Discovered in the early 1900’s, electrospinning is a long-known polymer processing technique that has recently been rediscovered. It allows for the creation of nanofibers (fibers with a diameter well in the realm of nano dimensions) that can be collected to form a non-woven fabric. The resulting material can be applied to create many products including medical devices, tissue engineering scaffolds, clothing and filtration media. The new ways of applying this technology are having an increasing impact on the fields of nanotechnology and biomaterials.
What is electrospinning?
The electrospinning process is based upon simple concepts and is shown schematically in Figure 1. An electrically charged polymer solution is fed through a small opening or nozzle, usually a needle or pipette tip. Because of its charge, the solution is drawn as a jet towards an oppositely charged collecting plate, usually a metal screen, plate, or rotating mandrel. During the jet's travel, the solvent gradually evaporates, and a charged, solid polymer fiber is left to accumulate on the collecting plate. The charges on the fibers eventually dissipate, as they are neutralized by the surrounding environment. The final product of the process is a non-woven fiber mat that is composed of tiny fibers with diameters on the order of nanometers. See Figure 3 where a human strand of hair is superimposed on a matrix of electrospun nanofibers.
The events that occur at the nozzle, as the electric field deforms the fluid contained within the nozzle and forces the formation of a stream, have been studied and explained. When the voltage is initially applied to the solution, the droplet at the nozzle forms a hemispherical surface. As the electric field is increased, the surface undergoes a shape change from hemispherical to spherical and eventually to conical. These changes are due to the competition between the increasing solution charge and its surface tension.The final conical shape has come to be known as the Taylor cone (after Sir Geoffrey Ingram Taylor). When the applied voltage is sufficient to induce enough charge to overcome surface tension, a stream is ejected from the tip of the Taylor cone (see Figure 2).
The first patent (U.S. Patent 1,975,504) in electrospinning was granted to Formhals in 1934 for a process that produced fine fibers from a cellulose acetate solution. He was later granted related patents (U.S. Patents 2,116,942; 2,160,962; and 2,187,306) in 1938, 1939, and 1940. Since this time, most of the research effort has focused on exploring what types of materials can be spun using this process and how small one can make the individual fiber diameters.
In the last decade, electrospinning has been rediscovered. Interest in this technology has risen dramatically, as evidenced by the 15-fold increase in scientific publications between 2002-2008. Research has concentrated on critical analysis of the electrospinning process, the effects of solution concentration and spinning voltage on fiber morphology, the thermal properties of electrospun polymers and the transport properties of the fibers and fiber mats. Using high-speed photography, attempts have been made to explain, in detail, the travel of the fiber jet from the nozzle to the collecting plate. Mathematical models have been developed describing the jet travel.
Electrospinning is advantageous for many reasons. Properties of the fibers can be designed in advance and controlled to a high degree. The fibers are very thin and have a high length to diameter ratio, thereby providing a very large surface area per unit mass. Only a small amount of material is required, and there is very little waste. The process is versatile: fibers can be spun onto any shape using a wide range of polymers.
However, for manufacturing products with a tightly defined set of characteristics as required in implantable medical devices, the influence of the 30 odd parameters involved in the electrospinning process must be well understood and the process must be controlled in a very precise manner. Nicast has the facility along with the knowledge of the science and process necessary to meet these stringent requirements.