micro-manufacturing engineering and technology || micro-/nano-fibers by electrospinning technology

16Micro-/Nano-Fibers

by Electrospinning

Technology: Processing,

Properties and ApplicationsIoannis S. Chronakis

INTRODUCTION

Human beings have used fibers for centuries. In5000 BC, our ancestors used natural fibers such aswool, cotton silk and animal fur for clothing. Massproduction of fibers dates back to the early stages ofthe industrial revolution. The firstman-made fiber –viscose – was presented in 1889 at the World Exhi-bition in Paris. Developments in the polymer andchemical industries – as well as in electronics andmechanics – have led to the introduction of newtypes of man-made fibers, especially the first syn-thetic fibers, such as nylon, polypropylene and poly-ester. The needs and further progress allowed theproduction of high functionality fibers (antistatic,flame resistant, etc.) and high performance fibers(carbon fibers in 1960 from viscose and aramidfibers in 1965) that showed high strength, a highmodulus and great heat resistance. These fibers areused not only in clothing but also in hygienic pro-ducts, in medical and automotive applications, ingeo-textiles and in other applications.

Traditional methods for polymer fiber produc-tion includemelt spinning, dry spinning, wet spin-ning and gel-state spinning. These methods relyon mechanical forces to produce fibers by extrud-

ing a polymermelt or solution through a spinneretand subsequently drawing the resulting filamentsas they solidify or coagulate. Thesemethods allowthe production of fiber diameters typically in therange of 5 to 500 microns. At variance, electro-spinning technology allows the production offibers of much smaller dimensions. The fibersare produced by using an electrostatic field [1].

Electrospinning is a fiber-spinning technologyused to produce long, three-dimensional, ultra-fine fibers with diameters in the range of a fewnanometers to a few microns (more typically100 nm to 1 micron) and lengths up to kilo-meters (Fig. 16-1). When used in products, theunique properties of nano-fibers are utilized,such as extraordinarily high surface area per unitmass, very high porosity, tunable pore size, tun-able surface properties, layer thinness, high per-meability, low basic weight, ability to retainelectrostatic charges and cost effectiveness,among others [2].

While electrospinning technology was developedand patented by Formhals [3] in the 1930s, it wasonly about fifteen years ago that actual developmentswere triggered by Reneker and co-workers [4].

C H A P T E R

264

Interest today is greater than ever and this cost-effec-tive technique hasmade its way into several scientificareas, suchasbiomedicine, filtration, electronics, sen-sors, catalysis and composites [5,6]. Electrospinningis a continuous technique and is hence suitable forhighvolumeproductionofnano-fibers.The ability tocustomize micro-/nano-fibers to meet the require-ments of specific applications gives electrospinningan advantage over other, larger-scale, micro-/nano-production methods. Carbon and ceramic nano-fibers made of polymeric precursors further expandthe list of possible uses of electrospun nano-fibers [7].

WORKING PRINCIPLEAND CONFIGURATION OFELECTROSPINNING PROCESSING

Electrospinning is increasingly being used to pro-duce ultra-thin fibers from a wide range of poly-mer materials. This non-mechanical, electrostatictechnique involves the use of a high voltage elec-trostatic field to charge the surface of a polymer-solution droplet, thereby inducing the ejection ofa liquid jet through a spinneret (Fig. 16-2). In atypical process, an electrical potential is appliedbetween a droplet of a polymer solution held atthe end of a capillary tube and a grounded target.When the electric field that is applied overcomesthe surface tension of the droplet, a charged jet ofpolymer solution is ejected. On the way to thecollector, the jet will be subjected to forces thatallow it to stretch immensely. Simultaneously, thejet will partially or fully solidify through solventevaporation or cooling, and an electricallycharged fiber will remain, which can be directedor accelerated by electrical forces and then col-lected in sheets or other useful shapes.

A characteristic feature of the electrospinningprocess is the extremely rapid formation of thenano-fiber structure, which occurs on a millisec-ond scale. Other notable features of electrospin-ning are a huge material elongation rate of theorder of 1000 s�1 and a reduction of the cross-sectional area of the order of 105 to 106, whichhave been shown to affect the orientation of thestructural elements in the fiber.

The Electrospinning Mechanism

In spite of the simple set-up for electrospinning,the actual spinning mechanism is quite complex.Although extensive studies have been conductedto explore the mechanism, some aspects and phe-nomena are not yet fully understood.Formation of the Taylor Cone and SubsequentFluid Jet. When the high voltage field is applied,the droplet of polymer solution at the tip of theneedle will become highly electrified and thecharges induced will be evenly distributed overthe polymer solution surface. The droplet will

FIGURE 16-1 (a) SEM image of poly(ethylene terephthal-ate) (PET) nano-fiber web. The nano-fibers were electro-spun from a PET solution in THF:DMF. The diameter of thefibers is about 200 nm. (b) PET nano-fiber web – compari-son with human hair [1].

CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology 265

experience two types of electrostatic forces: elec-trostatic repulsion between the charges on thesurface and Coulombic forces in the externalfield. Under the influence of these two forces, thedroplet will be elongated and finally distorted intoa so-called Taylor cone. As the voltage increases,the electrostatic forces will become stronger andeventually overcome the surface tension, and acharged jet of fluid will be ejected.

Both electrostatic and fluid dynamic instabil-ities can contribute to the basic operation of theprocess.

Reznik et al. [8] experimentally and numeri-cally studied the shape evolution of small dropletsattached to a conducting surface that was sub-jected to relatively strong electric fields. Threedifferent scenarios of droplet shape evolutionare distinguished, based on numerical solutionof the Stokes equations for perfectly conductingdroplets:

1. In sufficiently weak (subcritical) electric fields,the droplets are stretched by the electric Max-well stresses and acquire steady-state shapeswhere equilibrium is achieved by means of sur-face tension.

2. In stronger (supercritical) electric fields theMaxwell stresses overcome the surface ten-sion, and jetting is initiated from the droplettip if the static (initial) contact angle of thedroplet with the conducting electrode isas < 0.8p; in this case, the jet base acquiresa quasi-steady, nearly conical, shape with avertical semi-angle of b � 30�, which is sig-nificantly smaller than that of the Taylor cone(bT = 49.3�).

