Degree of substitution via NMR spectroscopy and ninhydrin test. H-NMR spectra of unmodified gelatin and GelMA were obtained using a Bruker AV 500 spectrometer. The samples were dissolved in deuterium oxide and the experiment was performed at 35 °C. To obtain the degree of substitution (DoS), the spectra of gelatin and GelMA were normalized against the signal of phenylalanine (6.9 -7.5 ppm). Then, the signal of lysine methylene groups in both spectra was integrated. The calculation was done according to Equation .