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DOI: 10.1177/0040517511424524 2012 82: 129 originally published online 19 October 2011Textile Research Journal

Rajkishore Nayak, Rajiv Padhye, Illias Louis Kyratzis, Yen Bach Truong and Lyndon ArnoldRecent advances in nanofibre fabrication techniques

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Original article

Recent advances in nanofibre fabricationtechniques

Rajkishore Nayak1,2, Rajiv Padhye1, Illias Louis Kyratzis2,Yen Bach Truong2 and Lyndon Arnold1

Abstract

Over the past decade, there has been a tremendous increase in the demand for polymeric nanofibres which are promising

candidates for various applications including tissue engineering, protective clothing, filtration and sensors. To address

this demand, researchers have turned to the development of various techniques such as electrospinning, meltblowing,

bicomponent spinning, forcespinning and flash-spinning for the fabrication of polymeric nanofibres. However, electrospin-

ning is the widely used technique for the fabrication of continuous nanofibres. The ability to fabricate nanofibrous

assemblies of various materials (such as polymers, ceramics and metals) with possible control of the fibre fineness, surface

morphology, orientation and cross-sectional configuration, gives electrospinning an edge over other processes. Although

several researches have been done in electrospinning, understanding some of the other processes is still in infancy. In this

perspective article, we summarize the fundamentals of various techniques for the fabrication of nanofibres. This paper also

highlights a gamut of recent advances in the techniques for nanofibre fabrication.

Keywords

Nanofibre, polymer, needle, solution, melt, electrospinning

Introduction

Recently, various engineering fields have given muchattention to nanoscale materials e.g. nanofibres in thefibre industry. Nanofibres are fibres with a diameter of100 nm or less,1 notable for their characteristic featuressuch as large surface-area-to-volume ratio, extremelysmall pore dimensions and superior mechanical proper-ties.2 Due to these features, nanofibres have a widerange of applications in areas such as high performancefiltration, battery separators, wound dressing, vasculargrafts, enzyme immobilization, electrochemical sensing,composite materials, reinforcements, blood vessel engi-neering and tissue engineering.3–5

The existing fibre spinning technologies cannot pro-duce robust fibres with diameter smaller than 2 mm dueto limitations in the process. The process widely used forthe fabrication of nanofibres is electrospinning,6–7

because of its simplicity and suitability for a variety ofpolymers, ceramics and metals. Other processes includemeltblowing, flash-spinning, bicomponent spinning,forcespinning, phase-separation and drawing. In mostof these processes, the fibres are collected as nonwoven

random fibre mats known as nanowebs, consisting offibres having diameters from several nanometers tohundreds of nanometers.

Although several research publications have beenreported on nanofibre fabrication, it has been recognizedthat a comprehensive and systematic review covering allthe techniques and recent developments has not beenconducted which is essential for future developments inthe area. In this paper, a comprehensive reviewof varioustechniques used to produce nanofibres, along with thelatest developments in these techniques is reported.The article also highlights some of the advantages anddisadvantages of each production process. In addition,

1RMIT University, Australia.2CSIRO, Australia.

Corresponding author:

Rajiv Padhye, RMIT University, 25 Dawson Street, Brunswick, Melbourne

3056, Australia

Email: [email protected]

Textile Research Journal

82(2) 129–147

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schematic diagrams of various methods for nanofibreproduction, and the features of nanofibres that can beproduced using each technique, are illustrated in thisreview.

Electrospinning

The most common method for the production of nano-fibres is electrospinning. The origin of electrospinningas a viable technique for the production of nanofibrescan be traced back to Formhals patent in 1934, on theproduction of artificial filaments using high electricfield.8 The work was based on the effect of electrostaticforce on liquids i.e. when a suitably electrically chargedmaterial is brought near a droplet of liquid held in a finecapillary, it forms a cone shape and small jets may beejected from the tip of the cone if the charge density isvery high. Electrospinning process can be classified intotwo groups according to the method of preparing thepolymer, namely: solution electrospinning and meltelectrospinning.

In solution electrospinning, factors affecting theproperties of the electrospun web, characteristic featuresof the web, and various applications of the web, havebeen thoroughly reviewed by many researchers.9–13

Despite various attempts, the solution electrospinningprocess suffers from many drawbacks such as low pro-ductivity (up to 300mg/hr14); requirement of additionalsolvent extraction process; and environmental concernssince toxic solvents are used. Although the melt elec-trospinning process is free from the above constraints,limited work has been done on it. This is because of thedifficulties inherent in finer fibre formation, higher visc-osity of molten polymer and the electrical dischargeproblem associated with the application of high voltageto polymeric melt.

Several researchers have modified the basic setupfor solution electrospinning to increase the productiv-ity.15–20 The basic principle to obtain higher productiv-ity in solution electrospinning is based on increasing thenumber of jets by adopting different techniques. Thesetechniques can be summarized as: (a) multi-jets fromsingle needle, (b) multi-jets from multiple needles, and(c) needleless systems.19

Multi-jets from single needle

Generally, in single needle electrospinning (SNE) asingle-jet is initiated from the Taylor cone with theapplication of electric field. Multi-jets were observedfor the first time by Yamashita et al.21 during electro-spinning from a single home-made stainless steel needlewith a grooved tip mounted on a glass syringe. Beadlessmembranes of polybutadiene (PB) were successfullyprepared by growing multi-jets at the needle tip of an

SNE setup. The formation ofmulti-jets on an SNE setupwas attributed to two possible mechanisms: significantdiscrepancy in electric field distribution and some degreeof solution blockage at the needle tip. In anotherapproach, a curved collector was used by Vaseashta22

for the formation of multi-jets from multiple Taylorcones in an SNE system.

Multi-jets can be obtained from a single needle bysplitting the polymer jet into two separate sub-jets onits path to the collector.16,23 Jet-splitting (i.e. a sequenceof secondary jets emanating from the primary jet) hasbeen observed under certain conditions, where fluid jetsinteract with the large axial electric fields.24–27 Althoughthe mechanism of jet-splitting is yet to be fully analysed,the experimental investigation of controlled jet-splittingwill be a fascinating challenge for increasing the produc-tivity of SNE systems.

