Micro-/Nano-Fibers by Electrospinning Technology: Processing, Pr

Introduction

Human beings have used fibers

for centuries. In 5000 BC, our ancestors used natural

fibers

such as wool, cotton silk and animal fur for clothing. Mass production of

fibers

dates back to the early stages of the industrial revolution. The first man-made

fiber

– viscose – was presented in 1889 at the World Exhibition in Paris. Developments in the polymer and chemical industries – as well as in electronics and mechanics – have led to the introduction of new types of man-made

fibers,

especially the first synthetic

fibers,

such as nylon, polypropylene and polyester. The needs and further progress allowed the production of high functionality

fibers

(antistatic, flame resistant, etc.) and high performance

fibers

(carbon

fibers

in 1960 from viscose and aramid

fibers

in 1965) that showed high strength, a high modulus and great heat resistance. These

fibers

are used not only in clothing but also in hygienic products, in medical and automotive applications, in geo-textiles and in other applications.

Traditional methods for polymer fiber production include melt spinning, dry spinning, wet spinning and gel-state spinning. These methods rely on mechanical forces to produce fibers by extruding a polymer melt or solution through a spinneret and subsequently drawing the resulting filaments as they solidify or coagulate. These methods allow the production of fiber diameters typically in the range of 5 to 500 microns. At variance, electrospinning technology allows the production of fibers of much smaller dimensions. The fibers are produced by using an electrostatic field [1].

Electrospinning is a fiber-spinning technology used to produce long, three-dimensional, ultra-fine fibers with diameters in the range of a few nanometers to a few microns (more typically 100 nm to 1 micron) and lengths up to kilometers (Fig. 16-1). When used in products, the unique properties of nano-fibers are utilized, such as extraordinarily high surface area per unit mass, very high porosity, tunable pore size, tunable surface properties, layer thinness, high permeability, low basic weight, ability to retain electrostatic charges and cost effectiveness, among others [2].

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Figure 16-1.

(a) SEM image of poly(ethylene terephthalate) (PET) nano-fiber web. The nano-fibers were electrospun from a PET solution in THF:DMF. The diameter of the fibers is about 200 nm. (b) PET nano-fiber web – comparison with human hair [1].

While electrospinning technology was developed and patented by Formhals [3] in the 1930s, it was only about fifteen years ago that actual developments were triggered by Reneker and co-workers [4]. Interest today is greater than ever and this cost-effective technique has made its way into several scientific areas, such as biomedicine, filtration, electronics, sensors, catalysis and composites [5] and [6]. Electrospinning is a continuous technique and is hence suitable for high volume production of nano-fibers. The ability to customize micro-/nano-fibers to meet the requirements of specific applications gives electrospinning an advantage over other, larger-scale, micro-/nano-production methods. Carbon and ceramic nano-fibers made of polymeric precursors further expand the list of possible uses of electrospun nano-fibers [7].

Working Principle and Configuration of Electrospinning Processing

Electrospinning is increasingly being used to produce ultra-thin fibers from a wide range of polymer materials. This non-mechanical, electrostatic technique involves the use of a high voltage electrostatic field to charge the surface of a polymer-solution droplet, thereby inducing the ejection of a liquid jet through a spinneret (Fig. 16-2). In a typical process, an electrical potential is applied between a droplet of a polymer solution held at the end of a capillary tube and a grounded target. When the electric field that is applied overcomes the surface tension of the droplet, a charged jet of polymer solution is ejected. On the way to the collector, the jet will be subjected to forces that allow it to stretch immensely. Simultaneously, the jet will partially or fully solidify through solvent evaporation or cooling, and an electrically charged fiber will remain, which can be directed or accelerated by electrical forces and then collected in sheets or other useful shapes.

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Figure 16-2.

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

A characteristic feature of the electrospinning process is the extremely rapid formation of the nano-fiber structure, which occurs on a millisecond scale. Other notable features of electrospinning are a huge material elongation rate of the order of 1000 s⁻¹ and a reduction of the cross-sectional area of the order of 10⁵ to 10⁶, which have been shown to affect the orientation of the structural 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 conducted to explore the mechanism, some aspects and phenomena are not yet fully understood.

Formation of the Taylor Cone and Subsequent Fluid Jet

When the high voltage field is applied, the droplet of polymer solution at the tip of the needle will become highly electrified and the charges induced will be evenly distributed over the polymer solution surface. The droplet will experience two types of electrostatic forces: electrostatic repulsion between the charges on the surface and Coulombic forces in the external field. Under the influence of these two forces, the droplet will be elongated and finally distorted into a so-called Taylor cone. As the voltage increases, the electrostatic forces will become stronger and eventually overcome the surface tension, and a charged jet of fluid will be ejected.

Both electrostatic and fluid dynamic instabilities can contribute to the basic operation of the process.

Reznik et al. [8] experimentally and numerically studied the shape evolution of small droplets attached to a conducting surface that was subjected to relatively strong electric fields. Three different scenarios of droplet shape evolution are distinguished, based on numerical solution of the Stokes equations for perfectly conducting droplets:

1. In sufficiently weak (subcritical) electric fields, the droplets are stretched by the electric Maxwell stresses and acquire steady-state shapes where equilibrium is achieved by means of surface tension.

