## 4 Hybrid Melt Electrospinning Writing-Fiber Reinforcement of Hydrogel Constructs
### Reinforcement Mechanism in Melt Electrowritten Fiber-Hydrogel Composites
However, theoretical stiffness value was calculated larger than the experimental one. It demonstrates the complexity of theoretical modeling of polymer-hydrogel composites, which demands more in-depth studies. [PERSON] et al. stated a similar hypothesis for reinforcement mechanism of hydrogels by MEW fiber network [48]. As stiffness of the composite relies on the MEW fiber networks, a detailed study was performed by controlling fiber spacing of 400 \(\upmu\)m and 800 \(\upmu\)m, and grid patterns with 0-90\({}^{\circ}\) and 0-60-120\({}^{\circ}\) orientations. The constructed melt electrowritten PCL fiber networks were infiltrated with GelMA, and GelMA/hyaluronic acid-methacrylamide (HAMA) hydrogels and the stiffness of the structures were evaluated. It was proposed that the hydrogels had significant lateral to axial strain ratio due to high Poisson's ratio values. However, the composites showed low Poisson's ratio, since highly organized fiber networks suppressed lateral deformation of the hydrogels. This synergistic reinforcement mechanism was the same as that reported by [PERSON] et al. [109]. In another study, the high order finite element method was used to simulate the mechanical characteristics and elastic modulus of composites [110]. Composites with varying fiber spacing were used for the analysis. Simulation analysis revealed that decreasing the fiber spacing increased the compressive moduli of the composite due to higher reinforcing filler ratio, which was a similar synergistic reinforcement mechanism with the aforementioned studies [48; 109]. The simulation results presented higher stiffness value for fiber networks as compared to experimental data, although the experimental and theoretical data for hydrogel alone and fiber-reinforced hydrogel samples were similar. [PERSON] et al. performed two different finite element (FE) analyses in order to investigate the mechanism behind the reinforcement of hydrogel through fiber networks [111], as summarized in Figure 4. In this regard, melt electrowritten PCL network with different fiber spacing and GelMA hydrogel were manufactured. Subsequently, a compression test was performed to obtain stress-strain data that were used as an input for FE analysis. Afterward, a melt electrowritten fiber network was manufactured, and GelMA hydrogel was infiltrated into its gaps, and the FE analysis results were validated with experimental data. In the first analysis, continuum FE model was examined by employing the idealized geometry of the composite, which is unidirectional lamina. Continuum FE model exhibited the expansion of hydrogel inside the composite. Besides, the diminishing effect of fiber network on hydrogel movement was observed. For individual hydrogel and fiber network structures, continuum the FE model results were similar to the experimental data in terms of compressive stress-strain behaviors. However, theoretical stiffness of the composite with higher fiber volume fraction was significantly lower than the experimental data in this model, although the stiffness values for composites with lower fiber volume fraction were similar. In second analysis, micro-FE model was performed by employing the micro-computed tomography (\(\mu\)-CT) images of the composite's geometry. The micro-FE model presented similar results with the experimental data in terms of deformation of fiber scaffold and composites. The results indicated that addition of hydrogel into fiber scaffold increased stiffness of the overall composite several folds due to the prevention of fiber network buckling through the resistance of hydrogels. Several inferences were made from those FE models. Continuum FE model stated that the reinforcement of the composites with low fiber fraction volume was governed by lateral expansion of the hydrogel, which put the fibers under tension. This hypothesis was similar to the previously mentioned studies. However, those  
Figure 4: General modeling overview for continuum and micro-finite element (FE) models.