CHAPTER 1: Electrical Spinning to Electrospinning: a Brief History

Electrospinning technology has progressed a long way, from certain fundamental concepts to substantial industrial applications. Electrospinning is an electrohydrodynamic process resulting in the movement of a fluid by means of the application of an electrostatic field. This movement of liquids by electrostatic force was first observed by William Gilbert, sometime personal physician to Queen Elizabeth I of England and president of the Royal College of Physicians. In about 1600, Gilbert¹  first recorded the movement of liquid under the influence of a triboelectric field produced by rubbing amber. Due to friction-induced charge separation, the amber is electrified. The rubbing or friction of some material with another material to produce a separated charge was widely known, having said to have been first observed by the Greek philosopher, Thales of Miletus.²  The use of amber (in ancient Greek ἤλεκτρον or ēlektron) gave rise to the coinage by Gilbert of the word ‘electricus’, which in turn lead to the terms ‘electric’ and ‘electricity’. When the rubbed amber was placed in proximity to a water droplet placed on a dry surface, the water droplet distorted under the influence of electric charge into a diagnostic conical shape that later became known as the Taylor cone.³  William Gilbert observed:

Indeed it plainly does draw the body itself in the case of a spherical drop of water standing on a dry surface; for a piece of amber applied to it at a suitable distance pulls the nearest parts out of their position and draws it up into a cone; otherwise, if it were drawn by means of the air rushing along, the whole drop would have moved.¹ 

Before the invention of reliable methods of generating low-voltage, high-current electricity, a considerable amount of fundamental work on electricity using high-voltage electrostatics was undertaken by workers such as Otto von Guericke, Stephen Gray, Charles Francois Dufay and Georg Mathias Bose.

In 1749, French clergyman and physicist Jean-Antoine Nollet⁴  observed another electrohydrodynamic phenomenon, namely how electrifying water can cause the formation of droplets, a process now known as electrospraying. None of this early work resulted in the development of recognizable processing technology for either electrospraying or electrospinning. The efforts of these early workers were focused on the description and understanding of these phenomena. The concept of an artificial fiber dates from this period, being proposed by the natural philosopher Robert Hooke. Hooke⁵  said that to succeed in producing artificial fibers from an unnamed ‘glutinous composition’, “very quick ways of drawing it out into small wires for use could be found”.

There were also parallel attempts to store the charge. The storing of electrostatic charge was first experimentally achieved using a glass jar (known as a Leyden jar), coated inside and out with metal foil. The Leyden jar form of the capacitor was developed by Pieter van Musschenbroek, Professor of Physics at Leiden University, but the credit for performing this experiment is often attributed to the German scientist, Ewald G. von Kleist, in 1745.⁶ 

In electrospinning, electrostatic charge provides the motive force for the production of fibers. The speed of a fiber in flight is commonly between 60 and 100 m s⁻¹, but the mass rate of production from a single spinneret is typically measured in mg per hour. The electrostatic charge that is engendered in the fluid by applying a high voltage causes the fluid to overcome the surface tension that holds a pendant droplet in shape. Ultimately the surface of the droplet fails and droplets or – if there is sufficient molecular cohesion – a fiber are ejected from the droplet. This form of use of electrostatic charge is applied in various fields in addition to electrospinning. Examples include photocopying, laser printing, electrostatic painting and acoustic transducers, including high-fidelity loudspeakers, particle separators, and electrostatic filters.⁶ 

The establishment of the electrohydrodynamic process of electrospinning as a commercial application is the result of work by a number of researchers and manufacturers. It should be noted that the work by the academic community has been of most use in understanding the parallel efforts of commercial companies to develop the spinning process. Electrospinning is but one of a number of artificial fiber manufacturing processes that were developed during the 19th century.