2. In stronger (supercritical) electric fields the Maxwell stresses overcome the surface tension, and jetting is initiated from the droplet tip if the static (initial) contact angle of the droplet with the conducting electrode is α_s < 0.8π; in this case, the jet base acquires a quasi-steady, nearly conical, shape with a vertical semi-angle of β ≤ 30°, which is significantly smaller than that of the Taylor cone (β_T = 49.3°).

3. In supercritical electric fields acting on droplets with a contact angle in the range 0.8π < α_s/<π, there is no jetting and almost the whole droplet jumps off: this is similar to gravity or drop-on-demand dripping.

The droplet-jet transitional region and the jet region proper are studied in detail for the second case using quasi-one-dimensional equations, taking into account the inertial effects and additional features such as the dielectric properties of the liquid (leaky dielectrics). The flow in the transitional and jet region is matched to that in the droplet. This is used to predict the current−voltage characteristic, I = I(U), and the volumetric flow rate, Q, in electrospun viscous jets, given the potential difference applied. The predicted dependence, I = I(U), is nonlinear due to the convective mechanism of the charge redistribution superimposed on the conductive (ohmic) mechanism. Realistic current values I = O(10² nA) have been predicted for U = O(10 kV) and fluid conductivity σ = 10⁻⁴ Sm⁻¹.

Thinning of the Fluid Jet

Beyond the conical base, immediately at the end of the capillary tip, the jet continues to become thinner. This jetting mode is known as the electrohydrodynamic cone jet. The jet will initially travel in a straight line towards the collector but will eventually become unstable. To the naked eye, it looks like the jet splits into multiple jets and it was thought before 1999 that this was the main reason for the small diameter of the electrospun fibers. However, when the jet is examined with a high speed camera, it can clearly be seen that the splaying is actually one single fiber rapidly bending or whipping, causing the fiber to make lateral excursions that grow into spiraling loops.

Jet splitting does occur, but it is not as common as previously thought and it is not the dominant process that occurs during spinning. Bending or whipping is caused by a phenomenon called bending instability and can occur in electrified fluid jets. Every loop then grows larger in diameter and the jet becomes thinner. New bending instabilities arise when the jet is thin enough and enough stress relaxation of the viscoelastic stress has taken place. This is called the second instability region and is very similar to the first instability region but acts on a much smaller scale. A tertiary-bending instability has also been documented. Each cycle of bending instability can be described in three steps:

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 perimeter of the loop is large enough and the diameter of the jet is small enough, the conditions of the first step of the cycle are fulfilled. The next cycle of bending instability then begins.

Several research groups have attempted to explain the bending instability by mathematical models.

Electrospinning Processing Parameters – Control of the Micro-Nano-Fiber Morphology

The fiber morphology has been shown to be dependent on process parameters, namely solution properties (system parameters), process conditions (operational parameters) and ambient conditions [1] and [2].

Solution Properties

Solution properties are those such as molecular weight, molecular weight distribution and architecture of the polymer, and properties such as viscosity, conductivity, dielectric constant and surface tension. The polymer solution must have a concentration high enough to cause polymer entanglements, yet not so high that the viscosity prevents polymer motion induced by the electric field. The resulting fibers’ diameters usually increase with the concentration of the solution according to a power law relationship. Decreasing the polymer concentration in the solution produces thinner fibers. Decreasing the concentration below a threshold value causes the uniform fiber morphology to change into beads [9]. The main factors affecting the formation of beads (Fig. 16-3) during electrospinning have been shown to be solution viscosity, surface tension and the net charge density carried by the electrospinning jet. Higher surface tension results in a greater number of bead structures, in contrast to the parameters of viscosity and net charge density, for which higher values favor fibers with fewer beads. This reduction in thickness is due to the solution conductivity, which reflects the charge density of the jet and thus the elongation level. The surface tension also controls the distribution and the width of the fibers, which can be decreased by adding a surfactant to the solution. Adding a surfactant or a salt to the solution is a way of increasing the net charge density and thus reducing the formation of beads. Finally, the choice of solvent(s) directly affects all of the properties mentioned and is of major importance to the fiber morphology.

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Figure 16-3.

Example of bead formation during electrospinning: SEM micrographs of poly(propyl carbonate) (PPC) beads prepared by electrospinning a PPC solution in dichloromethane [9].

Process Conditions

The parameters in the process are spinning voltage, distance between the tip of the capillary and the collector, solution flow rate (feed rate), needle diameter and, finally, the motion of the target screen. Voltage and feed rate show different tendencies and are less effective in controlling fiber morphology as compared to the solution properties. Too high a voltage might result in splaying and irregularities in the fibers. A bead structure is evident when the voltage is either too low or too high. However, a higher voltage also leads to a higher evaporation rate of the solvent, which in turn might lead to solidification at the tip and instability in the jet. Morphological changes in the nano-fibers can also occur upon changing the distance between the syringe needle and the substrate. Increasing the distance or decreasing the electrical field decreases the bead density, regardless of the concentration of the polymer in the solution.

Ambient Conditions

Ambient conditions include factors such as humidity and temperature, air velocity in the spinning chamber and atmospheric pressure. Humidity primarily controls the formation of pores on the surface of the fibers. Above a certain threshold level of humidity, pores begin to appear and, as the level increases, so does the number and size of the pores.

