microfabricated drug delivery systems

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Advanced Drug Delivery Reviews 55 (2003) 315328www.elsevier.com/locate/addr

Microfabricated drug delivery systems: from particles to poresa,b a,b ,*Sarah L. Tao , Tejal A. Desai

a

Department of Bioengineering, University of Illinois at Chicago, 851 S. Morgan Street, Chicago, IL 60607, USAbDepartment of Biomedical Engineering, Boston University, 44 Cummington Street, Boston, MA 02215, USA

Abstract

Microfabrication techniques which permit the creation of therapeutic delivery systems that possess a combination of

structural, mechanical, and perhaps electronic features may surmount challenges associated with conventional delivery of

therapy. In this review, delivery concepts are presented which capitalize on the strengths of microfabrication. Possible

applications include micromachined silicon membranes to create implantable biocapsules for the immunoisolation of

pancreatic islet cellsas a possible treatment for diabetesand sustained release of injectable drugs needed over long time

periods. Asymmetrical, drug-loaded microfabricated particles with specific ligands linked to the surface are proposed for

improving oral bioavailability of peptide (and perhaps protein) drugs. In addition, microfabricated drug delivery systems

ranging from transdermal microneedles to implantable microchips will be discussed.

2002 Elsevier Science B.V. All rights reserved.

Keywords: Silicon; Microtechnology; Microfabrication; Therapeutic; Drug delivery; Microparticles

Contents

1. Introduction ............................................................................................................................................................................ 316

2. Controlled release drug delivery systems............ .................... ................... .................... .................... .................... ................... . 316

3. Microfabrication technology ................... .................... .................... ................... .................... .................... ................... ........... 317

4. Microneedles for transdermal drug delivery.................. ................... .................... .................... .................... ................... ........... 317

4.1. Microfabrication of silicon microneedle arrays ................... .................... ................... .................... .................... ................. 317

4.2. Transdermal transport studies.............. .................... .................... .................... ................... .................... .................... ....... 318

4.3. Other microneedles .................. .................... .................... .................... ................... .................... .................... ................. 318

5. Implanted microchip for localized drug delivery ................. .................... .................... ................... .................... .................... .... 319

5.1. Microchip design .................. .................... .................... ................... .................... .................... ................... .................... . 319

5.2. Irreversible metallic valves ................. .................... .................... ................... .................... .................... .................... ....... 319

5.3. Reversible polymeric valves ................... ................... .................... .................... ................... .................... .................... .... 319

5.4. Current developments............ .................... .................... ................... .................... .................... ................... .................... . 320

6. Bioadhesive microparticles for oral drug delivery ................... ................... .................... .................... ................... .................... . 320

6.1. Silicon dioxide microparticles .................... .................... ................... .................... .................... ................... .................... . 321

6.1.1. Fabrication of microparticle body.. ................... .................... .................... ................... .................... ................... ..... 321

6.1.2. Fabrication of microparticle reservoir ................... ................... .................... .................... ................... .................... . 321

6.1.3. Surface modification chemistry .................. ................... .................... .................... .................... ................... ........... 321

6.2. Poly(methyl methacrylate) microparticles....... .................... .................... ................... .................... .................... ................. 322

*Corresponding author. Tel.: 1 1-617-358-3054; fax: 1 1-617-353-6766.

E-mail address: [email protected] (T.A. Desai).

0169-409X/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-409X(02)00227-2
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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316 S.L. Tao, T.A. Desai / Advanced Drug Delivery Reviews 55 (2003) 315328

6.2.1. Fabrication of microparticle body.. ................... .................... .................... ................... .................... ................... ..... 322

6.2.2. Fabrication of microparticle reservoir ................... ................... .................... .................... .................... ................... . 322

6.2.3. Surface modification chemistry .................. .................... ................... .................... .................... ................... ........... 322

6.3. Release mechanism .................. .................... .................... .................... ................... .................... .................... ................. 323

6.4. Microparticle bioadhesion to intestinal epithelium ................. .................... .................... ................... .................... .............. 323

6.4.1. Lectins .................... .................... ................... .................... .................... .................... ................... .................... .... 3236.4.2. Caco-2 binding .................. .................... ................... .................... .................... ................... .................... .............. 323

