Polymer Nanofibers Assembled by Electrospinning

Current Opinion in Colloid and Interface Science 8 (2003) 64–75

Polymer nanofibers assembled by electrospinning

Audrey Frenot, IoannisS. Chronakis*

IFP Research, Swedish Institute for Fiber and Polymer Research, P.O. Box 104, S-431 22 Molndal,¨ Sweden

Abstract

Electrospinning is a process by which polymer nanofibers (with diameter lower than 100 nm and lengthsup to kilometres ) can be produced using an electrostatically driven jet of polymer solution (or polymer melt). Simple alignment of electrospun nanofibers constructs unique functional nanostructures such as nanotubes and nanowires. Significant progress has been made in thisarea throughout the pastfew yearsand thistechnology hasbeen exploited to a wide range of applications.Mostof the recent work on electrospinning has focused either on trying to understand deeper the fundamental aspects of the process in order to gain control of nanofiber morphology, structure, surface functionality, and strategies for assembling them or on determining appropriate conditionsfor electrospinningof variouspolymersand biopolymers. 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Electrospinning; Electrospun; Nanofibers; Nanostructured material; Nanotechnology; Polymer fibers; Biopolymer fibers; Blends; Electrified fluid jet

1. Introduction pinning from a mixture of rigid rod polymersand flexible polymers is also feasible. The electrospun nan- Unlike conventional fiber spinning techniques (wet ofiberscan even be aligned to constructunique func- spinning, dry spinning, melt spinning, gel spinning), tional nanostructures such as nanotubes and nanowires. which are capable of producing polymer fiberswith Furthermore, depending on the specific polymer being diametersdown to the micrometer range, electrostatic used, a wide range of fabric properties such as strength, spinning, or ‘electrospinning’ is a process capable of weight and porosity, surface functionality etc. can be producing polymer fibersin the nanometer diameter achieved. Thisnovel fiber spinningtechnique provides range. Electrospinning is a novel and efficient fabrica- aswell the capacity to lace together a variety of types tion process that can be utilised to assemble fibrous of polymers, fibers, and particles to produce ultrathin polymer matscomposedof fiber diametersranging from layers. Small insoluble particles can be added to the several microns down to fibers with diameter lower than polymer solution and encapsulated in the dry nanofibers. 100 nm (Fig. 1). This electrostatic processing method Soluble drugsor bacterial agentscan be added and uses a high-voltage electric field to form solid fibers electrospun into nonwoven mats. from a polymeric fluid stream (solution or melt) deliv- Although the process of electrospinning has been ered through a millimeter-scale nozzle. known for almost 70 years and the first patent was Nanofibersare the ultra-fine solidfibersnotable for issued to Formhals in 1934 (US Patent, 1-975-504), their very small diameters (lower than 100 nm), their polymeric nanofibersproduced by electrospinninghave large surface area per unit mass and small pore size. become a topic of great interest for the past few years. Due to the inherent propertiesof the electrospinning Reneker and Chun, who revived interest in this technol- process, which can control the deposition of polymer ogy in the early 1990s, has shown the possibility to fibers onto a target substrate, nanofibers with complex, electrospin w1x a wide range of polymer solutions in and seamless three-dimensional shapes could be formed. 1996. Larrondo and Manley had performed a similar Construction of nanoscale composite fibers by electros- work on polymer meltsin 1981 w2x. Typically, electros- *Corresponding author. Tel.: q46-31-706-63-00; fax: q46-31- pinning isapplicable to a wide range of polymerslike 706-63-65. those used in conventional spinning, i.e. polyolefine, E-mail address: [email protected] (I.S. Chronakis). polyamides, polyester, aramide, acrylic as well as bio-

1359-0294/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-0294Ž03.00004-9 A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 65

Fig. 1. (a) Electrospun PET from a solution of 13 wt.% in 1:1 dichloromethane and trifluoroacetic acid. Comparison with human hair; (b)

