Abundant tannic acid modified gelatin/sodium alginate biocomposite hydrogels with high toughness, antifreezing, antioxidant and antibacterial properties.

The acidity of high tannic acid (TA) content solution can destroy the structure of protein, such as gelatin (G). This causes a big challenge to introduce abundant TA into the G-based hydrogels. Here, the G-based hydrogel system with abundant TA as hydrogen bonds provider was constructed by a "protective film" strategy. The protective film around the composite hydrogel was first formed by the chelation of sodium alginate (SA) and Ca. Subsequently, abundant TA and Ca were successively introduced into the hydrogel system by immersing method. This strategy effectively protected the structure of the designed hydrogel. After treatment with 0.3 w/v TA and 0.06 w/v Ca solutions, the tensile modulus, elongation at break and toughness of G/SA hydrogel increased about 4-, 2-, and 6-fold, respectively. Besides, G/SA-TA/Ca hydrogels exhibited good water retention, anti-freezing, antioxidant, antibacterial properties and low hemolysis ratio. Cell experiments showed that G/SA-TA/Ca hydrogels possessed good biocompatibility and could promote cell migration. Therefore, G/SA-TA/Ca hydrogels are expected to be used in the field of biomedical engineering. The strategy proposed in this work also provides a new idea for improving the properties of other protein-based hydrogels.