections.immunization with viral (50/50) or 2. Stronger, longer and markedly 1997glycoprotein-loaded branched oligoesters of biphasic antibody responsesmicrospheres or immunity- (D,L)LA were observed when immunized withstimulating complexes compared with BHV-1-loaded branched chain(ISCOMs) were studied in ISCOMs. All had oligo-LA microspheres.long-term (31 week) incorporated glycoproteins 3. Indirect evidence of proteinexperiment. of bovine incorporation into both

herpesvirus-1 (BHV-1). microspheres and ISCOMs.

The effect of the PLGA/PEG Microparticles of 1. Linear release profiles and 265blend ratio on the release PLGA (50/50) and PEG constant release rates werekinetics of the microspheres blends with entrapped calculated for FITC–IgG-loaded 1997produced by double model drugs (FITC–IgG particles with PEG wt% Ä5%emulsion/solvent extraction and FITC–dextran) 2. FITC–dextran-loaded particlestechnique with high efficiency had a biphasic release profileIn vitro studies showed initial and rates varied with % PEG.burst effect dependent on 3. Feasibility of release profilePLGA/PEG blend ratios and modulation by PLGA/PEGrelease rate increased in direct blend ratio shown.relation to the PEG content forup to 28 days.

Non-invasive in vivo PaHAs were made into 1. MRI makes it possible to 266monitoring of drug release and sandwich-like tablets monitor water content, tabletpolymer erosion from and implanted. shape and response of the 1997biodegradable polymers by Nitroxide radicals used biological system.electron paramagnetic resonance as model drug-releasing 2. MRI gives direct proof for in(EPR) spectroscopy and NMR substances containing vivo mechanisms of polymerimaging (MRI). 15N and 14N. erosion.

Biodegradable polymeric A scleral plug made of 1. Ganciclovir was released 267device for sustained intravitreal biodegradable PLGA throughout 10 week period.release of ganciclovir in (Mwt 121 000) with 2. ÍGanciclovirË in vitreous 1997pigmented rabbits. In vivo 25% ganciclovir was maintained in therapeutic rangerelease of ganciclovir was implanted. to treat CMV retinitis for 12evaluated by spectrophotometry Biocompatibility was weeks.and ÍganciclovirË measured determined by several 3. No significant retinal toxicityusing HPLC. techniques. observed.




Biodegradable microspheres Polylactide (PL) and 1. Morphology of the particles was 268were prepared by spray drying polylactide-co-glycolide investigated.for an in vitro evaluation of (PLG) (with Mwt 2. In vitro dissolution varied with 1997the release profile of acycloguanosine 109 000–3000). The Mwt of the polymer.and an in vivo study of process gave good. 3. In vivo study showed sustainedhumoral drug concentration encapsulation of drug release obtained.after intravitreal administration acycloguanosine.in rabbits.

Microspheres were made using P(L)L, P(D,L)L, 1. The hydrophobicity of PHB 269a double emulsion/evaporation P(D,L)LG, PHB and microspheres compared with PL/PLGmethod. The particles were PHBV polymers were particles was confirmed 1997characterized for size distribution, used and BSA was and the generation of adrug loading, release incorporated in the significant immune responsekinetics, surface morphology microspheres. was delayed using PHAs.and hydrophobicity. The 2. The effect of the Mwt of theinfluence of these factors on PHB was also investigated.the dynamics of immuneresponse following i.m.administration was studied.

The characterization and Microspheres of 1. 95% average yield of L-158 809 270evaluation of in vitro and in P(D,L)L blend microspheres (10% w/w). Averagevivo release mechanism of formulation were made microsphere diameter 1–3 lm. 1997biodegradable microspheres by spray-drying 2. A significant ‘burst’ effectcontaining an antihypertensive technique. (Ä15%) was observed in alldrug (L-158 809). Characterization of release experiments. FollowedIn vitro study based on microspheres and by almost zero-order releasemicrospheres of blends of high- polymer blends carried kinetics.and low-Mwt polymer. out using DSC, SEM, 3. In vivo studies with two differentIn vivo study studies evaluated confocal laser formulations showed a strongby (L-158 809) antagonist AT1 microscopy and size shift of angiotensin IIfunction against the shift of the analysis. dose–response curve.normal dose–response curve of 4. The system is described as ablood pressure induced by multiparameter controlledangiotensin II. release system in which the drug

is molecularly dispersed.

I .6 Summary of research papers on biodegradation, biocompatibility and cytology studies of biodegradablepolymers used for controlled drug delivery


Microparticle/polymer Four commercially available 1. Rate of release of protein 271degradation rates of PLG and PLG polymers of dependent onin vitro release of model known composition and Mwt (i) Mwt of polymer 1994protein (ovalbumin) were were used to prepare (ii) copolymer compositionassessed by three methods. microspheres. (iii) protein loading.

1. Surface morphology using Each method confirmed 2. Generally the rate of polymerSEM. degradation increased with degradation and the rate of

2. Weight loss. high glycolide content protein release showed good3. Molecular mass using GPC. and low Mwt of PLGs. correlation.

The effects of polymer P(D,L)lactide-co-glycolide 1. Initially non-porous PLGA became 272degradation on drug release (50 : 50 PLGA) was more porous with degradationfrom the bulk were studied used with the model drugs and there were concomitant 1995using (i) 5-fluoro-uracil (5-FU), increases in permeability and(i) SEM to look at morphology (ii) cyclosporin (CyA). transport parameters of drugs.

changes, 2. Sharp transitions at Mwt Á5000(ii) transport properties and and porosity 70–80% were

mechanism (TM). attributed to changes intransport mechanism.

