What are Nanofibers?
Fibers with diameters in the nanometer range are referred to as nanofibers.
While some definitions limit this to fibers with diameters under 100 nm, the broader accepted definition in the textile and nonwoven industry includes fibers with diameters less than 1 micron (1000 nanometers). To put this in perspective, one nanometer is equivalent to one billionth of a meter, making nanofibers many orders of magnitude smaller than an average human hair (Figure 1).
Figure 1: Nanofibers (<1 μm) web laid on human
How are Nanofibers Formed?
One of the commonly utilized methods for the production of nanofibers is electrospinning.
The electrospinning process has a few key elements: a power supply, a reservoir for containing the polymeric solution, and a grounded collector. In the typical polymer solution electrospinning apparatus (Figure 2), a high voltage power supply is used to supply the necessary voltage to a conical metal reservoir filled with the desired polymer solution. When the electrostatic repulsive forces become sufficient to overcome the surface tension of the polymeric solution, a “Taylor cone” is formed, and the fluid jet is ejected. As the polymeric solution jet travels further from the tip of the metal reservoir, it becomes more destabilized, creating what is known as a bending instability. The combination of electrostatic and mechanical forces pulling the polymer solution from the metal reservoir to the grounded collector result in stretching of the polymer fiber, which leads to the collection of fibers with submicron (Manipulation of the components of the electrospinning apparatus and process conditions as well as the polymers and solvents employed in this technique allow the nanofiber product to be tailored to specific applications. In addition, various additives can be mixed into the polymer solution prior to electrospinning and incorporated into the resulting polymeric product, leading to novel nanofiber materials with a diverse array of properties.
Other methods employed for nanofiber production include polymer melt electrospinning, in which the polymeric solution is substituted with a polymeric melt; the melt blown process involves the production of fibrous webs or articles directly from molten polymers using high-velocity air or another appropriate force to attenuate the filaments. Another example is the Islands-In-The-Sea method of nanofiber formation. This technique first involves the formation of bi-component polymer fibers in which one component is soluble in a particular solvent and the other component is not. Following formation, the resulting fibers are rinsed or submerged in the chosen solvent, selectively dissolving one component and generating polymer nanofibers of the remaining component.
Figure 2: General Electrospinning Apparatus
When compared to conventional fibers, nanofibers offer numerous advantages, most notably high surface area, small pore size and high pore volume.
If the nanofibers are produced using a water soluble polymer, the high surface area can result in extremely fast dissolving fibers, which can be used in actives delivery systems. For hydrophilic nanofibers, the fibers can wick fluid rapidly for better absorption. No matter what characteristics the polymer may have, converting it to nanofibers will enhance them, potentially resulting in improved product performance and higher efficiency.