The precise mechanism behind the formation of pores and texturing on the surface is complex and is thought to be dependent on a combination of breath figure formation and phase separation. Breath figures are imprints formed due to the evaporative cooling during evaporation of the solvent, which results in condense solvent drops on the surface and, later, pores. Surface porosity (Fig. 16-4) can also be achieved by selective removal of one of the components in the polymer blend after spinning. The pores formed on the fiber surface can be used, for example, to capture nano-particles, act as a cradle for enzymes or increase the surface area for filtration applications.

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Figure 16-4.

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

Baumgarten studied the spinning velocities in addition to the effect of the flow rate, voltage, gap and the surrounding atmosphere [10]. He was able to determine the spinning velocity using the power balance:

(1)where V is the potential, I is the current and and are the mass flow rate and the spinning velocities, respectively. The calculation showed velocities close to the velocity of sound in air. Other researchers calculated velocities of the fibers reaching the collector to be 140 to 160 m/s. Obviously, these speeds must depend on the process parameters and solution used.

Increasing the solution temperature is also a method for speeding up the process, but it might cause morphological imperfections, such as the formation of beads. Furthermore, the regulation of scale and bifurcation-like instability in electrospinning are intriguing problems that remain to be solved. Regulatory mechanisms for controlling the radius of electrospun fibers at the different states are clearly illustrated in the work by He et al. [11].

Electrospinning Set-Ups and Tools

Novel Set-ups

The traditional set-up for electrospinning has been modified in a number of ways during the last few years in order to be able to control the electrospinning process and tailor the structure of micro-nano-fibers.

Yarin and Zussman achieved upward electrospinning of fibers from multiple jets without the use of nozzles; instead using the spiking effect of a magnetic liquid [12]. The concept (Fig. 16-5) consists of a bath filled with a layer of magnetic liquid (a). This liquid is covered by the solution to be spun (b). An electrode is submerged into the magnetic fluid (d). A counter-electrode (c) as a collector is placed a certain distance above this bath. A strong permanent magnet or electromagnet (f) is placed under the bath around the electrode. When a magnetic field is applied, the spiking effect causes some of the polymer solution to protrude into the electrical field applied between the electrodes (c and d). The protrusion is sufficient to initiate multiple jets of polymeric fibers traveling towards the collector. The production rate was reported to be about 12 times that of a conventional set-up. This approach also avoids clogging problems.

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Figure 16-5.

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

Using supercritical CO₂-assisted electrospinning, polymer fibers of high molecular weight polydimethylsiloxane (PDMS) and poly(D,L-lactic acid) (PLA) were produced by means of only electrostatic forces and without the use of a liquid solvent. The fibers were formed between two electrodes in a high pressure view cell. This supported the idea that the supercritical CO₂ reduces the polymer viscosity sufficiently to allow fibers to be pulled electrostatically from an undissolved bulk polymer sample.

Electrospinning in a vacuum is also a novel set-up. Compared to electrospinning in air, a vacuum allows higher electric field strength over large distances and higher temperatures compared to what can be achieved in air, which influences both the spinning process and the morphology of the fibers that are produced. Other attempts have been made to incorporate vibration technology in polymer electrospinning. The idea is to produce finer nano-fibers under lower applied voltage by vibration technology. Other electrospinning set-ups are discussed in a recent review by Teo and Ramakrishna [6].

Set-ups Involving Dual Syringes

A set-up was developed for electrospinning involving a dual syringe spinneret (Fig. 16-6). The development enables spinning highly functional nano-fibers such as hollow nano-fibers, nano-tubes and fibers with a core-shell structure [13]. A recent study describes the formation of hollow nano-tubular fibers in a single step using electrospinning and sol-gel chemistry. The method exploits electrohydrodynamic forces that form coaxial jets of liquids with microscopic dimensions. A high voltage is applied to a pair of concentric needles used to inject two immiscible liquids that lead to the formation of a two-component liquid cone that elongates into coaxial liquid jets and forms hollow nano-fibers.

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Figure 16-6.

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

Set-ups Controlling the Orientation and Alignment of Micro-nano-fibers

A number of set-ups that allow control over the orientation of fibers have been developed. The orientation is crucial for different applications of nano-fibers and opens new opportunities for manufacturing yarn, micro-nano-wire devices, etc. Most of the set-ups are based on rotating collection devices.

A technique called dry rotary electrospinning involves the organization and alignment of electrospun nano-fibers into planar assemblies [14]. The technique (Fig. 16-7) involves a rotating disc as a grounded collector that stretches the coils into aligned rings. The dry fibers in a ring shape can be collected into linear strands to form a nano-fibrous yarn.

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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).

Another method for controlled deposition of oriented nano-fibers uses a micro-fabricated scanned tip as an electrospinning source [15]. The tip is dipped in a polymer solution to gather a droplet as a source material. A voltage applied to the tip causes the formation of a Taylor cone and, at sufficiently high voltages, a polymer jet is extracted from the droplet. By moving the source relative to a surface, thus acting as a counter-electrode, oriented nano-fibers can be deposited and integrated with micro-fabricated surface structures. This electrospinning technique is called a scanned electrospinning nano-fiber deposition system. In addition to achieving uniform fiber deposition, the scanning tip electrospinning source can produce self-assembled composite fibers of micro- and nano-particles aligned in a polymeric fiber.