3. Change corresponds to degradationdeveloping pore network, whichbecomes the dominant pathway.

4. Shift in TM cause of typical trimodal,final, rapid releaseprofiles from bulk degradationpolymers.




In vitro biodegradation study of Spherical BSA-loaded 1. Surface erosion of microspheres. 273BSA-loaded PHBV/PCL microspheres. microporous matrix-type (a) Limited in HB with increase inIncubated in Hank’s microspheres of Pore diameter, coalescence 1996buffer (pH 7·4) (HB), newborn PHBV (11% HV)/20% and pits forming.calf serum (NCS), 1·5% PCL blend were made (b) Micropores disappeared inpancreatin (P) and synthetic using an O/W Pancreatin and diameter ofgastric juices (SGJ) over 30 emulsion/solvent Particles decreased. Fracturing ofdays and measuring %wt loss, evaporation technique. Surface then matrix break-upgravimetric changes and Greatest %wt loss was easier.surface morphology using with newborn calf (c) In SGJ, little surface erosionSEM. serum and decreased as but flaking and bulk erosion

follows: broke up matrix.NCS ¿P ¿SGJ ¿HB. (d) In NCS, spherical shape keptIncreased theoretical % but diameter reduced. Bulk erosionloading only increased %wt with macropores extending intoloss with SGJ and HB. spheres which appeared hollow.

2. Exogenous enzyme activityassumed to enhance biodegradationin NCS and P comparedwith simple ester hydrolysis.

PL microspheres biodegradable Two samples of P(D,L) 1. Varying the weight ratio of two PLs in 274in days can be used for chemoembolization lactide (PL) of different Microspheres is an efficient waypurposes. Microspheres Mwt were combined to control in vitro degradation 1996made by oil-in-water PL (M

n65 000) for high kinetics.

emulsion/evaporation. Final mechanical strength and 2. Morphology changes seen withpolymer content was compared PL (M

n3500) for rapid SEM.

with initial composition of the oil biodegradability. 3. Mwt data and increased acidity ofphase. DSC analysis was used incubation medium were used toto determine if a monophase determine kinetics ofblend was formed. degradation and progress of ester

group hydrolysis.

In vitro release of LHRL P(D,L)LGA microcapsules 1. Using an in vitro test system, 275agonist from injectable produced to that successfully predicted inmicrocapsules of PLGA tested optimize conditions for vivo drug release, PLGA weight 1996to clarify differences in the release of leuprorelin loss and Mwt decrease delayed.release with in vivo systems. an LHRH agonist. 2. In vivo accumulation of PLGApH, ÍsaltË and osmolarity of degradation products and ratio ofdispersion medium were found LA : GA in copolymer most effectto change drug release. on PLGA weight loss.

Cell response of cultured P(R)-3-HB is known to 1. High-ÍPHBË (¿10 lg mlÉ1) phagocytosis 276macrophages, fibroblasts and biodegrade. A study of a dose dependent, associatedco-cultures of rat Kupffer cells new block copolymer with cell damage in macrophages 1996and rat hepatocytes on short-chain produced small but not in fibroblasts.PHB (M

n2300). crystalline, short-chain 2. Low-ÍPHBË macrophages not

PHB, ideal for a activated either, so biocompatible.biocompatibility study. 3. No cytotoxicity or changes in

albumin secretion shown withco-cultures.

In vitro biodegradation study of A PHBV (11% HV)/20% 1. Only 5%, 10% and 15% BSA-loaded 277PHBV/PCL microspheres. PCL blend loaded particles in SGJ and HB showedIncubated in Hank’s buffer (pH with BSA was made increased %wt loss with BSA loading. 19977·4) (HB), newborn calf into microspheres using 2. Surface erosion of microspheresserum (NCS), 1·5% pancreatin a W/O/W double in each medium.and synthetic gastric juices emulsion/solvent (a) Limited in HB, with increase incontaining 10% pepsin (SGJ) evaporation technique. pore diameter, coalescence andover 30 days and measuring Greatest %wt loss was pits forming.%wt loss, gravimetric changes with newborn calf (b) In pancreatin, increases degradationand surface morphology using serum and decreased as with loss of spherical shape andSEM. follows: partial loss of structure to surface

NCS ¿P ¿SGJ ¿HB and bulk.(c) In SGJ, significant and bulk

erosion but no loss of shape.(d) In NCS, surface and bulk

erosion, total loss of shape andstructure after 30 days.



I .7 Summary of current research into other biomedical uses for biodegradable polymers


The effect of wetting on P(L)LA Porous discs of the 1. Prewetting with EtOH increased 278and P(DL)LGA foams for polymer foams of volume void filled after 48 h fromtissue culture. Two-step P(L)LA and P(DL)LGA 23% to 79% for PLLA and from 59% 1994immersion in ethanol for 1 h (85/15 and 50/50). to 97% for PLGA (85/15) discs.and then water was used 2. Water entry even after 1 h wasto wet discs. close to plateau value for all prewet

polymers tested.3. Use in uniformly seeding 3D

biodegradable polymer substratesfor cell and tissue culturerealized.

