Darrell H Reneker describes electrospinning. Courtesy of Akron University.

Electrospinning works by ejecting liquid through a needle at the end of a cone. By applying an electric field, interaction between the charges in the liquid and the field provides the tensile force that would be exerted by spindles and reels in conventional spinning. Meanwhile the surface tension of the liquid – if it is sufficient – stops the ejected liquid breaking up into droplets. The result is long, extremely narrow fibres. While the textile industry has used the process since the 1930s, its potential for producing fibres with nanoscale diameters only came to light in the 1990s.

The fabric of future healthcare

Like any other spun yarn, electrospun nanofibres can be woven, and the resulting nanoporous fabric can have huge advantages. Porous materials allow diffusion of molecules – useful for a number of applications, among them drug delivery. In 2006 Pattama Taepaiboon, Uracha Rungsardthong and Pitt Supaphol in Thailand were first to publish on the potential of electrospun hydrogel polymers for drug delivery through the skin. Their study of drug-loaded poly(vinyl alcohol) (PVA) electrospun mats not only showed that the chemical integrity of the drugs was unimpeded by electrospinning, but provided insights into the effect of drug solubility on the morphology of mat formed, as well as on the drug release characteristics. Ten years later use of electrospun mats for drug delivery remains a hot topic of research.

As well as enabling molecular diffusion, the porosity of woven electrospun nanofibres provides a higher surface area. This can encourage cell attachment in nerve regeneration, as reported by Zhiyuan Zhang and colleagues at Shanghai Jiao Tong University School of Medicine in China. Alternative aids for the healing of nerve injuries are keenly sought to avoid the sensory loss caused by harvesting nerves for autografts from other regions of the body. “Electrospinning is an easy and versatile technique that has recently been used to fabricate fibrous tissue-engineered scaffolds which have great similarity to the extracellular matrix on fibre structure,” explain Zhang and colleagues in their report. They also suggest that similarities in the electrospun structures and naturally occurring materials in the body may provide topographical signals for directing cellular functions. Making the most of the versatility of electrospinning, the researchers produced a composite scaffold that harnesses the advantages of two polymers. More recently, Xiaofeng Lu, Zhiyuan Zhang and colleagues at Shanghai Jiao Tong University School of Medicine and the Provincial Hospital Affiliated to Shandong University introduced multiwalled carbon nanotubes into their electrospun nerve conduit. The multiwalled carbon nanotubes provide strength and tunable conductivity, as well as being biocompatible and having low toxicity.

Spinning a sensitive yarn

The high surface area of any nanostructure provides an increased number of adsorption sites, a phenomenon that has been widely exploited in sensing. While surface area is one characteristic valuable for sensors, other attributes play a role too. Nanomaterials with a core-shell structure of different semiconductors have shown enhanced sensing performance due to the resulting energy bands. With this in mind, Hyoun Woo Kim at Hanyang University and Sang Sub Kim at Inha University in South Korea and their colleagues produced core-shell nanofibres. Noting the advantages of p-n junction sensors for stabilising chemisorbed gases, they made pCuO/nZnO core–shell nanofibres by atomic layer deposition of ZnO on electrospun CuO fibres. The team showed that the nanofibres detected reducing gases at concentrations of less than one part per million, with the shell thickness proving to be key.

Technology for revolution

Despite the wide-ranging potential applications, one limitation of traditional electrospinning so far has been the throughput. Using more than one needle leads to interference between the needles’ electric fields and clogging. That means the individual needles need cleaning systems. Instead Jani Holopainen, Toni Penttinen, Eero Santala and Mikko Ritala at the University of Helsinki in Finland developed a simple needleless electrospinning technique. “As compared with many other needleless setups, the equipment is simple and straightforward to use as there are no moving parts except the syringe pump and solution itself,” they explain in their paper. The setup uses a twisted metal wire as a spinneret that can spin out a free-flowing polymer solution from multiple cones simultaneously. The resulting fibres are collected around a cylinder.

The technology for the mass production of spun natural textiles made it one of the dominant economic sectors in the industrial revolution. It was concentrated in the Ruhr Region and Upper Silesia in Germany, New England in the US and Lancashire in the north of England, where the success or failure of the industry spelled feast or famine for hundreds of thousands. Today synthetic fabrics have eaten into some of the market once occupied by natural textiles, but it seems that the ease of spinning fibres at the nanoscale has brought its own kind of revolution to nanotechnology. As Reneker and Chun rightly predicted in their 1996 report, “Fibres in the nanometre diameter range, which have been largely overshadowed by the development of larger diameter synthetic textile fibres, are likely to find many new applications.”