Bioinspired Strategy to Reinforce Hydrogels via Cooperative Effect of Dual Physical Cross-Linkers

Many biological tissues including cartilage and tendons are composed of polymeric hydrogels, which exhibit high toughness and rapid self-recovery. However, developing hydrogels with both high toughness and rapid recovery remains a challenge. Inspired by the nacre of abalone shell, two non-covalent cross-linkers (Al3+ and diethylenetriamine [DETA]) with different relaxation times are introduced into a polyacrylic acid network. Compared with mono cross-linked hydrogels, the dual physical cross-linked hydrogel exhibits both high toughness (work of extension at fracture up to 8.0 MJ m(-3)) and rapid self-recovery ability without loss of extensibility. Strong carboxylate-DETA ionic cross-links with longer relaxation time endow the hydrogels with high strength and help to localize the reformation of carboxylate-Al3+ coordinate bonds; weak carboxylate-Al3+ coordinate bonds with shorter relaxation time dissociate and reform to dissipate energy. Therefore, the hydrogels can dissipate massive energy effectively without any residual strain. More notably, the toughness and hysteresis of hydrogels can be completely recovered in 20 min. This finding unravels a new path to reinforce the hydrogels by the cooperation of different non-covalent interactions, which can be applied in more material systems.