3. In supercritical electric fields acting on dropletswith a contact angle in the range 0.8p < as/<p,there is no jetting and almost the whole dropletjumps off: this is similar to gravity or drop-on-demand dripping.

FIGURE 16-2 Schematic illustration of the conventional set-up for electrospinning. The insets show a drawing of theelectrified Taylor cone, bending instability and a typical SEM image of the non-woven mat of PET nano-fibers deposited onthe collector. The bending instability is a transversal vibration of the electrospinning jet. It is enhanced by electrostaticrepulsion and suppressed by surface tension.

266 CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology

The droplet-jet transitional region and the jetregion proper are studied in detail for the secondcase using quasi-one-dimensional equations,taking into account the inertial effects and addi-tional features such as the dielectric propertiesof the liquid (leaky dielectrics). The flow in thetransitional and jet region ismatched to that in thedroplet. This is used to predict the current�vol-tage characteristic, I = I(U), and the volumetricflow rate, Q, in electrospun viscous jets, giventhe potential difference applied. The predicteddependence, I = I(U), is nonlinear due to the con-vective mechanism of the charge redistributionsuperimposed on the conductive (ohmic) mecha-nism. Realistic current values I =O(102 nA) havebeen predicted for U =O(10 kV) and fluid con-ductivity s = 10�4 Sm�1.Thinning of the Fluid Jet. Beyond the conicalbase, immediately at the end of the capillarytip, the jet continues to become thinner. Thisjetting mode is known as the electrohydrody-namic cone jet. The jet will initially travel in astraight line towards the collector but willeventually become unstable. To the naked eye,it looks like the jet splits into multiple jets andit was thought before 1999 that this was themain reason for the small diameter of the elec-trospun fibers. However, when the jet is exam-ined with a high speed camera, it can clearly beseen that the splaying is actually one singlefiber rapidly bending or whipping, causing thefiber to make lateral excursions that grow intospiraling loops.

Jet splitting does occur, but it is not as com-mon as previously thought and it is not thedominant process that occurs during spinning.Bending or whipping is caused by a phenome-non called bending instability and can occur inelectrified fluid jets. Every loop then growslarger in diameter and the jet becomes thinner.New bending instabilities arise when the jet isthin enough and enough stress relaxation of theviscoelastic stress has taken place. This iscalled the second instability region and is verysimilar to the first instability region but acts ona much smaller scale. A tertiary-bending insta-bility has also been documented. Each cycle of

bending instability can be described in threesteps:1. A smooth, straight or slightly curved segment

starts to bend.2. The segment of the jet in each bend elongates

and a spiral of growing loops develops.3. As the perimeter of the loops increases, the

diameter of the jet decreases. When the perim-eter of the loop is large enough and the diam-eter of the jet is small enough, the conditions ofthe first step of the cycle are fulfilled. The nextcycle of bending instability then begins.Several research groups have attempted to

explain the bending instability by mathematicalmodels.

ELECTROSPINNING PROCESSINGPARAMETERS – CONTROLOF THE MICRO-NANO-FIBERMORPHOLOGY

The fiber morphology has been shown to bedependent on process parameters, namely solu-tion properties (system parameters), process con-ditions (operational parameters) and ambientconditions [1,2].

Solution Properties

Solution properties are those such as molecularweight, molecular weight distribution and archi-tecture of the polymer, and properties such asviscosity, conductivity, dielectric constant andsurface tension. The polymer solution must havea concentration high enough to cause polymerentanglements, yet not so high that the viscosityprevents polymer motion induced by the electricfield. The resulting fibers’ diameters usuallyincrease with the concentration of the solutionaccording to a power law relationship. Decreas-ing the polymer concentration in the solutionproduces thinner fibers. Decreasing the concen-tration below a threshold value causes the uni-form fiber morphology to change into beads [9].The main factors affecting the formation of beads(Fig. 16-3) during electrospinning have beenshown to be solution viscosity, surface tension


and the net charge density carried by the electro-spinning jet. Higher surface tension results in agreater number of bead structures, in contrast tothe parameters of viscosity and net charge density,for which higher values favor fibers with fewerbeads. This reduction in thickness is due to thesolution conductivity, which reflects the chargedensity of the jet and thus the elongation level.The surface tension also controls the distribution

and the width of the fibers, which can bedecreased by adding a surfactant to the solution.Adding a surfactant or a salt to the solution is away of increasing the net charge density and thusreducing the formation of beads. Finally, thechoice of solvent(s) directly affects all of the prop-ertiesmentioned and is ofmajor importance to thefiber morphology.

Process Conditions

The parameters in the process are spinning voltage,distance between the tip of the capillary and thecollector, solution flow rate (feed rate), needlediameter and, finally, the motion of the targetscreen. Voltage and feed rate show different ten-dencies and are less effective in controlling fibermorphology as compared to the solution proper-ties. Too high a voltagemight result in splaying andirregularities in the fibers. A bead structure is evi-dent when the voltage is either too low or too high.However, a higher voltage also leads to a higherevaporation rate of the solvent, which in turnmight lead to solidification at the tip and instabilityin the jet. Morphological changes in the nano-fibers can also occur upon changing the distancebetween the syringe needle and the substrate.Increasing the distance or decreasing the electricalfield decreases the bead density, regardless of theconcentration of the polymer in the solution.

Ambient Conditions

Ambient conditions include factors such ashumidity and temperature, air velocity in the spin-ning chamber and atmospheric pressure. Humid-ity primarily controls the formation of pores onthe surface of the fibers. Above a certain thresholdlevel of humidity, pores begin to appear and, asthe level increases, so does the number and size ofthe pores.

The precise mechanism behind the formationof pores and texturing on the surface is complexand is thought to be dependent on a combinationof breath figure formation and phase separation.Breath figures are imprints formed due to theevaporative cooling during evaporation of the

FIGURE 16-3 Example of bead formation during electro-spinning: SEM micrographs of poly(propyl carbonate) (PPC)beads prepared by electrospinning a PPC solution in dichlor-omethane [9].


solvent, which results in condense solvent dropson the surface and, later, pores. Surface porosity(Fig. 16-4) can also be achieved by selectiveremoval of one of the components in the poly-mer blend after spinning. The pores formedon the fiber surface can be used, for example,

to capture nano-particles, act as a cradle forenzymes or increase the surface area for filtra-tion applications.