Multi-jets from multiple needles

Some work has been done to increase the productivityof electrospinning using multiple needle electrospinning(MNE) systems.28–31While designingMNE systems, theneedle configuration, number of needles and needlegauge are the important factors to be taken into con-sideration. Needles can be arranged in linear configura-tion or two-dimensional configurations such aselliptical, circular, triangular, square and hexagonal.For example, a linear configuration with four needleshas been designed to fabricate nanowebs and to scale-upthe production.32 The nanofibres were unevenly depos-ited on fibrous substrates because of the distortedelectric field on the multineedle setup. Similarly, Dinget al.33 fabricated biodegradable nanofibrous mats byMNE setup with four movable syringes and a rotatablegrounded tubular collector. Uniform thickness of nano-fibrous mats with good dispersibility was achieved. Inanother attempt, where seven and nine needles werearranged in a linear configuration, it was observedthat the behaviour of central jets and border jets wasdifferent.34

Tomaszewski and Szadkowski17 designed a linearconfiguration of 26-needles with a nozzle gauge of3mm. It was found that only a few border needles onboth sides formed fibres, while the needles near thecentre were inactive. The failure of the central needleswas attributed to the low inter-nozzle distance causingelectric field shielding near the central needles.34 In thelinear multineedle configuration, the use of an auxiliaryelectrode (known as extractor) was essential similar tothe high-compactness multi-jet electrospraying.18

Several investigations have also been reported in thefield of MNE with two-dimensional configuration. Forinstance, multi-jet spinning heads with concentric andelliptic arrangements were designed to achieve higher

130 Textile Research Journal 82(2)



productivity and to improve process stability.17 Theconcentric head was found to be the best type withrespect to both the efficiency and quality of the process.Theoron et al.34 investigated a complex multineedledesign with needles arranged in a 3�3 matrix. It wasobserved that the jets were repelled from their neigh-bours by the mutual columbic forces. The jets at theedge of the arrangement were strongly bent outwardswhereas the inner jets were squeezed along the line onwhich the spinning nozzles were located.

Yamashita et al.35 designed varieties of multi nozzleheads to address some issues related to electrospinningproductivity and nozzle clogging. Nanofibres withincreased production were achieved with these designs.Varesano et al.36 investigated a 4�4 matrix arrangementwith inter-nozzle distance of 1 cm and observed that itwas impossible to collect completely dried nanofibresfrom the inner jets. The effect of placing anon-conductingtextile substrate between the jets arranged in two lineararrays on the stability of the jets, distribution of thedeposition zones and nanofibre morphology was investi-gated.37 It was found that the presence of the non-conducting element increased the intensity of jet pertur-bation resulting in defects in the nanofibrous mat. Theshielding of the non-conducting substrate reduced themutual interaction of the charge bearing jets.

Kim et al.31 designed an electrospinning system con-sisting of five nozzles arranged in two arrays with 8mminter-nozzle distance. Nanofibres were fabricated usingan extra cylindrical electrode connected to the nozzlesof an electrospinning process to reduce the edge effectcaused by mutual repulsion of initial jets from adjacentnozzles. Higher productivity with reduced area of thespun mats was achieved in the modified setup. A sevenneedle setup with regular hexagon arrangement havinga central needle was reported by Yang et al.38 It wasreported that with a spacing of 10mm, the middle jetbehaved similar to a standard single-jet electrospinningsetup, whereas the peripheral needle-jets bent outwardsdue to the resultant electric forces. Kim and Park39

patented an advanced bottom-up electrospinningsystem that consisted of a nozzle block with multiplenozzles. The versatility of the setup allowed fabricationof composite nanofibre webs from several polymersolutions.

Recently, Yang et al.40 designed a shield ringenhanced equilateral hexagon distributed multineedlespinneret to produce fibres with special structures at ahigh production rate. The spinneret consisted of threeneedle sets distributed in an equilateral triangle arrange-ment, with a coaxial shield ring nearer the needle tip tocreate a uniform electric field. The simulation resultsshowed that the outer needles assisted in creating amore uniform electric field near the inner tips of theneedles by restricting the path of inner jets.

Generally, MNE systems require large operatingspace and careful design of the relative spacing betweenthe needles in order to avoid strong charge repulsionbetween the jets. The spacing between the needlesdepends on nozzle gauge as well as the solution proper-ties to be electrospun. A nonuniform electric field onneedle tips at different positions, needle clogging,instability problems (such as dripping or non-workingneedles) and uneven fibre deposition are some of the keylimitations of MNE systems.40

Multi-jets from needleless systems

Needleless electrospinning systems are becomingpopular as the productivity can be substantiallyimproved by provoking numerous polymeric jets fromfree liquid surfaces.41 The basic principle of formationof multi-jets from a needleless system is as follows: thewaves of an electrically conductive liquid self-organizeon a mesoscopic scale and finally form jets whenthe applied electric field intensity is above a criticalvalue.42 The pioneering work on needleless electrospin-ning was reported by Yarin and Zussman19 (Table 1,Figure 1). The work was based on a combination ofnormally-aligned magnetic and electric fields actingon a two-layer system, where the lower layer was a fer-romagnetic suspension and the upper layer was apolymeric solution. Numerous steady spikes of poly-ethylene oxide (PEO) were generated at the free surfaceof the magnetic-fluid by the application of a magneticfield. With the addition of the polymer layer and appli-cation of high voltage, some perturbations were visibleat the free surface of the polymer layer. After a thresh-old voltage, multiple jets were ejected towards thegrounded electrode and fibres were collected on a glassslide.

Jirsak et al.43 investigated the formation of multi-jetsfrom the free surface of a liquid uploaded in a slowlyrotating horizontal cylinder. The success of the methodlead to commercialization by Elmarco Company(Liberec) under the brand name Nanospider

TM

.Dosunmu et al.44 presented a novel needleless electro-spinning method based on a cylindrical porous poly-ethylene (PE) tube for the fabrication of nanofibresfrom multiple jets of nylon 6 solution. The polymersolution was electrified at the bottom of the tube andpushed by air pressure through the walls of the poroustube. The top of the tube was fitted to a pressurized airsupply device. The average fibre diameter (lengthweighted) obtained was similar to those from a single-jet electrospinning system but with a broaderdistribution.

