[ Electrospinning History ] [ The Electrospinning Process ]

A diagram showing the basic elements used in an electrospinz aparatus

The History of Electrospinning

In the late 1500s Sir. William Gilbert set out to describe the behaviour of magnetic and electrostatic phenomena. He observed that when a suitably electrically charged piece of amber was brought near a droplet of water it would form a cone shape and small droplets would be ejected from the tip of the cone: this is the first recorded observation of electrospraying.

The process of electrospinning was patented by J.F Cooley in February 1902 (U.S. Patent 692,631) and by W.J. Morton in July 1902 (U.S. Patent 705,691).

In 1914 John Zeleny, published work on the behaviour of fluid droplets at the end of metal capillaries. His effort began the attempt to mathematically model the behaviour of fluids under electrostatic forces.

Further developments toward commercialisation were made by Anton Formhals, and described in a sequence of patents from 1934 (U.S. Patent 1,975,504) to 1944 (U.S. Patent 2,349,950) for the fabrication of textile yarns. Electrospinning from a melt rather than a solution was patented by C.L Norton in 1936 (U.S. Patent 2,048,651) using an air-blast to assist fibre formation.

In 1938 N.D Rozenblum and I.V Petryanov-Sokolov, working in Prof. N.A. Fuks' group at the Aerosol Laboratory of the L.Ya Karpov Institute in the USSR, generated electrospun fibres , which they developed into filter materials known as "Petryanov filters". By 1939, this work had led to the establishment of a factory in Tver' for the manufacture of electrospun smoke filter elements for gas masks. The material, dubbed BF (Battlefield Filter) was spun from cellulose acetate in a solvent mixture of dichloroethane and ethanol. By the 1960s output of spun filtration material was claimed as 20 million m2 per annum.

Between 1964 and 1969 Sir Geoffrey Ingram Taylor produced the theoretical underpinning of electrospinning. Taylor’s work contributed to electrospinning by mathematically modelling the shape of the cone formed by the fluid droplet under the effect of an electric field; this characteristic droplet shape is now known as the Taylor cone. He further worked with J. R. Melcher to develop the “leaky dielectric model” for conducting fluids.

In the early 1990s several research groups (notably that of Reneker who popularised the name electrospinning for the process) demonstrated that many organic polymers could be electrospun into nanofibers. Since then, the number of publications about electrospinning has been increasing exponentially every year.

Since 1995 there have been further theoretical developments of the driving mechanisms of the electrospinning process. Reznik et al. (2004) describes extensive work on the shape of the Taylor cone and the subsequent ejection of a fluid jet. The work by Hohman et al. (2001) investigates the relative growth rates of the numerous proposed instabilities in an electrically forced jet once in flight. Also important has been work by Yarin et al. (2001) endeavouring to describe the most important instability to the electrospinning process, the bending (whipping) instability.

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The Electrospinning Process

Electrospinning can be viewed as a special case of electrospraying. The latter is a method of atomising fluids and has found extensive use in the field of chemistry as part of mass spectrometry, and in industrial applications (e.g. coating of car parts that have complex shapes).

As with electrospraying, the feedstock for electrospinning is usually directly connected to a high voltage power supply to raise the electrostatic potential of the fluid. Polymer solutions or melts with a high degree of molecular cohesion due to intermolecular interactions are used as the feedstock. Increasing the electrostatic potential increases the surface charge of the liquid. Normally the shape of a volume of fluid is dictated by its surface tension. However, when the fluid is charged the surface charge acts in the opposite manner to surface tension, resulting in the fluid changing shape, forming the structure known as the Taylor cone (Taylor, 1964).

If the surface of a conductor forms a sharp point, the electric stresses will concentrate on that point. In a Taylor cone, there is a sharp point at the tip of the cone, so this concentration of electric stresses leads to the ejection of a fluid jet due to the increased electrical attraction at the tip. This fluid jet carries a charge, so it will be drawn in the direction of the local electrostatic field. After a certain amount of flight time this jet will become vulnerable to a number of instabilities. Careful control of these instabilities ensures successful fibre formation. For instance, the axisymmetric Plateau-Rayleigh instability, which causes atomisation in the electrospraying process, must be avoided to ensure fibre formation. Included in the electrospinning process is the off-axis bending instability that is largely responsible for the narrow fibre diameter obtained during electrospinning (Reneker et al., 2000).

The off-axis bending instability occurs due to small perturbations in the straight line trajectory of the fibre, which generate a force perpendicular to the primary axis due to the self-repulsion of the charged jet when perfect symmetry is lost. This force is very small and is initially countered by the viscoelastic nature of the polymer solution. The viscous component will resist the motion generated by this force and the elastic component will work to restore the perturbed fibre to its original position. However, at some point the perturbation forces become larger than the resistance, at which point the bending instability begins and increases throughout the rest of the jet’s flight, causing stretching of the jet.

The high surface area of these narrow fluid jets allows rapid solvent loss, leading to the conversion of the fluid jet into a solid fibre within the short flight time. It has been observed that polymer molecules within these fibres have a high degree of crystallinity (Wang et al., 2006). This has been explained both by the alignment of polymer molecules due to the electrostatic field and by the high draw ratio of the fibre while in flight.

Study of the electrospinning process can be intrinsically difficult due to the large number of parameters that are involved in the electrospinning process (see Table). An example of how much effect small changes in these parameter can cause is the addition of ionic salts. It has been shown that as little as 0.25% wt. of ioic salts can drastically reduce the mass transfer rate due to a phenomina known as the "virtual orifice" (Stanger et al. 2009).

Table showing the parameters affecting the electrospinning process (Stanger et al. 2009)

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