Baumgarten studied the spinning velocities inaddition to the effect of the flow rate, voltage, gapand the surrounding atmosphere [10]. He wasable to determine the spinning velocity using thepower balance:

IV ¼ m:s v

: 2s

2ð1Þ

whereV is the potential, I is the current and _ms and_vs are the mass flow rate and the spinning veloci-ties, respectively. The calculation showed velocitiesclose to the velocity of sound in air. Other research-ers calculated velocities of the fibers reaching thecollector to be 140 to 160 m/s. Obviously, thesespeeds must depend on the process parameters andsolution used.

Increasing the solution temperature is also amethod for speeding up the process, but it mightcause morphological imperfections, such as theformation of beads. Furthermore, the regulationof scale and bifurcation-like instability in electro-spinning are intriguing problems that remain to besolved. Regulatory mechanisms for controlling theradius of electrospun fibers at the different statesare clearly illustrated in the work by He et al. [11].

ELECTROSPINNING SET-UPSAND TOOLS

Novel Set-ups

The traditional set-up for electrospinning hasbeen modified in a number of ways during the lastfew years in order to be able to control the elec-trospinning process and tailor the structure ofmicro-nano-fibers.

Yarin and Zussman achieved upward electro-spinning of fibers from multiple jets without theuse of nozzles; instead using the spiking effect of amagnetic liquid [12]. The concept (Fig. 16-5) con-sists of a bath filled with a layer of magnetic liquid(a). This liquid is covered by the solution to bespun (b). An electrode is submerged into the

FIGURE 16-4 SEM images of (a) porous poly(L-lactide)(PLA) nano-fibers prepared by electrospinning a solutionof PLA in dichloromethane [5]. (b) Poly(propyl carbonate)(PPC) nano-fibers with a porous surface electrospun from aPPC solution in dichloromethane [9].


magnetic fluid (d). A counter-electrode (c) as acollector is placed a certain distance above thisbath. A strong permanent magnet or electromag-net (f) is placed under the bath around the elec-trode. When a magnetic field is applied, the spik-ing effect causes some of the polymer solution toprotrude into the electrical field applied betweenthe electrodes (c and d). The protrusion is suffi-cient to initiate multiple jets of polymeric fiberstraveling towards the collector. The productionrate was reported to be about 12 times that of aconventional set-up. This approach also avoidsclogging problems.

Using supercritical CO2-assisted electrospin-ning, polymer fibers of high molecular weightpolydimethylsiloxane (PDMS) and poly(D,L-lac-tic acid) (PLA) were produced by means of onlyelectrostatic forces and without the use of a liquidsolvent. The fibers were formed between two elec-trodes in a high pressure view cell. This supportedthe idea that the supercritical CO2 reduces thepolymer viscosity sufficiently to allow fibers tobe pulled electrostatically from an undissolvedbulk polymer sample.

Electrospinning in a vacuum is also a novel set-up. Compared to electrospinning in air, a vacuumallows higher electric field strength over large dis-tances and higher temperatures compared to whatcan be achieved in air, which influences both thespinning process and the morphology of the fibersthat are produced. Other attempts have beenmade to incorporate vibration technology in poly-mer electrospinning. The idea is to produce finer

nano-fibers under lower applied voltage by vibra-tion technology.Other electrospinning set-ups arediscussed in a recent review by Teo and Ramak-rishna [6].

Set-ups Involving Dual Syringes

A set-up was developed for electrospinninginvolving a dual syringe spinneret (Fig. 16-6).The development enables spinning highly func-tional nano-fibers such as hollow nano-fibers,nano-tubes and fibers with a core-shell structure[13]. A recent study describes the formation ofhollow nano-tubular fibers in a single step usingelectrospinning and sol-gel chemistry. Themethod exploits electrohydrodynamic forces thatform coaxial jets of liquids with microscopicdimensions. A high voltage is applied to a pairof concentric needles used to inject two immisci-ble liquids that lead to the formation of a two-component liquid cone that elongates into coaxialliquid jets and forms hollow nano-fibers.

Set-ups Controlling the Orientationand Alignment of Micro-nano-fibers

A number of set-ups that allow control over theorientation of fibers have been developed. Theorientation is crucial for different applications ofnano-fibers and opens new opportunities formanufacturing yarn, micro-nano-wire devices,etc. Most of the set-ups are based on rotatingcollection devices.

FIGURE 16-5 Schematic representation of the upward electrospinning set-up [12].


A technique called dry rotary electrospinninginvolves the organization and alignment of elec-trospun nano-fibers into planar assemblies [14].The technique (Fig. 16-7) involves a rotating discas a grounded collector that stretches the coils intoaligned rings. The dry fibers in a ring shape can becollected into linear strands to form a nano-fibrous yarn.

Another method for controlled depositionof oriented nano-fibers uses a micro-fabricatedscanned tip as an electrospinning source [15].The tip is dipped in a polymer solution to gathera droplet as a source material. A voltage appliedto the tip causes the formation of a Taylor coneand, at sufficiently high voltages, a polymer jet isextracted from the droplet. By moving the sourcerelative to a surface, thus acting as a counter-electrode, oriented nano-fibers can be deposited

and integrated with micro-fabricated surfacestructures. This electrospinning technique iscalled a scanned electrospinning nano-fiber depo-sition system. In addition to achieving uniformfiber deposition, the scanning tip electrospinn-ing source can produce self-assembled compositefibers of micro- and nano-particles aligned in apolymeric fiber.