Another needleless electrospinning system for fabri-cating polyvinyl alcohol (PVA) nanofibres by using aconical metal wire coil as the spinneret has been

Nayak et al. 131



Table 1. Modifications of basic electrospinning process for the production of nanofibres

Process Features Advantages Disadvantages

1. Needless electrospinning19

Polymer used: PEO

(Mw ¼ 600000),

Fibre diameter:

200–800 nm

Production rate:

12 times of the

conventional

electrospinning

Wider fibre

diameter

distribution

2. Bubble electrospinning55,71

Polymers used: PVP

(Mw ¼ 40000 g/mol)

and PEO (Mw ¼

500000 g/mol)

Solution concentra-

tion for PVP ¼

30 wt% and

PEO ¼ 2 wt%

High production

rate, high efficiency,

free from clogging,

ease of operation,

simple process

and low cost

Solvent recovery

problem

3. Electroblowing61

Polymer used:

Hyaluronic

acid (HA) 2–3%

(w/v); Fibre

diameter:

40–120 nm

High production

rate, and simple

process

Solvent

recovery

problem

4. Cylindrical porous hollow tube64

Polymer used:

PVP (MW 360000);

Fibre diameter:

300–600 nm

High

productivity

Complex

design of the

equipment

(continued)




reported.14 The needleless approach produced finernanofibres on a much larger scale compared to conven-tional electrospinning. Kumar et al.45 developed an elec-trospinning setup for the formation of multiple jets withcontrolled fibre repulsion using a plastic filter. Apartfrom increased throughput, this setup reduced fibrerepulsion as compared to a multineedle setup. Thefibre repulsion was reduced by controlling emitter vol-tage and emitter/collector distance. It was found thatthe plastic filter setup produced fibres with lower aver-age diameter and better uniformity.

Needleless electrospinning comprising a rotatingdisk and a cylinder nozzle, for the fabrication of PVAnanofibres was reported by Niu et al.42 Under identicaloperating conditions, the fibres produced from the disknozzle were finer than those from the cylinder nozzle.The disk nozzle needed a relatively low voltage to initi-ate the fibre formation. The cylinder nozzle showed ahigher dependence on the applied voltage and polymerconcentration.

Tang et al.46 designed a new needleless electrospin-ning apparatus where the polymer solution was

splashed on the surface of a metal roller spinneret. Onthe application of high voltage, several spinning jetswere ejected from the surface of the polymeric solution.Solidified nanofibres were deposited on the collector,similar to the conventional electrospinning process.Lukas et al.47 increased the productivity of electrospin-ning by simultaneously provoking innumerable poly-meric jets from a sufficiently large liquid surface. It wasfound that the system started to be self organized above acritical value of the applied electric field. Reneker et al.48

carried out preliminary investigations on multiple jetsobtained from a single droplet of PEO solution. Threejets were initiated by the application of low voltagewhichwere stabilized by the asymmetric arrangement of one ormore charged electrode(s).

Recently, Liu et al.49 used a flat aluminum plate col-lector to initiate multi-jets from a single droplet. It wasdemonstrated that multi-jets could be easily initiated ina controlled manner at suitable solution concentration,high voltages and flow rates when the droplet experi-ences several cycles of dripping. The auto-initiation ofdouble jets was observed by means of a high-speed

Table 1. Continued


5. Microfluidic manifold66

Polymer used: PVP;

Fibre diameter:

85–350 nm

Simple process, flexible

control over channel

size, rapid prototyping

and the ability to

spin multiple fibres

in parallel through

arrays of individual

microchannels

Solvent recovery

problem

6. Roller electrospinning69

Polymer used: PU

(Mw ¼ 2000 g/mol)

Avg. fibre diameter:

144 nm

Production rate

higher, simple

process

Solvent recovery

problem, low

molecular weight

polymers are difficult

to be spun

Nayak et al. 133



camera and a possible mechanism was outlined. Besidesthe above works, some patents50–54 have been publisheddealing with multi-jet electrospinning systems from asingle droplet.

Other potential approaches inelectrospinning

In addition to the needle andneedleless systemsdiscussedabove, electrospinning can also be classified into a con-fined feed system (CFS) and unconfined feed system(UFS) based on the manner in which the solution ormelt is dispensed. In CFS, the polymer solution or meltis injected at a constant rate whereas in UFS it flowsunconstrained over the surface of another material.The advantages of CFS are: restricted flow rate (neededfor maintaining continuous stable electrospinning), uni-form fibre diameter and better quality fibre. However,CFS increases the system complexity (as a controlsystem is required for each jet or multi-jets) and isprone to clogging. CFS includes the electrospinning sys-tems that use a syringe pump, whereas UFS includesdifferent systems such as bubble electrospinning;55 elec-troblowing; electrospinning by using: porous hollowtube, microfluidic manifold and roller electrospinning.In the following section various UFS have beenillustrated.

Bubble electrospinning

Liu and He55 explored the feasibility of mass productionof nanofibres by bubble electrospinning. The device con-sisted of a high voltage DC generator, a gas pump, avertical liquid reservoir having a top opening, a gastube installed at the bottom centre of the reservoir,a thin metal electrode fixed along the centre line of thegas tube and a grounded collector (Table 1, Figure 2).The gas tube and the electrode were inserted through thebottom of the reservoir and were connected with thegas pump and the DC generator, respectively. One orseveral bubbles were formed on the free surface of thesolution, when the gas pump was turned on slowly. Theshape of each bubble changed from spherical to conical(similar to a Taylor cone56) as the DC voltage wasapplied. Multiple jets were ejected from the bubbleswhen the applied voltage exceeded a threshold value.

The polymer jets in bubble electrospinning alsoexhibited an instability stage similar to that in the con-ventional electrospinning. The fibres produced were amixture of straight, coiled and helical fibres along witha few beaded and thick fibres. The number of bubbleswas affected by the gas pressure, solution viscosity,nozzle diameter and height between the nozzle tip andliquid surface. Later, Liu and coworkers57–59 investi-gated the effect of applied voltage on fibre diameter

and morphology in bubble electrospinning. Theaverage fibre diameter increased with the applied vol-tage, which is quite different from the results in basicelectrospinning. Yang et al.60 investigated the effect ofapplied voltage on fibre diameter of PVA nanofibres inbubble electrospinning and showed that higher voltagefavours smaller diameter.

Electroblowing

Electroblowing is an electrospinning process assistedwith air blowing. The method comprises preparation ofapolymer solutionbydissolving the polymer in a solvent,feeding the polymer solution through a spinning nozzleapplied with high voltage, injecting compressed airthrough the lower end of the spinning nozzle and collect-ing the fibres in the form of a web on a suitable groundedcollector. In electroblowing, two simultaneously appliedforces (an electrical force and an air-blowing shear force)interact to fabricate the nanofibres from the polymericfluid. Nanofibres of both thermoplastic and thermoset-ting resins can be produced by electroblowing.

