Synthesis of GelMA. The methacrylation of gelatin was performed using methacrylic anhydride (Supplementary Information Figure ). The successful modification of the gelatin was confirmed through 1 H-NMR (Supplementary Information, Figure ). The GelMA spectra presented two signals of vinylic protons at 5.8 ppm and 5.55 ppm and a signal of methylene protons at 2.03 ppm, which corresponded to the methacrylic groups grafted and were not found in the unmodified gelatin. In addition, the signal of the lysine methylene groups (3.12 ppm) decreased considerably in the spectra of GelMA compared to that of gelatin, indicating that these residues were modified during the reaction. The DoS, defined as the percentage of modified amino groups, of the product obtained after purification was determined via 1 H-NMR and a ninhydrin assay (Supplementary Information, Figure ). There were no statistically significant differences between the DoS obtained from 1 H-NMR (67 ± 3%, n = 5) and the ninhydrin test (68 ± 3%, n = 3).