## 4 Hybrid Melt Electrospinning Writing-Fiber Reinforcement of Hydrogel Constructs
### Reinforcement Mechanism in Melt Electrowritten Fiber-Hydrogel Composites  
Hydrogels are excellent candidates for scaffold fabrication with the potential for resembling the microenvironments of human body and encapsulation of different cells in their highly hydrated  
Figure 3: Different hybrid manufacturing approaches by employing melt electrospinning writing (MEW). structures. Their tunable physicochemical properties, such as growth and differentiation factor ingredients, could strongly affect cellular behavior [104; 105; 106; 107]. However, their structures are not as mechanically strong as the ECM of soft tissues, including fibrous proteins. Their lack of mechanical instability also limits the proper cellular functionality. The enhancement of mechanical strength can be achieved by increasing the concentration of polymer content or the crosslinking degree in the hydrogel, which may negatively affect cell viability, proliferation, migration, and differentiation [104; 105; 106; 107; 108]. The incorporation of a hydrogel within geometrically varied micro-fibers produced by MEW has fulfilled the required mechanical properties to mimic the function of fibrous ECM of soft tissues [109; 110; 111; 112; 113; 114; 115; 116]. Mainly, the construction procedure of hybrid hydrogel-MEW composites has two steps, which are the fabrication of fibers via MEW and infiltration of hydrogel in manufactured fibers [109; 110; 111; 112; 113; 114; 115; 116; 117]. The mechanism behind reinforcement of hydrogel matrices by melt electrowritten fibers was investigated in different studies with several hypotheses. [PERSON] et al. [109] elucidated the reinforcement mechanism while using gelatin methacrylate (GelMA) hydrogel infiltrated into highly arranged networks of PCL microfibers. The composite was manufactured for musculoskeletal tissue engineering application, and the reinforced structure showed a significant increase in stiffness compared to hydrogel structure. It was revealed that the reinforced structure's mechanical strength was similar to that of native cartilage tissue. The hydrogel reinforced with fiber structure showed higher stiffness, rather than hydrogel scaffold and, interestingly, higher than the fiber scaffold without hydrogel. This result demonstrated the synergistic effect of reinforcing the composites. Mainly, lateral expansion of the hydrogel leads to the conversion of axial loads into lateral loads, which can be covered by fiber networks as tension in hybrid composites. Therefore, an increase in stiffness of the composite was correlated with horizontal expansion of the hydrogels while applying stress to the neighboring fibers under tension. Moreover, comparing the compressive loading responses also assessed the effect of fiber diameter on the stiffness of the composites. Low and high fiber diameter networks were manufactured via MEW and FDM, respectively. The composite structure with high fiber diameter showed similar stiffness with the structure without hydrogel. This means that the axial loads did not cause an elongation of the thick fibers. On the other hand, for the composites with thin fibers, the fibers that were elongated as a response to axial loading and hydrogel supported the structural integrity. In addition, the stiffness difference was observed between the groups of fiber networks with and without hydrogels. These observations indicated that a synergistic reinforcement effect was only observed only in the composites with small fiber diameters that were manufactured via MEW. The stiffness of the composite after compressive loading was further demonstrated through a mathematical model [109]. To calculate construct stiffness, fiber radius, the number of fibers and elastic modulus of the polymer were used as directly proportional variables, while the axial strain of fiber and composite and construct radius were used as reversely proportional ones. The mathematical model revealed that hydrogel expands with axial compressive loading and causes exposure of the MEW fibers to tensile loads.