The need for large quantities of reliable fiber for the production of woven and embroidered goods during the Victorian period was an early driver for the production of artificial fibers. At that time, machine embroidery was mainly used for decorative furnishings. In 1828, Josué Heilmann developed commercial machine fiber combing and stressed the importance of improvements to profitability that this process offered for large-scale production; the machine was patented in 1829 in England. Joshua Wordsworth, the owner of a weaving mill who took out a patent on the machine, and in turn established a parallel arrangement with Manchester silk manufacturer Louis Schwabe for fabricating embroidered goods. Louis Schwabe devised a process for spinning glass fiber, but was unable to find a more suitable spinning material for fabric production,⁷  although he put considerable work into “carrying out experiments which would lead to the discovery of a substance which would form a homogeneous mass possessing the quality of ductility and susceptible of being drawn out through fine holes, or otherwise, into filaments or fibers possessing suitable strength and other properties to adapt it for manufacturing purposes”.

The invention of a potential spinning formulation by George Audemars⁸  in 1855, of suitable strength and suitable for the manufacturing process, had been demonstrated in concept by Schwabe. Audemars dipped needles into a nitrocellulose solution, then drew it into air, obtaining a long durable thread as the solvent evaporated. The manually applied dragging force pulling the fibers from the polymer solution was low, but the process was very slow and hence only suitable for small-scale production. Cellulose was first nitrated to a high degree by the Swiss chemist Christian Schönbein, in 1845, who patented his process for the production of the explosive known as guncotton. In 1846 Louis-Nicolas Ménard and Florès Domonte dissolved cellulose nitrate in a mixture of ether (ethoxyethane) as the solvent and ethanol as a diluent to produce a solution that became known as collodion. The expedition of generating fibers continued. Count Hilaire De Chardonnet, a French engineer and industrialist, forced a solution of cellulose derivatives through small holes like a spinneret, then dried these in air and further treated the fibers with a coagulating medium: for this work he became known as the ‘father of the artificial silk industry’.⁹  His artificial silk was sold as ‘Chardonnet silk’. Chardonnet silk was the first artificial semi-synthetic fiber. The invention was exhibited by Chardonnet in 1889, but pre-dated in 1855 by Joseph Wilson Swan, the British chemist who produced fibers for electric light filaments from ‘parchmentized’ cotton, and who went on by 1881 to develop and patent a method for extruding nitrocellulose to form conducting fiber filaments – this material was used to produce incandescent light bulbs by the Edison & Swan United Electric Light Company.

The next significant development was cuprammonium silk. In 1857, Eduard Schweizer investigated the solubility of cellulose in an ammoniacal solution of copper hydroxide (cuprammonium hydroxide solution). This observation was applied to the fiber production process in 1890 by L. H. Despaissis, who regenerated a solution of cellulose into a solid¹⁰  by treating a cuprammonium solution of cellulose in a coagulating bath. Although the process was patented, it was not successfully commercialized. Two years later, in 1892, Dr Max Fremery and Johan Urban used a development of the process to make lamp filaments: the first profitable outcome of manufacturing cellulose fibers from a cuprammonium solution. At first only short discontinuous lamp filament fibers were produced. It took significant further work to produce continuous long fibers. Early in the market in 1908 with this material was J. S. Bemberg with Bemberg silk. Bemberg silk fiber was spun through a stretch spinning system developed by Dr Edmund Thiele and Emil Elsässer, in which a thick fiber was extruded, then stretched to a final thickness. Asahi Kasei in Japan is the only manufacturer still producing Bemberg fiber. Having started production of this fiber in 1931, by 2015 they had a production capacity of 17 000 tons per year. In parallel with the development of the stretch spinning system, another process for spinning the cellulose fiber known as ‘viscose’ was developed. This fiber-producing process was patented in 1893 by Cross and Bevan in England.

Over the last century, the basic process for producing the fibers has been evolved through many iterations. Wood pulp was reacted with sodium hydroxide solution and then the solution was treated with carbon disulfide to produce cellulose xanthate. This procedure was followed by addition of further caustic soda to dissolve cellulose xanthate into a spinnable viscous solution. The viscous solution¹¹  is pumped through a spinneret into a coagulation medium to produce a solid fiber. This fiber or thread is further treated with dilute sulfuric acid to decompose the xanthate and to regenerate the material into cellulose. As rayon, this regenerated cellulose fiber entered the USA textile market in 1920 and the manufactured artificial fiber is still widely used in numerous commercial applications.