6.5. Future developments .................... .................... ................... .................... .................... ................... .................... .............. 324

7. Nanoporous immunoisolating biocapsules .................... .................... ................... .................... .................... ................... ........... 324

7.1. Allotransplantation ................... .................... .................... .................... ................... .................... .................... ................. 324

7.2. Biocapsule design ................. .................... .................... ................... .................... .................... ................... .................... . 325

7.3. Biocompatibility and cytotoxicity studies.... .................... ................... .................... .................... .................... ................... . 326

8. Conclusions ............................................................................................................................................................................ 326

Acknowledgements....... .................... .................... .................... ................... .................... .................... .................... ................... . 327

References ................... .................... .................... .................... ................... .................... .................... ................... .................... . 327

1. Introduction administration. However, these types of dosages are

not easily able to control the rate of drug delivery or

The application of micro- and nanotechnology to the target area of the drug and are often associated

the biomedical arena has tremendous potential in with an immediate or rapid drug release. Conse-

terms of developing new diagnostic and therapeutic quently, the initial concentration of the drug in the

modalities. Over the last several years, microfabrica- body peaks above the level of toxicity and then

tion technology has been applied to the successful gradually diminishes over time to an ineffective

development of a variety of health care-related level. The duration of therapeutic efficacy then

products including diagnostic (lab-on-a-chip) sys- becomes dependent on the frequency of administra-

tems and techniques and apparatus for high through- tion, and half-life of the drug, and high dosages of

put screening of new drug candidates [1,2]. While the non-targeted drugs are often administered to achieve

majority of research has focused on the development an effective blood concentration [8]. In recent years,

of miniaturized diagnostic tools, researchers have increasingly sophisticated and potent drugs havemore recently concentrated on the development of been developed by the biotech industry. For many of

microdevices for therapeutic applications. Micro- these new protein-based and DNA-based com-

and nanofabrication techniques are currently being pounds, the therapeutic concentration range is often

used to develop implants that can record from, sense, small, toxicity is observed for concentration spikes,

stimulate, and deliver to biological systems. Mi- or the therapeutic concentration range varies with

cromachined neural prostheses, drug delivery mi- time, which renders traditional methods of drug

cropumps, tissue scaffolds, and stents [36] have all delivery ineffective [9]. An immense amount of

been fabricated using precision-based microtech- interest has been increasingly placed on controlled

nologies. Drug delivery remains an important chal- release drug delivery systems to maintain the thera-

lenge in medicine [7] and microfabrication tech- peutic efficacy of these drugs. There are a number of

niques may be used to develop novel drug delivery mechanisms that can provide such controlled releasedevices with capabilities not possible with current of drugs, including transdermal patches, implants,

systems. This paper will review some of the current bioadhesive systems, and microencapsulation [7].

and future approaches that utilize microfabrication Newer drug delivery technologies currently attracting

technology for drug delivery. attention include inhaled and sustained release inject-

able peptide/protein drugs from biodegradable poly-

mers.

2. Controlled release drug delivery systems Nevertheless, the ideal drug delivery system has

not yet been attained. Therefore, interdisciplinary

Conventional dosage forms, such as oral delivery approaches are being sought. These alternative ap-

and injection, are the predominant routes for drug proaches should, at least conceptually, address those


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S.L. Tao, T.A. Desai / Advanced Drug Delivery Reviews 55 (2003) 315328 317

unsolved issues that still engage the drug delivery injection is the administration of drugs across the

scientist. These include drug targeting (of peptides, skin. This approach seeks to avoid any degradation

proteins, and DNA) to improve cancer chemotherapy of the molecules in the gastrointestinal tract and

and cardiovascular treatment, improved control of first-pass effects of the liver associated with oral

the rate of release from implanted dosage forms, drug delivery as well as the pain of intravenouspulsatile drug delivery, closed-loop systems that can injection [1014]. It also offers the possibility to

sense a change in the bodies biochemistry leading to continuously control the delivery rate over extended

modulated rate or extent of drug delivered, and the periods of time [14]. However, conventional trans-

