Cold water fish skin

The synthesis of GelMA and PAMAM-MA followed protocols described previously [26 (link), 35 (link)]. Gelatin powder from cold-water fish skin (Sigma–Aldrich, USA) was dissolved in phosphate-buffered saline (PBS; HyClone, USA) at a concentration of 100 mg/mL at 50 °C. Methacrylic anhydride (MA, Aladdin, China) was added to the gelatin solution at a rate of 0.5 mL/min (20%, w/v). The mixture was reacted for 3 h under stirring conditions at 50 °C. The reaction was then terminated by the addition of excessively warm PBS. The GelMA solution was dialyzed against deionized water for 7 days at 40 °C to remove excess residual small molecules, and then the GelMA solution was filtered, freeze-dried and stored at − 20 °C. PAMAM (Weihai CY Dendrimer Technology Co., Ltd., China) was modified with MA in accordance with the same protocol above to synthesize PAMAM-MA.

Bovine gelatin (type B, gel strength of 225 g Bloom), porcine gelatin (type A, gel strength of 300 g Bloom), and cold-water fish skin (without gel strength) were commercially purchased (Sigma-Aldrich, Brazil). Cannon–Fenske viscometer evaluated gelatin's viscosity-average molecular weight (MW) (Schott Geraete, GMBH-D65719, Germany). The intrinsic viscosity (η) was calculated by Huggin’s equation, Eq. (1). Then, the molecular weight was estimated using the Mark-Houwink equation, Eq. (2). $$\frac{{\eta }_{\mathit{SP}}}{c}=(\eta )+k{(\eta )}^{2}c$$ $$\left[\eta \right]=K{(MW)}^a$$ where η_SP/c is the reduced viscosity (mL g⁻¹), η_SP is the specific viscosity which compares the viscosity of the gelatin in solution to that of the solvent (dimensionless), c is the gelatin concentration (g mL⁻¹), k is the Huggins constant (dimensionless), K and a are constants, which depends on the system solvent-polymer (K = 0.16 mL g⁻¹ and a = 0.82)²⁷ .