Electrospinning

Electrospinning is currently the only technique that allows the fabrication of continuous fibers with diameters down to a few nanometers. The method can be applied to synthetic and natural polymers, polymer alloys, and polymers loaded with chromophores, nanoparticles, or active agents, as well as to metals and ceramics. Fibers with complex architectures, such as core–shell fibers or hollow fibers, can be produced by special electrospinning methods. It is also possible to produce structures ranging from single fibers to ordered arrangements of fibers. Electrospinning is not only employed in university laboratories, but is also increasingly being applied in industry. The scope of applications, in fields as diverse as optoelectronics, sensor technology, catalysis, filtration, and medicine, is very broad.

At first glance, electrospinning gives the impression of being a very simple and, therefore, easily controlled technique for the production of fibers with dimensions down to the nanometer range. First, polymers will be surveyed as fiber forming materials. Later, materials such as metals, ceramics, and glasses will be considered as fiber precursors. In a typical electrospinning experiment in a laboratory, a polymer solution or melt is pumped through a thin nozzle with an inner diameter on the order of 100 µm.
Electrospinning technique uses electrical field to produce fibers with diameter ranging from nano-scale to a few microns. This technique has become a popular approach among nanofiber production techniques, owing to its simplicity, speed, efficiency, and low preparation cost. The applied voltage causes a cone-shaped deformation of the drop of polymer solution in the direction of the counter electrode. In electrospinning, the cone angle is about 30°. On the way to the counter electrode, the solvent evaporates (or the melt solidifies), and solid fibers with diameters ranging from micrometers to nanometers are precipitated with high velocities (of 40 ms-1 or more) on the counter electrode. Upon closer inspection, it becomes clear that the electrospinning process is very complex. The jet, for instance, only follows a direct path towards the counter electrode for a certain distance, but then changes its appearance significantly. The jet is moved laterally and forms a series of coils, the envelope of which has the form of a cone opening towards the counter electrode. On occasion, beads, rather than fibers, are formed during electrospinning; fibers with beads arranged like pearls on dimensions of the fibers formed depend on a large set of parameters, for example, the properties of the polymer itself (such as molecular weight, molecular-weight distribution, glass-transition temperature, and solubility), as well the properties of the polymer solution (such as viscosity, viscoelasticity, concentration, surface tension, and electrical conductivity). The vapor pressure of the solvent and the relative humidity of the surroundings can also have a significant impact. Furthermore, the properties of the substrate, the feed rate of the solution, and the field strength and geometry of the electrodes (and therefore, the form of the electric field) play a major role in fiber formation. If higher voltages are applied, a jet is formed from the deformed drop, which moves towards the counter electrode and becomes narrower in the process. Electrospun nanofibers have attracted many interests for different applications in recent years because of their unique properties such as high surface to volume ratio and high porosity. These nanofibers have various applications including membranes and filtration sensors and biomedical applications such as drug delivery, wound dressing, tissue engineering and bio sensing. With the increase in the number of electrospinning companies over the last years, electrospinning is progressively moving from laboratory bench processes to industrial scale processes.

Nanofiber

Electrospun fibers are considerably thinner than a human hair. To help the imagination, we carry out some example calculations for fibers with diameters on the order of micrometers or nanometers. If the fiber diameter is 10 µm, fibers with a total length of 13 km can be produced from 1 g of polyethylene. In contrast, a diameter of 100 nm leads to fibers with a total length of 130 000 km. In the first case, the specific surface area of the fibers is 0.4 m2g-1, while in the second case, it is 40 m2g-1. In fiber technology, the unit denier, which specifies the mass of a fiber with a length of 9000 m, is often used as a measure of fiber fineness. For a fiber of 10 µm in diameter, the fineness is 1 denier, and for a fiber of 100 nm in diameter, it is 10-4 denier.

Today, nanofibers from synthetic or natural polymers can be fabricated in a controlled manner with dimensions down to a few nanometers, and functionalized by the addition of drugs, or of semiconductor or catalyst nanoparticles. They can be employed in numerous applications with great benefit. Hybrid fibers composed of metals and ceramics are attainable, as are nanofibers with a solid or liquid core and a solid shell.
Electrospinning generally affords smooth fibers with a circular cross section; only in exceptional cases does the cross section differ from this form. For a variety of applications, for example, tissue engineering, filtration, catalysis, drug delivery, and nanofiber reinforcement, it could be advantageous if the fiber surfaces were not smooth or were porous. For example, pores function as anchoring points for cells in tissue engineering, increase the surface area in filtration or catalysis, modify the wetting properties and, hence, the matrix–fiber coupling in fiber strengthening, and alter the kinetics of drug release. Pores can also influence the kinetics of biodegradation of bioerodible nanofibers.
In fact, it is now possible to generate different nanofiber topologies during the electrospinning process by choosing particular solvents or solvent mixtures, by varying the humidity, or by using polymer mixtures. If, for instance, phase separation into polymer-rich and polymer-poor regions occurs upon evaporation of the solvent (this can generally be estimated from phase diagrams), there is a high probability that pores will form in the solid in the polymer-poor region. Thereby, the extent of pore formation is determined by the relative proportions of both phases. The use of solvent mixtures allows the selective adjustment of porosity.
For nonwovens produced by electrospinning, the fiber arrangement (1D, 2D, or 3D) and pore structure are of great importance. Using a standard laboratory apparatus for electrospinning (consisting of circular spin nozzles and a flat counter electrode), the fibers are deposited onto the plane defined by the counter electrode in a statistical orientation. A very open mesh is generated, and the nonwoven is fabricated by the layer-by-layer deposition of such planar arrangements. The developing nonwoven can serve as a surface coating (of porous filtering papers, for example), it can modify the surface of a solid substrate (to alter the wetting properties, for example), or it can also be used as a self-supporting nonwoven (as a template for tissue engineering, for example).
Electrospinning is not limited to the production of nonwovens with a random planar fiber orientation. The orientation of nanofibers along a preferred direction is of interest for structural reinforcement with nanofibers or for tissue engineering to give the cells a preferred growth direction.
Parallel fibers can, for example, be obtained by the use of rapidly rotating cylindrical collectors, which either serve as counter electrode or are combined with an electrode. The collectors usually have the shape of a cylinder, but can also be narrow or wheel-shaped. Parallel fibers can also be produced with special electrode arrangements consisting of two parallel flat plates or with frame-shaped electrodes. A very high degree of orientation can be achieved with this method. Another possibility is the use of a quadratic arrangement of four electrodes, which leads to a cross-shaped deposition of nanofibers. High degrees of orientation can also be achieved if the distance between the electrodes is reduced to the centimeter or millimeter range and if either the spinning electrode or the counter electrode has the shape of a fine tip. To obtain fiber arrangements oriented in 3D in nonwovens, the techniques used for the production of conventional nonwovens composed of macroscopic fibers may be applied (needling, water-jet treatment). This topic has not yet been discussed with respect to nonwovens made of nanofibers.