Electrospun PEO fibers from a solution of 5 wt.% PEO in 50 wt.% H2 O and 45 wt.% ethanol. The diameter of the fibers lies between 50 and 500 nm. (Audrey Frenot, unpublished work). polymers like proteins, DNA, polypeptides, or others ing challenges. The current utilisation of nanofibers like electric conducting and photonic polymers. The mainly for biomedical applications, for developing nano- high specific surface area and small pore size of elec- electronic machines and for designing potential struc- trospun nanofibers make them interesting candidates for turesin order to develop functional nanostructured a wide variety of applications. For instance nanofibers materials, is another active research area that covered in with a diameter of 100 nm have a ratio of geometrical thisreview. The major developmentsare highlighted surface area to mass of approximately 100 m2yg. Anoth- covering the publicationsperiod from 1999 onwards. er interesting aspect of using electrospun fibers is that the fibers may dissipate or retain the electrostatic charges 2. The electrospinning process depending on the electrical propertiesof the polymer. The chargescan be manipulated aswell by electrical The electrospinning equipment for polymer solution fieldsand the electrical polarity of the fibersisaffected and melt are well presented in the papers w3–7●● ,8x and by the polarity of the applied voltage. Nanofiberspro- w2x, respectively. Electrospinning is easily realised by vide also a connection between the nanoscale world and applying a high voltage to a capillary filled with the the macroscale world, since the diameters are in the polymer fluid to be spun with help of an electrode. The nanometer range and the lengthsare kilometres.There- fore, current emphasis of research is to exploit such propertiesand focuson determining appropriate conditionsfor electrospinningvariouspolymersand biopoly- mersfor eventual applicationsincluding multifunctional membranes, biomedical structural elements (scaffolding used in tissue engineering, wound dressing, drug delivery, artificial organs, vascular grafts), protective shields in speciality fabrics, filter media for submicron particles in separation industry, composite reinforcement, and structures for nano-electronic machines among others. In thisreview we concentrate on current understanding of the electrospinning process and those parameters, which influence the propertiesof the fibersproduced from it. We also address issues of nanofiber structures and characteristics and discuss models proposed for processing instabilities. Due to the growing interest on nanofibers processed with specific bulk morphologies and surface topologies, we focus on such recent advanc- Fig. 2. Scheme of an electrospinning equipment. NB, several raw es and discuss some general considerations and remain- material sources can be used (i.e. several capillaries). 66 A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 resulting fibers are collected on a grounded plate, which electrostatic force overcomes the surface tension and a can be covered with a fabric for example. The process charged jet of fluid isejected from the tip of the Taylor is schematically illustrated in Fig. 2. The groups working cone. The discharged polymer solution jet undergoes a on electrospinning have adopted different solution for whipping process wherein the solvent evaporates, leav- the polymer feed. Some have simply opted for placing ing behind a charged polymer fiber, which ishighly the capillary perpendicularly, letting the polymer fluid stretched and reduced in diameter asit travelsbefore it dropping with help of gravitation and lying the collector laysitselfrandomly on a grounded collecting metal underneath w9x. Sometimesthe capillary can be tilted at screen (the jet region). The rapidly whipping jet, the a defined angle to control the flow w5,10,11x. In other fluid instability, is an essential element of the process ●● cases, the capillary was horizontal w12 x and a pump that cause bending and stretching of the jet. In the case was used to initiate the droplet, the pump being also of the melt the discharged jet solidifies when it travels w ●●x used in the case of vertical feeding 3,4,6,12 . The in the air and iscollected on the grounded metal screen. electrode can be inserted either in the polymer fluid or Splaying occursin a region in which the radical forces placed onto the tip of the capillary if a syringe with a from the electrical chargescarried by the jet, become metal needle isused. larger than the cohesive forces within the jet, and the The collector is usually a plane metal sheet or a grid single jet divides into many charged jets (with approx- that can be covered with a fabric. Kim and Reneker imately equal diametersand charge per unit length ) used a different set-up in a work on polybenzimidazole before fibers‘landing’ on the collector (the collection ( ) w x PBI 13 . Actually, they used a rotating cylinder region). Although little iscurrently known about this covered with a grounded aluminium sheet. Rangpukan splaying process of the primary jet into multiple fila- and Reneker have also reported the development of ments, it is thought to be responsible—together with the w x electrospinning from molten polymers in vacuum 14 . elongation due to the acceleration of the jet by electrical Electrospinning from polymer melts in vacuum is advan- forces—for the unusually small diameter fibers which tageous because higher electric field strength over large can produced by electrospinning w1x. distances and higher temperature can be used than in As mentioned, electrospun fibers can be electrostati- air. In the work by Theron et al. an electrostatic field- cally charged andyor the charge can be manipulated. assisted assembly technique was described, combined The charge can be removed, when it isnot wanted, by with an electrospinning process used to position and exposure to ions carried in the air (or the charge can align individual nanofiberson a tapered and grounded sometimes be neutralised during contact with the collec- wheel-like bobbin w15●●x. The bobbin isable to wind a tor). If the fibersare electrically conducting, the charge continuously as-spun nanofiber at its tip-like edge. The can also be adjusted by conduction through the fiber alignment approach hasresultedin polyethylene oxide w1x. Tsai and Schreuder-Gibson, discuss the effect of (PEO)-based nanofibers with diameters ranging from electrospinning material and conditions upon the residual 100 to 300 nm and lengthsof up to hundredsof microns. electrostatic charge of polymer nanofibers w17x. Poly- The equipment developed by the University of Mas- ( ) sachusetts Darmouth in collaboration with Massachu- merssuchPEO, polycaprolactone a polyester , poly- setts Institute of Technology had also a different carbonate, and polystyrene have been evaluated for their configuration: parallel-plate geometry. Indeed a ‘point- charge induction and charge retention characteristics and plate’ geometry leadsto curved electric field linesclose are ranked according to their inherent polarity. to the capillary, resulting thus in a non-uniform electric Besides circular fibers, a variety of cross-sectional field. The parallel-plate configuration allowsthe control shapes and sizes can be obtained from different polymer ( ) of the curvature w6x. Moreover, they fed the solution to solutions during electrospinning Fig. 3 . Actually multiple spinnerets placed in a radial geometry. The Koombhongse, Wenxia and Reneker obtained branched collector wasrotating. fibers, flat ribbons, bent ribbons, ribbons with other Beneath, a brief description of the electrospinning shapes, and fibers that were split longitudinally from processisplaced (Fig. 2). Electric field issubjectedto larger fibers from electrospinning a polymer solution w x the end of a capillary tube that containsthe polymer 9 . Studies on the properties of fibers with these cross- fluid held by its surface tension. This induces a charge sectional shapes from a number of different kinds of on the surface of the liquid. Mutual charge repulsion polymersand solventsindicatesthatfluid mechanical causes a force directly opposite to the surface tension effects, electrical charge carried with the jet, and evap- w16x. As the intensity of the electric field is increased, oration of the solvent all contributed to the formation the hemispherical surface of the fluid at the tip of the of the fibers. The influence of a skin on the jets of capillary tube elongatesto form a conical shapeknown polymer solutionsaccountsaswellfor a number of asthe Taylor cone (the base region). With increasing properties observed. We address the issues of the elec- field, a critical value isattained when the repulsive trospinning processing variables in the next paragraph. A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 67

Fig. 3. Scanning electron micrographs showing flat ribbons formation during electrospinning of PET from a solution of 13 wt.% in 1:1 dichloromethane and trifluoroacetic acid. Left magnification =1000, right: an enlarged image with magnification =3500. (Audrey Frenot, unpublished work).