ability to implant xenogeneic cells in humans to dermal drug delivery is severely hindered by the

more naturally control a range of diseases, including outer 1020 mm of skin, a barrier of dead tissue

diabetes and neurodegenerative conditions. called the stratum corneum [15]. The development of

microneedles for transdermal drug delivery came

about as an approach to enhance the poor permeabili-

ty of the skin by creating microscale conduits for3. Microfabrication technology

transport across the stratum corneum [14]. The

development of microneedles that are long andThe use of traditional microfabrication techniques, robust enough to penetrate this layer of skin, but

the same processing techniques used to manufactureshort enough to avoid stimulating nerves has the

microelectronic chips, is a recent and alternativepotential to make transdermal delivery of drugs more

method of creating drug delivery platforms. Micro-effective [15].

electronic process engineering was a discipline that

developed due to the rapid growth of the integrated4.1. Microfabrication of silicon microneedle arrays

circuit industry. Traditionally, microelectromechani-

cal systems (MEMS) research has been used toBy adapting microfabrication technology, three-

produce functional devices on the micron scale, suchdimensional arrays of sharp-tipped microneedles can

as sensors, switches, filters, and gears, from silicon,be made for transdermal drug delivery [10,14,15]. To

the dominant material used throughout the IC indus-fabricate microneedles, a deep reactive ion etching

try. These devices are fabricated by the repeated process is commonly used. In this process, aapplication of unit process steps such as thin-filmchromium masking material is deposited onto silicon

deposition, photolithography, and etching. Such mi-wafers and patterned into dots that have a diameter

crofabrication techniques allow for the precise con-approximately equal to that of the base of the desired

trol over surface microarchitecture, topography, andneedles. When placed in the reactive ion etcher, the

feature size.wafers are exposed to carefully controlled plasma of

Although research on microfabricated devices forfluorine and oxygen, which causes a deep vertical,

biomedical applications (BioMEMS) has rapidlyetch and slight lateral underetching. The regions on

expanded in recent years, relatively few researchersthe wafer that are protected by chromium remain and

have concentrated on therapeutic applications ofeventually form the microneedles. Etching is allowed

microfabrication technology such as drug delivery.to proceed until the masks are undercut and fall off,

The use of microtechnology to tailor the size, shape,

leaving behind an array of silicon spikes [14]. Thereservoir number, reservoir volume, unidirectionalaspect ratio of the microneedles can be adjusted by

openings and surface characteristics of the drugsimply modifying the ratio of flow rates of SF and

6delivery vehicle in conjunction with appropriateO . Hollow silicon needles can also be fabricated

2surface chemistry are potentially influential in theusing deep reactive ion etching in an inductively

area of controlled release.coupled reactive ion etcher. The deep etch creates

arrays of holes through the silicon wafer (the needle

lumen) and the microneedles are formed by reactive

4. Microneedles for transdermal drug delivery ion etching around these holes [15].

Arrays of solid silicon microneedles have been

One alternative to oral delivery and intravenous fabricated with individual needles measuring 150 mm


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in length, 80 mm in diameter at the base, with a level for as long as 5 h [14]. Hollow microneedles

radius of curvature less than 1 mm [14]. Hollow were also capable of insertion into the skin without

needles have also been microfabricated with similar any extensive damage to the microneedles or skin

dimensions, but containing hollow bores anywhere [15]. In addition, the improved design of these

from 5 to 70 mm in diameter, depending on design needles increased skin permeability further still [15].[15]. To test their durability, the solid needles were

inserted into skin with gentle pushing, an approxi-4.3. Other microneedlesmate force of 10 N. All but a few percent of the

microneedles remained intact. Even in these fewMore recently, the original microneedle design hasneedles, only the top 510 mm was damaged

been further refined to provide better control over[14,15]. Additionally the array of microneedles coulddrug delivery. Silicon microhypodermic needles havealso be removed without additional damage andbeen fabricated in combination with heat-controlledcould also be reinserted into skin multiple times.bubble pumps [15]. Hollow metal microneedles have

been fabricated by defining molds in epoxy and4.2. Transdermal transport studies filling them by electrodepositing metal [15]. Similar-