Using a frame as a countered electrode alsoallows an oriented deposition of fibers [16]. Thesame effect can be accomplished by placing twoelectrodes parallel to each other that are separatedby a void [17]. Amodifiedmethod for electrospin-ning that generates uniaxially aligned arraysof nano-fibers over large areas has also beenreported. A collector composed of two conductivestrips separated by an insulating gap of variablewidth was used. Directed by electrostatic

FIGURE 16-6 Schematic illustration of the set-up used to co-electrospin compound core-shell nano-fibers [13]. It involvesthe use of a spinneret consisting of two coaxial capillaries through which two polymer solutions can simultaneously beejected to form a compound jet.


interactions, the charged nano-fibers are stretchedto span across the gap and become uniaxiallyaligned arrays. Two types of gaps were demon-strated: void gaps and gaps made of a highly insu-lating material. When a void gap was used, thenano-fibers could readily be transferred onto thesurfaces of other substrates for various applica-tions. When an insulating substrate was involved,the electrodes could be patterned into variousdesigns on the solid insulator. In both cases, the

nano-fibers could be conveniently stacked intomulti-layered architectures with controllablehierarchical structures.

Zussman et al. reported an approach to ahierarchical assembly of nano-fibers into cross-bar nano-structures [18]. The polymer nano-fibers are created through an electrospinningprocess with diameters in the range of 10–80 nm and lengths up to centimeters. When theelectrostatic field and the polymer rheology of

FIGURE 16-7 The rotary electrospinning apparatus: (a) schematic illustration of the set-up used for electrospinning nano-fibers as uniaxially aligned arrays. (b) Schematic illustration of the effect of the rotating speed on the formation of fibers [14].(c) Aligned poly(vinylidene–Huoride) (PVDF) nano-fibers (Chronakis et al., unpublished results).


the nano-fibers are controlled, they can beassembled into parallel periodic arrays. Theseauthors also observed failure of nano-fibersowing to a multiple necking mechanism, some-times followed by the development of a fibrillarstructure, during electrospinning using a rotat-ing tapered accumulating wheel (electrostaticlens). This phenomenon was attributed to astrong stretching of solidified nano-fibers bythe wheel, if its rotation speed became too high.Necking has not been observed in the nano-fibers collected on a grounded plate.

Various other set-ups have been reported in theproduction of oriented, continuous nano-fiberssuch as using copper wires spaced evenly in theform of a circular drum as a collector and the useof a rotating wheel. In particular, the work byTheron and co-workers described an electrostaticfield-assisted assembly technique that was com-bined with an electrospinning process used toposition and align individual nano-fibers on atapered and grounded wheel-like bobbin [19].The bobbin is able to wind a continuous as-spunnano-fiber at its tip-like edge. The alignmentapproach resulted in nano-fibers with diametersranging from 100–300 nm and lengths of up tohundreds of microns.

CHARACTERISTICS AND DESIGNCONSIDERATIONSOF ELECTROSPUN MICRO-NANO-FIBERS

Molecular Orientation

In traditional fiber spinning, the molecular orien-tation obtained by stretching the fibers after theirformation is critical for their strength. The molec-ular orientation of electrospun fibers has alsobeen a subject of various studies.

Dersch and co-workers studied the intrinsicstructure of polyamide (nylon 6) and PLA electro-spun fibers [16]. They found that the fibers do notdiffer a great deal from as-spun thicker fibersobtained by melt spinning and showed rather dis-ordered crystals and different degrees of crystalorientations. The orientation seems to be almost

absent in the PLA fibers and to be locally strong,yet inhomogeneous, in the polyamide fibers.However, stretching the PLA fibers did lead toan increased orientation of the crystals along thefibers’ axis. On the other hand, the electrospin-ning of polyethylene oxide (PEO), for example,causes some molecular orientation but a poorlydeveloped crystalline micro-structure.

Collecting electrospun fibers onto a high speedrotating drum can enhance the molecular orien-tation up to an optimal speed, after which theorientation can decrease slightly [20]. In thereport of a study using a high speed winder, itwas suggested that a critical winding speed existsthat just matches the ‘natural’ velocity of the fiber(due to electrohydrodynamic forces) and thatadditional drawing of the fiber should occur forhigher winding speeds. This work concluded thatthe degree of molecular orientation, which devel-ops only due to electrohydrodynamic forces and,hence, would be expected in non-woven electro-spun fabrics, is quite low.

Shapes and Sizes

In addition to circular fibers, a variety of cross-sectional shapes and sizes can be obtained fromdifferent polymers during electrospinning.Koombhongse and co-workers actually obtainedbranched fibers, flat ribbons, ribbons of othershapes and fibers that were split longitudinallyfrom larger fibers in electrospinning a polymersolution [21]. Studies of the properties of fiberswith these cross-sectional shapes from a numberof different kinds of polymers and solvents indi-cate that effects of the fluid mechanics, the elec-trical charge carried with the jet and evaporationof the solvent all contributed to the formation ofthe fibers.

Sung and Gibson used polycarbonate inanother study [22]. Electrospun fibers created inthis process showed a wrinkled structure that wasfound to depend on the rate of evaporation of thesolvent from the surface related to the rate ofevaporation from the core. Indeed, as the solventon the surface evaporated and a ‘skin’ formed, thesolvent entrapped in the core diffused into the


ambient atmosphere and caused what they calleda ‘raisin-like structure’. A rapid evaporation ofsolvent from the jet that creates a skin, as men-tioned above, can in fact give rise to hollow fibersthat can collapse into a ribbon.

Alterations of Secondary Structureand Functionality

The electrospinning process is highly versatile andallows not only the processing of many differentpolymers into polymeric nano-fibers but also theco-processing of polymer mixtures and mixturesof polymers and low molecular weight non-vola-tile materials. This is done simply by using ternarysolutions of the components for electrospinningto form a combination of nano-fiber functionali-ties. Polymer blends, core-shell structures andside-by-side bicomponent electrospinning aregrowing research areas that are connected withthe electrospinning of multi-component systems.The targets are either to create nano-fibers of an‘unspinnable’material or to adjust the fiber mor-phology and characteristics.

The option of spinning a polymer blend renderspossible the creation of core-shell nano-fibersthrough phase separation as the solvent evapo-rates. Another method for creating a structure ofthis kind is to co-electrospin two different poly-mer solutions through a spinneret consisting oftwo coaxial capillaries (see Fig. 16-6). Nano-fibers with hollow interiors are used in severalapplications, such as nano-fluidics and hydrogenstorage. Electrospun tubular fibers can also beused as sacrificial templates.

