Dual Ionically Cross-linked Double-Network Hydrogels with High Strength, Toughness, Swelling Resistance, and Improved 3D Printing Processability.

We report a dual ionic cross-linking approach for the preparation of double-network hydrogels with robustness, high strength, and toughness, sodium alginate/poly(acrylamide- co-acrylic acid)/Fe3+ (SA/P(AAm- co-AAc)/Fe3+), in a facile "one-step" dual ionic cross-linking method. We take advantage of the abundant carboxyl groups on alginate molecules and the copolymer chains and their high coordination capacity with multivalent metal ions to obtain hydrogels with high strength and toughness. The optimal SA/P(AAm- co-AAc)/Fe3+ (SA 2 wt % and AAc 5 mol %) hydrogels showed a remarkable mechanical performance with 3.24 MPa tensile strength and 1228% strain, both of which remained stable with 76% water content and were highly swelling resistant in an aqueous environment. The hydrogels possessed high fatigue resistance, self-recovery, pH-triggered healing capability, shape memory, and reversible gel-sol transition facilitated by pH regulation. Moreover, they show three-dimensional (3D) printing processability by properly adjusting the solution viscosity. The approach may provide a convenient way of obtaining hydrogels having high strength and toughness with a number of desirable properties for a broad range of biomedical applications.