Preparation and characterization 1. Preparation of P(L)LA 1. Salt weight fractions (wt%). 279of P(L)LA foams. membranes of (a) 50–60 asymmetric membranes–particleParticulate-leaching method controlled porosity, size had no effect. 1994used to prepare very porous surface/volume ratio (b) 70–90 homogeneous membranes—poresbiodegradable polymer and crystallinity. interconnected. Membranemembranes using 2. Sieved particles of properties independent of salt used but(i) casting of polymer/salt NaCl, Na tartrate and Na dependent on particle size.

membrane citrate salts were used. 2. Porosity increased with salt wt%(ii) dissolution of the salt. 3. Median pore diameter increased as

size of salt particle increased.4. Polymer/salt composite

membrane quenched or annealedto adapt crystallinity.

5. Foams 99·9% salt-free and withporosities up 0·93 obtained.

Fabrication of pliable, porous, PLGA/PEG blends were 1. A wide range of shear moduli, 280biodegradable foams to made by solvent-casting/ porosities and median poreengineer soft tissue. The effects particulate-leaching diameters were obtained. 1996of four different processing technique. Parameters : 2. Initial salt weight fraction and blendparameters on pliability and PLGA copolymer ratio, ratio have most significant effectpore morphology of PLGA/PEG blend on physical/mechanical scaffoldbiodegradable scaffolds. ratio, initial salt weight properties.

fraction, salt particle 3. Enhanced pliability of 3D foamssize. of PLGA/PEG shown by

tube formation.4. Potential uses : regeneration of

soft tissue (skin), intestine/vascular grafts.

Synthesis of polymer network UV photopolymerization 1. Glassy and transparent networks. 281scaffolds from L-lactide and of triacrylated 2. Gel content approximately 90%.PEG and their interaction with lactic acid oligomer from DP of lactide and amount and 1997cells. Scaffolds with cell glycerol base (GL) plus Mwt of PEG had no effect.adhesion resistance, ligand monacrylated PEG 3. All networks showed relativelyimmobilization and biodegradation GL–PEG cross-linked low swelling in water becausecharacter for use in tissue polymer networks of crosslinking.engineering. Non-toxic obtained. 4. No melt endotherms but T

gmacromolecules used. indicated PEG phase mixing.

5. PEG shown in surface and ifhigh-Mwt PEG incorporatedmore. More hydrophilicity insurface/less cell adhesion.

In vivo biocompatibility study Synthetic polymer 1. Early re-inflammation ¼surgical 282in rats of a biodegradable matrix with dense top implantation trauma.matrix used as a cell-seeded layer and porous underlayer 2. Neovascular and fibrous tissue 1994skin substitute for large-scale of P-ether/P-ester ingrowth in porous underlayerskin defects and skin substitute. (PEO : PBT) copolymer within 2–4 weeks for all matrices.N.B. Previous study on cell called PolyactiveTM 3. Copolymer and PLLA showsubstrate properties and and also PLLA were increased fragmentation andphysicochemical character of investigated. liquifaction.matrix. Implant for large body 4. After 1 year small polymersurface area and study fragments retrieved from site.biocompatibility at 2, 4, 13, 26, 5. No systematic effects on animal52 weeks. organs, so has potential for use as

skin substitute.




Design and physicochemical Biodegradable PLA and 1. Opacity increased during 283properties of new biodegradable PCA polymers produced hydrolysis.temporary wound from research programme 2. Variation in water vapour 1995dressings, used in the treatment BMFT/FRG, 01KG8809/7. permeance with method ofof burns, which meet the Films were made of the measurement.objectives of skin substitute in copolymer and 3. Mechanical propertiesapplication, safety and comfort. characterized and tested characterized; maximum elongation

for degradation. (37¡C) ¿2000% and very lowelasticity modulus.

4. Aptitude for wound dressingestablished.

Development of polymeric P-e-CL or blends of P-e-CL 1. 30% MePEG in PCL: decreased 284surgical paste formulation for with methoxypoly(ethylene T

mby 5¡C, decreased tensile

local delivery of taxol (an glycol) (MePEG) strength from 153 to 27 N cmÉ2. 1996inhibitor for tumour growth). In of Mwt 350 were increased PCL crystallinityvitro release of taxol was incorporated with taxol from 42% to 51%.carried out using HPLC assay and characterized. In 2. Release profile of taxol from PCLfor taxol. Antiangiogenic vitro conditions : 37¡C in unchanged.activity assessed using a chick phosphate-buffered 3. Uptake of water increased withCAM. saline at pH 7·4. added MePEG but taxol release

rate decreased.4. CAM assay showed

antiangiogenic activity.

In vitro and in vivo study of PD, LLA–PEG-PD, LLA 1. Sustained release of taxol from 285biodegradable polymeric paste and blends of low-Mwt copolymer paste over 2 months byformulations for local delivery PD, LLA and PCL diffusion and erosion. 1996of taxol. Release of taxol into melted and mixed with 2. The blend released taxol mainlyPBS albumin buffer was taxol to produce pastes. by erosion and there was burstmeasured by HPLC. Characterization of followed by slow release.

polymers and pastes 3. Both pastes significantly reducedcarried out. tumour growth in mice after 16

days.4. Pastes with faster in vitro release

rates more effective inhibitors oftumour growth.

APPENDIX II PATENTS

A basic search was carried out using the World PatentIndex (WPINDEX), Derwent Information Ltd, on theSTM network for relevant patents on biodegradablepolymers. The following appendices are based on appli-

cations, e.g. packaging, pharmaceutical and agriculturaluses, and also developments in microbial and chemicalsynthesis, processing and blending.

II .1 Packaging, hygiene and non-pharmaceutical applications

Year Reference Title Author(s)/organization(s)

1990 90-317488 Biodegradable film used as wrapping material for Chuko Kasei Kogyo KKfoods—obtained by applying aqueous emulsion ofpoly(hydroxybutyric/valeric acid) copolymeron glass plate and heating.