The electrospinning technique also provides thecapacity to lace together a variety of types of nano-particles or nano-fillers to be encapsulated into anelectrospun nano-fiber matrix (Fig. 16-8) [23].Several functional components (e.g. nanometer-sized particles, nano-fillers, carbon nano-tubes,drugs, enzymes and DNA) can be dispersed inthe initial polymer solutions, which are then elec-trospun to form composites in the form of contin-uous nano-fibers and nano-fibrous assemblies.

Another interesting aspect of nano-fiber pro-cessing is that it is feasible tomodify not only their

morphology and their (internal bulk) content butalso their surface structure in order to carry var-ious chemically reactive functionalities. Thus,nano-fibers can be easily post-synthetically func-tionalized, for example by using plasma modifi-cation, physical or chemical vapor deposition(PVD, CVD) and chemical modifications such ascross-linking or grafting. By varying the proces-sing parameters, it is also possible to producefibers with unique surface features and secondarystructures such as micro-textured/nano-porousfibers and micro-nano-webs.

APPLICATIONS OF ELECTROSPUNFUNCTIONAL MICRO-NANO-FIBERS

Electrospun micro-nano-structures are a class ofnovel materials that is exciting because of severalof the unique characteristics discussed above.Significant progress has been made in this fieldin the last few years, and the resulting micro-nano-structures may serve as a highly versatileplatform for a broad range of important techno-logical applications in areas such as biomedicine,pharmacy, sensors, catalysis, filter, composites,ceramics, electronics and photonics. Some of themost recent developments in their processingand the relevant applications that are consideredare presented below.

Biomedical Applications

Tissue Engineering. Electrospun 3D nano-fibrous structures meet the essential design crite-ria of an ideal tissue engineered scaffold basedupon their unique action in supporting and guid-ing cell growth [24]. Most studies confirm thatthe electrospun nano-fibrous structure is capableof supporting cell attachment and proliferation(Fig. 16-9) [25]. The structure features a morpho-logical similarity to the extracellular matrix ofnatural tissue, which is characterized by a widerange of pore diameter distribution, high porosityand effective mechanical properties.

Nano-fibers have been studied for engineeringcardiovascular tissues such as heart tissue con-structs and blood vessels. Ramakrishna’s group


published several articles on the use of nano-fibersas a scaffold for blood vessels and looked at theinfluence of fiber diameter, orientation and otherparameters on cell proliferation [26]. Nano-fibersmade of poly(L-lactid-co-e-caprolactone) P(LLA-CL) or poly(ethylene terephthalate) (PET) wereprimarily used.

Biomimetism towards human ligament hasbeen considered, and the effects of fiber alignmentand direction of mechanical stimuli on the extra-cellular matrix (ECM) generation of human liga-ment fibroblast (HLF)was studied [27]. An elasticbiodegradable material in a tubular formwas pro-duced by combining polylactide with cross-linked

elastin [28]. The tubular material obtainedshowed excellent mechanical properties equal tothose of blood vessel and peripheral nerve tissue.

Nano-fibers are potential structures for bonetissue engineering. Yoshimoto et al. used poly(e-caprolactone) (PCL) scaffolds to grow mesen-chymal stem cells (MSCs) derived from bone mar-row [29]. Polylactide combined with cross-linkedelastin shows a potential for neural applications[28]. The regeneration of peripheral nerve axonswas observed in transplantation using a rat modelwith sciatic trauma. Silk-like polymers with fibro-nectine functionality (extracellularmatrix proteins)have been electrospun tomake biocompatible films

FIGURE 16-8 SEM images of (a) electrospun poly(ethylene terephthalate) (PET) nanofibers containing encapsulatedmolecular imprinted 17b-estradiol nano-particles (50% of the nano-fibers content) [23]. (b) Electrospun polyurethane(PU) nano-fibers coated with SiC ceramic nano-particles (Chronakis et al., unpublished results).


for use in prosthetic devices intended for implanta-tion in the central nervous system [30].Wound Dressings and Healing. Electrospunnano-fibrous membranes can be used in the pro-duction of novel wound dressings. These mem-branes are particularly important because of theirfavorable properties, such as high specific surfacearea, combined with antibacterial and drugrelease functionality. Recent studies support thatnano-fibrous dressings promote hemostasis, havebetter absorptivity, semi-permeability and con-formability and allow scar-free healing [31]. Thenano-fibrous membrane also shows controlledevaporative water loss, excellent oxygen perme-ability and promoted fluid drainage ability, but itcan still inhibit exogenous microorganism inva-

sion because its pores are ultra-fine. Histologicalexaminations also indicate that the rate of epithe-lialization is increased and that the dermisbecomes well organized when wounds are cov-ered with electrospun nano-fibrous membrane.

A nano-fiber mat made of fibrinogen, a solubleprotein that is present in blood, has been pro-duced by electrospinning [32]. The mat could beplaced and left on a wound, thereby minimizingblood loss and encouraging the natural healingprocess. Fibrinogen increases the ‘stickiness’ ofclotting cells, thickens the blood and promotesthe formation of fibrin (the stringy protein thatforms the basis of blood clots). Electrospinningcan also be used to create biocompatible, thinfilms with a useful coating design and a surfacestructure that can be deposited on implantabledevices in order to facilitate the integration ofthese devices in the body.Drug Carrier and Delivery Systems. Electro-spun fiber mats have also been explored as drugdelivery vehicles, with promising results. Theapplication of electrostatic spinning in pharma-ceutical applications resulted in dosage forms withuseful and controllable dissolution properties.For instance, hydroxy propoxy methylcellulose(HPMC), a cellulose derivative commonly usedin pharmaceutical preparations, together withthe drug has also been tested [33]. Poly-(L-lacticacid) (PLLA) and poly(D,L-lactide-coglycolide)(DLPLGA) nano-fibers are other polymers thathave been electrospun with an encapsulated drugand have shown promising drug release properties.Incorporation of an antibiotic in fibers developedfor scaffold applications has also been reported.The combination of mechanical barriers based onnon-woven nano-fibrous biodegradable scaffoldsand their capability for local delivery of antibioticsmakes them desirable for applications in the pre-vention of post-surgical adhesions and infections.

