## 4 Hybrid Melt Electrospinning Writing-Fiber Reinforcement of Hydrogel Constructs
### Reinforcement Mechanism in Melt Electrowritten Fiber-Hydrogel Composites
According to quantitative reverse transcriptase-polymerase chain reaction (PCR) analysis, a physiological compressive loading of 20% strain and 1 Hz induced the up-regulation of expression of genes encoding the ECM proteins of chondrocyte. In another study, to recapitulate viscoelastic and stress relaxation characteristics of the articular cartilage, high negatively charged star-shaped poly(ethylene glycol)/heparin (sPEG/Hep) was used as hydrogel and then reinforced with the PCL fiber network having a 0\({}^{\circ}\)-90\({}^{\circ}\) grid pattern with different pore sizes that emulated collagen fibers in terms of anisotropic and nonlinear features [110]. Treating the surface with NaOH treatment increases the wettability of the fiber network. When compared to alone sPEG/Hep hydrogel and MEW fiber network, the compressive modulus was several folds higher in fiber-reinforced hydrogel composite. Since the compressive modulus of the composite was measured as being higher than the summation of compressive modulus of hydrogel and fiber network alone, which indicated the synergistic reinforcement effect. Moreover, composite structure exhibited similar viscoelastic nature of the articular cartilage. Similar to the results of compressive modulus, only the fiber-reinforced hydrogel composite exhibited similar stress-relaxation behavior with the human articular cartilage, and the enhancement of ECM protein expression under hydrostatic pressure was observed [110]. Another study that was related to articular cartilage tissue engineering was conducted by employing reinforcement of hydrogels through melt electrowritten bi-layered microfiber network [113]. Different zones of articular cartilage tissue were resembled by two layers designed with different fiber patterning strategy in order to obtain zonal mechanical characteristics of the native cartilage tissue. The GelMA hydrogel was cast into the PCL fiber network, which was made up of dense structure with 0-45-90-135\({}^{\circ}\) crossed diagonal pattern mimicking a superficial tangential zone (STZ) and uniform 0-90\({}^{\circ}\) box structure mimicking middle and deep zone (MDZ) of the articular cartilage. A significant mechanical strength difference was observed between the reinforced and non-reinforced hydrogel structures. The presence of a STZ layer enhanced the compressive modulus several folds as compared to the composite without STZ layer as the presence of STZ layer provided distribution of compressive load to the whole composite uniformly. Human chondrocytes encapsulated in GelMA hydrogel reinforced with bi-layered PCL fibers were tested under two different conditions: static conditions with chondrogenic differentiation factor and mechanical strength induced condition without addition of any differentiation factor. The results revealed that both conditions showed cartilage differentiation by the similar production of sulfated glycosaminoglycan (GAG) and collagen II, which indicated the activation of signaling factors under proposed mechanical condition. The reinforcement of hydrogel by fiber networks was also used for a variety of other soft tissues. Fiber networks with different geometries have been constructed to meet the requirements of differentmechanical properties of soft tissues. For instance, since soft tissues function under high tensile loads, a flexible soft tissue bearing high tensile loads were manufactured by embedding stretchable curvy PCL fiber networks into a hydrogel for tensile load-bearing, mechanical anisotropy, and flexibility [114]. A combination of poly (ethylene glycol) diacrylate (PEGDA), GelMA, and alginate were employed as hydrogel matrix. Since the transfer of the applied loads depends on the interaction between soft matrix and reinforcing fibers, the surface of the fibers was functionalized by photo-crosslinkable acryl groups, which provided covalent bonding between fiber network and PEGDA and GelMA hydrogels.