Wang et al.61 were one of the first to modify an elec-trospinning apparatus by the attachment of an air blow-ing system to fabricate nanofibres from hyaluronic acid(HA) (Table 1, Figure 3). In the setup, high positivevoltage was supplied between the spinneret and thegrounded collector plate. The air temperature and blow-ing rates were achieved by controlling the power outputof the heater and the flow rate of air respectively. The airflow was also used to control the cooling rate of the fluidjet and the rate of solvent evaporation. The factorsaffecting the fibre morphology and diameter were theair blowing rate, polymer concentration, solution feed-ing rate, electric field strength and type of collector.

Kim et al.62 prepared polyacrilonitrile (PAN) fibreswith diameters ranging from several nanometers tohundreds of nanometers by electroblowing a 20wt%solution of PAN in dimethyl formamide (DMF). Theapparatus consisted of a storage tank for polymersolution, a spinning nozzle, an air nozzle adjacent tothe lower end of the spinning nozzle for injectingcompressed air, a source of high voltage and a groundedcollector. The equipment had a higher productivity overthe conventional electrospinning.

Arora et al.63 produced nanowebs of PP with averagediameter of 850–940 nm by electroblowing. In thesetup, compressed and heated air was supplied fromair nozzles positioned around the sides of the spinningnozzle. The air forwarded the newly issued polymericsolution from the nozzle and attenuated to nanofibreswhich were collected on a grounded porous collectionbelt. Other materials used in the process were boththe addition and condensation polymers such as:polyamide (PA); polyester; polyolefins; polyacetal;




polyalkylene sulfide; polyarylene oxide; polysulfone;cellulose ether and ester; polyvinylchloride (PVC);polymethylmethacrylate (PMMA); polystyrene (PS);polyvinylidene fluoride; polyvinylidene chlorideand PVA.

Electrospinning by porous hollow tube

Varabhas et al.64 used a polytetrafluroethylene (PTFE)tube with porous wall for increasing the productivity ofthe electrospinning process (Table 1, Figure 4). The tubecontaining the pores with an average diameter of 20–40mmwas oriented horizontally. Holes of 0.5mmdiameter,spaced 1 cm apart from each other, arranged in two rowsparallel to the axis of the tube and penetrating 1mm intothe wall were drilled along the bottom of the tube. Inorder tomaintain an equal electrical potential in the vici-nity of each hole, a wire electrode made out of a squarewire mesh having 5mm spacing between the wires wasinserted inside the tube.

The porous tube was suspended on the frame of aPVC pipe with an adjustable distance of 12–15 cmabove the grounded aluminum foil collector. The solu-tion of polyvinylpyrrolidone (PVP) in ethanol (15wt%)was pushed at low air pressure (1–2 kPa) through thetube wall with a voltage of 40–60 kV. During electro-spinning, each hole produced one jet which finallybecame a continuous long fibre. As the jet traveled adistance of a few centimeters, the bending instabilitybecame dominant and the jet formed an expandingcoil. The production rate obtained by this method wasabout 3–50 times higher than the SNE system which wasdependant on the number of rows of holes, spacingbetween the holes and the collector geometry.

Electrospinning by microfluidic manifold

Srivastava et al.65,66 designed a microfluidic device tofabricate multicomponent nanofibres and to scale-upthe production of electrospinning (Table 1, Figure 5). Apolydimethylsiloxane (PDMS) based multilayer micro-fluidic device capable of spinning several hollow fibres inparallel was used for the production of nanofibres. Twolayers of microchannels (four spinnerets in each layer)were used to flow PVP solution as sheath material. Anarray of spinnerets was used for the fabrication of thecore consisting of heavy mineral oil or pyrrole. Each ofthe eight outlet spinnerets was provided with constantpressure by two layers of nonintersecting stacked micro-channels arranged in a branching tree pattern. Thesheath polymer solution was introduced through themicrochannels at the bottom layer and the core materialwas introduced through the top microchannels. Duringthe electrospinning, mutually interacting electrified jetsunderwent bending instabilities and were repulsed from

their neighbours due to columbic repulsion. Uniformnanofibre mats were produced at the rate of 0.1 g/hrwith the inter-nozzle distance of approximately 8mm.

Srivastava et al.67 also produced bicomponent Janusnanofibres using a PDMS based multiple outlet micro-fluidic device capable of spinning eight nanofibres inparallel. The ability of the microfluidic device to synthe-size multiphase nanofibres with controlled distributionof functionalities (which is otherwise difficult to obtain),provides a useful technology to the next generation ofsmart materials. The microfluidic manifold method hasmany advantages over conventional electrospinningsuch as rapid prototyping, ease of fabrication andparallel electrospinning within a single, monolithicdevice. The fabrication of complex networks of micro-channels is easily accomplished using PDMS basedmicromolding technology.

Roller electrospinning

Fabrication of nanofibres by electrospinning usingrotational setup can be dated back to the 1980s.68

Roller electrospinning process was first developed byJirsak et al.43 in 2005 at Technical University of Liberec(Czech Republic). The mechanism of formation ofTaylor cones on the surface of roller was described byLukas et al.47 Cengiz and Jirsak69 studied the effect oftetraethylammoniumbromide (TEAB) salt on thespinnability of polyurethane (PU) nanofibres by rollerelectrospinning (Table 1, Figure 6). The roller electro-spinning setup consisted of a rotating cylinder to spinnanofibres directly from the polymer solution. An alumi-nium rotating roller was partially immersed in the PUsolution contained in a polypropylene (PP) dish. Therotating roller was applied with high voltage and the col-lector was grounded. It was found that the saltconcentration had an important effect on conductivity,viscosity, fibre diameter and morphology.

Later, Cengiz and coworkers70 also investigated theinfluence of solution properties on fibre diameter ofPVA using the device mentioned above. It was foundthat the electric conductivity and surface tension of thesolution did not affect both the throughput and fibrediameter significantly. Molecular weight had an impor-tant influence on spinnability whereas solution concen-tration influenced the throughput and properties ofnanofibres.

Melt electrospinning

The initial work on melt electrospinning was carried outby Lorrand and Manley.72 Generally, the setup for meltelectrospinning consists of a provision for melting thepolymer and other parts similar to solution electrospin-ning. The polymers for melt electrospinning can be

Nayak et al. 135



heated by different means such as heating oven,73 heatguns,74–75 laser melting devices76–77 and electric heat-ing.78. The fabrication of nanofibres of various polymerssuch as polylactic acid (PLA),73 polylactide,76 PP,74,75,78

polyethylene terephthalate (PET),78 (polyethyleneglycol)-block-(poly-e-caprolactone) (PEG47-b-PCL95)and (poly-e-caprolactone) (PCL),75 by melt electrospin-ning has been reported by several researchers.