1991 91-261627 Biodegradable mouldable material for disposal by Koyama, M., Suzuki, M. & Tokiwa, Y.burying—comprises natural high-Mwt substances, Agency of Ind. Sc.& Tech.Chuo ;e.g. starch thermoplastic resin and Kagaku Ltdoptional filler.

1991 91-354911 Non-woven fabric composite for hygiene articles, Sturm, V. & Utz, K.etc.—by extruding layer of molten polymer onto AOE Plastic GMBH; BP Chem.spun-bonded web or staple fibre web under Plastec.GMBHreduced pressure.




1992 92-105979 Decoration, e.g. wreath, or packaging for short- Mans, L. H. A.term use—has ribbon made of biodegradable Mans, L.H.A.Holdingsplastic for easy disposal.

1992 92-222619 Novel microbiologically degradable plastic Agency of Ind. Sc.& Tech.Chuo ;moulding for afforestation—comprises Chem. Ind.Co. Ltd.biodegradable aliphatic polyester and calciumand/or magnesium carbonate, for disposableoutdoor goods including cutlery.

1992 92-309206 Non-woven material for disposable nappies, bed Emirze, A., Eschwey, H., Giesen-underlays, etc.—comprises thermoplastic spun Wiese, M., Grill, M., Kauschke, M.,endless filaments, consists of at least 50 wt% Klein, B. & Seidler, H.biologically degradable poly(caprolactone) of Freudenberg FA CarlMwt 35 000–70 000.

1993 93-036358 Biodegradable, liquid-impervious films used as Toms, D. & Wnuk, A. J.back sheets for nappies, etc.—comprises blend of Procter & Gamble Co.interpenetrated network of destructured starchand copolymer, preferably vinyl alcohol copolymer, and an aliphatic polyester.

1993 93-059674 Paper economy with paper handkerchiefs— Weigele, R.involves handkerchiefs stored in container that Weigele , R.can always be refilled.

1993 93-120869 Separable laminates used as wrapping materials Toppan Printing Co. Ltdfor foods, etc.—comprise biodegradable plasticlayer laminated between layers.

1993 93-182613 Structure having controlled water resistance— Brunger, P. & Kemmish, D. J.comprises a core material having dry but not wet Zeneca Ltd; Monsanto Co.strength and a coating of a water-resistantbiodegradable polymer.

1993 93-345009 Liquid-impervious biodegradable films—comprising Adams, J. M., Chang, P. I.,interpenetrated network of destructured starch McBride, R. K. & Ray, C. D.with ethylene–acrylic acid or –vinyl alcohol Tredegar Ind. Inc.copolymer, with aliphatic polyester.

1994 94-000953 Biodegradable carrier for denitrifying bacteria Groten, R., Heidecke, G.,in water purification—made of spun fleece of Mansbart, T. & Siekermann, V.poly(caprolactone) filaments, optionally blended Freudenberg FA Carlwith other biodegradable polymer.

1994 94-018030 Heat-moulding composition—contains starch Fleche, G., Gosset, S. & Videau, D.component, biodegradable polyester and salt of Roquette Freres SAhydroxycarboxylic acid, givingbiodecomposable water-resistant articles.

1994 94-114861 Biodegradable paper container—comprises paper Toppan Printing Co. Ltdlaminate with biodegradable plastic layer

1994 94-201936 Biodegradable packaging material especially for Kammerstetter, H. & Schroeter, J.liquid or solid foodstuffs—having improved oxy- Buck Werke GMBH & Co.gen, aroma and/or water vapour blocking properties.

1994 94-249165 Poly(hydroxyalkanoate) film product—by melt Waddington, S. D.extrusion onto chilled roller to form film, Zeneca Ltd; Monsanto Co.followed by heating thereof to crystallize thepolymer.

1994 94-338741 Biodegradable fibre preparation used for Zeneca Ltdagricultural and fishery material, etc.—by meltspinning a mixture of poly(beta-hydroxyalkanoate)

1994 94-338742 Biodegradable multifilament used for fishery Zeneca KKmaterial—composed of multifilament comprisingpoly(beta-hydroxyalkanoate), having good heatresistance.




1995 95-067312 Bonding articles together using Kemmish, D. J.poly(hydroxyalkanoate)s—which are Zeneca Ltd; Monsanto Co.biodegradable, useful in packaging, cartonsealing, sanitary towels, disposable nappies,hospital equipment, etc.

1995 95-092171 Laminate manufactured by vacuum deposition of Utz, H.functional layer between two films—using non- Fraunhofer Ges Foerderungmetallic transparent material acting as a barrier Angewandtenand bonding layer, eliminating need forlaminating adhesive and allowing recycling.

1995 95-117776 Toilet material for pets, giving good Suzuki Sogyo KKbiodegradability—including deodorant and/orantibacterial agent, biodegradable resin andoptional aromatic material.

1995 95-215202 Manufacturing biodegradable cellulose and/or Kemmish, J. D., Montador, J. H. &cellulose acetate film having good water barrier Kemmish, D. J.properties—comprises applying aqueous Zeneca Ltd; Monsanto Co.suspension of at least partly amorphous particlesof poly(hydroxyalkanoate) to cellulose and/orcellulose acetate film and heating.

