GelMA was synthesized following previously published protocols . 10 g gelatin (type A, gel strength ~300 g bloom, from porcine skin, 0.266 mmol NH2 groups, 1 equiv.) was dissolved in 100 mL DPBS. The solution was heated to 50 °C until complete dissolution of gelatin. Then, 3.17 mL methacrylic anhydride (2.13 mmol, 8 equiv.) was added dropwise and the mixture was stirred at 50 °C for 2 h, followed by dilution with 100 mL DPBS. The solution was transferred into dialysis tubes (MWCO: 12-14 kDa) and dialyzed against deionized water at 40 °C for 12 cycles of 8 h each. After lyophilization, the product was obtained as a white solid and stored at -20 °C.