The nozzle size; temperatures of the spinneret and thespinning region;73 polymer molecular weight,78 shearand extensional viscosities;79 and the polymer flowrate78 were found to be the important factors for produ-cing submicron fibres in melt electrospinning. In themajority of the cases, the melt electrospun web consistsof fibres both in nanometer and micrometer scale. Forexample, it has been demonstrated that most of themeltblown fibres had diameters in the range of 10–20mm with some fibres 250–500 nm,74 and 1–30 mm withsome random scattered fibres of 247 nm80 in the web.The diameter of the melt electrospun fibre is large inthe initial duration of the experiment which decreasessubstantially after a few seconds.75,81

Melt electrospinning has several advantages oversolution electrospinning such as no recycling/removalof toxic solvents, high throughput rate as there is noloss of mass by solvent evaporation, ease of fabricatingpolymeric fibre blends and suitability to the polymers(such as PE, PP and PET) having no appropriate solventat room temperature.73 In spite of the above advan-tages, melt electrospinning has some limitations suchas the requirement of a high temperature meltingsystem, an electric discharge problem associated withthe melt and low conductivity of the melt. The difficultyin obtaining submicron fibres in melt electrospinningcan be attributed to the high viscosity of the melt; andthe rapid solidification of the polymer in the regionbetween the needle tip and the collector. Various meltelectrospinning apparatuses used for the production ofnanofibres are demonstrated in (Table 2, Figure 1–5).

Other techniques without electrostatic

force

Apart from the techniques discussed above which aremainly based on the application of electrostatic force forthe fabrication of nanofibres, several other approachessuch as meltblowing, flash-spinning, bicomponentspinning, forcespinning, phase-separation and drawing,are already used for the fabrication of nanofibres. Thesetechniques are highlighted in the flowing section.

Meltblowing

Meltblowing is a simple, versatile and one step processfor the production of materials in micrometer and

smaller scale. The technology of meltblowing was firstdeveloped in the 1950s at the Naval ResearchLaboratory of United States.84 In the meltblowing pro-cess, a molten polymer is extruded through the orifice ofa die. The fibres are formed by the elongation of thepolymer streams coming out of the orifice by air-dragand are collected on the surface of a suitable collector inthe form of a web. The average fibre diameter mainlydepends on the throughput rate, melt viscosity, melttemperature, air temperature and air velocity. A briefreview of the meltblowing process and the factors affect-ing the properties of the web have been reported byvarious researchers.85–87

The difficulty in fabricating nanofibres in meltblow-ing is due to the inability to design sufficiently smallorifice in the die and the high viscosity of the polymericmelt. Nanofibres can be fabricated by special die designswith a small orifice, reducing the viscosity of the poly-meric melt and suitable modification of the meltblowingsetup. For example, Ellison et al.88 produced meltblownnanofibres of different polymers by a special designedsingle-hole die with small orifice using the processingconditions used in industry (Table 3, Figure 1).Similarly, the special die design by Podgorski et al.89

where the polymer nozzles were surrounded by air noz-zles produced nanofibres with diameters ranging from0.74 to 1.41 mm (Table 3, Figure 2).

Bhat et al.90 produced nanofibres of 50–2000 nm dia-meter by meltblowing with a special die design and sui-table modification of the processing conditions.Nanofibres of various polymers with average diameterof 500 nm have been reported by Wente,91 without anyreport of fibre size distributions. A special stacked platedie design with an orifice of 0.125mm diameter was usedto fabricate nanofibres (average diameter of 300 nm).92

The use of small orifices made by an electric dischargemachine for the production of super-hydrophobic nano-fibres and microfibres has been reported.88

In another approach, Brang et al.93 fabricated nano-fibres by meltblowing using a modified die with plateedge profile having very large length to diameter (L/D)ratio and small orifice diameter (Table 3, Figure 3). Thepolymer feed rate was very low (0.01 g/hole/min) duringthe meltblowing. The decrease in productivity with lowfeed rate was compensated by increasing the density ofthe spin holes (100 holes per inch). Heated and pressur-ized air was applied to attenuate the filaments upon exitfrom the die and then collected on a moving conveyor.The equipment was able to fabricate fibres of thermo-plastic polymers with diameter less than 0.5 mm at aproduction rate of 1.5 kg/meter/hr.

Bodaghi et al.94 fabricated meltblown nanofibres bychanging the rheology of the polymers (Table 3, Figure4). The meltblowing apparatus consisted two extruderswith different barrel diameter to create different shear




Table 2. Melt electrospinning setup for the production of nanofibres


1. Melt electrospinning (Electrical heating73)

Polymer used: PLA;

Avg. Fibre diameter:

800 nm

Simple setup,

solvent-free

approach

Mostly amorphous

fibre, thermal

degradation

2. Melt electrospinning (Electrical heating82)

Polymer used: poly-

ethylene glycol-block-

poly-e-caprolactone;

Fibre diameter: 16 �

10.7 mm for molten

fibres and 560 � 90 nm

for solid fibres

Defect free, con-

tinuous and consistent

fibre; Non-toxic

method

Presence of few poor

quality molten fibre

3. Coaxial melt electrospinning83

Fabricated PCM based

nanofibre (sheath TiO2

and core octadecane);

Avg. Fibre diameter:

150 nm

One step process for

encapsulation;

Production of compo-

site nanofibres;

Suitable for wide range

of materials

Complex setup

4. Melt electrospinning (heating gun75)

Polymer used: PP and

PEG-b-PCL and PCL

blend; Fibre diameter:

35 � 1.7 mm (PP-no

additive) and 840 �

190 nm (PP with visc-

osity reducing agent); 2

� 0.3 mm (blend-no

additive) and 270 �

100 nm (blend-with gap

method)

Production of blended

nanofibre

Coiling and buckling

instabilities of the jet

near to the collector

(continued)

Nayak et al. 137



rates. A spin pack, coupled with both the extruders,directed high velocity air towards the meltblown fibresto attenuate and split them into nanoscale. The processwas suitable for many melt-spinnable commercial poly-mers, copolymers and their blends such as polyesters,polyolefins (PE and PP), PA, nylons, PU, PVC, PVAand ethylene vinyl acetate. The meltblown fibres hadsignificantly reduced average diameter and enhancedsurface area to mass ratio compared to conventionalmeltblown fibres. Hills inc. (West Melbourne) producedmeltblown nanowebs from low viscosity (or high meltflow index 1500–1800) homopolymers with averagediameter of 250 nm.95 Apart from the low viscosity,smaller diameter orifices with higher L/D ratio (500+)and low flow rate assisted in nanofibre fabrication.

