## 5 Other Hybrid Approaches with MEW  
Melt electrowritten fibers post-processed with other materials could be classified as hybrid manufacturing. This approach relies on the initial production of MEW scaffold, which is tuned into relatively hydrophilic surface by plasma, NaOH treatment, or polymer coating. Subsequently, these microfibers are basically coated with cell attractive biomaterials, such as calcium phosphate (CaP), collagen, and fibronectin. For example, NaOH treatment to PCL scaffolds causes the formation of carboxyl and hydroxyl groups, which allows for the reproducible CaP coating. CaP coated fibers showed successful attachment of cells, and the results were found promising for bone tissue engineering [82; 121]. [PERSON] et al. proposed the utilization of the CaP coated PCL melt electrowritten fibers to develop a human bone environment in a murine host model to monitor the metastasis of human breast tumor to bone tissue [121]. The primary human osteoblast cells (hOB) successfully attached on the hybrid structure for seven days, which was then successfully implanted into immunodeficient mice. Afterwards, the formation of humanized bone ossicles in mice was confirmed. After the injection of human breast cancer cells into blood circulation of the mice, it was observed that the cancer cells metastas and formed lesions on bone tissues. This proposed model system was suggested to be useful for investigating the mechanism of human breast cancer metastasis to human bone in a murine host. In another study, the formation of endosteal bone-like tissue, which has an important role in hematopoietic stem cell proliferation, migration, and differentiation found in the inner wall of long bones surrounding bone cavity on CaP coated MEW PCL scaffolds were demonstrated [82]. The expression of endosteal and osteogenic specific markers of both primary human osteoblasts and placenta-derived mesenchymal stem cells under non-osteogenic and osteogenic conditions demonstrated a realistic design of tissue engineering strategies with physiologically relevant 3D porous ECM microenvironment for the generation of tissue-specific niches. [PERSON] et al. [91; 122] fabricated melt electrowritten PCL composites by further coating with proteins and monitored the enhancement of cell attachment and cell functioning. Besides, the relation between the pore size and formation of neovascularization like structures was analyzed while using melt electrowritten structures with 100, 200, and 350 \(\upmu\)m pore size coated with fibronectin [122]. Figure 7 shows HUVECs attachment on the coated fiber networks after one day incubation period. The formation of a neovascular structure was observed after one week incubation of HUVEC cells on 350 \(\upmu\)m pore size scaffolds. In the other study, collagen-functionalized star-shaped polyethylene oxide based prepolymers (sP(EO-stat-PO)) coated PCL scaffolds showed decreased cell attachment, due to a decrease in the non-specific protein adsorption on the surface [91]. MEW is used in combination with other scaffold generation methods to enhance their biological and mechanical properties. It was combined with FDM to generate a roughened surface that enhanced initial attachment, proliferation, and Ca\({}^{2+}\) deposition of the osteoblast-like cells [123]. In another study, melt electrowritten structures with different geometries were designed for attachment of osteoblasts on one side, and keratinocytes and connective tissues on the other side. The fibrous structures were combined with a core PCL film that was prepared via casting to prevent possible infections for applications of oral and maxillofacial surgery [124]. In this study, the design of construct was found cytocompatible with improved cell proliferation and showed bacteria-tightness over two weeks. Melt electrospun scaffolds could be functionalized by integrating with solution electrospinning [125; 126].