## 1 Background  
Scaffolds are considered as one of the key elements in tissue engineering (TE), providing a porous three-dimensional (3D) support structure for cells [1; 2; 3; 4]. The interconnected porosity assists cell migration and nutrients and oxygen flow within the structure [5; 6]. The fabricated scaffold must meet the principle criteria of a non-toxic, biocompatible structure with controlled biodegradability while maintaining sufficient mechanical properties that are comparable to native tissue to hold the structure's integrity [7; 8; 9]. In recent years, additive manufacturing (AM), also known as 3D-printing, has been widely used to fabricate well-defined structures with necessary environmental factors to enhance cell adhesion, proliferation, differentiation, and extracellular matrix (ECM) secretion [10; 11; 12]. This technique leads to the production of user-defined geometries with highly controlled porous complex macro-structures in a layer-by-layer approach that was based on the computer-aided design (CAD) model. AM techniques benefit from several technological advantages, among which reproducibility of the process, a broad range of choices in materials, and lower cost as compared to the other conventional methods are highlighted [13; 14; 15]. There are numerous approaches in AM, and the most common onesare extrusion-based printing or fused deposition modeling (FDM) [16; 17], stereolithography (SLA) [18], ink-jet printing [19], and selective laser sintering (SLS) [20]. Electrospinning is a very popular technique, in which the material is deposited on a collector by the aid of a strong electric field through a fine nozzle. This technique shows great potential in the field of TE, owing to its simplicity, low cost of apparatus, and the ability to combine different polymers with improved properties [21]. Among the main approaches in electrospinning, solution electrospinning (SE) is a well-known method that has been used to fabricate 3D porous scaffolds for several decades in the field of TE [22; 23; 24; 25; 26; 27; 28]. SE setup has different parts as a syringe connected to a fine nozzle, a syringe pump that applies pressure, a high voltage supplier, and a collector. In principle, a continuous jet of polymer solution would be formed while using applied electrical field from the nozzle to the collector, while the solvent will be removed during or after deposition by self-evaporation. The polymer jet is narrowed due to a balance between the electrostatic repulsions of the ions and the surface tension that willing to minimize the surface charges. By a further increase of the applied voltage, the filament at the spineret tip turns into a conical shape jet stream called Taylor cone [29; 30; 31]. Several challenges, such as toxicity of solvents, solubility, and miscibility of polymers, instability of mass flow, and evaporation rates of the solvents have been already addressed in the literature [29; 32; 33; 34]. It should be noted that some attempts have been made to address the instability challenge of SE and, hence, to control the filament placement, including near-field electrospinninging [35], focused electrical field [36], using rotating disc collector [37] and high speed cylindrical collector [38], patterning the collector's conductivity [39], and direct-write electrospinninging [40]. However, the issue of using toxic solvents is yet to be addressed. In addition, the fabrication of an aligned 3D structure with large dimensions is still a challenge due to the accumulation of residual electrical charges in the deposited filaments [41]. Melt electrospinningning (ME) is a green, solvent-free method with a higher surface quality of the resultant filament as compared to solution electrospinning. By this means, no ventilation and further recovery and the removal of the toxic solvent is required, which reduces the cost of the process. Moreover, the toxicity concerns caused by toxic solvents would be eliminated. In addition, some polymers that cannot be dissolved in any solvent can be processed by ME.