Design of mechanically strong and tough alginate hydrogels based on a soft-brittle transition

This study reveals the water-induced mechanical transition of alginate hydrogels and the corresponding mechanism, through which, mechanically strong and tough hydrogels are fabricated. It is found that in the water content (C_W, wt%) range of 62 < C_W < 93, the alginate hydrogels exhibit soft material properties with low modulus and fracture stress, but high extendability. However, within range of 19 < C_W < 48, ultrahigh modulus and fracture stress but low strain are observed for the hydrogels, suggesting brittle behavior. At intermediate C_W, the maximum fracture energy is obtained. The soft-brittle transition of alginate hydrogels arises from the aggregation of polymer chains upon dehydration, which transforms the inactive hydrogen bonding interaction sites into active ones. Through facile control of the dehydration degree, a very wide range of supramolecular interactions and hence of mechanical properties can be accessible, which renders the alginate hydrogels ideal candidates for further development of load-bearing materials.