3. Electrospinning processing variables and instabil- increasing solution concentration according to a power ities models law relationship. In addition, electrospinning from solu- tionsof high concentration hasbeen found to produce The following parameters and processing variables a bimodal distribution of fiber sizes, reminiscent of affect the electrospinning process: (i) System parameters distributions observed in the similar droplet generation such as molecular weight, molecular-weight distribution process of electrospray. Moreover they found evidence and architecture (branched, linear etc.) of the polymer that electrostatic effects influence the macroscale mor- ( and solution properties viscosity, conductivity and sur- phology of electrospun fabrics and may result in the ) ( ) face tension , and ii Process parameters such as formation of heterogeneousor three-dimensional electric potential, flow rate and concentration, distance structures. between the capillary and collection screen, ambient Meanwhile, both electrostatic and fluid dynamic insta- ( parameters temperature, humidity and air velocity in bilitiescan contribute to the basicoperation of the ) w x the chamber and finally motion of target screen 18 . process w3,7●●● ,9,20 ,21,22 ●●●●● ,23 ,24 ,25 ,26x. The bend- For instance, the polymer solution must have a concen- ing instabilities that occur during electrospinning have tration high enough to cause polymer entanglements yet been studied and mathematically modelled by Reneker not so high that the viscosity prevents polymer motion ●● et al. w7 ,9,21x. After the jet travelsstraightforward, induced by the electric field. The solution must also unsteadiness appears under the form of loops. The jet have a surface tension low enough, a charge density does not only bend but it also forms lateral excursions high enough, and a viscosity high enough to prevent the that grow into spiralling loop. Every loop grows larger jet from collapsing into droplets before the solvent has w ●●x evaporated. Morphological changescan occur upon in diameter and the jet becomesthinner 7 . New decreasing the distance between the syringe needle and bending instabilities arise when the jet is thin enough the substrate. Increasing the distance or decreasing the and enough stress relaxation of the viscoelastic stress ( ) electrical field decreases the bead density, regardless of hastaken place. An envelope cone Fig. 4 definesthe the concentration of the polymer in the solution. Applied region of these instabilities. w ●●x fieldscan, moreover, influence the morphology in peri- Reneker et al. 7 chose to model the instabilities odic ways, creating a variety of new shapes on the with viscoelastic dumbbells connected together (Fig. 5). surface. The interactionsbetween the beadsforming the dumb- Deitzel et al. have evaluated systematically the effects bellswere following Coulomb’slaw. The other para- of two important processing parameters, spinning volt- meterstaken into account were the electrical forcesdue age and solution concentration, on the morphology of to the electric field, surface tension and the fact that the the fibersformed w19x. They found that spinning voltage spring connecting the beads had a Maxwellian viscoe- isstrongly correlated with the formation of bead defects lastic resistance to elongation, like the jet. Aerodynamic in the fibers, and their measurements can be used to forcesand gravity effect were neglected aswell as signal the onset of the processing voltage at which the solvent evaporation. This was taken into account in a bead defect density increases substantially. Solution later study w21x. In these articles, it is stated that concentration has also been found to most strongly electrospinning process and instabilities can be consid- affect fiber size, with fiber diameter increasing with ered asparticular examplesof the Earnshawtheorem in 68 A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75

imentscan be carried out. The approximation governs both long wavelength axisymmetric distortions of the jet, aswell aslong wavelength oscillationsof the centerline of the jet. Three different instabilities were identified: the classical (axisymmetric) Rayleigh instability and electric field induced axisymmetric and whipping instabilities. At increasing field strengths, the electrical instabilities are enhanced whereas the Rayleigh instability is suppressed. Which instability dominates depends strongly on the surface charge density and the radiusof the jet. In their later work, they have usedthe stability theory to develop a quantitative method for predicting when electrospinning occurs w23●x. First, a method for calculating the shape and charge density of a steady jet as it thins from the nozzle was presented and wasshownto capture quantitative featuresof the experiments. Then, this information was combined with the stability analysis to predict scaling laws for the jet behaviour and to produce operating diagramsfor when electrospinning occurs, both as a function of experimental parameters. Predictions for how the regime of elec- trospinningchangesasa function of the fluid conductivity and viscosity were also presented. Furthermore, Vao Soongnern, Doruker and Mattice analysed static and dynamic properties of amorphous polyethylene nanofibers by using a Monte Carlo simu- w ●x Fig. 4. Schematic of the electrospinning equipment showing the enve- lation technique 27,28 . They looked at how confine- lope cone. With courtesy of the American Institute of Physics w7●●x. ment and curvature of the surface of the nanofibers affectschain propertiescompared to the bulk of a thin electrostatics. This means that one can not get a stable structure if the elements of this one interact only by Coulomb’slaw. There isa tendency of the chargesin the polymer fluid to reduce their Coulomb interaction energy by moving the fluid in a complicated path. However, the challengesahead lay in extending this level of fundamental knowledge to include non-linear effects arising from viscoelasticity of the fluid and additional fluid phenomena that occur downstream of the initial instability. This was attempted in a later study by the same group, where they propose that in liquids with a non-relaxing elastic force, that force also affects the shapes of the electrospun fibers w22●●x. They showed that the Taylor cone doesnot representa unique critical shape but half angles are much closer to this new shape. Another group by Hohman, Shin, Rutlege and Brenner also studied electrospinning, with regard to electrically forced jet and instabilities, and proposed a stability theory for electrified fluid jets w23●●● ,24 ,25 ,26x. A series of papers demonstrate that an essential mechanism of electrospinning is a rapidly whipping fluid jet. The authorsanalysedthe mechanicsof thiswhipping jet by studying the instability of an electrically forced fluid jet with increasing field strength. An asymptotic approxi- Fig. 5. Bending jet modelled by a system of beads connected by mation of the equationsof electrohydrodynamicswas viscoelastic elements. With courtesy of the American Institute of developed so that quantitative comparisons with exper- Physics w7●●x. A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 69