Micro-nano-fibers as Supportfor Enzymes and Catalysts

Electrospun micro-nano-fibers are an attractiveclass of supports for enzymes and catalysts dueto their ultra-thin sizes and large surface areas.

FIGURE 16-9 (a) SEM image showing fibroblast (humanMRC-5) extension wrapping around the electrospun nano-fiber. (b) TEM image showing a fibroblast extension in closecontact with electrospun Artelon� nano-fiber [25].


Reneker and co-workers demonstrated the possi-bility of using nano-fibers for the immobilizationof enzymes, showing catalytic efficiency for bio-transformations [34]. Enzyme-modified nano-fibers of PVA and PEO achieved by loading theenzymes, i.e. casein and lipase, into the polymersolutions have also been reported. The membraneswith encapsulated enzymes were six times morereactive than cast films from the same solutions.

Investigations have been made of the catalyticactivity of nano-fibers obtained by incorporatingcatalysts. For instance, the incorporation of pal-ladium (Pd) nano-particles has been studied indetail using carbonized and metal oxide nano-fibers [35].

Generation of Micro-nanomechanicaland Micro-nano-fluidic Devicesthrough Electrospinning

As mentioned earlier, electrospun micro-nano-fibers can serve as sacrificial templates for thegeneration of micro-nano-structures with hollowinteriors. Czaplewski and co-workers preparednano-fluidic channels [36]. The channels obtainedwere elliptical and presented no sharp corners, asin conventional lithographic techniques, whichpromotes a smoother fluid flow through them.Furthermore, the spin-on glass is optically trans-parent and compatible with chemical analysis,thereby opening applications in biomolecular sep-aration and single molecule analysis. They alsodemonstrated the use of these templates for thefabrication of micro-electromechanical devices,such as nano-scale mechanical oscillators.

Deposits of oriented poly(methyl methac-rylate) nano-fibers, combined with contactphotolithography, created silicon nitride nano-mechanical oscillators with dimensions in theorder of 100 nm. The fibers were used as etchmasks to pattern nano-structures in the surfaceof a silicon wafer. The oriented polymeric nano-fiber deposition method that was used in this ex-periment offers an approach for rapidly formingarrays of nano-mechanical devices, connected tomicro-mechanical structures, that would be diffi-cult to form using a completely self-assembled

or completely lithographic approach. This ap-proach may provide a useful method for realizingnano-scale device architectures in a variety ofactive materials.

Furthermore, magnetite nano-particles wereincorporated as a colloidally stable suspension intopolyethylene oxide or polyvinyl alcohol solutions[37]. After electrospinning, the nano-particleswerealigned along the fibers’ axis. These nano-fibersexhibited superparamagnetic behavior anddeflected when subjected to a magnetic field atroom temperature. A micro-aerodynamic deceler-ator based on permeable surfaces of nano-fibermats was reported by Zussman and Yarin [38].Thematswerepositionedon light,pyramid-shapedframes. These platforms fell freely through the air,apex down, at a constant velocity. The drag of thiskind of passive airborne platform is of significantinterest in a number of modern aerodynamicsapplications including, for example, dispersionof ‘smart dust’ carrying various chemical andthermal sensors, dispersion of seeds, and move-ment of small organisms with bristle appendages.

Micro-nano-fibers in Sensors

Recent advances in micro-nano-technology andthe electrospinning technique offer great potentialfor the construction of cost-effective, next-gener-ation chemical and biosensor devices. The highsurface area per volume unit makes electrospunmicro-nano-structures great candidates for a vari-ety of sensing applications as they can offer highsensitivity and response time. These sensorscan find applications in medical diagnosis andenvironmental and bioindustrial analysis, amongothers [1,23].

Conducting electroactive polymers haveremarkable sensing applications because of theirability to be reversibly oxidized or reduced byapplying electrical potentials. For biosensingapplications, conducting electroactive polymerscombine the role of a matrix immobilizationtemplate and the generation of analytical sig-nals. The most common conducting electroac-tive polymers include polypyrrole, polyanilineand polythiophene and are characterized by an


electronic conductivity of up to 104 W�1. Resis-tive-type sensors made from undoped or dopedpolyaniline nano-fibers outperform conven-tional polyaniline on exposure to acid or basevapors, respectively [39].

Electrospinning of lead zirconate titanate, Pb(ZrxTi1-x)O3 (PZT) fibers should be mentionedbecause of its technological importance in thefield of sensors, electronics and non-volatile fer-roelectric memory devices. PZT is one example ofone-dimensional nano-structures, the smallestdimension structures for efficient transport ofelectrons and optical excitation, that can be usedas building blocks in a bottom-up assembly indiverse applications in nano-electronics and pho-tonics [40]. Wang et al. showed that ultra-finePZT fibers could be synthesized from metallo-organic compounds simply by using metallo-organic decomposition (MOD) and vacuum heattreatment electrospinning techniques [40].

Other developments are electrospun nano-fibers of polyvinylpyrrolidone (PVP) containingthe urease enzyme that show a potential as a ureabiosensor and nano-fibers coated with metal oxi-des (TiO2,MoO3) for the detection of toxic gases.Molecular imprinted nano-fibers with selectivemolecular recognition ability and a chemosensormaterial with a high surface area obtained byelectrospinning a fluorescent conjugated polymerhave also been developed [23,41].

Micro-nano-fibers in Electricand Electronic Applications

Electrospun nano-fibers with electrical and elec-tro-optical activities have received a great deal ofinterest in recent years because of their potentialapplication in nano-scale electronic and optoelec-tronic devices, such as nano-wires, LEDs, photo-cells, etc. Lead zirconate titanate (PZT) and car-bon nano-fibers are two typical and challengingexamples of one-dimensional nano-structuresthat can be used as building blocks in bottom-upassembly in diverse applications in nano-electron-ics and photonics [40].

Studies support that electrospinning can be asimple method for fabricating a one-dimensional

polymer field-effect transistor (FET), whichforms the basic building block of logic circuitsand switches for displays [42, 43]. In addition,the excellent adherence of the nano-fibers toSiO2 and to gold electrodes may be useful inthe design of future devices. By means of electro-spinning processing, extremely low dimensionalconducting nano-wires have been made from,e.g., polyaniline or polypyrrole for use in nano-electronics (Fig. 16-10) [44].