ly, polysilicon microneedles have been fabricatedQuantification of transdermal transport of various with reusable molds [15,16]. Polysilicon micronee-

molecules with and without inserted microneedle dles are likely to be most cost effective and have thearrays was used to assess any increase in skin potential to produce single use disposable platformspermeability leaving (Fig. 1). Insertion of the mi- [15]. These types of needles, combined with acroneedles increased permeability only 1000-fold pressurized reservoir to generate a drug deliverybecause the microneedles or the silicon plate may pump, have already been incorporated into a wear-have blocked access to the microscopic holes [14]. able drug infusion system to deliver insulin [17].When the microneedles were removed after 10 s, Furthermore, by modifying needle dimension andpermeability increased by 10 000-fold [14]. Removal design to incorporate multiple channels and ports,after 1 h increased skin permeability by 25 000-fold optimized microhypodermic needles and micro-

[14]. Elevated permeability after microneedle inser- probes can be developed for cellular, local tissue, ortion was found to remain at approximately the same systemic delivery [15].

Fig. 1. (A) Scanning electron micrograph of microneedles made by reactive ion etching technique. (B) Microneedle tips inserted across

epidermis. The underside of the epidermis is shown, indicating that the microneedles penetrated across the tissue and that the tips were not

damaged. Arrows indicate some of the microneedle tips. Reproduced with permission from Ref. [14].


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Fig. 2. Photographs of a prototype microchip: the electrode-containing front side and the back side with openings for filling the reservoirs.

Scale bar 10 mm. Reproduced with permission from Ref. [18].

implanted for drug delivery applications need not be5.4. Current developments

removed.

Current developments on microchip systems con-

sist of integrating active components. Combining

battery clocks, reference electrodes and biosensors6. Bioadhesive microparticles for oral drugwith the microchip could provide a single packagedeliveryfor implantation. Another logical extension of the

controlled-release microchip would be the develop-Oral drug delivery is one of the most preferredment of a passive, polymer microchip that contains

methods of drug administration due to its non-inva-no electronics, power sources, or microprocessorssive nature. However, it is generally not a viable[18]. Each of the reservoirs would be covered by a

method for peptide and protein delivery. The humancap of degradable material, or a nondegradableGI tract resists absorption of peptides, proteins, andmaterial of known permeability for the drug, or leftother large molecules until they are broken downuncapped. Time and rate of release from the reser-into smaller molecules. The acidic environment ofvoir would then be dictated by the degradation ratethe stomach combined with an array of enzymes andof the cap or diffusion of the drug [18]. This type ofphysical barriers in the intestines either destroy orpolymeric microchip device would have the addition-prevent absorption of nearly all macromolecules.al advantage of being biodegradable, and once

Fig. 3. (A) A Ag/AgCl and IrO valve electrode in the same micromachined drug delivery cavity. Both electrodes are 303 30 mm. (B)x

SEM micrograph of artificial muscle grown on TEM gold grid coated with poly-HEMA in holes (38.5 mm3 38.5 mm) of a drug reservoir.

Reproduced with permission from Ref. [20].


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This has led to the development of oral delivery sents the opportunity to create multiple reservoirs of

systems that can potentially enhance the delivery of desired size to contain not just one, but many drugs /

peptides utilizing mechanisms such as use of protec- biomolecules of interest [28].

tive coatings [22], targeted delivery [23], permeation

enhancers [24], and protease inhibitors [25]. 6.1. Silicon dioxide microparticlesBioadhesive drug delivery systems have also

generated considerable interest due to their potential Microparticles have been successfully fabricated

for prolonging the residence time at the site of drug from silicon dioxide using microfabrication protocols

action or adsorption. Localization of the delivery [28]. These particles are then easily modified by

system at a given target site would intensify its silane chemistry to introduce to sites for the attach-

contact with the mucosal epithelial barrier, thereby ment of biological molecules.