beadsare formed. The net charge densityinfluencesin the same way. Actually, adding salt (i.e. NaCl) isa way to increase this density. Deitzel et al. investigated as well the formation of beadsand found out that the spinning voltage influences mainly the formation of beadswhile the polymer concentration haseffect on the fiber size w19x. They also noticed that at high concentration a bimodal distribution of the fiber sizes was created. Wilkesat the Virginia Polytechnic Institute w18x has studied the effect of solution concentration, capillary– screen distance, electric potential at the tip and flow rate on electrospinning Esthane 5720, a segmented polyether urethane. He found that bead-like structure Fig. 6. Example of bead formation during electrospinning of 13 wt.% appeared when the capillary–screen distance decreased, PET in 1:1 dichloromethane and trifluoroacetic acid (Audrey Frenot, while the average fiber diameter wasincreased.When unpublished work). increasing the concentration, the average diameter rised and the bead-like structure turned into blobs at lower film. Major findingsof thisstudywere (i) fibersof capillary–screen distance. A too high concentration different sizes, i.e. consisting of different numbers of leaded to failure of the fibersinsteadof being drawn, parent chains, exhibit almost identical hyperbolic density as the viscosity was too high. This can also be caused profiles at the surfaces, (ii) the end beadsare predomi- by a too high flow rate at lower concentration. Finally, nant and the middle beadsare depleted at the free increasing the potential decreased the fiber diameter. surfaces, (iii) there isan anisotropy in the orientation Demir et al. studied the electrospinning behaviour of of bondsand chainsat the surface, (iv) the centre of an elastomeric polyurethane urea copolymer in solution mass distribution of the chains exhibits oscillatory w34x. They observed that mainly the morphology of behaviour acrossthefibersand (v) the mobility of the electrospun fibers is strongly correlated with viscosity, chain in nanofiber increasesasthediameter of the equivalently concentration and temperature. Actually, nanofiber decreases. They concluded that these nanofi- the morphological imperfectionslike formation of fibers ber trends, with its strongly curved surface, are similar with beadsor curly fiberscan be improved by increasing to the results seen in simulations by the same technique the solutiontemperature. Thisisalsoan important of free-standing thin film, where there is no curvature advantage from the aspect of industrial applications at the surface. since high temperature makes the electrospinning process quick. 4. Polymer nanofibers with specific bulk morpholo- Biopolymerssuchaspoly (lactic acid)(PLA) fibers gies and surface topologies have been electrospun by Bognitzki et al. w4x. The fibers had an average diameter of approximately 1 mm. They Electrospinning process is applicable to many poly- used tetraethyl benzylammonium chloride (TEBAC) to mers, as a melt or a solution. Reneker’s group has obtain fiberswith lower diameter. Thisisdue to the done work on many different polymers, for example: increased surface tension and electrical conductivity that polyethylene terephtalate w1x; PBI w13x; polystyrene w8x, TEBAC procures. Adding PEO to the solution resulted poly(2-hydroxyethyl methacrylate), polyvinylidene in the same phenomenon because of an increase of fluoride, and poly(ether imide) w9x; styrene–buta- surface tension only. PLA and polyvinilpyrrolidone diene–styrene triblock copolymer w29x and blends are materials that have also been used by these poly(ferrocenyldimethylsilane) w30x. They have even authors w3x. spun DNA fibers with a diameter of approximately 30 For someapplicationsit isdesirableto control not nm w31x and silk for which there is a patent w32x. They only the diameter of the fibersand their internal mor- have used PEO in order to study more deeply the phology (and thustheir intrinsicproperties ) but also electrospinning process and the influential parameters. their surface structure. An interesting feature of nanofi- They also studied w33x the formation of beadson the bersisthat they displayan internal phasemorphology fibers (Fig. 6). The bead morphology hasbeen well that is evidently controlled by rapid phase separation documented for electrospun fibers and is affected by and rapid solidification. By selective removal of one processing conditions w19,33x. Beadscan be considered component, fine porousfibersand functional nanofibers as capillary break-up of the jets by surface tension. with specific surface topologies are accessible. Forma- Thus, charge density carried by the jet, surface tension tion of such co-continuous phase morphologies was and viscoelastic properties of the solution are significant studied in a work by Bognitzki et al. where they used parameters. The higher the solution viscosity is, the less electrospinning of PLA to manufacture polymerymetal 70 A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 hybrid nano- and mesotubes w12●●x. The aim wasto be dimethyl formamide. They observed that surface segre- able to control not only the fiber diameter but also the gation of fluorine in electrospun fibers is similar in surface structure, which can be important for filtration magnitude to that measured for the thin film surface. applicationsor formation of functional nanotubes.The Fluorinated surfaces are inherently of interest for stain tubeswere formed with a processtheycall TUbesby resistant and water repellent materials and for coating fiber template (TUFT) process. This process consists in applications. coating the PLA nanofiberswith firstpoly (p-xylylene) Furthermore, chemical crosslinking method can be by chemical vapour deposition and then with the desired used to crosslink the pre-formed nanofibers. In a recent metal (aluminum in that case). Then the PLA