In meltblowing, the sudden cooling of the fibre as itleaves the die can prevent the formation of nanofibres.This can be improved by providing hot air flow in thesame direction of the polymer around the die. The hotair stream flowing along the filaments helps in attenuat-ing them to smaller diameter. The viscosity of polymericmelt can be lowered by increasing the temperature,but there is a risk of thermal degradation at hightemperature.

Template melt-extrusion

Li et al.96 combined the extrusion technology with thetemplate method for the production of polymericnanofibres of thermoplastic polymers. In this process(Table 3, Figure 5), the molten polymer was forcedthrough the pores of an anodic aluminum oxide mem-brane (AAOM) and then subsequently cooled down toroom temperature. A special stainless steel appliancewas designed to support the thin AAOM, to bear thepressure and to restrict the molten polymer movementalong the direction of the pores. The appliance

containing the polymer was placed on the hot plate ofa compressor (with temperature controlled functions)followed by the forcing of the polymeric melt (indicatedby the arrow). The hot plate was stopped after twohours of heating and the pressure was maintaineduntil the system cooled to room temperature.

Isolated nanofibres of PE were obtained by theremoval of the AAOM with sodium hydroxide/ethanol(20wt%). Finally, the nanofibres were broken downfrom the bulk feeding film by ultrasound (in ethanolfor 5min.) to form isolated fibres. The diameter of thePE fibres ranges from 150 to 400 nm (diameter ofAAOM pores ¼ 200 nm) and the length of fibres corre-spond to the length of the pores in AAOM (i.e. 60 mm).

Flash-spinning

In the flash-spinning process, a solution of fibre formingpolymer in a liquid spin agent is spun into a zone oflower temperature and substantially lower pressure togenerate plexi-filamentary film-fibril strands. A spinagent is required for flash-spinning which: 1) shouldbe a non-solvent to the polymer below its normal boilingpoint, 2) can form a solution with the polymer at highpressure, 3) can form a desired two-phase dispersionwith the polymer when the solution pressure is reducedslightly, and 4) should vaporize when the flash isreleased into a substantially low pressure zone.

The flash-spinning process was described by Bladesand White of DuPont in 1963 and since then severalpatents have been filed. Weinberg et al.97 producednanofibres of polyolefins with fibre length of 3–10 mmand at a production rate which is at least two orders ofmagnitude higher than the conventional electrospinningusing flash-spinning (Table 4, Figure 1). The nonwovenfibrous webs produced had significantly differentmorphology (i.e. complex interconnecting networks or

Table 2. Continued


5. Melt electrospinning (LASER heating76)

Polymer used:

Polylactide; Avg. fibre

diameter: ~1 mm

Free from electric dis-

charge problem of the

conventional melt

electrospinning;

Suitable for polymers

with relatively high

melting point; Reduced

thermal degradation as

local and instantaneous

heating is possible

Amorphous fibres;

unstable fibre forma-

tion with higher laser

output power




Table 3. Processes for the production of nanofibres based on polymeric melt


1. Schematic of the meltblowing die:

(a) sectional view and

(b) end-on view of the top and bottom piece88

Polymers used:

Polybuta styrene,

PP and PS; Avg.

fibre diameter: less

than 500 nm

Nanofibres feasible

at commercial

processing

condition

Dispersion of

spherical particles

among the fibre

mat, fibre breakup

between the die

and collector

2. Die design for providing airflow

in meltblowing by Podgorski89

Polymer used: PP;


210 nm-37.5 mm

High production

rate

High variation of

fibre diameter

3. Meltblowing setup93

Polymer used: PP,

PET, PA, PE, PLA,

Co-PA, PFE; Avg.

fibre diameter:

most of the fibres

less than 500 nm

High production

rate

Complex setup

needed

4. Meltblowing setup94

Polymer used:

polyesters, poly-

olefins (PE and PP),

PA, PU, PVC, PVA

and ethylene vinyl

acetate; Avg. fibre

diameter: 940nm

along with some

microfibres

Favorable for many

polymers; High

production rate

Complex process

(continued)

Nayak et al. 139



webs of large and small polyolefin filaments or fibressimilar to spider webs) than those produced by othertechnologies.

Flash-spinning is more suitable for difficult-to-dissolve polymers such as polyolefins and high molecu-lar weight polymers. The spinning temperature shouldbe higher than the melting point of polymer and theboiling point of solvent in order to effect solvent eva-poration prior to the collection of the polymer. Theflash-spinning process does not produce fibrous websconsisting completely of nanofibres.

Bicomponent spinning

Bicomponent spinning is a two step process thatinvolves spinning two polymers through the spinningdie (which forms the bicomponent fibre with island-in-sea (IIS), side-by-side, sheath-core, citrus or segmented-pie structure) and the removal of one polymer.98

Although bicomponent fibres of different cross-sec-tional shapes and geometries with micrometer diametercan be produced with the existing fibre forming techni-ques, fabricating smaller diameters especially in nan-ometers is a real challenge.

The production of webs of IIS structure (nylon 6island and PLA sea) by the spunbonding process andthe subsequent removal of sea for the production ofmicro and nanofibres have been reported.99 Hill Inc.produced nanofibres of 300 nm diameter from the IISstructure.100 Lin et al.101 fabricated side-by-side bicom-ponent nanofibres of elastomeric (polyurethanes) andthermoplastic (PAN) polymers, using a microfluidicdevice as the spinneret in electrospinning. The siliconemicrofluidic spinneret consisted of three capillary chan-nels: two for the inlet of polymer solutions and the otherfor outlet. They observed self crimping of PAN after thePU was removed from the bicomponent fibre by dissol-ving in tetrahydrofuran (THF).

Bicomponent spinning can be used for the fabrica-tion of smaller nanofibres by sacrificing one of the poly-mer components as well as to create multicomponentnanofibres. Several researches have reported bicompo-nent polymeric nanofibres of sheath-core structure bythe electrospinning process using a coaxial two-capillaryspinneret.102–104 The use of melt coaxial electrospinningfor the fabrication of core-shell nanofibres havingpotential for temperature sensors105 and compositesbased on phase change materials (PCM)83 have beenreported. The segmented-pie structure forms microand nanofibres (diameter 500 nm to 2 mm) with noncir-cular cross section. Recently, a new modified coaxialelectrospinning process has been developed to preparepolymer fibres from a high concentration solution ofPVP.106 This process involved a pure solvent concentri-cally surrounding polymer fluid in the spinneret and wasable to produce fibres with a smooth surface morphol-ogy and good structural uniformity.