increasing the drug concentration gradient due to

intense contact [26,27]. Rather than having an im- 6.1.1. Fabrication of microparticle body

planted controlled-release microchip for local drug To fabricate the particle body an etch stop layer

delivery, such systems could also be fabricated for was created by growing a thermal oxide under wet

specific targeting. By developing inorganic or poly- conditions. Low-pressure chemical vapor deposition

meric reservoir-containing particles on the micron was then used to deposit a sacrificial layer of poly-

scale, and grafting bioadhesive agents on their crystalline silicon atop the thermal oxide by low-

releasing side, these particles could be adapted for pressure chemical vapor deposition. Next, a layer of

use as a bioadhesive controlled release oral drug low temperature silicon dioxide (LTO) was depos-

delivery system. This type of system could improve ited to form the device layer. Negative lithography

the effectiveness of treatment by first, targeting and was carried out to mask define the particle shape. A

localizing a drug at a specific site, inhibiting dilution reactive ion etch (RIE) with SF and O was used to6 2

of the drug in body fluids, thereby helping to define the actual LTO particles and any remaining

maintain the drug concentration at the optimal photoresist was then removed in negative photoresist

concentration between effective and toxic levels. remover.

Micromachined platforms, when combined with

complementary approaches, may address some of the 6.1.2. Fabrication of microparticle reservoirshortcomings of current oral delivery systems for Positive lithography was carried out using infrared

peptides and proteins by combining several features (IR) backside alignment to define the wells. The

into a single drug delivery platform. First, one can wafers were then time-etched in buffered oxide

achieve control over the size and shape of the etchant to carve out the wells. Any remaining

delivery device. Unlike other spherical drug delivery photoresist was removed. The welled microdevices

particles, microfabricated devices may be designed were then released into solution by etching the

to be flat, thin, and disc-shaped to maximize contact sacrificial polysilicon layer with KOH. The KOH

area with the intestinal lining and minimize the side solution was diluted with deionized water and fil-

areas exposed to the constant flow of liquids through tered to isolate the micro devices. As seen in Fig.

the intestines. The size of the particles can be 4A, the particles were fabricated with well-defined

selected to be small enough to have good contact features across the entire wafer. Released microparti-with the undulations of the intestinal wall and large cles are shown in Fig. 4B. The particles were

enough to avoid endocytosis of the entire particle. uniform and semi-transparent due to their poly-

While endocytosis of nanoparticles has been pro- crystalline nature.

posed as a method to enhance transport of large

molecules across the intestinal barrier, this process 6.1.3. Surface modification chemistry

can destroy the macromolecule. Secondly, one can Chemistries used to link biologically active mole-

selectively attach bioadhesive agents onto the device cules, such as proteins, on to the releasing side of the

surface using relatively simple surface chemical silicon dioxide microparticles have also been de-

modification strategies. Finally, micromachining pre- veloped using traditional silane chemistry and car-


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Fig. 4. (A) An array of welled SiO microparticles. (B) Released microparticles.2

bodiimide coupling reagents. First, amine groups are modified by changing the spin rate or by spinning on

formed on the surface of unreleased microparticles additional layers of PMMA. Additionally, altering

through hydroxylation of the silicon, followed by the masked area of the wafer easily changes the

silanization in 2% v/ v APTES at room temperature shape and surface area of the particles.

for at least 1.5 h. Biological molecules that contain

carboxyl groups were coupled with carbodiimide and 6.2.2. Fabrication of microparticle reservoir

N-hydroxysuccinimide to form a reactive inter- A second positive lithography was carried out to

mediate ester that is susceptible to attack by amines. expose the intended reservoir areas in the PMMA.

This reaction can be applied to self-assemble a This area was then etched 12 mm deep using an

monolayer of covalently coupled biological mole- oxygen plasma and any remaining photoresist was

cules to the surface of the microparticles. removed. The dimensions of the reservoir can be

altered by changing the masked area and their depth

6.2. Poly(methyl methacrylate) microparticles can be modified by changing the time and/or flow

rate of plasma in the RIE (Fig. 5). By creatingSuch a system has also been developed based on smaller wells, a series of multiple wells can be

the polymer polymethylmethacrylate (PMMA). etched into the particles to create separate reservoirs

PMMA is an ideal material for such a MEMS-based for a combination of drugs or permeation enhancers.