The use of electricity for spinning fibers was attempted by Professor Charles Vernon Boys in 1888. Boys is noted for his invention of a number of scientific instruments. While making a torsion balance to determine the gravitational constant he required a ligament to suspend the moving part of the instrument. His first attempt to make such a fiber was recognizably electrospinning.¹²  He used a small insulated dish which was connected to a high-voltage electrical supply, and successfully spun from various melts such as beeswax, shellac, and collodion. This he described as “the old, but little known experiment of electrical spinning”. Unfortunately, none were strong enough for the purpose in hand, and he eventually achieved success by attaching a silica billet to a crossbow quarrel, heating the billet to soften it, and then firing the crossbow down the length of his laboratory: this is reasonably assumed to be the first successful attempt to produce a nano-scale fiber.

In the early 20th century, John Francis Cooley, an inventor and professional electrician of Penn Yan, New York, took out a UK patent (GB 6385) on 5th April 1900. His patent was titled Improved methods of and apparatus for electrically separating the relatively volatile liquid component from the component of relatively fixed substances of composite fluids.¹³  On 4th February 1902 and 24th November 1903 Cooley took out another two patents (US692631 and US745276). These patents covered methods and the apparatus for dispersion of fluids by means of electrostatic force. Direct current was used to charge the fluids, and spinnable fluids were dropped in between the electric field source and the grounded collector and fibers were collected by reeling onto a glass rod (Figure 1.1).

Cooley observed that when his diluent–solvent mixture (ether–ethanol) was modified with the addition of a third liquid (benzole), the spinnability of the fluid was enhanced. He also described spinneret systems for coaxial spinning, air-assisted spinning and rotating collectors (Figure 1.2).

W. J. Morton, an American physician working on the use of electricity to cure ailments, known as electrotherapy, patented a method for the dispersion of fluids (US 705691) in 1902. Dispersion of the fluid was assisted by indirect and direct electric charging methods. He did not attempt to commercialize the system, saying his “cobweb-like mass” of fibers “may be put to any industrial use”. Most electrospinning methods use the direct charging method proposed by Morton.

Professor Kiyohiko Hagiwara, of Kyoto College of Industrial Arts, Japan, worked on the manufacture of artificial silk and made significant contributions to rayon manufacturing technologies. In Japan, rayon manufacturing was advanced after the expiry of the patent for the basic viscose process for producing artificial silk in Europe.¹⁴  Hagiwara charged the colloidal liquid viscose solution before spinning the fibers. The electric charge was employed here to retain the aligned molecular structure of fibers, limiting the tendency towards fiber aggregation. The fiber obtained had better luster and Hagiwara patented the use of electricity during artificial production as US patent 169965 in January 1929. He also tried to increase the thickness of the fibers produced by combining electrostatic force with air pressure to increase the flow rate of spinnable fluid through the spinneret. His method used an 80 Hz oscillating voltage, presumably to avoid charge build-up in the product.

Anton Formhals made a large number of highly significant contributions to the development of electrospinning technology. He had 22 patents to his credit, taken out between 1929 and 1944, which covered numerous aspects of the electrospinning process. Formhals invented the saw-tooth emitter for distribution of spinnable fluid. The saw-tooth emitter (Figure 1.3) resembled a circular saw blade that dipped into the spinnable fluid, and emitted fibers from the saw-tooth points where the electrostatic field was strongest, and the curved surface of the liquid promoted the fracture of the droplet. The fibers were collected on a charged rotating collector.

Formhals developed a multi-head spinneret (Figure 1.4), which was filled up individually with spinnable fluids from an overhead tank and produced short fibers known as staple fibers gathered on a rotating notched-disc counter electrode. Further modifications for the collection of short fibers used an air blast. In another patent¹⁵  he proposed a conical, tapered nozzle for distributing spinnable fluid.

Electrospun staple fiber manufacturing was also proposed by Ernest K. Gladding while he was working for E. I. du Pont de Nemours and Company. In his patent (US 2168027), Gladding proposed his method for producing staple fibers. Unlike Formhals' method, where the short fibers were collected on a charged counter electrode in the method depicted in Figure 1.5, the collecting electrode is not conducting, with the counter electrode placed behind the collector. This means that the loosely attached fibers on the collector belt are easier to remove onto reels. Here also multi-head spinnerets or nozzles were used to produce fiber strands. The mechanism for collecting fibers in a multi-head spinneret machine is shown in Figure 1.5.

