## 3 Opportunities for MEW in Tissue Engineering  
MEW is a recent milestone in additive manufacturing techniques for scaffold fabrication. Applying different biocompatible and biodegradable materials enables great opportunities for a wide range of applications in tissue engineering [4; 80] including scaffolds for soft tissues like skin [81], endosteum [82], nerve [83], and cardiac tissue engineering [84]. Tubular structures that showed promising results for different tissues from bone, neural, and vascular applications were also fabricated with the MEW method [64; 65]. A wide range of polymers has been used in ME for tissue engineering applications. Polycaprolactone (PCL) [50; 66], polylactic acid (PLA) [85], poly-t-lactic acid (PLLA) [86], poly (ethylene glycol) (PEG) [62], polyurethane (PU) [87], polymethyl methacrylate (PMMA) [88], polypropylene (PP) [89], and their blends are the most commonly reported materials. So far, some novel polymers were implemented in this field as well to address the disadvantages of mostly used polymers, like hydration and hydrophobicity. However, polymers in exposure to  
Figure 2: The effect of collector speed (**A**) on final structure at constant pressure, and (**B**) on fiber diameter at different air pressures [72] Copyright 2018 by Wiley-VCH Verlag GmbH & Co. KGaA Weinheim. Reprinted by permission of John Wiley and Sons. water-containing environments as culture medium would absorb a considerable amount of water, and consequently lose their mechanical properties. To address this issue, [PERSON] et al. [90] synthesized poly (t-lactide-co-\(\varepsilon\)-caprolactone-co-acryloyl carbonate) (poly(LLA-\(\varepsilon\)-CL-AC)), a photo-crosslinkable terpolymer. [PERSON] et al. coated PCL with a hydrophilic hydrogel that increased its hydrophilicity for a long-term duration in a pH-independent strategy [91]. [PERSON] and coworkers [92] manufactured melt electrospun scaffolds from a hydrophilic class of polymers, so-called poly(2-oxazoline)s (POx). They synthesized poly(2-ethyl-2-oxazoline (PEtOx) and then revealed its potential for scaffold fabrication by MEW. Melt electrospinning writing was implemented for another polymer with piezoelectric properties. [PERSON] et al. [93] applied Poly(vinylidene diluoride) (PVDF) for a melt electrospun fibrous mesh structure with the manipulation of process parameters to achieve a perfect highly-ordered structure. To date, the number of studies working on MEW with new approaches is significantly increasing [34; 94]. [PERSON] et al. demonstrated the scale-up ability of MEW by the integration of multi-print heads to their setup [95]. [PERSON] and [PERSON] [96] studied the effect of scaffold's architecture on human skeletal stem cell behavior after optimizing their MEW setup. Different scaffold structures were fabricated in random orientation and controlled fiber patterns. The patterned structures were formed by depositing the fibers with an angle of 90, 45, and 10 degrees of every second layers. The effect of scaffold morphology on mechanical strength and cellular behavior were monitored. The results indicated that structure with 90\({}^{\circ}\) showed better mechanical properties, as well as cell spreading and osteogenic differentiation. [PERSON] et al. [71] regulated the pressure and collector speed in their MEW setup during printing and achieved multimodal and multiphasic gradient scaffolds. Cell spheroids seeded in the scaffolds porosities attached and formed aggregates due to the low fiber diameter of the gradient scaffold and proper interconnectivity. In another study by [PERSON] and colleagues [97], cell spheroids from adipose-derived stromal cells were seeded on a mesh-like structure. Some ultra-fine threading was printed at the bottom of the scaffold by manipulating the processing parameters in order to hold the cell spheroids. The melt electrowritten scaffold induced the adipogenic lineage differentiation, and it provided an environment to obtain a sheet-like structure.