# Electrospinning of Chitosan for Antibacterial Applications--Current Trends
## 2 Electrospinning  
### Definition and Technical Setup  
The electrospinning method was introduced by [PERSON] in 1900 [9]. Electrospinning has the distinct advantage of being able to create nanofibers with diameters ranging from micrometers to nanometers with ease and flexibility. Nanofibers are desirable because they have a high surface-area-to-volume ratio, providing more binding sites to electrostatically connect with other particles. Nanofibers may also offer great tensile strength and flexibility in their surface properties [10]. Furthermore, compared to conventional materials, the effective porosity of electrospun nanofibers with numerous interconnected tiny holes increases the infiltration rate and contaminant rejection ratio [11]. All of these advantages make nanofibers an ideal option for technical, medical, and biological applications [12; 13; 14; 15; 16]. The needle-based and needle-free electrospinning methods can be roughly distinguished as follows. The needle-based electrospinning machine mainly consists of four components--a high voltage source, a calibrated tube with a needle, a syringe pump and a metal collector where nanofibers are deposited [17] (see Figure 1a). The needleless methods, as an alternative to needle-based methods, make use of different techniques, such as the wire-based technique (see Figure 1b). In this electrospinning method, a wire is coated with a polymer solution or melt and nanofibers are formed through a strong electric field and deposited on a substrate such as polypropylene (PP), which covers a second wire [18]. In all electrospinning methods, a surface charge density is generated from a polymer solution by applying an electric field between the needle and the collector, resulting in a stretched, suspended droplet forming a cone, the Taylor cone, on the plane of the spinneret [19]. The charged liquid jet is ejected from the tip of the cone. As soon as the electric field overcomes the surface tension of the liquid, a fine fiber is formed that is drawn to the collector with the opposite polarity. With continuous feeding and replacement of the polymer solution by the syringe pump, endless filaments are formed. The instability of the jet as a consequence of its surface charge interaction with the external field favors its bending in a whipping mode, rather than axial propagation, accompanied by solvent evaporation, in a way that the obtained mat comprises finer, extremely stretched, dry nanofibers [20; 21]. A significant influence on the fiber orientation, fiber diameters, texture, morphology and surface geometry is exerted by many operating parameters, such as the surface tension, viscosity, and crystallinity of the electrospinning solution; the solvents used; the electrical conductivity and voltage; the temperature and humidity in the chamber; the distance between spinneret and collector; and the molecular weight of the polymer [22; 23]. To obtain fibers through electrospinning, a critical concentration is required. Below the threshold, there are not enough chain entanglements to overcome Coulombic repulsion  
Figure 1: Electrospinning setup for (**a**) needle-based and (**b**) needle-free electrospinning methods. Adapted from reference [18], originally published under a CC-BY 4.0 license. inside the ejected jet, resulting in the production of sprayed droplets and beads due to the high surface tension of the polymer solution. The decrease in the surface tension of the solution promotes electrospinning and the formation of uniform and homogeneous fibers at lower electric fields [23]. The fiber diameter can be influenced by operating parameters as well. The applied voltage allows charge transfer from the needle tip to the collector, and raising it may alter the fiber diameter and pore size of the nanofibers. The flow velocity of the polymer solution may affect both the size and form of the fiber. Accelerating the flow rate, for example, increases the pore size by increasing the fiber diameter.