Charles Ladd Norton was born in Springfield, Massachusetts, in 1870. He graduated from the Electrical Engineering and Physics Department of MIT in 1893. He worked on fire-resistant materials and the exploitation of X-rays as a medical diagnostic tool with Francis Williams, at the Boston City Hospital. In his 1936 patent (US 2048651) and his posthumous 1940 patent (US 2185417), Norton described a method and apparatus for forming filamentous or fibrous material by electrospinning from a melt (Figure 1.6). Currently, this process is attracting commercial attention for wound dressings, filtration, textiles, nanomedicine and regenerative medicines.¹⁶  This process produces fibers without any risk of solvent carryover into the finished product.

The materials suggested for processing included gums, pitches, synthetic rosins and fused glass. The materials are held inside a refractory container, and one pole of a high-voltage electric source is connected to the container and the other pole of the source to a target screen, which is a flat plate or a cylinder. The distance between collector and container was changed depending on the material to be spun. Norton maintained the distance between 1 foot and 6 feet (0.3–1.8 m) for high melting point materials and for low melting point materials such as rosins, the distance was changed to 20 feet (6 m) or more. Fibers were formed by a combination of electrostatic repulsion and air blast.¹⁷  The material was extracted from the container through its tilted lip or spout, to encourage the formation of fine fibers rather than droplets or coarse fibers.

In the USSR the industrial production of air filters started in late 1939. The commercial production was the result of research performed in 1938. In 1938 two scientists, Igor’ Vasil'evich Petryanov-Sokolov and Natalya D. Rosenblum, generated electrospun fibers from a nitrocellulose feedstock, which they developed into what came to be known as ‘Petryanov filters’. This work won them the Stalin Prize, and rapidly led to the establishment of a works in Tver, manufacturing electrospun smoke filters used in gas masks. The Tver works is the first industrial-scale electrospun fiber factory. The material was spun from cellulose acetate in a solvent mixture of dichloroethane and ethanol.¹⁸  These filters are for protection against aerosols and airborne particulates.¹⁹  The USSR production capacity²⁰  of this filter reached 6.5 kg hours⁻¹. This work is particularly notable because it marks the first use of electrospun fiber as a non-woven, thus doing away with the difficulties associated with handling near invisible and highly charged fibers in further production processes.

F. W. Manning, a Canadian resident in the USA, took out a patent in 1943 ²¹  on producing beds of randomly orientated fibers described as “by the deposition and induration of disrupted filaments in a promiscuous and intersected condition”. His method was not intended to produces yarns but rather non-woven fabrics for uses as “sanitary napkins, surgical dressings, filtering purposes, leather substitutes, blankets, draperies, rugs, upholstering, insulating and most purposes for which woven fabrics are now used”. This neatly sidesteps the difficulties of handling highly charged nano-scale fibers for manufacture into handleable yarns. Manning's design used Cooley-type indirect charging, with the fibers cast onto the collector belt through a venturi.

H. R. Child patented an electrospinning process in 1944 while working for Eastman Kodak.²²  To address spinneret blocking, Child proposed the use of glass to make a non-conducting spinneret. Fiber collection is assisted by a directed current of warm air.

W. C. Heubner of Dayton, Ohio, USA, suggested preheating the spinning dope to speed up the solvent losses in flight.²³  He also suggested a modification to the spinneret to produce hollow fibers. He used a teardrop-shaped electrode inside his rotating drum collector to concentrate the electrostatic field to ease fiber removal.

In 1966, H. L. Simons, working for the Kendall Company in the USA, described in a patent the production of aligned fibers,²⁴  claiming that previous textile non-wovens are limited in application because they must be robust enough to cope with the bonding stage required to make the finished product, but by implying a minimum areal weight of “12 to 16 grams per square yard” he proposed to better this level by direct fiber deposition. The patterned deposition characteristic of his process is due to the use of a ‘segmented receiver’ with areas of different electric potentials. This is achieved using a rotating drum with a perforated surface as a collector – the fiber landing on the metal rather than the voids.