1995 95-162679 Increasing transparency of moulded articles or Bueler, F. S., Fanelli, R.,sheets of thermoplastic starch or starch–polymer Treutlein, R. & Zechner, T.blend—by treating granulate, articles or sheets EMS Inventa AGwith silicone oil, animal or plant fat or oils orsolution of synthetic or natural polymer.

II .2 Pharmaceutical and agricultural patents

Year Reference Title Authors(s)/organization(s)

1979 79-15983B Microcapsule and microspherule preparation for Sandoz SAmedicaments, etc.—by addition of phase separationagent to solution of polymer with solution ordispersion of active compound at É100 to É40¡C.

1979 79-68042B Microsphere production from particle dispersion in Fong, F. W.polymer solution—by adding phase separation agent Sandoz Inc.at low temperature.

1986 86-049030 Microcapsules for controlled release of Sandow, J. & Seidel, H. R.regulatory peptide(s)—containing poly(D-3-hydroxy Hoechst AGbutyric) acid as biodegradable carrier.

1984 84-301706 Slow release composition for treating aquatic Dunn, R. L. & Lewis, D. H.plants—containing herbicide, etc. in strands of Stolle Res.& Dev.polymeric, e.g. poly(lactic acid), fibre.

1986 86-226722 New composite materials useful as hard tissue Dorman, L. C. & Meyers, P. A.prosthetics—comprise synthetic biodegradable Dow Chem.Co.polymers and unsintered calcium phosphate biomaterials,with optional pore-forming agent,and polymerized in situ .

1986 86-306804 Biodegradable polymer with low free acid Miyaggawa, T., Ogawa, Y.,content—and used in drug microencapsulation is Okada, H. & Yamamoto, M.e.g. hydroxy-acid ester or poly(cyano-acrylic- Takeda Chem. Ind. Ltd ; Wako Pureester). Chem. Ind. Ltd

1989 89-066688 Polymer-encapsulated microsphere preparation— Lewis, D. & Sherman, J. D.89-150631 by adding core material to polymer solution in Stolle Res.Dev.Corp.

non-solvent for core, adding synthetic orvegetable oil and quenching with secondnon-solvent.




1989 89-146427 Microcapsule production containing soluble protein Sandow, J. K. & Schmeidel, R.or peptide—using mixture of poly(hydroxybutyric Hoechst AGacid) and poly(lactide-co-glycolide).

1991 91-324928 Biocompatible microspheres for parenteral Spenlehauer, G., Veillard, M. &administration—containing biodegradable and Verrechia, T.biocompatible polymer and surfactant, used for Rhone Poulenc Rorer SAtreating inflammation, infections and cancer.

1992 92-060919 Auxiliary material for fixing artificial joint or Terumo Corp.filling bone defects—comprises biodegradableor bio-absorbable material and leaves structureto allow formation of fresh bone.

1992 92-176586 Controlled release microparticles containing Canal, T., Carli, F., Lovrecich, M.polysaccharide gelling agent, etc.—comprising & Lourecich, M. L.biodegradable polymer, interfacial agent, Vectorpharma Int . SPAamphiphilic polymer and active substance,especially calcitonin, etc.

1992 92-391146 Synthetic bone–tooth filler for artificial limb Fuse, T., Machino, H., Matuura, K.,and denture material—comprises water-insoluble, Niijima, K., Otani, S. &biodegradable coating applied to substrate with Yanagisawa, S.porous surface layer. Mitsubishi Kasei Corp.; Res.Dev.Corp. Japan

1993 93-153559 Dispenser, especially for controlled release of Buschmann, E., Kiessling, U.,sexual pheromone(s)—used in plant protection, Neumann, U. & Renz, G.comprises pheromone-impermeable container sealed BASF AGwith pheromone-permeable foil.

1993 93-164265 Slow release fertiliser—has multiple alternate Nippon Steel Chem.Co.;coatings of a biodegradable polymer (I) and a Nippon Steel Corp.water-soluble polymer (II).

1994 94-135531 Particles of crystallizable polymer coated with Clauss, J., George, N.,surfactant of phospholipid to maintain Horowitz, D. M., Hunter, B. K.amorphous state—for making shaped articles, e.g. & Sanders, J. K. M.fibres, for controlled release of pharmaceutical Zeneca Ltd; Monsanto Co.or agrochemical or for removal of solvent, etc.

1994 94-183125 Sustained release microspheres requiring no Kino, S., Mizuta, H. & Osajima, T.surgical implant—contain hydrophobic Yoshimoto Pharm. Ind. KKantipsychotic encapsulated in biodegradablepolymer, allowing prolonged therapeutic effect byinfrequent administration.

1994 94-193872 Preparation of drug-releasing biodegradable Miettinen-Laehde, S. S. &compositions used for antibiotics, etc.—by Toermaelae, P. O.ultrasonically melting biodegradable polymer Orion-Yhtymae OYmatrix and pharmaceutical substance.

1995 95-041205 Sustained release preparation of water-soluble Kirin Brewery KKpeptide hormone—comprising fine tube with speci-fied diameter made of water-insoluble biode-gradable, high-Mwt substance, containing core.

1995 95-044929 Biodegradable urethral stent—substantially shorter Viherksoski, E.than the urethra in which it is totally installed.

1995 95-351303 Polymer microspheres useful for drug delivery Coombes, A. G., Davis, S. S. &and targeting, etc.—prepared by mixing solution Schacht, E.of water-soluble polymer and solution of Univ .Ghent ;conjugate of poly(ethylene glycol) and Univ.Nottinghamevaporating first solvent.