bioadhesive system because it is biocompatible and Since the PMMA is adherent to the surface of silicon

already used in many biomedical applications, is by linkage to the native oxide layer, the wafer was

commonly used as a resist in photolithographic soaked in basic solution to break this bonding and

applications, and contains a functional methyl ester immediately release the particles.

group for potential surface modification. The current

prototype is a square particle 150 mm across and 3 6.2.3. Surface modification chemistry

mm thick containing square reservoirs 80 mm across Heterogeneous modification of PMMA with N-

and 2 mm deep. lithioethylenediamine as the aminolyzing agent leadsto amination of its ester groups [29,30]. This reaction

6.2.1. Fabrication of microparticle body layer of amine sites tethered to the PMMA backbone

To fabricate these particles, PMMA was spun on by stable amide groups. Amine groups were placed

to clean silicon wafers. Positive lithography was on the surface of the releasing side of PMMA

used to expose the area between the particles and microparticles by reacting unreleased microparticles

isolate the particle bodies. The unmasked area was with N-lithioethylenediamine for 10 min. Biological

then etched completely through in the reactive ion molecules were linked to these amine sites using the

etcher using an O plasma and any remaining resist same traditional carbodiimide coupling reagents used2

was removed. The thickness of the particles can be for silicon dioxide microparticles.


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Fig. 5. Arrays of 150 mm3 150 mm PMMA particles with (A) 50 mm3 50 mm, (B) 80 mm3 80 mm, (C) 100 mm3 100 mm, and (D)

multiple 28 mm3 28 mm wells.

6.3. Release mechanism 6.4.1. Lectins

Lectins are a class of carbohydrate binding pro-

These reservoirs can be filled with pico- to teins or glycoproteins of non-immune origin. They

nanoliters of a polymeric solution with microinject- have been deemed a second generation bioadhesive

ors. Water quickly evaporates from these reservoirs because they bind to cell-surface glycoconjugates in

leaving behind the drug contained in polymer which a complementary way analogous to ligandreceptor

acts as a timed-release plug. Using a specific type of interactions. Tomato lectin is of special interest inpolymer predetermines the time and rate of release of intestinal targeting due to its stability in low pH

drug from the reservoir; for example, a hydrogel that environments and its non-toxicity. Furthermore, it

swells in response to a specific pH, solvent or has been shown to bind selectively to the small

temperature or a polymer with a known dissolution intestine epithelium [31], resist digestion in the

rate. Different polymers with various dissolution alimentary canal of rats and bind to rat intestinal villi

rates can be used in each reservoir to obtain con- without disruption of the lectin integrity [32].

trolled release of separate compounds.

6.4.2. Caco-2 binding

6.4. Microparticle bioadhesion to intestinal In vitro studies using a Caco-2 model of the

epithelium intestinal epithelium were used to demonstrate thespecific binding of the tomato lectin-conjugated

The current microparticle model is designed to PMMA particles. The tomato lectin-conjugated par-

specifically target the intestinal epithelium. The ticles showed, on average, a marked increase in

microparticles were first rendered bioadhesive by binding almost five times greater than the binding of

forming avidin-coupled surfaces using the chemistry unmodified particles over an entire period of 2 h

previously described. Commercially available (Fig. 6). Furthermore, the modified particles once

biotinylated lectins were then attached to the mi- bound, remained bound whereas only 20% of origi-

croparticle surface by taking advantage of the strong nally bound unmodified particles remained bound

interaction and affinity of avidin and biotin in nature. after 20 min. Currently, in vivo studies are being


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The disease manifests itself as hyperglycemia. In-

sulin remains the mainstay of virtually all type 1 DM

and many type 2 DM patients and in most cases is

administered subcutaneously. However, the kinetics

of insulin administered by this route do not mimicthe normal rapid rise and decline of insulin secretion

in response to ingested nutrients.