J. E. Owens and S. P. Scheinberg, working for du Pont de Nemours and Company, proposed a method called ‘flash spinning’ for making electrospun fibers.²⁵  The liquid raw material is heated “far above the boiling point of the solvent and at a pressure usually near the critical pressure of the solvent”. When the liquid leaves the spinneret, the solvent evaporates very rapidly to form what Owens and Scheinberg call a “plexifilamentary” non-directional fiber. Their adjustable spinning distance collector system is held at a non-zero controlled potential above earth. Oscillating baffles are deployed to evenly disperse the fiber by influencing an electrostatic field in the fiber flight path. They also seek to minimize corona discharge, to help achieve a uniform mat deposition. In the early 1970s this work was extended by Hollberg and Owens²⁶  by using the electrostatic wind from the spinning nozzle and a variation of the oscillating baffle system to improve the uniformity of mat deposition.

W. Simm, working for Bayer Aktiengesellschaft in Germany, devised a multiple spinning zone apparatus for making Petryanov-type filters, offsetting the position of the spinnerets to even out fiber deposition.²⁷  Multiple spinneret systems sometimes do not give an even deposition of fibers due to mutual repulsion between the fiber streams. The spinnerets are in the form of a rotating ring dipping into the spinnable dope. A curved liquid surface is required to initiate fiber production; the curve puts the liquid surface under tension, and in addition maximizes the electrostatic field strength, causing a stress concentration that results in rupture of the surface. Simm suggests that a “free flowing material” powder can be added to the surface of the fiber.

A group led by G. E. Martin at Imperial Chemical Industries in the UK patented a method for making prosthetic devices “such as an internal organ” using an earthed spinneret and a collector belt connected to a Van de Graaf triboelectric generator.²⁸  The method allows a number of fiber collection methods to make a layered structure typical of human tissues. Martin suggests the employment of shaped collector substrates, such as a face mask to produce finished articles, with the advantage of reducing difficulties associated with post-deposition fiber handling.

A. Bornat, also working for Imperial Chemical Industries, and the University of Liverpool, has proposed a method to electrospin prosthetic components.²⁹  The method uses the earthed spinneret technique previously described and a number of spinnerets to deposit fiber onto a rotating drum collector.

The collector has a rotating internal metal rod. The outer layer is collapsible to allow removal of the fiber product. The fiber can then be sintered to improve the strength of the prosthesis. Bornat later proposed that the fiber tube can be curved while still on the outer sheath and before sintering to fix the shape.

C. Guignard, working at the Batelle Memorial Institute in Geneva, Switzerland, devised a melt spinning method.³⁰  The solid polymer is deposited on a charged belt and passed through a heater. The polymer melt then passes under a secondary earthed belt, and the liquid polymer is drawn up as fiber onto another belt acting as the collector.

B. Kliemann and M. Stoll, working for Rhodia AG in Germany, proposed the extrusion of a polymer melt as a feed material for electrospinning.³¹  The molten strand is charged by a wire electrode inside the extrusion nozzle. The patent notes the charge on the fiber persists for up to 300 days after manufacture. It is likely that this persistent charge is due to the formation of an electret as the polymer change state while under the influence of a strong electrical field.

T. V. How, working with the University of Liverpool and Ethicon Inc. USA,³²  suggests varying the rotation rate of a drum collector between 2000 and 20 000 rev min⁻¹ to control orientation of the fiber product. This is presumably to match the surface speed of the drum to that of the terminal speed of the fiber (typically ranging from 60 to 100 m s⁻¹). The spinneret scans along the mandrel at speeds between 2 and 40 cm s⁻¹ to lay the fiber lay in a controlled fashion to vary the mechanical properties of the resulting tubular structure.

Bornat suggests controlling alignment of fibers in tubular structures³³  by means of an electrode array made of parallel bars placed at a small displacement from the collecting drum. This array is set at a voltage 4–5 kV less than the drum, the drum being the charged component and the spinneret is earthed.

