Ionic Crosslinking Improves the Stiffness and Toughness of Protein Hydrogels

Protein-based hydrogels are promising materials for biomedical and materials science applications. However, engineering hydrogels with both high stiffness and high toughness, a key requirement for many applications, remains challenging. Recently, by using the denatured crosslinking method, we developed highly stiff and tough protein hydrogels based on the polyprotein (FL)₈ via introducing chain entanglements into the hydrogel network, which allow for stiffening the hydrogel without sacrificing toughness. These hydrogels exhibited a Young's modulus of ∼0.7 MPa and breaking strain of ∼100% in tensile tests. To further enhance their stretchability and toughness, here we report the engineering of a protein/alginate hybrid hydrogel, in which the protein and alginate networks are covalently joined. Alginate was chemically modified with tyramine to introduce phenol groups, allowing the modified alginate to be photochemically crosslinked together with the polyprotein (FL)₈ to form a hybrid network hydrogel. Using calcium-mediated ionic crosslinking, we demonstrated the feasibility to tune the Young's modulus and breaking strain of these hydrogels by controlling the degree of tyramine modification of alginate. Our results showed that incorporating noncovalent ionic crosslinking into the hydrogel network increased the hydrogel's stretchability from ∼100% to over 200% without compromising stiffness, significantly improving the hydrogel's toughness. This work expands the mechanical tunability of protein hydrogels and the repertoire of strategies for engineering hydrogels with a broad range of mechanical properties.