Other approaches

In addition to the above mentioned techniques, severalother innovative methods such as template synthesis,self-assembly, phase-separation, drawing have beenreported for nanofibre fabrication.107 In template synth-esis, nanofibres of polymers,metals, semiconductors andceramics are formed within the numerous cylindricalpores of a nanoporous membrane (5–50mm thickness)by oxidative polymerization accomplished electrochemi-cally or chemically. In electrochemical synthesis one sur-face of a membrane is coated with a metal film whichworks as an anode for the polymer whereas in chemicalsynthesis the membrane is immersed in a solution ofthe monomer and its oxidizing agent. The templatesynthesis process has been used to prepare nanofibresof PAN, PCL, polyaniline, polypyrrole and poly(3-methylthiophene).108,109

Table 3. Continued


5. Template melt-extrusion96

Polymer used: PE;


150 nm–400 nm,

fibre length: 60 mm

Less variation in

fibre diameter

Lengthy process;

Very short length

fibre with some

breaks




Table 4. Other approaches for nanofibre production


1. Flash-spinning97

Polymer used: PP; Fibre

diameter: 500 nm

High production rate Short fibre length

(3 to 10 mm)

2. Drawing114,115

Polymer used:

polytrimethylene

terephthalate; Fibre

diameter: 60 nm

Simple and one step

process, longer fibres

Lower productivity,

nonuniform fibre size

3. Application of pressurized gas118

Polymer used: nylon,

polyolefins, polyimides,

polyesters, fluoro-poly-

mers; and spinnable fluids

include molten glassy

materials, molten pitch,

polymeric melts, polymers

that are precursors to

ceramics; Fibre diameter:

less than 3000 nm

Versatile process for

many spinnable fluids,

polymeric melts and

solutions for nanofibre

production

Complex machines and

equipments necessary

4. Forcespinning119,120

Polymer used: PEO, PLA,

bismuth, PP, PS, acryloni-

trile-butadiene-styrene

and polyvinyl pyrrolidone

Simple process, free

from high electric field

and solvent and high

production rates

Sometimes heating to

very high temperature

is necessary

5. Jet-blowing setup123

Polymer used: PTFE,

polyaramide, PMMA,

organic polymers and their

blends; Fibre diameter:

10 nm to 50 mm

Environmental advan-

tages as it is non-toxic,

uses chemically inert

gas and no deleterious

solvents are used;

Superior fibre proper-

ties; Formation of

composites

Complex process

(continued)

Nayak et al. 141



Self-assembly is a manufacturing method wheresmall molecules are used as basic building blockswhich add-up to give nanofibres.110 The small moleculesare arranged in a concentric manner which upon exten-sion in a normal plane produce the longitudinal axes ofthe nanofibres. In self-assembly the final (desired) struc-ture is ‘encoded’ in the shape of the small blocks, ascompared to traditional techniques (such as lithogra-phy) where the desired structure must be carved outfrom a large block of matter. Self-assembly is thusreferred to as a ‘bottom-up’ manufacturing technique,whereas lithography is a ‘top-down’ technique. Thesynthesis of molecules for self-assembly often involvesa chemical process called convergent synthesis. Thisprocess requires standard laboratory equipment andis limited to specific polymers.

In self-assembly, the shape and properties of nanofi-bres depend on the molecules and the intermolecularforces that bring the molecules together. Nanofibres ofvarious polymeric configurations such as diblock copo-lymers; triblock copolymers; triblock polymers (of pep-tide amphiphile and dendrimers); and bolaform (ofglucosamide and its deacetylated derivatives) can beassembled by this process. Nanofibres from diblockcopolymers and triblock polymers were prepared byLiu et al.71 and Yan et al.111 respectively by self-assembly.

In phase-separation, the gel of a polymer is preparedby storing the homogeneous solution of the polymer atthe required concentration in a refrigerator set at thegelation temperature.112 The gel is then immersed indistilled water for solvent exchange, followed by theremoval from the distilled water, blotting with filterpaper and finally transferring to a freeze-drying vesselleading to a nanofibrematrix. The phase-separation pro-cess was used for the fabrication of nanofibre matricesof poly-L-lactic acid and blends of poly-L-lactic

acid-polycaprolactone.113 Although the phase-separa-tion process is very simple, it is only limited to the labora-tory scale.

In the drawing process114,115 (Table 4, Figure 2) amillimetric droplet of a solution is allowed to evaporateafter it is deposited on a silicon dioxide (SiO2) surface.The droplet becomes more concentrated at the edgebecause of evaporation due to capillary flow. A micro-pipette is dipped into the droplet near the contact linewith the surface and then withdrawn at a speed of 100mm/s, resulting in a nanofibre being pulled out. Thepulled fibre is then deposited on another surface bytouching it with the end of the micropipette. Fromeach droplet, nanofibres can be drawn for severaltimes. Nanofibres of sodium citrate were formed bydissolving it in chloroauric acid through the drawingprocess.116 The drawing process is suitable for viscoe-lastic materials which can undergo strong deformationswhile being cohesive enough to support the stressesdeveloped during pulling. This process is simple butlimited to laboratory scale as nanofibres are formedone by one.

Muthuraman et al.117 fabricated high quality nano-fibres from various polymeric fluids by using the edge ofa flat plate as a source electrode onto which polymericsolution was poured as droplets. The droplets under-went a gravity-assisted flow and collected on thegrounded collector when the voltage was applied.Three configurations of the flat plate such as parallel-plate, edge-plate and waterfall geometry were used forthe experiments. Nanofibres with uniform diameterwere produced by the needleless system. This novelapproach worked in a remarkably similar manner toconventional electrospinning, is free from the problemsof clogging and has a high potential for scale-up.

Reneker et al.118 fabricated nanofibres by usingpressurized gas where an expanding gas jet supplied

Table 4. Continued


6. Melt-spinning setup124

Polymer used: polyolefins,

polyesters, PA, vinyl poly-

mers, polystyrene-based

polymers, bio-polymers,

polycarbonates, cellulose

esters, acrylics, acrylics,

fluoro-polymers and

chlorinated polyethers.

