2.4. Compressive Modulus Test

Before the compressive test, the hydrogel samples were immersed in PBS buffer to swell for 24 h; after that, a biopsy puncher was used to cut the sample into the shape of a cylinder (the diameter was 3 mm and the thickness was 1 mm). The mechanical testing of hydrogel disks was performed using a universal tester (GB 4201, Instron, UK) with 50 N load cell, the constant crosshead speed was 0.5 mm/min, and the sample number in each group was five. The compressive modulus was determined as the slope of the linear region corresponding to 5–15% strain in the stress–strain curve.