Berry³⁴  describes a method that produces larger diameter fibers with a degree of orientation around the circumference of the cylinder, and embedded in a random orientated mat of smaller diameter fibers. He does this by means of two additional electrodes intended to bifurcate the fiber strand, meaning that one strand of the fiber has a longer flight path than the other, and will hence be drawn thinner. The bimodal fiber thicknesses result in a tube that resists buckling.

Doshi and Reneker's historic paper³⁵  marks a step-change in the rate of academic publishing and patenting. The process has gradually gained momentum in the USA, as well as Europe and the Pacific Rim. According to Barhate et al.,³⁶  more than 20 industries are currently engaged in the production of electrospun fiber-based filter production processes alone.

Today, electrospinning is a vibrant research field in materials science and engineering and is a promising lab- and industrial-scale technique to spin micro- and nano-scale materials. A SciFinder database search on 19 November 2017 found that over 4000 studies had been published since January 2017, and that there had been a significant increase (∼272 times) in the number of published reports between 2000 and 2017 (Figure 1.7). The outcome of these research studies is an expanding frontier that provides numerous options regarding spinning technologies, materials to spin, and applications. Depending on the target application, polymers, ceramics, or metals, as well as their composites, can be spun from the melt, solution, or emulsion, with each technique providing a distinct advantage. Melt electrospinning, for instance, ensures safety and high throughput, while solution electrospinning is more versatile with regards to electrospun materials and is more economical with respect to energy consumption. Compared to melt electrospinning, the solution process affords fibers with better optoelectronic and mechanical properties. Researchers usually resort to emulsion electrospinning if high melting point materials or composites with immiscible components are to be spun. Also, recent innovations in the field now enable encapsulation of functional materials into spun fibers to design nano-scale materials with wide-ranging implications for biomedicine, catalysis, the environment, and energy (Figure 1.8 and Table 1.1).

Currently, materials science-orientated research into electrospinning has evolved beyond fabricating textile yarns into diverse fields (Figure 1.8). In the biomaterials field, electrospinning is now explored to fabricate vectors for drug delivery,^37–41  platforms to heal wounds,^41–45  and scaffolds for tissue engineering.^46–50  Polymers including poly(lactic acid),^51–54  poly(lactic-co-glycolic acid),^55–60  poly(vinyl alcohol),^61,62  poly(vinylpyrrolidone),^63,64  polycaprolactone,^65–67  polyurethane,^68,69  and poly(3-hydroxybutyrate)^70–72  are classic materials that are electrospun into scaffolds for diverse biomedical applications. It is now feasible to encapsulate cells, growth factors or drugs into these polymers and electrospin the resulting composite into nano-scale scaffolds for tissue engineering. Recently, Ramakrishna demonstrated this concept, electrospinning a polycaprolactone-based core–shell nanofiber scaffold that encapsulates nerve growth factor, or bovine serum albumin for neural tissue engineering.⁷³  Ramakrishna and coworkers employed an emulsion electrospinning technique to design this random or aligned nanofibrous scaffold that guides the cultured cells to grow in a predetermined orientation. Release of the nerve growth factor from the scaffold enables rat pheochromocytoma cells to grow, differentiate, and express critical biomarkers. Characteristically, biodegradable polymers such as poly(lactic acid) and polycaprolactone are poor platform technologies for tissue engineering because they lack cell recognition sites, a property that limits cell affinity and adhesion. Recent innovative developments in electrospinning now endow these polymers with improved cell affinity and adhesion properties, positioning them as promising nanofibrous scaffolds for tissue engineering. An example is the enhanced mineralization, cell adhesion, and cell differentiation properties observed in poly(lactic acid) or polycaprolactone electrospun fibers when an ossein–hydroxyapatite complex, a natural bone extracellular matrix mimic, is incorporated into the polymer matrix before electrospinning the resulting composite.⁷⁴  It was shown that composites containing a bone extracellular matrix enhances the mechanical properties, for instance, the initial modulus and breaking stress, of the electrospun nanofiber.^75,76  A typical example is the incorporation of hydroxyapatite into tussah silk fibroin to form a composite, which is spun into nanofibers with improved initial modulus and breaking stress compared to neat tussah silk fiber.⁷⁶  Also, the nanofibers from the composite are more biocompatible towards osteoblast-like MG-63, and are more efficient in promoting cell adhesion, proliferation, and biomineralization than natural tussah silk fiber.⁷⁵  Also, it is now possible to tune critical properties such as morphology, Young's modulus and tensile strength via developing composites of polymers and other materials such as ceramics as precursors to electrospun biomaterial scaffolds. A recent report proved that ultrafine nanofiber spun from a composite of poly(l-lactic-co-glycolic acid), 10% tussah silk, and 1% graphene oxide has a smaller diameter and higher Young's modulus and tensile strength than that from poly(l-lactic-co-glycolic acid).⁷⁵  Also, the presence of the tussah silk and graphene oxide endows the scaffold with functionalities that promote mouse mesenchymal stem cell adhesion and proliferation as well as enabling biomineralization and biodeposition.

