## 4 Hybrid Melt Electrospinning Writing-Fiber Reinforcement of Hydrogel Constructs
### Reinforcement Mechanism in Melt Electrowritten Fiber-Hydrogel Composites
(**A**) Uniaxial compression test for the investigation of reinforcement mechanism of the composite. (**B**) Continuum FE model on a quarter of an idealized composite architecture (**C**) Schematic \(\mu\)-CT representation of the micro-FE model for the real composite architecture at different deformation levels. (From reference [111], reprinted with permission from Springer Nature). studies did not consider different fiber spacing while explaining the mechanism. On the other hand, micro-FE model underlined the significance of load transfer through the interconnecting regions of the fibers. Reinforcement of the composites with high fiber volume fraction through buckling inhibition by the resistance of hydrogels was also highlighted. Thus, FE analysis would be effective in optimizing the structure's architecture based on desired mechanical properties. As an alternative to the heuristic approach, which relies on experimental trials, numerical modeling could be employed for design of the architecture and optimization of manufacturing parameters. In this regard, [PERSON] et al. introduced the design of soft network composite with different mechanical and biological characteristics by modeling and manufacturing the composite, accordingly [112]. By provided compressive modulus and Poisson's ratio value of the hydrogel as an input to numerical model, compressive modulus of the composite was calculated. Based on the numerical results, 0-90\({}^{\circ}\) grid patterns with varying pore size and fiber thickness were determined for the fiber network design. Theoretical compressive modulus value obtained from the numerical model was validated by comparing the experimental results with that of different zones of the articular cartilage tissue model having different mechanical features. For this aim, the PCL fiber network with different fiber thicknesses and pore size was manufactured via MEW, and GelMA hydrogel was filled within the scaffold gaps, as shown in Figure 5. The initial layers of scaffold were printed with the PCL fibers, including hydroxyapatite nanoparticles (nHA), in order to mimic calcified zone, which is present between the native articular cartilage tissue and subchondral bone. Reinforcement of the hydrogels by fiber network was determined through uniaxial compression test. Compressive modulus values obtained by numerical modeling were in agreement with the experimental mechanical testing results. Figure 5: (**A**) Schematic view of the designed composite for cartilage application tissue having a different composition, geometry, and mechanical properties. (**B**) Micro-CT images of PCL fiber networks. (**C**) Scanning electron microscopy (SEM) images showing different zones of the composite scaffold. **(D)** SEM images of polycaprolactone (PCL) fibers with and without hydroxyapatite nanoparticles (nHA). **(E)** General view of the composite structure. Reprinted from [112] Copyright (2018) with permission from Elsevier. Biological and Mechanical Aspects of Reinforced Composites in Different Tissue Engineering Applications  
Depending on mechanical requirements of the target tissue, such as stress-strain relations, anisotropy, viscoelasticity, and flexibility, different improvements have been made on the MEW fiber-hydrogel composite. Within this framework, fiber networks with varying polymer types, fiber thicknesses, fiber spacing, and geometries have been designed and combined with several hydrogels for different tissue engineering applications. In a simple approach, MEW PCL scaffold's porosity and crosslinking degree of GelMA was evaluated based on the composite stiffness and recovery for articular cartilage tissue [109]. Chondrocytes that were encapsulated within GelMA hydrogel were homogeneously distributed throughout the PCL construct. The cells kept their spherical morphology and showed enhanced viability within the reinforced GelMA hydrogels.