core is work by Ding et al., nanoscale poly(vinyl alcohol) removed by taking advantage of the ability to degrade (PVA) fiber (100–500 nm) aggregateswere prepared of PLA. Thisshowsagreat potential for the production with an electrospinning technique and then chemically of novel tubular devices. Most possible TUFT process crosslinked w40x. The results showed that the properly will gain significance with improvement of the control crosslinked PVA fiber aggregates had better antiwater of tube diameters, structuring and functionalization of solubility and mechanical properties than the non-cross- the tubes, which should be possible by adjusting the linked PVA fiber aggregates. parametersfor the preparation of template fibersby ( electrospinning. In another work by Caruso et al. poly L- 5. Molecular nano-electronic machines lactide) electrospun fibers were coated with amorphous titanium dioxide using a sol–gel coating technique w35x. On removal of the thermally degradable polymer, hollow Electronic polymershave been electrospunwith the titania fiberswere produced. The sol–gel coating was purpose to be able one day to create nano-electronic w x able to mimic the finer detailsof the fiber, thereby machines 10,11,41 . For example, these small fibers forming noduleson the inner wallsof the tubes.Nano- can support arrays of nanomachines, and connect inte- tubes are also the topic treated in the presentation of Ko grated arrays of nanomachines to larger scale systems. et al. at the ASC 16th Annual Technical Conference Nanoscale fibers are also containing special arrange- w36x. In that work, they have co-electrospun a mixture ments of polymer molecules, often called crystallograph- of carbon nanotubes and PEO solution. This process ic defects, which can themselves, function as alignsthe carbon nanotubes (if well dispersed in the nanomachinesto translateand rotate polymer molecule w x polymer solution) in the resulting nanocomposites 1 . fibrils. According to models, the mechanical properties Using electrospinning Diaz-de-Leon has prepared of carbon nanotubescompositesare higher than usual nanofibersof doped polyaniline (an organic conducting composites. Nanotubes and nanowires can also be con- polymer) as itself or in blend with polystyrene or PEO w x structed by using an electrostatic field-assisted assembly and electrically characterise them 10 . The conductivity technique combined with an electrospinning process to is found to be very sensitive to the morphology of the position and align individual nanofibers on a tapered fibers (i.e. amount of defects, thickness) which isindeed and ground wheel-like bobbin w15●●x. related to the initial blend properties and solvent used. Cellulosic nanofiber membranes that present good Therefore, better chain alignment in the polymer blends wettability and absorbencypropertiesaswell asthe yieldshigher overall conductivity of the blended nano- ability to carry chemically reactive functionalitieswere fibers. It is notable also, that the nanofibers obtained reported by Liu and Hsieh w37x. In another work, Sung showed regions with polymer crosses; this polymer– et al. studied the electrospinning of polycarbonates and polymer junctionsare of interestin making nanoelec- their surface characteristic w38x. Electrospun fibers there- tronic junction devices. Conductive nanofibers were from show a wrinkled structure that was found to depend obtained from electrospinning using blends of spinnable on the evaporation rate of the solvent from the surface (e.g. poly(ethylene oxide)) and non-spinnable but con- related to the rate of evaporation from the core. Indeed, ductive (e.g. polyaniline doped with camphorsulfonic as the solvent on the surface has evaporated and a ‘skin’ acid) polymers, by MacDiarmid et al. w11,42●x. The rate has formed, the solvent entrapped in the core diffuses for the vapour phase de-dopingyre-doping of the elec- to the ambient atmosphere and causes what they call trospun fibers was at least one order of magnitude faster the ‘raisin-like structure’. They also believe that this is than to those for cast films. Again, this stresses the a reason for the formation of flat bands. enormous effect of increase in the surface-to-volume Surface functionalization of electrospun fibers has ratio and selected chemical properties accomplished by been also studied recently by Deitzel et al. w20●● ,39 x. electrospinning the polymer material into nanofibers. This has been accomplished by electrospinning a series The resulting nanofibers are conductive and are of of random copolymersof PMMA-r-TAN (polymethyl potential interest for applications in micro- and optolec- methacrylate with varying tetrahydroperfluorooctyl acry- tronics, for example, nanowires, LEDs, photocells, etc. late concentration) from a mixed solvent of toluene and Moreover, the Korea Institute of Science and Technology A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 71 ownsa patent on the fabrication of a lithium secondary tide-co-glycolide membranesfor anti-adhesionapplica- battery comprising a fibrous film made by electrospin- tions w49x. Among them, there was also a presentation ning w43x. Another interesting progress has been the from Reneker’sgroup on encapsulationof particlesinto development of a hybrid solar cell utilising electrospun electrospun polymeric nanofibers w50x. Thiswas conductive polymers‘doped’ with photovoltaic dyesand achieved by adding insoluble particles to the polymer nano-crystalline semiconductor particles w44x. Ziegler et solution. These particles were gold, pollen spore, algin- al. have prepared a flexible photovoltaic membrane by ate, and silver sulfadiozine for burns treatment, for utilising electrospun polyacrylonitrile (PAN) nanofiber example. They also added biomaterials for wound heal- dyed with copper phthalocyanine, while nano-crystalline ing or other substances that functionalized the fiber, e.g.