Efforts to address the short-comings of current

subcutaneous administration of insulin, including the

use of complex multidose regimens, have led to the

development of other dosage forms and routes of

administration such as needleless injectors, con-

stant infusion pumps, and inhaled insulin. These

newer approaches still suffer from the same general

issue plaguing current subcutaneous administration.Fig. 6. Binding of unmodified and lectin-conjugated PMMA

microparticles to Caco-2 monolayers. 7.1. Allotransplantation

conducted to determine the bioadhesive properties of A potentially useful approach, which has proven

these particles in the gastrointestinal tract of rats. effective in only a handful of cases, is the allotrans-

plantation of islets or whole pancreases from a6.5. Future developments suitable human donor into a diabetic recipient.

Researchers in Canada recently reported successful

By replacing the molecule attached to the mi- transplantation of islet cells in type 1 DM patients

croparticles, an array of cells and tissues can be [34]. Although the potential complications of im-

targeted. For example, if the lectin is substituted with munosuppressive therapy were reduced by avoiding

an antibody that selectively binds to tumor cells in the use of glucocorticoids, each transplant required

the colon, the microparticles could actively seek out two harvestings of islet cells from organ donors.cancerous masses in the colon and deliver anticancer Moreover recipients are still required to take immune

drugs directly. This would allow the high concen- suppressing drugs for the rest of their lives. These

tration of drug to be locally delivered while keeping immunosuppressive drugs are toxic and have po-

the systemic concentration at a low level. Moreover, tential adverse side effects, including cancer. For this

microfabricated polymer particles with the same reason, an islet or pancreas allotransplant is normally

targeting abilities, but small enough for injection, carried out only in conjunction with a kidney trans-

could be developed for direct delivery into the plant, for which immunosuppression is required in

circulatory system. any case.

Because of the toxicity of immune suppressing

drugs, and the shortage of organ donors, islet and

7. Nanoporous immunoisolating biocapsules pancreas allotransplantation appears to hold limitedpromise as a cure for diabetes. A method then is

Diabetes mellitus (DM) represents a serious medi- required to sequester the islets from the bodys

cal problem. In the US alone, it is the third leading immune system which is able to recognize and reject

cause of death. While the majority of patients have these xenogeneic cell grafts. For the past 20 years,

type 2 diabetes, about 10% of all patients diagnosed investigators have focused on a range of microen-

with DM are insulin-dependent (type 1). In both capsulation methods most commonly involving so-

cases, disease is caused by decreased circulating dium alginate and another polycationic substance

concentrations of insulin and decreased response of such as polylysine. These materials have been used

peripheral tissue to insulin (insulin resistance) [33]. in an attempt to create a semipermeable membrane


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capable of blocking immune molecules such as IgG,

cytokines, and cell-secreted antigens from reaching

the encapsulated xenogeneic islet cells while allow-

ing glucose and insulin to freely diffuse through the

barrier [35]. However, this approach has provengenerally unsuccessful due to mechanical rupture of

the membrane, biochemical instability, incompatibili-

ty with islet cell heterogeneity, and broad pore size

distributions [3539]. When the barrier between the

xenogeneic cells and the external bioenvironment is

compromised, these foreign cells are subject to

various endogenous cells and antibodies as well as

complement and a host of cytokines such as tumor

necrosis factor, all of which can inflict cell damage.

As a result, the use of polymeric microcapsules for

allotransplantation has been unsuccessful clinically

in the absence of immunosuppression [35,37,39].

7.2. Biocapsule design

Microfabrication techniques have been applied to

create a biocapsule for effective immunoisolation of

transplanted islet cells for treatment of diabetes [40].

The fabrication of nanochannels in the membrane Fig. 7. Micrograph of a biocapsule membrane with 24.5-nmpores.structure consists of two steps: (1) surface mi-

cromachining nanochannels in a thin film on the top

of a silicon wafer, and (2) releasing the membrane chemically, and mechanically stable and retrievable.

by etching away the bulk of the silicon wafer It is also expected that improved dynamic responseunderneath the membrane. These nanopore mem- of islets can be obtained due to the limited mem-

branes (Fig. 7) are designed to allow the permeabili- brane thickness (Fig. 8) compared with the thickness

ty of glucose, insulin, and other metabolically active of conventional polymeric membranes prepared from

products, while at the same time, preventing the alginate and polylysine (100200 mm) [41]. It is

passage of cytotoxic cells, macrophages, antibodies, crucial that rapid secretion kinetics, particularly in

and complement. The membranes are bonded to a

capsule that houses the pancreatic islet cells. Because

the difference in the size of insulin, which must be

able to pass freely through the pores and the size of

IgG immunoglobulins, which must be excluded, is

only a matter of a few nanometers, the highlyuniform pore size distribution provided by mi-

cromachine membranes is essential for effective

immunoisolation and therapeutic effect.