Other studies report the development of car-bon nano-fiber webs from the oxidation andsteam activation of a polyacrylonitrile (PAN)nano-fiber web for use as an electrode in a super-capacitor [43], poly(vinylidene fluoride) (PVDF)nano-fibers for applications as a separator or asan electrolyte in batteries [45] and fabrication of alithium secondary battery comprising a fibrousfilm made by electrospinning [46].

Electrospinning mixtures of ceramic particleswith polymers and subsequent pyrolysis of thepolymer to form pure ceramic nano-fibers is anarea of intense research [7]. Because of their largesurface-to-volume ratios and narrow-band opti-cal emission, these nano-fibers can be used asselective emitters for thermophotovoltaic applica-tions and as emitting devices in nano-scaleoptoelectronic applications.

Micro-nano-fibers in Filters

The efficiency of nano-fibers in filtration has beenstudied by several groups. Generally, the electro-spun webs have been found to be much moreeffective than other commercial high-efficiencyair filter media. Inmost cases, the nano-fiber websare applied on a substrate chosen to providemechanical properties, while the nano-fiber dom-inates the filtration performance. Electrospinningcan also be used to produce charged fibers for usein filtration media. Obviously, the charge induc-tion and charge retention characteristics arerelated to the polymer material used for electro-spinning.

Controlling the parameters of electrospinningallows the generation of micro-nano-fiber webswith different filtration characteristics. A study


done by Schreuder-Gibson and Gibson showedthat it is possible to tailor pore size, air permeabil-ity and aerosol filtration of elastic non-wovenmedia by applying very light-weight layers of elec-trospun elastic fibers to the coarser webs [47]. Ithas been found that a significant deformation ofthe elastic webs increases air flow, and it might bepossible to design controlled flow filters or airbags that are modulated by a pressure drop acrosselastic webs with correspondingly variable poros-ities. Many filtering applications of electrospunmicro-nano-fibers are related to air filtration,but liquid filtration can also occur [48].

Moreover, ion exchange materials (such asresins, membranes, etc.) have been widely usedin various industries for water deionization orsoftening, metal recovery, biological process,food and beverages, pharmaceuticals and fuel cellapplications. Polymer nano-fiber ion exchangersare new, promising materials as they have a muchhigher surface area than common ion exchangers.

Micro-nano-fibers in Textiles

Electrospun micro-nano-membranes composedof elastomeric fibers are of particular interest inthe development of several protective clothingapplications. The excellent ability to captureaerosols and the possibilities to incorporate anykind of active substances make electrospun nano-fiber materials potential candidates for use in pro-tective clothing and smart cloths responding tochanges in the surrounding environment. Muchwork is being done with the aim to develop gar-ments that reduce soldiers’ risks for chemicalexposure [49]. The idea is to lace several typesof polymers and fibers to make protective ultra-thin layers that would enhance, for example,chemical reactivity and environmental resistance.Such mats have been found to have a higher con-vective resistance to air flow while the transportof water vapor is much higher than in normalclothing materials. These products exhibitremarkable ‘breathing’ properties, which arenow required in clothing applications.

In some other uses of protective clothing, ther-mal and flammability properties are essential

[50]. Electrospun poly(methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA)) and its lay-ered silicate nano-composites have shown goodthermal stability, reduced flammability andincreased self-extinguishing properties. The pos-sibility of using sub-micron and nano-scale fibersand fibrous assemblies based on conductivePEDOT for wearable electronics has also beenexplored [14]. Finally, the development of elec-trospinning apparatuses for fiber orientation thatallow the fabrication of yarns is of considerableinterest [51] (Figs. 16-9 and 16-10).

Micro-nano-fibers as CompositeReinforcement

The strength of a composite material is effec-tively enhanced by fiber-based reinforcement.Thus, the high surface-to-volume ratio of nano-fibers significantly improves the stiffness andmechanical strength of the composites comparedto conventional fibers due to the increased inter-action between the fibers and the matrix [52].Another positive aspect is that the compositesare able to maintain their optical transparencyrelated to the small cross-section of the nano-fibers.

FIGURE 16-10 SEM micrograph of conductive polypyr-role nano-fibers. The nano-fibers were electrospun from asolution of ((PPy3)+(DEHS)�)� in DMF [44].


Attempts to Increase the ProductionRate of ElectrospunMicro-nano-fibersElectrospinning using Multiple Nozzles. Themost obvious way to increase the rate of produc-tion of micro-nano-fibers is to increase the num-ber of nozzles used in the spinning process. In apatent, Chu et al. described an electrospinningapparatus with multiple nozzles, as shown inFig. 16-11 [53]. The essential invention in thispatent was not the use of multiple nozzles butthe possibility to better control the jet forma-tion, jet acceleration and fiber collection forindividual jets. This was achieved using severaladditional electrodes to homogenize the electricfield that accelerates the jets from the nozzles tothe collector. The possibility of controlling boththe flow of the conducting fluid (polymer solu-tion or melt) and the properties of the electricfield for each jet was described as essential forproducing nano-fibers using multiple nozzles. Alater patent by the same group focused on con-trolling multiple fiber jets as opposed to individ-ual jets or adding the possibility of blowing atemperate gas in the fiber spinning direction.The gas flow gives a higher production rate thantraditional electrospinning, as well as lowerenergy consumption.

Electrospinning without Nozzles. Perhaps themost successful way to increase the electrospin-ning production rate that can be recognizedthus far is the Nanospider� technology, pat-ented by O. Jirsak et al. [54]. This technologyis now owned by ElMarco (Czech Republic).Instead of using capillaries as a spinneret forintroducing the polymer solution into the elec-tric field, a rotating charged electrode is usedthat is partly immersed into the polymer solu-tion. This set-up allows the creation of manyTaylor cones and hence many jets that travelupwards to a conveyor belt that can be coveredwith a material to be coated.

A great advantage of this method is the possi-bility to create multiple jets of nano-fibers with-out the risk of the nozzles clogging. Anotheradvantage of the technique is that the spinningdirection is upwards, which minimizes the riskof solution droplets forming in the product. Theprocess is schematically shown in Fig. 16-12.

