Formation of double-network (DN) hydrogels. (A–D) Diffusion of FITC-GelMA molecules into GGMA hydrogels over time. (A) 1hr, (B) 2hrs, (C) 3hrs, (D) 5hrs. Scale bars represent 1 mm. (E) Vertical fluorescence profile of the cross-section of hydrogels over time. (F) Compressive modulus, (G) failure strain, and (H) failure stress of DN hydrogels with varying 2nd crosslinking time. 0.5% GGMA hydrogels crosslinked for 120 seconds and 20% GelMA(DM: 14.7%) solutions were used for (A)–(H). (*) indicates significant difference (P < 0.05).