Using a frame as a countered electrode also allows an oriented deposition of fibers [16]. The same effect can be accomplished by placing two electrodes parallel to each other that are separated by a void [17]. A modified method for electrospinning that generates uniaxially aligned arrays of nano-fibers over large areas has also been reported. A collector composed of two conductive strips separated by an insulating gap of variable width was used. Directed by electrostatic interactions, the charged nano-fibers are stretched to span across the gap and become uniaxially aligned arrays. Two types of gaps were demonstrated: void gaps and gaps made of a highly insulating material. When a void gap was used, the nano-fibers could readily be transferred onto the surfaces of other substrates for various applications. When an insulating substrate was involved, the electrodes could be patterned into various designs on the solid insulator. In both cases, the nano-fibers could be conveniently stacked into multi-layered architectures with controllable hierarchical structures.

Zussman et al. reported an approach to a hierarchical assembly of nano-fibers into crossbar nano-structures [18]. The polymer nano-fibers are created through an electrospinning process with diameters in the range of 10–80 nm and lengths up to centimeters. When the electrostatic field and the polymer rheology of the nano-fibers are controlled, they can be assembled into parallel periodic arrays. These authors also observed failure of nano-fibers owing to a multiple necking mechanism, sometimes followed by the development of a fibrillar structure, during electrospinning using a rotating tapered accumulating wheel (electrostatic lens). This phenomenon was attributed to a strong stretching of solidified nano-fibers by the 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 the production of oriented, continuous nano-fibers such as using copper wires spaced evenly in the form of a circular drum as a collector and the use of a rotating wheel. In particular, the work by Theron and co-workers described an electrostatic field-assisted assembly technique that was combined with an electrospinning process used to position and align individual nano-fibers on a tapered and grounded wheel-like bobbin [19]. The bobbin is able to wind a continuous as-spun nano-fiber at its tip-like edge. The alignment approach resulted in nano-fibers with diameters ranging from 100–300 nm and lengths of up to hundreds of microns.

Characteristics and Design Considerations of Electrospun Micro-Nano-Fibers

Molecular Orientation

In traditional fiber spinning, the molecular orientation obtained by stretching the fibers after their formation is critical for their strength. The molecular orientation of electrospun fibers has also been a subject of various studies.

Dersch and co-workers studied the intrinsic structure of polyamide (nylon 6) and PLA electrospun fibers [16]. They found that the fibers do not differ a great deal from as-spun thicker fibers obtained by melt spinning and showed rather disordered crystals and different degrees of crystal orientations. 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 to an increased orientation of the crystals along the fibers’ axis. On the other hand, the electrospinning of polyethylene oxide (PEO), for example, causes some molecular orientation but a poorly developed crystalline micro-structure.

Collecting electrospun fibers onto a high speed rotating drum can enhance the molecular orientation up to an optimal speed, after which the orientation can decrease slightly [20]. In the report of a study using a high speed winder, it was suggested that a critical winding speed exists that just matches the ‘natural’ velocity of the fiber (due to electrohydrodynamic forces) and that additional drawing of the fiber should occur for higher winding speeds. This work concluded that the degree of molecular orientation, which develops only due to electrohydrodynamic forces and, hence, would be expected in non-woven electrospun fabrics, is quite low.

Shapes and Sizes

In addition to circular fibers, a variety of cross-sectional shapes and sizes can be obtained from different polymers during electrospinning. Koombhongse and co-workers actually obtained branched fibers, flat ribbons, ribbons of other shapes and fibers that were split longitudinally from larger fibers in electrospinning a polymer solution [21]. Studies of the properties of fibers with these cross-sectional shapes from a number of different kinds of polymers and solvents indicate that effects of the fluid mechanics, the electrical charge carried with the jet and evaporation of the solvent all contributed to the formation of the fibers.

Sung and Gibson used polycarbonate in another study [22]. Electrospun fibers created in this process showed a wrinkled structure that was found to depend on the rate of evaporation of the solvent from the surface related to the rate of evaporation from the core. Indeed, as the solvent on the surface evaporated and a ‘skin’ formed, the solvent entrapped in the core diffused into the ambient atmosphere and caused what they called a ‘raisin-like structure’. A rapid evaporation of solvent from the jet that creates a skin, as mentioned above, can in fact give rise to hollow fibers that can collapse into a ribbon.

Alterations of Secondary Structure and Functionality

The electrospinning process is highly versatile and allows not only the processing of many different polymers into polymeric nano-fibers but also the co-processing of polymer mixtures and mixtures of polymers and low molecular weight non-volatile materials. This is done simply by using ternary solutions of the components for electrospinning to form a combination of nano-fiber functionalities. Polymer blends, core-shell structures and side-by-side bicomponent electrospinning are growing research areas that are connected with the electrospinning of multi-component systems. The targets are either to create nano-fibers of an ‘unspinnable’ material or to adjust the fiber morphology and characteristics.

The option of spinning a polymer blend renders possible the creation of core-shell nano-fibers through phase separation as the solvent evaporates. Another method for creating a structure of this kind is to co-electrospin two different polymer solutions through a spinneret consisting of two coaxial capillaries (see Fig. 16-6). Nano-fibers with hollow interiors are used in several applications, such as nano-fluidics and hydrogen storage. Electrospun tubular fibers can also be used as sacrificial templates.

