The ninhydrin assay was also performed in order to obtain the DoS of GelMA as the percent of modified amines, using the method reported by Zatorski et al. For this, a calibration curve of unmodified gelatin was obtained using concentrations between 1 and 10 mg/mL. The gelatin was dissolved in PBS and 87.5 μL of each solution were placed in a 96-well plate. Then, 12.5 μL of a 20 mg/mL ninhydrin solution in ethanol was added to each sample. Next, the well plate was sealed with an optical sealing tape and was kept at 70 °C for 30 minutes. Finally, the absorbance was determined at 560 nm with a plate reader (Tecan Infinite M200, Männedorf, Switzerland). A solution of 10% GelMA was prepared in triplicate and read under the same conditions as the calibration curve. The DoS was determined as: