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Materials with stimuli-switchable properties are widespread in nature. Through biomimicry, stimuli-responsive hydrogels have received increasing research in recent years. In particular, hydrogels with stimuli-tunable mechanical properties provide an important platform for designing new materials with specific applications such as 3D printing, soft robot, and tunable adhesion. Herein, the state-of-the-art of hydrogels with stimuli-responsive toughness is reviewed in detail. Characteristic reinforcement mechanisms are fully discussed first, followed by systematical demonstrations of various smart hydrogels responding to various stimuli. In particular, special attention is paid to thermal, light, pH, and salt triggers because of their broad applications and easy implementation. Furthermore, the applications of these smart hydrogels are included. In addition to the overview of recent advances, we finally summarize the critical challenges of current discoveries and make an outlook for the following work, which we anticipate to lead to substantial progress of these responsive materials in the future.
Lin et al. (2021) studied this question.