The electrospinning technique also provides the capacity to lace together a variety of types of nano-particles or nano-fillers to be encapsulated into an electrospun 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 in the initial polymer solutions, which are then electrospun to form composites in the form of continuous nano-fibers and nano-fibrous assemblies.

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Figure 16-8.

SEM images of (a) electrospun poly(ethylene terephthalate) (PET) nanofibers containing encapsulated molecular imprinted 17β-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).

Another interesting aspect of nano-fiber processing is that it is feasible to modify not only their morphology and their (internal bulk) content but also their surface structure in order to carry various chemically reactive functionalities. Thus, nano-fibers can be easily post-synthetically functionalized, for example by using plasma modification, physical or chemical vapor deposition (PVD, CVD) and chemical modifications such as cross-linking or grafting. By varying the processing parameters, it is also possible to produce fibers with unique surface features and secondary structures such as micro-textured/nano-porous fibers and micro-nano-webs.

Applications of Electrospun Functional Micro-Nano-Fibers

Electrospun micro-nano-structures are a class of novel materials that is exciting because of several of the unique characteristics discussed above. Significant progress has been made in this field in the last few years, and the resulting micro-nano-structures may serve as a highly versatile platform for a broad range of important technological applications in areas such as biomedicine, pharmacy, sensors, catalysis, filter, composites, ceramics, electronics and photonics. Some of the most recent developments in their processing and the relevant applications that are considered are presented below.

Biomedical Applications

Tissue Engineering

Electrospun 3D nano-fibrous structures meet the essential design criteria of an ideal tissue engineered scaffold based upon their unique action in supporting and guiding cell growth [24]. Most studies confirm that the electrospun nano-fibrous structure is capable of supporting cell attachment and proliferation (Fig. 16-9) [25]. The structure features a morphological similarity to the extracellular matrix of natural tissue, which is characterized by a wide range of pore diameter distribution, high porosity and effective mechanical properties.

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Figure 16-9.

(a) SEM image showing fibroblast (human MRC-5) extension wrapping around the electrospun nano-fiber. (b) TEM image showing a fibroblast extension in close contact with electrospun Artelon^® nano-fiber [25].

Nano-fibers have been studied for engineering cardiovascular tissues such as heart tissue constructs and blood vessels. Ramakrishna's group published several articles on the use of nano-fibers as a scaffold for blood vessels and looked at the influence of fiber diameter, orientation and other parameters on cell proliferation [26]. Nano-fibers made of poly(L-lactid-co- var epsilon -caprolactone) P(LLA-CL) or poly(ethylene terephthalate) (PET) were primarily used.

Biomimetism towards human ligament has been considered, and the effects of fiber alignment and direction of mechanical stimuli on the extracellular matrix (ECM) generation of human ligament fibroblast (HLF) was studied [27]. An elastic biodegradable material in a tubular form was produced by combining polylactide with cross-linked elastin [28]. The tubular material obtained showed excellent mechanical properties equal to those of blood vessel and peripheral nerve tissue.

Nano-fibers are potential structures for bone tissue engineering. Yoshimoto et al. used poly( var epsilon -caprolactone) (PCL) scaffolds to grow mesenchymal stem cells (MSCs) derived from bone marrow [29]. Polylactide combined with cross-linked elastin shows a potential for neural applications [28]. The regeneration of peripheral nerve axons was observed in transplantation using a rat model with sciatic trauma. Silk-like polymers with fibronectine functionality (extracellular matrix proteins) have been electrospun to make biocompatible films for use in prosthetic devices intended for implantation in the central nervous system [30].

Wound Dressings and Healing

Electrospun nano-fibrous membranes can be used in the production of novel wound dressings. These membranes are particularly important because of their favorable properties, such as high specific surface area, combined with antibacterial and drug release functionality. Recent studies support that nano-fibrous dressings promote hemostasis, have better absorptivity, semi-permeability and conformability and allow scar-free healing [31]. The nano-fibrous membrane also shows controlled evaporative water loss, excellent oxygen permeability and promoted fluid drainage ability, but it can still inhibit exogenous microorganism invasion because its pores are ultra-fine. Histological examinations also indicate that the rate of epithelialization is increased and that the dermis becomes well organized when wounds are covered with electrospun nano-fibrous membrane.

A nano-fiber mat made of fibrinogen, a soluble protein that is present in blood, has been produced by electrospinning [32]. The mat could be placed and left on a wound, thereby minimizing blood loss and encouraging the natural healing process. Fibrinogen increases the ‘stickiness’ of clotting cells, thickens the blood and promotes the formation of fibrin (the stringy protein that forms the basis of blood clots). Electrospinning can also be used to create biocompatible, thin films with a useful coating design and a surface structure that can be deposited on implantable devices in order to facilitate the integration of these devices in the body.

