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Compiled by Don Rosato
Keyword Abstract: nanotechnology, electrospinning, nanofibers, solid nanofibers, hollow nanofibers, nylon, polyester, acrylic, polylactide, water soluble polymers, polyethyleneoxide, cellulose, polymer blends, polymers containing solid nanoparticles, polymers containing functional small molecules, barrier fabrics, scaffolding, implants, membranes, wound dressing materials, filtration membranes, specialty filters, agriculture, electronics, sensors, nanowires, nanocomposites, nanopipes
Overview
Not since the discovery of the silicon chip has there been this much excitement in the field of physics and material sciences. Innumerable universities have established nanocenters with many receiving industrial funding/sponsorship with a large number of these spawning nanomaterial related entrepreneurial businesses spun out as the fruits of academic research. An emerging area of such interest is nanofibers.
Electrospinning Solid Nanofibers
Electrospinning, a process first patented in 1934, is a procedure that makes use of electrostatic forces to produce ultra-thin polymer fibers, ranging in diameter from microns down to tens of nanometers. While not new, the technology has only recently gained wide interest, triggered by the nanotechnology phenomena and special needs of military, medical and filtration applications. Traditional methods of polymer fiber production which include melt, solution and gel state spinning, rely on mechanical forces to produce fibers. Polymer melt or solution is extruded through a spinneret and the resulting filaments subsequently drawn as they solidify or coagulate. Fibers, with diameters limited to the 5 to 500 micron range, can be produced using these methods. Alternatively, using electrospinning technology ultra-fine fibers with nanoscale diameters can be produced.
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Comparison of Advanced Carbon Fiber and Electrospun Continuous Nanofibers
(Source: CMRA, University of Nebraska
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Electrospinning, a nonmechanical fiber drawing method, makes use of electrostatic forces to draw a charged polymer solution or melt from an orifice to an oppositely charged collector. The polymer solution or melt to be spun is forced via pump through a syringe to form a pendant drop of polymer at the tip of a pipette or syringe needle. One electrode is placed into the solution/melt within the syringe and the other is attached to a collector. A strong electric field is applied to the pendent drop of a polymer solution or polymer melt. As the intensity of the electric field is increased, the hemispherical surface of the fluid at the tip of the capillary tube elongates to form a conical shape referred to as the Taylor cone. With increasing field, a critical value is reached in which the repulsive electrostatic force overcomes the surface tension of the solution or melt and a charged jet of fluid is drawn from the tip of the Taylor cone towards the counter electrode 'collector.' As the jet travels through the atmosphere, the solvent evaporates, or in the case of the melt is solidified, leaving behind a fiber which is gathered on the grounded collecting device.
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Electrospinning Process Technology
(Source: Wilkes Research Group, Virginia Tech) |
Depending upon the choice of processing parameters including the surface tension, viscosity, conductivity, and concentration of the solutions, the polymer molecular weights, the applied fields and electrode configurations, fibers with diameters down to a few nanometers can be produced. A wide range of polymers can be electrospun. These include nylon, polyesters, acrylics, polylactides, water soluble polymers such as polyethyleneoxide, cellulose derivatives, polymer blends, and also polymers containing solid nanoparticles or functional small molecules. Randomly laid nanofibers aggregates in non-wovens, membranes and bulk structures have very high surface area, small interstitial size and very high open porosity, resulting in high absorbency, and reactivity that makes them well suited for a variety of civilian and military applications. Uses of nanofibers in highly efficient barrier fabrics, biomedicine (including tissue scaffolding, implants, membranes, wound dressing materials and drug delivery), filtration membranes, specialty filters, agriculture, electronics, sensors, nanowires, and nanocomposites, and other areas are presently being developed.
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Nanofiber Filter Media
(Source: eSpin Technologies)
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Electrospinning Hollow Nanofibers
Researchers in Spain at the 'Universidad de Málaga' and in the U.S. at the University of Nebraska–Lincoln have developed a technique to prepare hollow fibers with nanometer scale diameters in a single step The simple technique makes use of electrohydrodynamic forces to form two liquid coaxial nanojets composed of a shell forming liquid of the outside and an inert immiscible liquid on the interior. The two liquids are injected through a pair of concentric needles to which a high voltage is applied, so that the liquids emerge from the needles coaxially with a small interior stream of one material surrounded by a shell of the second material. The interior liquid serves as a simple template around which the shell solidifies. The inner liquid evaporates once the fibers are collected. Product centricity, diameter, and wall thickness depends on the properties of the liquids, particularly surface tensions and viscosities, as well as on operating variables such as flow rates, flow ratios, and voltage. The technique was used to spin fairly uniform sized hollow silica fibers. The shells of tetraethylorthosilicate via sol-gel chemistry were formed around cores of ordinary liquids such as olive oil or glycerin. The technique also extends to a wide variety of other materials. A likely extension of this work is the use of dissolved polymers to serve as core liquids to make sheathed fibers. Applications are anticipated in tissue engineering, sensors. The hollow fibers might also act as plastic nanopipes for nanofluids transfer, drug delivery and related applications.
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Electrospinning Hollow Fibers
(Source: Universidad de M?aga/University of Nebraska-Lincoln
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