Control of pore sizes in the tens of nanometers has

recently been suggested as probably the most realis-

tic way to achieve immunoisolation [41,42]. The use

of unconventional biomaterials such as silicon and

silicon dioxide provides a means to encapsulate

pancreatic islet cells in devices that are thermally, Fig. 8. Insulin secretory profile through differing pore sizes.


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the first phase of insulin release, be maintained over

time to provide physiologic feedback control of

blood glucose concentrations.

Another key feature of an implanted biocapsule

system is the role of neovascularization. The mem-branes proposed for testing have outer openings of 2

by 2 mm while the inner diffusion channels have a

pore size of between 10 and 30 nanometers. Studies

have shown that neovascularization at the mem-

branetissue interface occurs in membranes having

pore sizes large enough to allow complete penetra-

tion by host cells (0.88 mm) [41]. Thus, it isFig. 9. IgG diffusion through microfabricated biocapsules of threeexpected that neovascularization can occur at thedifferent pore sizes.large openings while not penetrating into the

nanometer pores. This phenomenon has two key

advantages: (1) the ability to rapidly deliver insulin membrane to ensure nutrient exchange for encapsu-

into the blood stream through new blood vessel lated islet cells. These experiments show that no

growth while (2) limiting pore clogging or fouling. diffusion barrier is formed by the membrane for

glucose and insulin, while taking into account the7.3. Biocompatibility and cytotoxicity studies effect of rotation on mass transfer. In addition,

microfabricated biocapsule membranes can be tailor

Preliminary biocompatibility and cytotoxicity tests made to attain desired IgG diffusion kinetics (Fig.

have been carried out by examining the cell mor- 9). At the same time, the deselection of IgG requires

phology, growth, and function of test cell lines absolute pore dimensions below 18 nm. It is noted

placed in contact with arrays of membranes, with that the percent of IgG diffusion (concentration of

promising preliminary results. The biocompatibility IgG that passes through the membrane) was less than

was evaluated via direct contact tests by cultivating 0.4% after 24 h and 2% after over 150 h through the

several different cell lines such as macrophages, 18-nm membranes [4446].fibroblasts, and HeLa cells, as well as isolated It may be possible to design nanopore membranes

primary islets of Langerhans both on the wafer which achieve a more constant rate of drug delivery,

surface and within the porous wafer pockets. All avoiding the burst effect. By precisely controlling

cells were seeded on silicon culture wafers, observed pore size, pore length and pore density, the nanopore

via light microscope, stained for cell viability and membrane fitted into a polymeric capsule suitable for

functionality, and counted with a hemocytometer. All subcutaneous implantation can serve as a diffusion

cell types had normal growth characteristics, mor- barrier for a variety of biological drugs.

phology, and greater than 90% viability [43,44].

Results indicate that the insulin secretion by

encapsulated islets and subsequent diffusion through 8. Conclusions

the biocapsule membrane channels is similar to thatof unencapsulated islets for both 3 mm and 78 nm The race to find effective diagnostic and therapeu-

pore sized membranes, with insulin diffusion though tic tools is under way, as scientific and engineering

the membrane occurring within 10 min of stimula- disciplines uncover and elucidate more about the

tion. Fig. 8 shows the typical insulin release profile human pathologic condition than ever before. Al-

in response to stimulatory (16.7 mM) glucose though we are getting closer to the clinical applica-

medium over 1 h under static incubation for 78-, 66- tion of intelligent drug delivery devices, many

and 18-nm pore-sized membranes. This profile indi- challenges remain for the future. The convergence of

cates that insulin and glucose diffusion occur at microtechnology and biology will lead to new ap-

sufficiently high rates through the microfabricated proaches in drug delivery and may provide advan-


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