Electrospinning

The very first available records indicate that electrospinning may be the earliest method for the production of nanofibers. The origin of this process could be tracked back to the early 1930s when in 1934 Formhals patented his invention for production of artificial filaments using electric charges [18]. He electrospun cellulose acetate fibers using acetone as the solvent. In the 1960s, fundamental studies on the jet-forming process during electrospinning of fibers were initiated by Taylor and the importance of the conical shape of the jet was stated [19,20]. It was established that the conical shape of the jet is important because it defines the onset of the extensional velocity gradients in the fiber-forming process. The electrospinning of acrylic fibers, whose diameters ranged from 500 to 1100 nm, was described by Baumgarten [21]. He determined the spinnability limits of polyacrylonitrile/dimethylformamide solution and observed the dependence of fiber diameter on the viscosity of the solution. Larrondo and Mandley produced polyethylene (PE) and polypropylene (PP) fibers from the melts [22-24]. They found that meltspun fibers had relatively larger diameters than solvent-spun fibers. However, until the mid-1990s there was little interest in electrospun nanofibers. Research on these fibers was triggered by the work of Doshi and Reneker who studied the characteristics of the polyethylene oxide nanofibers by varying the solution concentration and applied electric potential [25]. Before this time, the process of forming fine fibers using electrical charges was commonly referred to as electrostatic spinning. Since then, the process attracted a rapidly growing interest triggered by the potential applications of nanofibers in fields other than filtration and the term "electrospinning" was coined and now is widely used in the literature. More recently, three major breakthroughs were made: the development of methods of uniaxial alignment of electrospun nanofibers; the method of producing continuous ceramic nanofibers; and the method of coaxial electro-spinning for production of hollow and sheath-core nanofibers [4,10,26-32].

Note, however, that electrospinning does not result in nanofibers, but rather a nonwoven web composed of nanofibers, perhaps limiting its general applicability in many textile uses. The main application of electrospun fibers continues to be in filtration and medical products [24,33-35]. One of the major producers of electrospun products in Europe and the U.S. is Freudenberg Nonwovens of Weinheim (Germany), which has been practicing electrospinning for over 20 years, producing electrospun filter media from a continuous web feed for ultra-high efficiency filtration markets [33]. Donaldson Company Inc. also has been using nanofiber web (Ultra-Web), consisting of fibers with sub-half-micron diameters for air filtration in commercial, industrial, and defense applications since 1981 [5,8]. Smaller companies are now beginning to electrospin nanofibers, including eSpin Technologies in Chattanooga, TN and Foster Miller, Inc. in Waltham, MA [7]. The potential application of electrospun web is as a protective membrane layer for providing protection from extreme weather conditions, enhancing fabric breathability, increasing wind resistance, and improving the chemical resistance of clothing to toxic chemical exposure [8,36]. Other potential uses of electrospun nanofibers are nanoreinforcement, tissue engineering, implants, drug delivery, as supports for enzymes and catalysts, for fabrication of high-performance lithium batteries, nanoscale electronic and optoelectronic devices, and for use as nanofluidic channels [37,38].

Because of the continuing interest in this technology, a separate section is devoted to this topic, and the rest of this discussion is focused on the development of nanofibers as well as webs composed of nano-or near-nano fibers using other methodologies.

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