The polymer solution (2) is applied to thecharged cylindrical electrode (3) as it rotatespartly immersed in the solution. Multiple fiberjets are formed from the surface of the electrodetowards the oppositely charged electrode (40).The fibers are drawn to the electrode (40), not

FIGURE 16-11 Schematic drawing of an apparatus for large-scale electrospinning of nano-fibers [53].


only by the electric force but also due to the actionof a vacuum chamber (5). The patent also coversrotating cylindrical electrodes with different pat-terned surfaces. Using this technology, ElMarcoclaims that theywill have a production capacity of3000 m2/day (1 m in width) (2006).

Another approach to spinning nano-fibers with-out nozzles is to use a porous tube of polyethylene(Fig. 16-13), as reported by Reneker et al. [55]. Byapplying air pressure to a polymer solution inside acylindrical porous tube, these authors were able toform multiple jets of polymer solution in the elec-tric field. With this porous tube, the productionrate could be increased to about 250 times thatof the corresponding production rate for a singleneedle.

Recently, a new technology with the use ofcentrifugal forces has been developed and pat-ented from Swerea IVF [56]. The process is sche-matically shown in Fig. 16-14.

Commercial Products

It should be noted that some companies alreadyhave commercial products based on electrospunnano-fibers. One of the first companies to start an

industrial production line of nano-fiber web isNanoTechnics Co., Ltd, in Korea. The companyoffers nano-fiber webs of PA6 and PA66 for appli-cation in filters and PAN for electrodes in batter-ies. Other companies that claim to be able to elec-trospin webs for use in filters are Hollingsworth& Vose, Germany, and eSpin Technologies,USA. Donaldson Company Inc., USA, is also an

FIGURE 16-12 (a) Schematic drawing of the Nanospider� technology [54]. (b) Picture of the Nanospider� technology inaction (from www.nanospider.cz).

FIGURE 16-13 Schematic illustration of the electrospin-ning set-up using a porous polyethylene tube [55].


important actor in the field of nano-fiber-basedfilters. Donaldson has several US and interna-tional patents that cover nano-fiber innovations,configurations and uses.

TECHNOLOGICALCOMPETITIVENESSAND OPERATION ECONOMICS

Electrospinning is a very simple and versatilemethod for creating polymer-based, high func-tional and high performance micro-nano-fibersthat can revolutionize the world of structuralmaterials. The process is versatile in that there isa wide range of materials that can be spun(Table 16.1). At the same time, electrospunmicro-nano-fibers possess unique and interestingfeatures. The ability to customize micro-nano-fibers to meet the requirements of specific appli-cations gives electrospinning an advantage overother larger-scale micro-nano-production meth-ods. Combining well-established technologies oftoday with the emerging field of electrospunmicro-nano-fibers can potentially lead to thedevelopment of new technologies and newmicro-nano-structured smart assembles and stim-ulate opportunities for an enormous number ofapplications. Thus, electrospinning technologycan provide a connection between the worlds ofthe nano-scale and the macro-scale.

Another advantage of this top-down micro-nano-manufacturing process is its relativelylow cost compared to that of most bottom-upmethods. The electrospinning method itself isenvironmentally friendly because it consumesonly a small amount of electrical energy. In spiteof the high potential difference (10,000–40,000 V) that is applied, only a small electricalcurrent flows through the nano-fibers (in theorder of nano-amperes). In addition, the electro-spinning method provides nano-fiber structuresthat imply a large reduction in material consump-tion. For instance, the formation of a true nano-coating (monolayer-like) will result in a thicknessof only a few nanometers, while current coatingshave a thickness of a few micrometers. Thismeans that the consumption of materials isalso about 1000 times less for nano-coatingswhile it results in the same surface properties asare obtained with micro-coatings. Moreoverthe resulting micro-nano-fiber samples are oftenuniform and continuous and do not require expen-sive purification (unlike submicrometer-diameterwhiskers, inorganic nano-rods and carbon nano-tubes). Overall, despite the existence of some com-mercial products, it is evident that an upscaling ofthe electrospinning process and productivityimprovements are essential features and meritmore effort to ensure full success in socio-economic terms.

FIGURE 16-14 Schematic illustration of the micro-nano-fiber formation from rotating disc [56].


TABLE 16.1 Examples of Some Polymer-Biopolymer Materials that have been Electrospunand Solvents Used (in Alphabetical Order)

Materials Solvents

ABS N,N-Dimethyl formamide (DMF)or tetrahydrofuran (THF)

Cellulose Ethylene diamine

Cellulose acetate Dimethylacetamide (DMAc)/Acetone or acetic acid

Ethyl-cyanoethyl cellulose ((E-CE)C) THF

Chitosan and chitin 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)

Dextran Water, DMSO/water, DMSO/DMF

Gelatin 2,2,2-Trifluoroethanol

Nylon Formic acid

Poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PMAPS) Ethanol/Water

Polyacrylonitrile (PAN) DMF

Polyalkyl methacrylate (PMMA) Toluene/DMF

Polycarbonate THF/DMF

Poly(ethylene oxide) (PEO) Water, ethanol, DMF

Polyethylene terephthalate (PET) Trifluoroacetic acid (TFA)/dichloromethane (DCM)

Polylactic-based polymers Chloroform, HFIP, DCM

Poly(e-caprolacone)-based polymers Acetone, acetone/THF, chloroform/DMF,DCM/methanol, chloroform/methanol,THF/acetone

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) 2,2,2-Trifluoroethanol

Polyphosphazenes Chloroform

Polystyrene 1,2-Dichloroethane, DMF, ethylacetate,methylethylketone (MEK), THF

Bisphenol-A polysulfone DMAc/Acetone

Polyurethane (PU) THF/DMF

Polyvinyl alcohol (PVA) Water

Polyvinyl chloride (PVC) DMF, DMF/THF

Poly(vinylidene fluoride) (PVDF) DMF/THF

Poly(vinyl pyrrolidone) Ethanol, DCM, DMF

Silk Hexafluoroacetone (HFA),hexafluoro-2-propanol, formic acid


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micro-manufacturing engineering and technology || micro-/nano-fibers by electrospinning technology

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