Drug Carrier and Delivery Systems

Electrospun fiber mats have also been explored as drug delivery vehicles, with promising results. The application of electrostatic spinning in pharmaceutical applications resulted in dosage forms with useful and controllable dissolution properties. For instance, hydroxy propoxy methylcellulose (HPMC), a cellulose derivative commonly used in pharmaceutical preparations, together with the drug has also been tested [33]. Poly-(L-lactic acid) (PLLA) and poly(D,L-lactide-coglycolide) (DLPLGA) nano-fibers are other polymers that have been electrospun with an encapsulated drug and have shown promising drug release properties. Incorporation of an antibiotic in fibers developed for scaffold applications has also been reported. The combination of mechanical barriers based on non-woven nano-fibrous biodegradable scaffolds and their capability for local delivery of antibiotics makes them desirable for applications in the prevention of post-surgical adhesions and infections.

Micro-nano-fibers as Support for Enzymes and Catalysts

Electrospun micro-nano-fibers are an attractive class of supports for enzymes and catalysts due to their ultra-thin sizes and large surface areas. Reneker and co-workers demonstrated the possibility of using nano-fibers for the immobilization of enzymes, showing catalytic efficiency for biotransformations [34]. Enzyme-modified nano-fibers of PVA and PEO achieved by loading the enzymes, i.e. casein and lipase, into the polymer solutions have also been reported. The membranes with encapsulated enzymes were six times more reactive than cast films from the same solutions.

Investigations have been made of the catalytic activity of nano-fibers obtained by incorporating catalysts. For instance, the incorporation of palladium (Pd) nano-particles has been studied in detail using carbonized and metal oxide nano-fibers [35].

Generation of Micro-nanomechanical and Micro-nano-fluidic Devices through Electrospinning

As mentioned earlier, electrospun micro-nano-fibers can serve as sacrificial templates for the generation of micro-nano-structures with hollow interiors. Czaplewski and co-workers prepared nano-fluidic channels [36]. The channels obtained were elliptical and presented no sharp corners, as in conventional lithographic techniques, which promotes a smoother fluid flow through them. Furthermore, the spin-on glass is optically transparent and compatible with chemical analysis, thereby opening applications in biomolecular separation and single molecule analysis. They also demonstrated the use of these templates for the fabrication of micro-electromechanical devices, such as nano-scale mechanical oscillators.

Deposits of oriented poly(methyl methacrylate) nano-fibers, combined with contact photolithography, created silicon nitride nano-mechanical oscillators with dimensions in the order of 100 nm. The fibers were used as etch masks to pattern nano-structures in the surface of a silicon wafer. The oriented polymeric nano-fiber deposition method that was used in this experiment offers an approach for rapidly forming arrays of nano-mechanical devices, connected to micro-mechanical structures, that would be difficult to form using a completely self-assembled or completely lithographic approach. This approach may provide a useful method for realizing nano-scale device architectures in a variety of active materials.

Furthermore, magnetite nano-particles were incorporated as a colloidally stable suspension into polyethylene oxide or polyvinyl alcohol solutions [37]. After electrospinning, the nano-particles were aligned along the fibers’ axis. These nano-fibers exhibited superparamagnetic behavior and deflected when subjected to a magnetic field at room temperature. A micro-aerodynamic decelerator based on permeable surfaces of nano-fiber mats was reported by Zussman and Yarin [38]. The mats were positioned on light, pyramid-shaped frames. These platforms fell freely through the air, apex down, at a constant velocity. The drag of this kind of passive airborne platform is of significant interest in a number of modern aerodynamics applications including, for example, dispersion of ‘smart dust’ carrying various chemical and thermal sensors, dispersion of seeds, and movement of small organisms with bristle appendages.

Micro-nano-fibers in Sensors

Recent advances in micro-nano-technology and the electrospinning technique offer great potential for the construction of cost-effective, next-generation chemical and biosensor devices. The high surface area per volume unit makes electrospun micro-nano-structures great candidates for a variety of sensing applications as they can offer high sensitivity and response time. These sensors can find applications in medical diagnosis and environmental and bioindustrial analysis, among others [1] and [23].

Conducting electroactive polymers have remarkable sensing applications because of their ability to be reversibly oxidized or reduced by applying electrical potentials. For biosensing applications, conducting electroactive polymers combine the role of a matrix immobilization template and the generation of analytical signals. The most common conducting electroactive polymers include polypyrrole, polyaniline and polythiophene and are characterized by an electronic conductivity of up to 10⁴ Ω⁻¹. Resistive-type sensors made from undoped or doped polyaniline nano-fibers outperform conventional polyaniline on exposure to acid or base vapors, respectively [39].

Electrospinning of lead zirconate titanate, Pb(Zr_xTi_1-x)O₃ (PZT) fibers should be mentioned because of its technological importance in the field of sensors, electronics and non-volatile ferroelectric memory devices. PZT is one example of one-dimensional nano-structures, the smallest dimension structures for efficient transport of electrons and optical excitation, that can be used as building blocks in a bottom-up assembly in diverse applications in nano-electronics and photonics [40]. Wang et al. showed that ultra-fine PZT fibers could be synthesized from metallo-organic compounds simply by using metallo-organic decomposition (MOD) and vacuum heat treatment electrospinning techniques [40].

Other developments are electrospun nano-fibers of polyvinylpyrrolidone (PVP) containing the urease enzyme that show a potential as a urea biosensor and nano-fibers coated with metal oxides (TiO₂, MoO₃) for the detection of toxic gases. Molecular imprinted nano-fibers with selective molecular recognition ability and a chemosensor material with a high surface area obtained by electrospinning a fluorescent conjugated polymer have also been developed [23] and [41].

