## 4 Melt Electrospinning
### New Techniques and Configurations  
Different configurations have been designed and developed to improve the most important steps of the process: melting the material and forming the initial jet. These configurations include the use of an extruder, a ceramic or electrical heater, an oil-circulating heating system, a glass syringe with a heat gun, a gas-assisted system, or a CO\({}_{2}\) laser heating system [11]. When choosing one of these, the possible degradation and the control of the viscosity must be considered. In addition, different collector configurations provide different morphologies. As an example, the PCL fiber diameter can be decreased from 2 \(\mu\)m to 270 nm [39] by using the gap method of alignment for collection. Since the fiber diameter is influenced by the size of initial jet (i.e., the Taylor cone size) and also by the geometrical confinement of the jet, some studies have been focused on finding out their relationship by using simulation and modelling techniques and comparing predictions with experimental results [48; 49]. One of the above-mentioned methods, wire melt electrospinning was specially developed for polymers with low melting point like poly (\(\varepsilon\)-caprolactone) (PCL), a semi-crystalline thermoplastic polyester. In a study carried out by [PERSON] et al. [14], PCL thin fibers were produced using this method and applying a strong electrostatic field gradient. Figure 4 shows schemes of the most relevant setups--syringe, edge and wire configuration. It is obvious that, among the different methods, the initial jet is much thinner in wire electrospinning, being feasible to get ultrafine fibers (Figure 4c). In this method, a wire is coated with the polymer to generate the thinner initial jet. Taylor cone and size and shape of the initial jet depend on the electrostatic field around the wire tip. The small and narrow region with a size around 200 \(\mu\)m provides high-enough electrostatic forces to overcome the surface tension and lead ultrafine fibers [14]. Recently, a free surface melt electrospinning platform was used. In this case, multiple spontaneous jets were formed from a flat plate (schematically shown in Figure 4b). Solidification point was introduced as a major factor contributing to fiber diameter reduction in an unconfined melt electrospinning. This free surface melt electrospinning technique was found as an effective strategy to control the fiber diameter aiming the solidification point and temperature profile in an electrohydrodynamic jet [50; 51]. Among the different configurations and techniques to obtain micro and nanofibers, melt electrowriting (MEW) has been developed to reproduce designed patterns in a similar way to those obtained by 3D printing using computer-aided designs (CAD) [52]. In melt electrowriting, both nozzle and collector are placed in an appropriate position and the extruded filament becomes directly deposited onto the collector following program instructions. The new deposited material is still in the melt state, which makes an efficient crossover with the previous layers before solidification possible. The collector is simultaneously moved in a distance corresponding to the height of one filament, so the layers can be stacked as shown in Figure 5. This layer-by-layer deposition is repeated until the desired 3D object is fabricated [53; 54]. In melt electrospinning a separation distance is established between the dispense header and the collector, while in direct electrospinning the extruded polymer is applied directly to the collector surface, or on previous layers of the object under construction. The application of a high voltage allows a greater reduction in the diameter of the filament and therefore the improvement of the resolution of the writing process. Figure 5c,d show representative examples of samples processed by electrowriting. [PERSON] et al.