3.1 Synthesis and identification of GelMA
The amino groups on gelatin were partly replaced by methacrylic acid to form GelMA with crosslinking ability (Bahcecioglu, Hasirci, Bilgen, & Hasirci, 2019). The synthetic GelMA was identified by1H-NMR (Figure S1). The characteristic resonance peak of the methacrylic acid group (5.5-6 ppm) appeared in GelMA, which is missing in the control. The substitution degree of GelMA was assessed to be 79.3 ± 2.7% by TNBS.