Micro-nano-fibers in Electric and Electronic Applications

Electrospun nano-fibers with electrical and electro-optical activities have received a great deal of interest in recent years because of their potential application in nano-scale electronic and optoelectronic devices, such as nano-wires, LEDs, photocells, etc. Lead zirconate titanate (PZT) and carbon nano-fibers are two typical and challenging examples of one-dimensional nano-structures that can be used as building blocks in bottom-up assembly in diverse applications in nano-electronics and photonics [40].

Studies support that electrospinning can be a simple method for fabricating a one-dimensional polymer field-effect transistor (FET), which forms the basic building block of logic circuits and switches for displays [42] and [43]. In addition, the excellent adherence of the nano-fibers to SiO₂ and to gold electrodes may be useful in the design of future devices. By means of electrospinning processing, extremely low dimensional conducting nano-wires have been made from, e.g., polyaniline or polypyrrole for use in nano-electronics (Fig. 16-10) [44].

Full-size image (41K)

Figure 16-10.

SEM micrograph of conductive polypyrrole nano-fibers. The nano-fibers were electrospun from a solution of ((PPy3)⁺(DEHS)⁻)_× in DMF [44].

Other studies report the development of carbon nano-fiber webs from the oxidation and steam activation of a polyacrylonitrile (PAN) nano-fiber web for use as an electrode in a supercapacitor [43], poly(vinylidene fluoride) (PVDF) nano-fibers for applications as a separator or as an electrolyte in batteries [45] and fabrication of a lithium secondary battery comprising a fibrous film made by electrospinning [46].

Electrospinning mixtures of ceramic particles with polymers and subsequent pyrolysis of the polymer to form pure ceramic nano-fibers is an area of intense research [7]. Because of their large surface-to-volume ratios and narrow-band optical emission, these nano-fibers can be used as selective emitters for thermophotovoltaic applications and as emitting devices in nano-scale optoelectronic applications.

Micro-nano-fibers in Filters

The efficiency of nano-fibers in filtration has been studied by several groups. Generally, the electrospun webs have been found to be much more effective than other commercial high-efficiency air filter media. In most cases, the nano-fiber webs are applied on a substrate chosen to provide mechanical properties, while the nano-fiber dominates the filtration performance. Electrospinning can also be used to produce charged fibers for use in filtration media. Obviously, the charge induction and charge retention characteristics are related to the polymer material used for electrospinning.

Controlling the parameters of electrospinning allows the generation of micro-nano-fiber webs with different filtration characteristics. A study done by Schreuder-Gibson and Gibson showed that it is possible to tailor pore size, air permeability and aerosol filtration of elastic non-woven media by applying very light-weight layers of electrospun elastic fibers to the coarser webs [47]. It has been found that a significant deformation of the elastic webs increases air flow, and it might be possible to design controlled flow filters or air bags that are modulated by a pressure drop across elastic webs with correspondingly variable porosities. Many filtering applications of electrospun micro-nano-fibers are related to air filtration, but liquid filtration can also occur [48].

Moreover, ion exchange materials (such as resins, membranes, etc.) have been widely used in various industries for water deionization or softening, metal recovery, biological process, food and beverages, pharmaceuticals and fuel cell applications. Polymer nano-fiber ion exchangers are new, promising materials as they have a much higher surface area than common ion exchangers.

Micro-nano-fibers in Textiles

Electrospun micro-nano-membranes composed of elastomeric fibers are of particular interest in the development of several protective clothing applications. The excellent ability to capture aerosols and the possibilities to incorporate any kind of active substances make electrospun nano-fiber materials potential candidates for use in protective clothing and smart cloths responding to changes in the surrounding environment. Much work is being done with the aim to develop garments that reduce soldiers’ risks for chemical exposure [49]. The idea is to lace several types of 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 convective resistance to air flow while the transport of water vapor is much higher than in normal clothing materials. These products exhibit remarkable ‘breathing’ properties, which are now required in clothing applications.

In some other uses of protective clothing, thermal and flammability properties are essential [50]. Electrospun poly(methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA)) and its layered silicate nano-composites have shown good thermal stability, reduced flammability and increased self-extinguishing properties. The possibility of using sub-micron and nano-scale fibers and fibrous assemblies based on conductive PEDOT for wearable electronics has also been explored [14]. Finally, the development of electrospinning apparatuses for fiber orientation that allow the fabrication of yarns is of considerable interest [51] ([Figure 16-9] and [Figure 16-10]).

Micro-nano-fibers as Composite Reinforcement

The strength of a composite material is effectively enhanced by fiber-based reinforcement. Thus, the high surface-to-volume ratio of nano-fibers significantly improves the stiffness and mechanical strength of the composites compared to conventional fibers due to the increased interaction between the fibers and the matrix [52]. Another positive aspect is that the composites are able to maintain their optical transparency related to the small cross-section of the nano-fibers.