1995 95-122881 Absorbable polymer composition for surgical and Arnold, S. C., Reilly, E. P. &medical devices, e.g. sutures—comprises Scopelianos, A. G.absorbable poly(ester anhydride), polylactone or Ethicon Inc.; Johnson & Johnsonpoly(iminocarbonate) and polysuccinimide asbio-absorbable reinforcing filler.




1996 96-040269 Degradable multilayer melt-blown microfibre Joseph, E. G. & Rutherford, D. R.webs used for surgical dressings, etc.— Minnesota Mining & MFG Co.comprise layers of polyolefin, PCL anddegradable resins.

1996 96-201507 Long-term fertiliser rods—consist of a shell Pluder, K.made of segments of polymer material which Buna GMBHbiodegrade at different rates and are filledwith plant nutrients.

1996 96-267854 Surgical device, e.g. staple or ligating clip— Arnold, S. C., Reilly, E. P. &comprises absorbable matrix made of e.g. Scopelianos, A. G.polyamide and discrete filler material Ethicon Inc.comprising polysuccinimide to increasestiffness of polymer, and device can withstandheavy loads.

1996 96-350294 Biodegradable resin composites used as Mitsubishi Gas Chem.Co. Ltdcontainers for medical products—comprise poly(3-hydroxybutyric acid) and poly(caprolactone)with specified capillary size.

1997 97-044600 Expandable intra-luminal stent—made from sheet Williams, M. S.materials curled into cylinder having over Advanced Cardiovascular Systemslapping edges, and incorporating externalprotrusions which engage apertures to hold thestent in the expanded condition.

1997 97-459058 Membrane for tissue or bone regeneration— Hutmacher, D. & Kirsch, A.contains at least three layers and is flexible Kirsch, A.and biocompatible.

II .3 Microbial and chemical methods, synthesis and degradation patents


1991 91-051341 Construction and modification of polyester bio Peoples, O. P., Sinskey, A. J. &polymers—by introduction of poly(hydroxy Sinskey, A. L.butyrate/alkanoate) genes into bacteria or Massachusetts Inst . Technologyplants.

1991 91-305113 Purification of biodegradable polyester(s)—by re Heya, T., Ogawa, Y. & Yamada, M.precipitation from organic solvent with water Takeda Chem. Ind. Ltdto remove low-Mwt contaminants.

1992 92-224054 Block copolyester useful for biodegradable Nippon Kayaku KKtransparent film—prepared by reacting poly(3-hydroxybutyric acid) with 6-hexanolide inpresence of acid catalyst for high purity.

1992 92-346704 Poly(hydroxybutyrate) production—by two-stage Kim, K. & Lim, K.culturing ofAlcaligenes eutrophus strain. Korea Synthetic Textile Co.

1992 92-398873 Transgenic plants for poly(hydroxyalkanoate) Bright, S. W. J., Byrom, D. &production—containing genes encoding enzymes Fentem, P. A.catalysing poly(hydroxyalkanoate) production. Zeneca Ltd; ICI plc

1993 93-058785 Transgenic plants producing Dennis, D. E., Poirier, Y. &poly(hydroxyalkanoate) polymer(s)—obtained by Somerville, C. R.transformation with DNA encoding 3- Univ .Michigan State ; Univ .ketothiolase, acetoacetyl-coA reductase and Madison JamesPHE synthase.

1993 93-058791 Production of poly(hydroxyalkanoate) Liebergesell M. & Steinbuchel, A.polymer(s)—by culturingChromatium vinosum Zeneca Ltdof transformants containingC. vinosum PHAbiosynthetic genes.




1993 93-404052 New hydroxyalkanoate polymer derivatives—with Yalpini, M.terminal alkenyl or alkyl group, andproduction of partially degraded hydroxyalkanoate polymer derivatives.

1994 94-012388 Biodegradable block copolymer for packaging, Doi, Y.agriculture or medical use—obtained by ring- Sumitomo Metal Ind. Ltdopening polymerization of lactone(s) inorganic solvents, giving good miscibility topoly(hydroxybutyrate) and poly(caprolactone).

1994 94-199818 New fibre-producing plants expressing hetero Maliyakal, J. & John, M.logous bioplastic, especially Monsanto Co.poly(hydroxyalkanoate)—contain enzymesinvolved in bioplastic synthesis under controlof fibre-specific cotton promoter.

1994 94-265917 Poly(3-hydroxybutyric acid) production by ring- Hori, Y., Nishishita, T. & Yamaguchi, A.opening polymerization—of beta-butyrolactone Tagasago Int . Corp.;in presence of tin compound catalyst. Takasago Perfumery Co. Ltd

1994 94-542560 Polyester composition. Hammond, T., Liggat, J. J.,Montador, J. H. & Webb, A.Zeneca Ltd

1995 95-012035 Biodegradable resin composition preparation for Sumitomo Seika Chem.Co. Ltdfilms or sheets—by blending biodegradableplastic with water-soluble thermoplastic resin.

1995 95-075207 New block copolymers with hydrophilic and Domb, A. J., Gref, R., Langer, R.,hydrophobic blocks—covalently bonded to Minamitake, Y. & Peracchia, M. T.multifunctional compound, used to make Massachusetts Inst . Technologyparticles for controlled and optimal targeteddelivery of therapeutic or diagnostic agents.

1995 95-162982 Genetically modified micro-organism producing Buttcher, V., Kossman, J. & Wesh, T.poly(hydroxyalkanoate)—includes foreign gene Inst .Genbioligische Forschungfor hexosyl transferase to allow use of Hoechst-Scheringin expensive sugar substrates, also producingspecific polysaccharide.