Simple and versatile

technique suitable

for many polymers

Thermal degradation

of polymers




the mechanical force required to create nanofibres(Table 4, Figure 3). In this process a polymeric solutionor melt is fed into an annular column having an exitorifice and is subjected to the action of a gas jet whichpushes the material through the orifice forming thefibres. After the fibres are ejected from the orifice,solidification can occur in many ways such as: cooling,chemical reaction, coalescence or solvent removal. Thepolymers used in this process include nylon, polyolefins,PA, polyesters and fluoro-polymers. Various factorsaffecting the fibre diameter are the temperature ofthe gas jet, flow rate of the gas and flow rate of thepolymeric material.

Recently, nanofibres of a wide range of materialswere fabricated by a new process called forcespin-ning.119,120 In this process (Table 4, Figure 4) the electricfield of electrospinning is replaced by centrifugal force.The process involves heating a fibre forming material ina heated structure and rotating the heated structure(with at least one nozzle) at very high speed to extrudenanofibres of the material. Rotational speed of theheated structure, nozzle configuration, collectionsystem and temperature are the key factors governingthe geometry and morphology of the nanofibres. Thelimitations of the electrospinning process such as veryhigh electric field, low productivity and high costof production are eliminated in forcespinning.Forcespinning also broadens the selection of materialsas both conductive and non-conductive materials can be

spun into nanofibres. A number of solid materials canbe melted and directly spun into nanofibres without anychemical preparation. Also, the process is free fromextra process of solvent recovery as no solvent is used.

Weitz et al.121 obtained nanofibres with diameterdown to 25 nm from polymeric solution during a stan-dard spin-coating process unexpectedly. The processinvolves the application of drops of a polymeric solu-tion onto a standard spin-coater followed by fastrotation. The Rayleigh-Taylor instability triggers theformation of thin liquid jets emerging from the outwarddriven polymeric solution, yielding solid nanofibres afterevaporation of the solvent. This is a simple, efficient andnozzle-free process for the fabrication of nanofibres froma variety of polymeric solutions.

Badrossamay et al.122 fabricated three-dimensionalaligned nanofibres by exploiting a high-speed rotatingnozzle to form a polymer jet which undergoes stretchingbefore solidification. In this process, known as rotaryjet spinning (RJS), fibre diameter, fibre morphology andweb porosity can be controlled by varying rotationalspeed, nozzle geometry and solution properties. Thesystem consisted of a reservoir (with two side wall ori-fices) attached to the shaft of a motor with controllablerotation speed. The polymer solution was continuouslyfed to the reservoir at a suitable rate to maintain aconstant hydrostatic pressure and continuous flow.The fibres were collected either on a stationary sur-rounding cylindrical collector or on coverslips held

Table 5. Comparison of various nanofibre fabrication techniques

Manufacturing process

Scope for

scaling-up Repeatability

Control

on fibre

dimension Advantages Disadvantages

Electrospinning (Solution) Yes Yes Yes Long and continuous

fibres

Solvent recovery issues,

low productivity, jet

instability

Electrospinning (Melt) Yes Yes Yes Long and continuous

fibres

Thermal degradation of

polymers, electric

discharge problem

Meltblowing Yes Yes Yes Long and continuous

fibres, high productivity,

free from solvent recovery

issues

Polymer limitations,

thermal degradation of

polymers

Template synthesis No Yes Yes Easy to change diameter

by using different

templates

Complex process

Drawing No Yes No Simple process Discontinuous process

Phase-separation No Yes No Simple equipments

required

Only work with selective

polymers

Self-assembly No Yes No Easy to get smaller

nanofibres

Complex process

Forcespinning Yes Yes Yes Free from very high vol-

tage, eco-friendly

Requirement of high

temperature at times

Nayak et al. 143



against the collector wall. The RJS technique has severaladvantages over electrospinning, such as: (a) norequirement of high voltage, (b) fibre fabrication is inde-pendent of solution conductivity, (c) it is applicableto polymeric emulsions and suspensions, and (d)higher productivity.

A new jet-blowing technique (Table 4, Figure 5) wasused for the fabrication of micro and nanofibres ofpolymers with high melt viscosity such as PTFE andPMMA.123 In the process, the mixture of a polymerand a pressurized gas were blown through the apertureof a nozzle having two segments with different dia-meters. Polymeric fibres having diameters in the rangeof 10 nm to 50 mm were produced by this method.

Huang et al.124 produced nanofibres by melt spinningwhere a polymeric melt of at least one thermoplasticpolymer was supplied to the inner surface of a heatedrotating distribution disc having a forward fibre dis-charge edge. The melt is then distributed into a thinfilm and attenuated by hot gas to produce polymericnanofibres (Table 4, Figure 6). The process of nanofibre(diameter less than 100 nm) production by rapid expan-sion of a supercritical solution into a liquid solvent(RESSLS) was developed by Meziani et al.125 TheRESSLS process is the modification of the traditionalrapid expansion of a supercritical solution (RESS)process used for the production of polymeric particlesand fibres.126–128

Comparisons of different processes

Table 5 summarizes the relative merits and demerits ofvarious processes employed for nanofibre fabrication.The table also highlights the potential for scale-up,repeatability and the ease of control of fibre dimensionsby these processes.

Conclusions

Over the past few years, there has been explosivegrowth in the fabrication techniques for nanofibrousmaterials because of their unique features and manyuseful applications. In this review, various techniquesfor fabricating nanofibres and recent developments innanofibre-related technologies have been illustrated.However, fundamental analysis of these fabricationtechniques is needed to develop nanofibres with thedesired properties on a commercial scale. Some of thetechniques are still in their infancy and much research isrequired for standardization and commercialization. Ofall these processes, electrospinning is so far the onlymethod with potential for commercial production, andthe major issue yet to be resolved is how to substantiallyscale-up the production to match the demands from arange of potential markets.

Because of its commercial potential, more than 50%of the research articles pertinent to nanofibre produc-tion are based on the electrospinning methods, as it iswidely used for fabrication of nanofibres of a widerange of polymers, ceramics and metals. Some of thetechniques such as meltblowing and bicomponent spin-ning are limited to few polymers. Other techniquessuch as drawing, phase-separation and self-assemblyare very difficult to control the fibre diameter. Thenew technique recently developed for nanofibre fabrica-tion known as forcespinning, may overcome some of thelimitations such as application of very high voltage, andsolvent recovery issues of the electrospinning processes.The safety, simplicity and versatility of forcespinningwill make it another practical method of nanofibrefabrication allowing the development of new classes ofnanofibres that are not feasible by electrospinning.

Funding

This research received no specific grant from any funding

agency in the public, commercial, or not-for-profit sectors.

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