TiO2 semiconductorparticlescan be embedded aswell soluble drugs, antibacterial agents. Medical application within the nanofibers. of electrospinning is also the subject of a patent claimed The feasibility of electrospinning photo-responsive by Reneker and co-workers w51x. They produced a skin polymeric materialswith desirablefunctionalitiesis mask by directly electrospinning fibers onto the skin expected to find wide application in future electronic surface in order to protect or heal eventual wounds. device fabrication. Control of the polymer solutions, Another patent w52x claimsthe production of electrospun blending with other polymersand electrospinningcon- fibers containing pH-adjusting compound for use in ditionscan provide a variety of pathwaysfor function- dressing for wound treatment or protection from contam- alization of interesting photo-responsive electrospun ination. Electrospun fiber mats were also explored as nanofibers. Drew et al. w41x have demonstrated success- drug delivery vehicles with promising results. Mats were ful engineering of polymeric membranesby electrospin- made either from PLA, PEVA, or from a 50:50 blend ning a photonic polymer system PAN blended with the of the two using tetracycline hydrochloride as a model light harvesting azo-dye, Congo red. The process of drug w53x. electrospun nanofibers can also be exploited for use in The use of electrospinning for tissue engineering is various applications in the sensor technology. A recent also another interesting field of application of polymer attempt isthe one reported by Zhang et al. where a high nanofibers. Electrospun nanofibrous structures meet the surface area chemosensor material has been obtained, essential design criteria of an ideal tissue engineered by electrospinning of fluorescent conjugated polymer scaffold based upon its acts to support and guide cell w45x. Blendsof PEO and fluorescentpoly ((p-phenylene growth. Most publications support that the electrospun ethynylene)-alt-(thienylene ethynylene)) were electros- nanofibrous structure is capable of supporting cell pun from a chloroform solution. Furthermore, Wang et attachment and proliferation. Cells seeded on this struc- al. have prepared fluorescent electrospun polymer film ture tend to maintain phenotypic shape and guided sensors valuable for the detection of explosives w46x. growth according to nanofiber orientation. Li et al. have developed an electrospun structure for tissue-engineering 6. Polymer nanofibers for biomedical applications applications, composed of poly(D,L-lactide-co-glycolide) PLGA fibersranging from 500 to 800 nm in The use of polymer nanofibers for biomedical appli- diameter w54x. The structure features a morphologic cationsisanother active research area. In a recent similarity to the extracellular matrix of natural tissue, publication, an electrospinning method was used by which ischaracterisedby a wide range of pore diameter Zong and co-workersto fabricate bioabsorbableamor- distribution, high porosity, and effective mechanical phouspoly (D,L-lactic acid) and semi-crystalline poly(L- properties. Matthews et al. studied how electrospinning lactic acid) nanofiber non-woven membranesfor can be adapted to produce tissue-engineering scaffolds biomedical applications w47x. They showed that the fiber composed of collagen nanofibers (a matrix composed diameter and the nanostructured morphology depended of 100 nm fiber) w55●x. They found that the structural on processing parameters such as solution viscosity (e.g. properties of electrospun collagen varied with the tissue concentration and polymer molecular weight), applied of origin, the isotope, and the concentration of the electric field strength, solution feeding rate and ionic collagen solution, which were used to spin the fibers. salt addition. Concentration and salt addition were found Moreover, blend componentswith collagen have been to have relatively larger effectson the fiber diameter used as well for electrospinning w56x. Type I collagen- than the other parameters. The Fiber Society Spring PEO fibersand non-woven fiber networkswere pro- 2001 Conference treated aswell widely the electrospin- duced by the electrospinning of a weak acid solution of ning of nanofibers for biomedical applications. A study purified collagen at ambient temperature and pressure. on the fabrication of a scaffold by electrospinning The process proposed by Huang et al. provides a biomaterialssuchasPLA, poly (glycolic acid), convenient, non-toxic, non-denaturing approach for the poly(ethylene-co-vinyl acetate)(PEVA), and Type I generation collagen-containing nanofibers (diameter collagen, wasreported by Bowling et al. w48x. Fang et range of 100–150 nm) and nonwoven fabricsthat have al. presented nano-structured electrospun poly-D,L-lac- potential application in wound healing, tissue engineer- 72 A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 ing, and as hemostatic agents. The same group has also ite–nylon 6 (NLS) nanocomposite w62x. They also produced elastin-mimetic fibers from peptide polymer exerted hierarchical control of morphology and form in order to mimic protein fibersthat are found in arterial through the combination of a nanostructured material wall w57●x. Electrospinning can even be used to create and a nanoscale fabrication technique. The electrospin- biocompatible thin films with useful coating design and ning process resulted in highly aligned montmorillonite surface structure that can be deposited on implantable layers (layer normal perpendicular to the fiber axis) and devicesin order to facilitate the integration of these nylon 6 crystallites (layer normal parallel to