In the energy landscape, electrospinning is increasingly explored to fabricate materials that efficiently harness energy. At the front line of this enterprise is the exploration of the electrospinning process to fabricate electrodes for solar cells. As an example, Wali et al.⁷⁷  incorporated tin(iv) oxide particles – an n-type semiconductor – into poly(vinylpyrrolidone), forming a polymer–inorganic oxide nanocomposite. By electrospinning this nanocomposite, Jose and coworkers obtained one-dimensional multiporous nanofibers, porous nanofiber, or nanowires, which they fabricated into a photoanode for dye-sensitized solar cells. The electrospun multiporous nanofibers exhibit a larger surface area than the other nanostructured fibers, and its dye-sensitized solar cell features 80% incident photon-to-current conversion efficiency, which is 22% higher than that of tin(iv) oxide particles. Also, the electron lifetime and electron diffusion length of dye-sensitized solar cells fabricated from the multiporous nanofiber is three times higher than that from pristine tin(iv) oxide particles.

Electrospinning has grown into a versatile technique to fabricate a broad range of materials for different applications. Titanium(iv) oxide, a semiconductor, was recently incorporated into polyvinylpyrrolidone to form a sol, which was electrospun into one-dimensional nanowires.⁷⁸  The use of these nanowires as the photoanode in a dye-sensitized solar cell resulted in increased cell performance compared to titanium(iv) oxide particles (P25). Specifically, the short-circuit current density (J_sc) and efficiency (η) are higher in the dye-sensitized solar cell fabricated with the nanowire (J_sc = 8.57 mA cm⁻² and η = 3.45%) than that with P25 (J_sc = 6.76 mA cm⁻² and η = 2.88%).⁷⁷  Another recent demonstration of the versatility of electrospinning is the incorporation of the semiconductor, kesterite (Cu₂ZnSnS₄), into polyvinylpyrrolidone or cellulose acetate to form a polymer–inorganic composite that is electrospun into nanofibers and used as counter electrodes in dye-sensitized solar cells.⁷⁸  Of fundamental importance is the effect of the polymer on the morphology of the electrospun fiber. Whereas poly(vinylpyrrolidone)-based fibers are smooth, single crystalline materials with diameters in the range of 100–150 nm, the cellulose acetate-based fibers are polycrystalline with diameters of 10–20 nm. Compared to a conventional platinum/fluorine-doped tin oxide counter electrode with power conversion efficiency (η) of 1.72%, those derived from the poly(vinylpyrrolidone)-based and cellulose acetate-based fibers are more efficient, as revealed by their respective η values of 3.10 and 3.90%.

Electrospinning is a versatile approach to the fabrication of functional materials with broad-ranging implications for diverse human needs (Table 1.1). Many of these functional materials are being harnessed for their commercial value and several companies, including academic spin-offs and small, medium, and large companies across the world, are at the forefront in driving this translation.

This next phase of the electrospinning story is beyond the scope of this chapter, but it is hoped that reading this chapter will give an indication to the reader of the breadth and depth of the development of the process. The following chapters will provide an insight into current developments in diverse electrospinning applications and will provide an overview of expected future developments in industrial applications.