Attempts to Increase the Production Rate of Electrospun Micro-nano-fibers

Electrospinning using Multiple Nozzles

The most obvious way to increase the rate of production of micro-nano-fibers is to increase the number of nozzles used in the spinning process. In a patent, Chu et al. described an electrospinning apparatus with multiple nozzles, as shown in Fig. 16-11 [53]. The essential invention in this patent was not the use of multiple nozzles but the possibility to better control the jet formation, jet acceleration and fiber collection for individual jets. This was achieved using several additional electrodes to homogenize the electric field that accelerates the jets from the nozzles to the collector. The possibility of controlling both the flow of the conducting fluid (polymer solution or melt) and the properties of the electric field for each jet was described as essential for producing nano-fibers using multiple nozzles. A later patent by the same group focused on controlling multiple fiber jets as opposed to individual jets or adding the possibility of blowing a temperate gas in the fiber spinning direction. The gas flow gives a higher production rate than traditional electrospinning, as well as lower energy consumption.

Full-size image (45K)

Figure 16-11.

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

Electrospinning without Nozzles

Perhaps the most successful way to increase the electrospinning production rate that can be recognized thus far is the Nanospider™ technology, patented by O. Jirsak et al. [54]. This technology is now owned by ElMarco (Czech Republic). Instead of using capillaries as a spinneret for introducing the polymer solution into the electric field, a rotating charged electrode is used that is partly immersed into the polymer solution. This set-up allows the creation of many Taylor cones and hence many jets that travel upwards to a conveyor belt that can be covered with a material to be coated.

A great advantage of this method is the possibility to create multiple jets of nano-fibers without the risk of the nozzles clogging. Another advantage of the technique is that the spinning direction is upwards, which minimizes the risk of solution droplets forming in the product. The process is schematically shown in Fig. 16-12.

Full-size image (28K)

Figure 16-12.

(a) Schematic drawing of the Nanospider™ technology [54]. (b) Picture of the Nanospider™ technology in action

(from www.nanospider.cz).

The polymer solution (2) is applied to the charged cylindrical electrode (3) as it rotates partly immersed in the solution. Multiple fiber jets are formed from the surface of the electrode towards the oppositely charged electrode (40). The fibers are drawn to the electrode (40), not only by the electric force but also due to the action of a vacuum chamber (5). The patent also covers rotating cylindrical electrodes with different patterned surfaces. Using this technology, ElMarco claims that they will have a production capacity of 3000 m²/day (1 m in width) (2006).

Another approach to spinning nano-fibers without nozzles is to use a porous tube of polyethylene (Fig. 16-13), as reported by Reneker et al. [55]. By applying air pressure to a polymer solution inside a cylindrical porous tube, these authors were able to form multiple jets of polymer solution in the electric field. With this porous tube, the production rate could be increased to about 250 times that of the corresponding production rate for a single needle.

Full-size image (18K)

Figure 16-13.

Schematic illustration of the electrospinning set-up using a porous polyethylene tube [55].

Recently, a new technology with the use of centrifugal forces has been developed and patented from Swerea IVF [56]. The process is schematically shown in Fig. 16-14.

Full-size image (40K)

Figure 16-14.

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

Commercial Products

It should be noted that some companies already have commercial products based on electrospun nano-fibers. One of the first companies to start an industrial production line of nano-fiber web is NanoTechnics Co., Ltd, in Korea. The company offers nano-fiber webs of PA6 and PA66 for application in filters and PAN for electrodes in batteries. Other companies that claim to be able to electrospin webs for use in filters are Hollingsworth & Vose, Germany, and eSpin Technologies, USA. Donaldson Company Inc., USA, is also an important actor in the field of nano-fiber-based filters. Donaldson has several US and international patents that cover nano-fiber innovations, configurations and uses.

Technological Competitiveness And Operation Economics

Electrospinning is a very simple and versatile method for creating polymer-based, high functional and high performance micro-nano-fibers that can revolutionize the world of structural materials. The process is versatile in that there is a wide range of materials that can be spun (Table 16.1). At the same time, electrospun micro-nano-fibers possess unique and interesting features. The ability to customize micro-nano-fibers to meet the requirements of specific applications gives electrospinning an advantage over other larger-scale micro-nano-production methods. Combining well-established technologies of today with the emerging field of electrospun micro-nano-fibers can potentially lead to the development of new technologies and new micro-nano-structured smart assembles and stimulate opportunities for an enormous number of applications. Thus, electrospinning technology can provide a connection between the worlds of the nano-scale and the macro-scale.

Table 16.1 Examples of Some Polymer-Biopolymer Materials that have been Electrospun and 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(-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

Another advantage of this top-down micro-nano-manufacturing process is its relatively low cost compared to that of most bottom-up methods. The electrospinning method itself is environmentally friendly because it consumes only a small amount of electrical energy. In spite of the high potential difference (10,000–40,000 V) that is applied, only a small electrical current flows through the nano-fibers (in the order of nano-amperes). In addition, the electrospinning method provides nano-fiber structures that imply a large reduction in material consumption. For instance, the formation of a true nano-coating (monolayer-like) will result in a thickness of only a few nanometers, while current coatings have a thickness of a few micrometers. This means that the consumption of materials is also about 1000 times less for nano-coatings while it results in the same surface properties as are obtained with micro-coatings. Moreover the resulting micro-nano-fiber samples are often uniform and continuous and do not require expensive purification (unlike submicrometer-diameter whiskers, inorganic nano-rods and carbon nano-tubes). Overall, despite the existence of some commercial products, it is evident that an upscaling of the electrospinning process and productivity improvements are essential features and merit more effort to ensure full success in socio-economic terms.

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