fiber axis). deviceswith the body. Silk-like polymer with fibronec- Of particular interest are electrospun membranes com- tine functionality (extracellular matrix proteins) have posed of elastomeric fibers, for the development for been electrospun for making biocompatible films used several protective clothing applications. A lot of work on prosthetic devices aimed to be implanted in the isdone with aim to develop garmentsthat reduces central nervous system w58●x. soldiers’ risks for chemical exposure w63x. The idea is to lace severaltype of polymersand fibersto make 7. Other functions of nanofiber structures protective ultra-thin layersthat would enhance for exam- ples chemical reactivity and environmental resistance. Non-woven textiles composed of electrospun fibers The US Army presented at the Eighth International have a large specific surface area and small pore size Conference on Textile Coating and Laminating itsway compared to commercial textiles, making them excellent to consider electrospinning w64,65x. They see a manner candidatesfor usein filtration and membrane applica- to produce membranesfor laminated fabricsaswell as tions. Thus, electrospinning has been exploited to pro- direct sprayed-on fibrous coating structures. They pre- duce nonwoven composites for application in filters pared microporousmembranesof PAN and PBI, and w59x. A nanofiber web wasproduced from nylon 66 on compared their convective airflow resistance and water a substrate made of either meltblown or spunbond vapour diffusion resistance with conventional fabrics polypropylene. The resulting product was then sand- PTFE wind barrier (like Gore-Tex). The electrospun wiched between material of the same substrate and point membraneshad both good water vapour transportprop- bonded in a calender to form the composite structure. erties and convective flow resistance, meaning that they The filtration propertiesof microparticleswere then will present remarkable ‘breathing’ properties, which are measured and showed efficiency similar to the best nowadaysrequired in clothing applications. commercial filterseven if only one-fifth filtering media was used. Electrospinning can also be used to produce 8. Conclusions and future perspectives charged fibersfor usein filtration media w17x. Obviously, the charge induction and charge retention characteristics Electrospinning is a very simple and versatile method are related to the polymer material used for of creating polymer-based high functional and high electrospinning. performance nanofibersthat can revolutionisethe world Kim and Reneker have investigated the use of PBI of structural materials. The process is versatile in that nanofibersasreinforcement in an epoxy and a rubber there isa wide range of polymer and biopolymer matrix (styrene–butadiene) w60x. They observed that the solutions or melts that can be spun. This assemble epoxy wastoughened by the nanofibersand thisrein- approach can be expanded even into a hierarchical forcing effect washigher than that of PBI fibroids assemble of produced nanofibers in other well-defined (whiskers-like particles). Moreover, the PBI nanofibers functional nanostructures, such as nanotubes and nano- can provide dramatic reinforcement of a rubber matrix wires, etc. The various recent approaches that have been (i.e. the Young moduluswasten timeshigher and the developed for scientific understanding, engineering con- tear strength wastwice aslarge asfor the unfilled trol and then potential implementation in devicesof rubber). In another work, the reinforcement effect of the electrospun nanofibers are summarised in this review. ultrathin fibersof nylon-4,6 with a semi-infinitelength Essential studies, nevertheless, are still required and prepared by electrospinning solutions of the polymer in many challengesremain to be faced. In particular, formic acid was clearly demonstrated by Bergshoef and integration of nanofibersinto usefuldevicesrequires Vancso w61x. These fibers can be applied to prepare nanofibersof well-controlled orientation, size,and other transparent composites with an epoxy matrix. They target characteristicsaswellasreproducibility locating noticed that at certain given process parameters, the jet them in specific positions and orientations. The ability of nylon-4,6 solution pulled from the capillary tip during to do so, however, remains a major challenge in the the electrospinning process can splay into several finer field. The design and construction of process equipment jets. The potential of using polymer nanocomposites as for controllable and reproducible electrospinning could the foundation for fabricating nanofibers structures, illus- determine the character of instabilities revealed, satisfy trated by the work of Fong et al. by demonstrating the potential implementation and act asstimulustoprovide dissolution and reprocessing of exfoliated montmorillon- new products. Moreover, to control over key perform- A. Frenot, I.S. Chronakis / Current Opinion in Colloid and Interface Science 8 (2003) 64–75 73 ance parameters, the clarification of the fundamental w9x Koombhongse S, Liu W, Reneker DH. Flat ribbons and other electrodynamics of the electrospinning process and cor- shapes by electrospinning. J Polym Sci, Polym Phys Ed 2001;39:2598 –606. relation to the polymer fluid characteristics must be w10x Diaz-de-Leon MJ. Electrospinning nanofibers of polyaniline reliably predicted and utilised. Work that address the and polyaniliney(polystyrene and polyethylene oxide) evaluation of the fluid instabilities, postulated to be blends. National Conference of Undergraduate Research, crucial for producing nanofibers, and precise