1995 95-311553 Production of biodegradable, e.g. poly(hydroxy Lampe, R. A., Noda, I. &butyrate), fibrils for non-wovens—by Satkowski, M. M.contacting melted or solvated liquid resin Proctor and Gamble Co.mixture with gas flow.

1995 95-367684 Degradation of waste polymers, especially poly Batz, H., Jendrossek, D., Sluka,(hydroxy fatty acid)s—by incubating in aqueous P. & Steinbuechel, A.medium in presence of micro-organisms; Boehringer Mannheim GMBHespecially isloate SK37, and/or heating inaqueous alkaline medium.

1996 96-160943 Poly(hydroxy acid) production using recombinant Liebergesell, M., Pries, A.bacteria—especially Pseudomonas orAlcaligenes Steinbuchel, A. & Valentin, H.spp, containingThiocapsa poly(hydroxy acid) Monsanto Co.synthesase gene.

1996 96-486372 Thermoplastic biodegradable ether ester direct Engelhardt, J., Koch, W.,production from cellulose—uses quaternary Kramer, H., Meister, F. & Michels, C.ammonium base in etherification with oxiran Wolff Walsrode AGbefore esterification with monocarboxylic acid,useful for moulding, film or fibre.

1997 97-120248 Thermoplastic biodegradable poly(saccharide Engelhardt, J., Fink, H., Kalbe, J.,ether ester) for fibres—prepared by esterifi Koch, R., Koch, W., Mueller, H. P.,cation of the polysaccharide with anhydride and Szablikowski, K., Weber, G.,reaction of free carboxyl groups with an epoxide, Weigel, P. & Mueller, H.for heat-stable products. Wolff Walsrode AG



Year Reference Title Author(s) /organization(s)

1997 97-226171 Surface-modified cellulose micro-fibrils for Cavaille, J. Y. N., Chanzy, H. D.,filler in composite—have hydroxyl Fleury, E. & Sassi, J. F.functions on surface esterified by at least Rhone Poulenc Rhodia AGone organic compound, containing at least onefunction capable of reacting with hydroxylgroups.

II .4 Blending, composition and plasticizer patents


1991 91-001725 Destructurized starch and hydrophobic, thermo Lentz, D. J., Sachetto, J. & Silbiger, J.plastic polymer blends—have improved Warner Lambert Co.dimensional stability, humidity resistance andtoughness, are biodegradable and used incapsules, foams, films, etc.

1992 92-024382 Hydroxyalkanoate polymer composition—contains Barham, P. J., Organ, S. J. & Webb, A.ammonium chloride as nucleating agent, Zeneca Ltd; Monsanto Co.avoiding problems with opacity and improvingbiodegradability.

1992 92-064909 Biodegradable plastic composition—containing Carlson, A., Galvin, T. J. & Webb, A.microbiologically produced copolymer of 3- Zeneca Ltd; Monsanto Co.hydroxybutyrate and 3-hydroxyvalerate.

1993 93-093341 Biodegradable blend of microbiologically Doi, Y. & Kumagai, Y.produced poly(hydroxybutyric acid)—with Sumitomo Metal Ind. Ltdchemically synthesized random copolymer ofhydroxybutyric acid optical isomers, forproduction of flexible film, sheet, etc.

1993 93-200640 Biodegradable polymer composition—contains PVA Denki Kagaku Kogyo KKand poly(3-hydroxybutyric acid).

1993 93-288377 Compatible for polymer blends—comprises A–B Ballard, D. G. H & Buckmann, A. J. P.block copolymer where A is at least 50% alkylene Zeneca Ltd; Monsanto Co.side chains and B is at least 70% hydroxybutyricacid residue side chains.

1994 94-083141 Plasticizer for biodegradable, highly crystalline Montador, J. H. & Webb, A.polyester(s)—comprising an esterified hydroxy Zeneca Ltd; Monsanto Co.carboxylic acid with multiple ester groups inthe molecule.

1994 94-183458 Polymer composition—containing an oligomer of a Bal, J. S. & Hammond, T.polymer selected from poly(hydroxyalkanoate), Zeneca Ltd; Monsanto Co.polylactide, poly(caprolactone) and copolymers.

1994 121:135566 Polyesters, e.g. PLA or 3-hydroxybutyric acid/3- Montador, J. H. & Webb, A.,valeric acid copolymer, are plasticized with an Zeneca Ltd; Monsanto Co.ester such as Ac tri-Bu citrate.

1995 95-022746 Polyester composition with improved impact Hammond, T., Liggatt, J. J.,strength and elongation—comprises a biodegradable Montador, J. H. & Webb, A.polyester and a specific plasticizer. Zeneca Ltd; Monsanto Co.

1995 95-022752 Polyester composition comprising two poly(hydroxy Liggatt, J. J.alkanoate) compounds—including one in Zeneca Ltd; Monsanto Co.(semi)crystalline form, as a nucleant, forpaper, fabric, hygiene articles, sustained drugor agrochemical release system, adhesive, etc.

1995 95-024214 Resin composition used to produce porous biodeg Hisanaka, T., Kuwaki, T.,radable film—comprising resin mixture of Matsumura, H., Muta, Y., Ochi, Y. &copolymer of D-hydroxybutyrate and D-hydroxy Shimada, T.valerate, poly(epsilon-caprolactone) and inorganic Zeneca Ltdfiller.




