## 2 Principles and Challenges of MEW  
The general MEW setup includes a high voltage supply, back pressure, material reservoir, and a collector. A heating system that is mostly grounded is integrated as an additional element as compared to SE setup. The syringe or nozzle is heated and connected to a high voltage generator [50; 51]. Electrical force, applying pressure, and surface tension are the forces that are applied to the molten polymer jet. By increasing the voltage difference and reaching a threshold value, it will be continuously extruded to the collector with a much smaller diameter than the nozzle [52]. For example, this method could reduce the fiber diameter from tens of microns down to 820 nm [47]. The collector or the syringe head could be movable in different directions [50; 53]. One of the complexities of MEW setup is exposing heating element to high voltage electric field and how to put an insulator shield between the heating system and the syringe that was connected to high voltage to prevent any electrical interference [53; 54]. Another challenge of applying high voltage to polymer melt is to fabricate a well-ordered structure as the tip-to-collector distance increases. The charge accumulation on the deposited fibers would cause fiber repulsion, which results in structural distortion [55]. However, researchers have melt electrospun scaffolds with 100 layers of filaments [47], and [PERSON] and his coworkers [41] recently performed a numerical analysis to overcome the repulsion of depositing fiber issue that is caused by the accumulation of excess charge. They could achieve a high-volume structure of more than 7 mm. A further key factor in MEW is the heat and charge distribution in the syringe and nozzle, and between the collector and nozzle, respectively, which determines the viscosity of the polymer and the electric force behavior [43; 44; 56]. Many parameters as printer's head/collector speed, electric field, back pressure, temperature, and tip-to-collector distance must be in harmony in order to get a highly ordered filament deposition as \"writing\". It is noteworthy to mention that the MEW setup itself plays an important role and, since the process is in its infancy, many efforts have provided setup modifications for better control and feasibility of the process to reach perfect structures. Heating components other than electrical heating jackets can be classified into heating guns [57; 58], lasers [59; 60], and circulating fluids (water and oil) [61] that were used to circumvent the possible difficulties of electrical shortcuts  
Figure 1: Schematic representation of solution electrospinning (**left**), and melt electrospinning writing (**right**). between the heater and electrical fields [41]. In order to apply pressure, the majority of studies used a pneumatic system in their setup with higher control on the applying force [51], rather than that syringe pump [62], screw-extruding, and mechanical feeding [46; 63]. The collectors are also divided into two general categories as static [53] and rotating [43]. The rotating cylindrical collectors enable various types of geometries, like tubular structures [3; 64; 65]. In addition, some pattern shaped collectors were defined to guide the filament deposition in a porous hollow assembly [66]. So far, other setups for ME have been proposed to overcome technical issues or improve the system. For instance, [PERSON] et al. [67] prepared a needeless melt electrospinning setup to avoid the corona discharge issue with the ability to increase the voltage difference up to 90 kV. [PERSON] and coworkers [68] established a new setup as wire melt electrospinning and a down-stream non-isothermal heating method as an extra heating source for melt electrospinning in order to reduce the fiber diameter [69]. Another method called bubble melt electrospinning was proposed by [PERSON] et al. [70] to eliminate the needle and reduce the fiber diameter.