Stimuli-responsive hydrogels as promising platforms for soft actuators

Stimuli-responsive hydrogels are well-known as tissue-like intelligent soft substances and have positive responses to various physical and chemical stimuli. Under external stimuli, stimuli-responsive hydrogels show some particular behaviors, such as swelling, shrinking, bending, deformation, dehydration, etc. These unique properties drive stimuli-responsive hydrogels to serve as a perfect candidate for preparing soft actuators. The objective of the review is to sum up the newest development of stimuli-responsive hydrogel-based soft actuators. The basic synthetic strategies for hydrogels are introduced briefly, including physical and chemical. Next, the stimuli-responsive properties of hydrogels are discussed in detail, involving physical, chemical, dual and multiple stimuli. A further highlight is put on the applications of stimuli-responsive hydrogel-based soft actuators for walkers, grippers, smart valves, soft robotics, artificial muscles, biomedicine, as well as pollutant detection and removal. Finally, we summarize the key issues and challenges identified and provide an outlook on the future work. This review is expected to inspire more researchers to join hydrogel soft actuators, contribute to understanding the design principles and explore more widespread and accessible applications.