1995 95-027727 Starch composition for biodegradable sheet— Tsutsunaka Plastic Ind.Co. Ltdincludes biodegradable aliphatic polyester,low-Mwt aliphatic polyester and reinforcingadditives.

1995 95-076327 Biodegradable thermoplastic mixture of starch Kakuschke, R., Rapthel, I.,ester and poly(alkylene glycol)—using low- Reichwald, K., Hermann, C. &Mwt polyether(s) and polyester(s) derived Runge, J.from amylopectin starch. Buna GMBH

1996 96-343504 Biodegradable composition used as plasticizing or Hagiwara, T., Hongo, H., Hori, Y.,compatibilizing compound—containing poly(3- Takahashi, Y. & Yamaguchi, A.hydroxybutyric acid) or copolymer and block Takasago Int . Corp.; Takasagocopolymer having a stereospecifically regular Perfumery Co. Ltdstructure.

1997 97-414566 Biodegradable emulsion binder for non-woven Iovine, C. P. & Walton, J. H.97-70811 fibre stock—containing poly(hydroxybutyrate/ Nat. Starch & Chem. Investment

hydroxyvalerate) copolymer, imparts good Holding Corp.mechanical properties and is readilybiodegradable.



APPENDIX III LIST OF ABBREVIATIONS

The abbreviations of individual monomers, polymersand copolymers are based on those used by the originalauthors and are identiÐed within the text. However,some more generally used abbreviations are as follows.

III .1 Chemical terms

PreÐxes : R-\ alkyl-, Me-\ methyl-, Et- \ ethyl-,Bu- \ butyl-, Pr- \ propyl-, Ac- \ acetyl-

dimethyl tin oxide DMTOdiethyl tin oxide DETOdibutyl tin oxide DBTOdioctyl tin oxide DOTOdi-n-butylphthalate DBPpropylene glycol PGethylene acrylic acid EAAtriethyl citrate TECglycolic acid GAL(]) or S(]) lactic acid LLAD([) or R([) lactic acid DLA3(or b)-butyrolactone 3(or b)BL3(or b)-hydroxybutyric acid 3-HBAR-3-hydroxybutyric acid R3HBA4(or c)-hydroxybutyric acid 4-HBA3-hydroxyvaleric acid 3-HVAmethyl methacrylate MMAhydroxyethyl methacrylate HEMAR-3(or b)-butyrolactone R3(or b)BLRS-3(or b)-butyrolactone RS3BL

III .2 Polymers and copolymers

poly(a- or 2-hydroxy acid) PaHApoly(glycolic acid) PGApoly(lactic acid) PLApoly(L-lactic acid) PLLApoly(D-lactic acid) PDLApoly(D,L- or R,S-lactic acid) P(D,L or R,S)LApolylactide PLpoly(e-caprolactone) PeCLpoly(valerolactone) PVLpoly(b-D,L-butyrolactone) P(b(D,L)BL)poly(hydroxyalkanoate) PHApoly(3- or b-hydroxyalkanoate) P(3 or b)HApoly(3-hydroxybutyrate) P3HBatactic poly(3-hydroxybutyrate) a-PHB

poly(R-3-hydroxybutyrate) PR3HBpoly(RS)-hydroxybutyrate P(RS)HBpoly(3-hydroxyvalerate) P3HV

poly(4-hydroxybutyrate) P4HBpoly(6-hydroxyhexanoate) PHHpolyethylene PElow-density polyethylene LDPEhigh-density polyethylene HDPEpolystyrene PSpoly(vinyl alcohol) PVApoly(ethylene oxide) PEOpoly(vinyl acetate) PVAcpolypropylene PPpoly(ethylene terephthalate) PETpoly(vinyl chloride) PVCpoly(methyl methacrylate) PMMApoly(ethylene glycol) PEGpoly(sebacic anhydride) PSApoly(p-dioxanone) PDSpoly(carboxyphenoxyhexane) PCPHpoly(1,4-ethylene adipate) PEApoly(cyclohexyl methacrylate) PCHMApoly(epichlorohydrin) PECpoly(carboxyphenoxyvaleric acid) PCPVpoly(glycolic acid-co-lactic acid) PGLApoly(lactic acid-co-glycolic acid) PLGApoly(D,L-lactic acid-co-glycolic acid) P(D,L)LGApoly(3-hydroxybutyrate-co-3-

valerate)PHBV

poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

P3HB-co-4HB

poly(R-3-hydroxybutyrate-co-e-caprolactone)

P(R)(3HB)ÈPeCL

poly(R-3-hydroxybutyrate-co-L-lactone)

P(R)(3HB)ÈL-PL

poly(e-caprolactone-co-lactone) P(eCL-co-LA)poly((e-caprolactone-co-glycolide) P(eCL-co-G)poly(lactic acid-co-ethylene glycol) P(LA-co-PEG)poly(LGA-co-ethylene glycol) P(LGA-co-PEG)urea formaldehyde resin UF

III .3 Starch /cellulose connected terms

starch Ststarch octanoate OCSTstarch dodecanoate DODSTcellulose ester CEcellulose acetate CAcellulose acetate butyrate CABcellulose tributyrate CTBcellulose acetate propionate CAPdegree of substitution DSmethyl cellulose MCethyl cellulose EtCbacterial cellulose BCcarboxymethyl cellulose CMC



water-soluble polymer WSP

III .4 Other terms

municipal solid waste MSWtotal organic carbon TOCdissolved organic carbon DOCtheoretical oxygen demand TODsemi-continuous activated sludge SCAScontinuous-Ñow activated sludge CAS

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