# Electrospinning of Essential Oils
## 2 Production of Polymeric Fibres by Solution Electrospinning  
Electrospinning is a nanofabrication technique that operates at high electric voltages to extrude polymeric fibres with a diameter down to the nanometre [27; 28; 29; 30]. Two main electrospinning approaches are available: solution electrospinningning when the starting polymer is in solution with an appropriate solvent or solvent mixture; melt electrospinning when the polymer is processed in the molten state and no solvents are utilised. This review focuses on solution electrospinningning, hereinafter referred to simply as electrospinningning.  
A typical electrospinningning apparatus takes the form of a high-voltage power supply, a spinneret (usually a metallic needle) attached to a syringe, a syringe pump, and a conductive collector (usually copper or aluminium) [27; 31]. The first step of the electrospinningning process consists in generating a pendant droplet of the polymer solution at the tip of the spinneret by pumping the liquid at a constant flow rate using the syringe pump. Then, an electric potential difference is generated between the spinneret, which is connected to the high voltage power supply, and the collector (usually grounded). With voltage increase, electric charges accumulate on the surface of the droplet till a critical voltage is reached (more than tens of kV). At this point, the electrostatic repulsion overcomes the surface tension and the viscoelastic forces of the polymer solution, and the droplet deforms into a conical shape, known as a Taylor cone. An electrified liquid jet is ejected from the apex of the Taylor cone and it is accelerated toward the collector. The jet moves in a nearly straight line in a region close to the spinneret tip (near-field region); it stretches, gradually decreases in diameter, and experiences bending or whipping instabilities away from the spinneret and in close proximity to the collector (far-field region). The instabilities, due to electrostatic repulsion of charges, determine further jet thinning and solvent evaporation with consequent jet solidification. Finally, solid fibres are deposited on the collector to form a non-woven mat.  
The fabrication of defect-free electrospun fibres with the desired size and structure is achievable by a fine optimisation of all process parameters [32; 33]. This includes control over the properties of the  
Figure 1: Schematic representations of the biosynthesis of: (**a**) isoprenoids, reproduced from Ref. [20] with permission from the Royal Society of Chemistry; (**b**) phenylpropanoids, reproduced from Ref. [21].  
polymer solution (viscosity, molecular weight of the polymer, conductivity and boiling point of the solvent system, presence of additives), the applied voltage, the flow rate, the spinneret-collector distance, the environmental conditions (temperature and humidity), and the collector geometry. For example, solvent volatility is instrumental to engineer in-fibre porosity and generate fibres with high specific surface area [2; 34; 35; 36]. Binary and ternary polymer solutions prepared with an appropriate selection of solvents and non-solvents (low boiling point solvents) have been investigated to induce phase separation events prior or during the electrospinning process and hence generate pores. In addition, the porosity of the overall electrospun mat (fibre packing density) can be adjusted by acting on the collector geometry, which in turn affects the external electric field and, consequently, the orientation and alignment of the fibres [27; 37; 38]. While deposition of randomly oriented fibres occurs on a planar, static collector, the uniaxial alignment of fibres can be obtained by a rotating mandrel, a pair of electrodes separated by air gap, or two parallel permanent magnets.