A process known as Electrospinning (ES) is considered to be one of the most promising processes in the field of nanotechnology. Electrospun nanofibers applications started with and have grown to include environmental solutions, energy storage to healthcare and biotechnology and then to products used for defense and security measures. Today there exist the possibilities of commercializing nanofibers for micro and nano devices. The global market for nanofibers could grow from US$43.2 mln in 2006 to US$176 mln in 2012.
There are a number of methods that can be used to produce nanofibers. These include island-in-sea, melt fibrillation, gas jet techniques to self-assembly and nanolithography. Each process presents various challenges. The choice of process for producing nanofibers depends on which fiber materials are to be produced, what type of fibers alignment is needed, which production rate to be achieved and the process investment cost. The ES process has gained the most attention not only because of its simplicity but also because it can produce nanofibers with different materials. The ES process is straightforward, producing very fine fibers from polymer solutions or from melts with the help of electrostatic forces.
Formhals has first patented a practical technique by which a charge jet of polymer solution could be deposited onto a collector under the influence of an electric field. The real potential of ES process was demonstrated by the work of Doshi and Reneker.
A solution of a polymer flows out (spinneret) of the tip of a capillary where a droplet forms under the influence of the surface tension of the solution. Instead of utilizing air or other devices, a sufficiently large electric charge (a critical voltage) is applied to the solution to cause repulsive electrostatic forces between polymer and solvent molecules to overcome the surface tension. Consequently, a jet of polymer shoots away from the tip of the spinneret (nozzle) only to be collected on the grounded target screen (collector). The collector stops the liquid jet. As the jet of charged polymer solution travels towards the collector, the solvent evaporates. During their traverse as the solvent evaporates, the polymer chains get entangled, and resist the jet from breaking-up, to form nanofibers. The properties of the nanofibers will be determined by a combination of the viscoelastic behavior of the polymer and the electrostatic forces between polymer and solvent molecules. One can manipulate several parameter of the ES process including voltage, distance, environment moisture control, rotation and translation of the collector.
Today nanofibers are at the forefront of nanotechnology. Their unique porous structures and large surface to volume area make them suitable for a wide variety of applications. ES process gets its edge over other processes due to its extraordinary flexibility, which allows it to form dynamic porous products with a wide variety of pore sizes and shapes. Porosity provides breathability, allows the encapsulation of active substances, and fiber alignments. One can change the ES nanofibers' porosity to suit the needs of an application, for example additional porosity can be introduced to enhance air circulation in protective clothing. On the other hand, fiber alignment can be modified to improve mechanical properties. Work on electrospun polyamide shows that high chain alignment along the nanofiber axis provides polyamides with the high tensile strength and axial tensile stiffness.
The ES process requires high voltage and the resulting electrospun nanofibers can retain residual charges. One could take advantage of this residual charge to create new products such as passive air purifying textiles. On the other hand, flushing out of the high charge from the electrospun fiber and the remaining solvent built up could be detrimental to the constant production of the nanofibers a process called "pressure assisted spinning" (PAS).
By cleverly controlling several parameters of ES process, University of California, Berkley researchers have developed a "near-field" electrospinning process (NFES) where they are able to control precisely the site and the deposition of the nanofibers. This new process made several advances. The polymer travels a shorter distance that reduces the voltage requirements by up to 50 times. Instead of a fixed screen, fibers are captured on a plate that replaces the fixed screen. Moving this in various patterns and at different speeds provides excellent control at resolutions comparable to those achieved with much more expensive and sophisticated lithography tools. Immersing the collector in a bath of liquid nitrogen produces highly porous fibers throughout with increased surface areas. These porous fibers are suitable for encapsulation of active substances.
In a very recent report, researchers have exploited the ES process using a variety of photocrosslinkable macromers to develop degradable fibrous scaffolds with a range of properties.
Electrospun nanofibers have been in use for commercial filter media such as air filtration and coalescence filtration for over years. Obvious advantages over more traditional filter media are longer filter life, filtration efficiency, easier maintenance and lower weight. The Donaldson Company Inc. has been using electrospun nanofibers for years to producing filters for most major filter applications. Based on ES process, Elmarco, of Czech Republic patented Nanospider� technology that can produce a web of nanofibers with diameters ranging from 50-300 nanometers. To take advantage of the nanofibers produced from Elamarco's ES process, recently, Cummins Filtration Inc. entered into an agreement with Elmarco to produce efficient filtration systems for motor vehicle and related applications.
Potential medical applications include efforts to fabricate electrospun polymer nanofiber scaffolds for nerves, tissues, skin and bone. Investigations have already proven that bioactive surfaces are improved when covered by a mesh of biopolymer nanofibers. In these cases cells adhesion and proliferation have been promoted. Among myriads of applications, nanofiber membranes are also studied for sensors, polymer batteries, polymer electrolyte membrane fuel cells and photovoltaic cells.
Electrospinning provides the most versatile process to produce nanofibers with a wide range of properties. Possible new products include aligned nanostructures with specific morphology that can bring about the adsorption and decomposition of hazardous molecules, products that can be used with biodegradable materials for tissue engineering, and for the encapsulation of stem cells for targeted delivery. Process modifications are providing better control over the products through spinning different polymers together to control pore size distribution, to reduce the residual charges and to provide fine control of the structure for electronic components, encapsulation and other sophisticated applications. In the future, industries will widely use the electrospinning technique.