character- Lexington: University of Kentucky; 2001. isation of the morphology and material properties of w11x MacDiarmid AG, JonesWE, NorisID, et al. Electrostatical- electrospun polymer fibers themselves are in early stages ly-generated nanofibersof electronic polymers.Synthetic Met 2001;119:27 –30. and systematic studies are still emerging. With these w12x Bognitzki M, Hou H, Ishaque M, et al. Polymer, metal, and variablesunder control, the challengesof fully quanti- ●● hybrid nano- and mesotubes by coating degradable polymer tative modelling of the electrospinning process should template fibers (TUFT process). Adv Mater 2000;12:637 – be feasible, including both stable and unstable operating 40. regimes, starting from the existing equations of electro- The paper introduce an efficient and simple general route, called TUFT process (tubesby fiber templates ), towardsnano- and dynamic theory. Productivity improvement of the elec- mesotubes using electrospun degradable polymer fiber templates. trospinning process is also an essential feature and merits Thisapproach offersa great potential for the designof novel more studies. tubular devices in the sense of control of dimensions and shape and even with respect to the broad range of materials that can be manufactured. References and recommended reading w13x Kim J-S, Reneker DH. Polybenzimidazole nanofiber produced by electrospinning. Polym Eng Sci 1999;39:849 –54. Papers of particular interest, published within the annual period of w14x Rangpukan R, Reneker DH. Development of electrospinning review, have been highlighted as: from molten polymersin vacuum. J Textile Apparel, Technol Manage 2001, vol. 1. Special issue: The Fiber Society Spring ● of special interest 2001 Conference, Raleigh NC. w x ●● of outstanding interest 15 Theron A, Zussman E, Yarin AL. Electrostatic fields-assisted ●● alignment of electrospun nanofibres. Nanotechnology w1x Reneker DH, Chun I. Nanometre diameter fibresof polymer 2001;12:384 –90. produced by electrospinning. Nanotechnology 1996;7:216 – A new approach was described for assembling nanofibers in parallel 23. arrayswhile being able to control the average separationbetween w2x Larrondo L, Manley RSJ. Electrostatic fiber spinning from the fibres using electrostatic forces during the manufacturing polymer melts. I. Experimental observations on fiber for- process. w x mation and properties. J Polym Sci Polym Phys Ed 16 Doshi J, Reneker DH. Electrospinning process and applica- 1981;19:909 –20. tions of electrospun fibers. J Electrostat 1995;35:151 –60. w x w3x Bognitzki M, Frese T, Steinhart M, Greiner A, Wendorff JH. 17 Tsai PP, Schreuder-Gibson H. Effect of electrospinning mate- Preparation of fibers with nanoscaled morphologies: elec- rial and conditions upon residual electrostatic charge of trospinning of polymer blends. Polym Eng Sci 2001;41:982 – polymer nanofibers. J Textile Apparel, Technol Manage 2001, 9. vol. 1. Special issue: The Fiber Society Spring 2001 Confer- w4x Bognitzki M, Wendorff JH, Greiner A. Submicrometer ence, Raleigh NC. w x shaped polylactide fibers by electrospinning. Polym Prep 18 WilkesGL. Available from: http: yywww.che.vt.eduyWilkesy (ACS, PMSE) 2000;82:115 –6. electrospinningyelectrspinning.html. 2001. w x w5x Buer A, Ugbolue SC, Warner SB. Electrospinning and 19 Deitzel JM, Kleinmeyer J, HarrisD, Tan NCB. The effect properties of some nanofibers. Textile Res J 2001;71:323 –8. of processing variables on the morphology of electrospun w6x Warner SB, Buer A, Ugbolue SC, Ruteldge GC, Shin MY. nanofibersand textiles.Polymer 2001;42:261 –72. w x M98-D01 A fundamental investigation of the formation and 20 Deitzel JM, Kleinmeyer D, Hirvanen JK, Tan NCB. Con- ● ( ) properties of electrospun fibers. Department of Textile Sci- trolled deposition of electrospun poly ethylene oxide fibers. ences, University of Massachusetts Dartmouth; Department Polymers2001;42:8163 –70. of Chemical Engineering and Material Sciences, Massachu- The onset of a chaotic oscillation of the electrospinning jet was setts Institute of Technology Cambridge, 2000. controlled by the deposition of sub-micron polymer fibers on a w7x Reneker DH, Yarin AL, Fong H, Koombhonge S. Bending substrate through use of an electrostatic lens element and collection ●● instability of electrically charged liquid jets of polymer target of opposite polarity. w x solutions in electrospinning. J Appl Phys 2000;87:4531 –47. 21 Yarin AL, Koombhongse S, Reneker DH. Bending instability In this article the reasons for the instability are analyzed and in electrospinning nanofibers. J Appl Phys 2001;89:3018 – explained using a mathematical model. The theory accounts for 26. the non-linear effectsthat are characteristicsof finite perturba- w22x Yarin AL, Reneker DH. Taylor cone and jetting from liquid tions, as well as for the rheological behaviour of viscoelastic ●● droplets in electrospinning of nanofibers. J Appl Phys liquids. The authors give the three-dimensional equations describ- 2001;90:4836 –46. ing the dynamics of the bending of electrospun jets, both in the An interesting theory of the stable shapes of droplets as affected nearly straight region where the instability grew slowly and in by an electric field isproposedin thispaper. The theory compared the region where the bending dominated the path of the jet. with data acquired in experimental work on electrospinning of w8x Torres B. Ultrafine fibers of polystyrene dissolved in tetrah- nanofibers from polymer solutions and melts. ydrofuran prepared using electrospinning method. National w23x Shin MY, Hohman MM, Brenner M, Ruteldge GC. 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