In vitro release behavior of HK from composite hydrogel. A: release behavior of S1, S3, an S6 hydrogel with the same hydrogel concentration (30 wt%) and initial drug loading amount (1 mg). B: release behavior of 30 wt% S4 hydrogel with different initial drug loading amount (1 mg and 2 mg). C: release behavior of 1 mg HK loaded S4 hydrogel with different hydrogel concentration (20 wt% and 30 wt%). Error bars represent the standard deviation (n = 3).