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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 2 5 structure. In this bone scaffold, an inner ring with encapsulated human umbili- cal vascular endothelial cells (HUVECs) act as the blood vessel tubule. The out- er ring with encapsulated human os- teoblast-like cells serve as the bone [25] . As shown in Fig. 2, an osteon-like unit with a double-ring morphology was created; it features a 2 mm outer di- ameter, 1.2 mm middle diameter, and a 500 μm inner diameter. Human um- bilical vein endothelial cells are locat- ed in the inner ring of the structure and MG63 osteoblast-like cells are located in the outer ring. The structure exhibits high cell densities seven days after cell encapsulation. For cartilage regeneration, oligo- mers of dopamine methacrylate (ODMA) have been intercalated into GelMA hy- drogel chains (ODMA-GelMA) to create a scaffold. These ODMA-GelMA hydrogels have been shown to promote in vivo mesenchymal stem cell adhesion and growth as well as cartilage regenera- tion after chrondoitin sulfate or TGF- b 3 loading [26] . In epidermal tissue engineering, GelMA hydrogels have been found to support keratinocyte growth, differen- tiation, and stratification into a recon- structed multilayered epidermis. These keratinocyte laden hydrogels can be used as epidermal substitutes and sub- strates for in vitro skin models [27] . GelMA hydrogels have also been used as scaffolds for wound healing. By electro- spinningGelMA into3D fibers, structures have been created that support cell growth, adhesion, migration, and pro- liferation. Using these materials, forma- tion of cutaneous tissues is accelerated [28] . In liver tissue, composites of GelMA, alginate, andcellulosenanocrys- tals have been constructed as bicellular liver lobule mimetic structures using microextrusion bioprinting. Figure 3 shows honeycomb architectures with 0.48 mm wall thickness that were creat- ed using the hybrid ink. This hybrid hy- drogel can be used in tissue engineering and for the culture of liver cells for drug discovery research [29] . In lung tissue, GelMA has been fab- ricated as a highly adhesive, biocom- patible, and biodegradable sealant that can stop air leaks and support tissue Fig. 2 — Characterization of osteon-like double-ring modules: (a, b) Phase-contrast images of a single unit and an assembly; (c) fluorescent images under the rhodamine (red) perfusion; (d-f) confocal images of the cell-laden micropatterned unit for 1, 4, and 7 days, (insert) related assembly; and (g-i) higher magnification of confocal images. M = MG63s; H = human umbilical vascular endothelial cells. Cell seeding density: 5 × 106 cells mL -1 . Scale bars: 500 (a-f) and 100 μm (g-i). Courtesy of Y. Zuo et al. [25] (a) (b) (c) (d) (e) (f) (g) (h) (i)