# Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward
## 2 Electrospinning  
Electrospinning is an efficient and scalable method of producing nanofibres with diameters between 40 and 2000 nm [37]. In its most basic form, the electrospinningning setup consists of a high-voltage power supply, a needle spinneret, and a grounded collector [38]. The high-voltage power supply is attached to the spinneret and the conductor to create an electric field between the two elements. The spinneret supplies the polymer solution at a fixed rate. As free charges within the base of the polymer solution interact with the electric field, force is transferred to the solution [37]. This interaction forms the jet, where the polymeric solution stretches because of its interaction with the electric field, forming fibres. The jet stretches as it moves toward the collector, causing a decrease in diameter and an increase in length. Splaying occurs when external forces overcome the cohesive force of the jet, causing the jet to break up into many smaller branches. This encourages more stretching and the evaporation of the solvent. Finally, the jet is stopped at the point of the collector, where fibres are collected onto a substrate. For polymer solutions made with volatile solvents, the fibres can be deposited directly onto a collector and form a nonwoven membrane throughout the electrospinning process. Stationary collectors allow for the random deposition of fibres, creating a porous structure that shares similar mechanical properties in both the longitudinal and transverse directions [38]. In contrast, rotating collectors encourage the alignment of nanofibres, producing a membrane with enhanced strength in the longitudinal direction. The degree of alignment may be adjusted through the speed of collector rotation. Increasing the speed of drum rotation will encourage the fibres to become highly aligned. For solutions that use non-volatile solvents, a coagulating bath may be used as a collector to remove the excess solvent and form fibres [37]. The successful formation and stretching of a polymer jet to produce nanofibres requires the manipulation of many elements toward coherence. A basic overview of the parameters impacting the electrospinningning process is shown in Figure 1. Firstly, the properties of the solution should be adjusted in order to achieve the optimal stretching of the jet [39]. The polymer must be soluble enough to be dissolved in a solvent at an adequate concentration. The concentration may vary depending on the polymer but generally must be high enough to achieve sufficient polymer entanglement. Polymer chains that are highly entangled will result in a more viscous solution than polymer chains that are shorter and dispersed throughout the solution. If the solution viscosity is too low, then the stretching of the jet will not occur, and electrospraying may occur instead of electrospinning. However, if the viscosity is too high, the solution may clog the spinneret, inhibiting electrospinning. Therefore, having an appropriate viscosity is vital to the electrospinning process. In polymer science, viscosity is typically measured via intrinsic viscosity, which relates the viscosity of the solution to the molecular weight and the shape of the polymer (Equation (1)) [2]. \[[\eta]=KM^{a}, \tag{1}\]  
In Equation (1), \([\eta]\) is the intrinsic viscosity, \(K\) is a constant, \(M\) is the molecular weight of the polymer, and \(a\) is a function of the polymer coil shape. Therefore, an increase in molecular weight will increase the intrinsic viscosity. Further, an uncoiled or relaxed polymer chain is associated with an increased intrinsic viscosity, while a curled polymer chain is associated with a decrease in viscosity. The choice of solvent can have a major impact on the configuration of the polymer, with a 'good' solvent allowing the polymer to fully relax within the solution, and a 'bad' solvent causing the polymer to adopt a curled configuration [2]. A good solvent will typically dissolve a greater concentration of polymer before reaching a gelation state.