2.1 Synthesis and identification of GelMA
The GelMA was synthesized in the following procedure (Xu et al., 2020). 8 g gelatin (type A, 300 bloom from porcine skin, Sigma-Aldrich, USA) was dissolved in 50 ml 0.25 M carbonate-bicarbonate buffer with pH 9.2 for 1 h. 0.8 ml of methacrylic anhydride (MA) (Sigma-Aldrich, USA) was slowly added into this solution, and allowed to react for 3 h at 700 RPM at 55℃. The reaction was terminated by the addition of 80 ml Dulbecco’s phosphate buffered saline (DPBS), and the resulting solution was dialyzed in deionized water with 12-14 kDa cut-off dialysis tubes at 37 ℃ for 7 days. The deionized water was changed every morning and night. The solution was filtered and cooled at -20 ℃ for 1 hour, -80℃ for 2 hours, lyophilized for 5 days and stored at -20 ℃ freezer until use.