## 4 Hybrid Melt Electrospinning Writing-Fiber Reinforcement of Hydrogel Constructs
### Reinforcement Mechanism in Melt Electrowritten Fiber-Hydrogel Composites
Periosteum, which is the reservoir for vascular components, and bone-forming cells has a critical role in the regeneration of complex multiphasic system. The periosteum is mechanosensitive tissue that is exposed to shear and traction loads during movement and based on the subjected force it regulates cell proliferation and differentiation. This multiphasic structure was mimicked as in a composite of melt electrowritten PCL tubular scaffold seeded with human bone marrow mesenchymal stem cells (BM-MSCs) and sPEG/Hep hydrogel system that was loaded with human umbilical vein endothelial cells (HUVECs) as a target for large bone defect repair. BM-MSCs were used as bone-forming cells, while the HUVECs cells were utilized for vessel formation. sPEG/Hep hydrogels were modified with RGD and loaded with VEGF. After the hydrogel infiltration in the tubular PCL scaffold, the cells were cultured in angiogenic conditions for seven days of in vitro culturing. Subsequently, the composites were implanted in femur side of mice. The explanted constructs after 30 days from implantation showed that HUVEC cells were not proliferated, but formed capillary-like structures and BM-MSC kept proliferation without differentiation. In addition, it was observed that the implanted human cells were gradually replaced by the host cells. This study demonstrated the efficiency of multiphasic hybrid design with a combination of different human cell types in the periosteum tissue engineering concept. In another study, the effect of the composition of soft matrix component and photoinitiator type for crosslinking within PCL microfibers on the re-differentiation of chondrocyte was investigated [PERSON] et al. (2018). GelMA type A or type B that were mixed with hyaluronic acid (HA) were photocrosslinked with either lithium acylphosphatase (LAP-visible light; 405 nm) or Irgacure 2929 (IC2929-at UV light; 365 nm). Interestingly, it was found that PCL fiber reinforcement increased the expression of ECM components of chondrocytes in hydrogel constructs. GelMA from type B photocrosslinked with IC2929 enhanced the formation of cartilage-like tissue as compared to others through promoting increased GAG production, which showed similar compressive strength as native articular cartilage. The MEW-hydrogel hybrid manufacturing approach has also allowed for the development of 3D in vitro tissue models, such as neural and tumor cultures [PERSON] et al. (2018); [PERSON] et al. (2018). For example, Glycine receptor transfected (GTR; ligand-gated ion channel) Ltk-11 cells encapsulated in Matrigel reinforced with melt electrowritten PCL scaffold. This model provided information regarding the functioning of the ion channels with electrophysiological measurements from a physiologically similar 3D system [PERSON] et al. (2018). In another study, patient-derived ovarian cancer cells were encapsulated in PEG hydrogel infiltrated into PCL fiber scaffold. It was used to mimic the tissue environment for the investigation of malignant behavior and tumor-promoting signals [PERSON] et al. (2018). Designed models showed clinically similar gene expression of ovarian cancer with the samples that were obtained from a high-grade serous ovarian cancer patient. These models may be helpful for screening the possible medicines or a combination of therapies and design for other clinical trials. It is clear that the MEW-hydrogel hybrid system is a promising technique for RM, TE, and disease models, yet two-step fabrication of the hybrid structures limits the control over-structure design and fiber writing. To overcome this limitation, [PERSON] et al. assessed a single-step construction approach by MEW of PCL and extrusion bioprinting of eMSC-laden GelMA hydrogel [105]. This approach allowed for the precise deposition of cells in a mechanically stable composite with controlled porosity and pore shape. It was confirmed that the applied high voltage during construction did not affect either the cell viability and metabolic activity or the ability to differentiate toward multiple lineages of eMSC.