Bioinspired sensors and actuators based on stimuli-responsive hydrogels for underwater soft robotics

Abstract

Various active soft materials have been developed for sensors and actuators inspired by the powerful, dexterous, and adaptive biological muscles found in octopus arms and elephant trunks. Stimuli-responsive hydrogels, a class of water-loaded polymers, exhibit large volume change and actuation strain upon environmental cues, enabling them to absorb and release water up to more than 90% of their total weight. These tissue-like, multifunctional, and multi-responsive hydrogels exhibit attractive sensing and actuation capabilities, qualifying them as potential candidates for artificial muscles used in next-generation underwater soft robotics. This chapter introduces a variety of stimuli-responsive hydrogels that can serve as soft sensors for local environment and strain monitoring, and as powerful actuators capable of rapidly generating high force. Furthermore, these materials can be designed towards self-sensing actuation as a step towards mimicking biological local adaptation and intelligence. Three primary approaches have been taken to engineer materials that closely resemble naturally occurring muscles. Firstly, adaptive coloration in artificial skins has been utilized to mimic the camouflage and environmental detection capabilities of artificial muscle tissue. Secondly, to better approach the high work density of natural muscles, the exerted force and deformation speed of the artificial muscles were significantly enhanced by controlling the storing-releasing mechanism of the hydrogels' elastic energy. Thirdly, in an effort to approach biological intelligence, hydrogels have been designed to serve as simultaneous sensors and actuators, enabling self-monitoring soft robots and underwater swimming robots with fully self-regulated motion, shape tracking, and propulsion. These recent progresses demonstrate the versatility of smart soft materials and their potential for producing autonomous soft robots with self-diagnostic capability, built-in feedback control, and higher level of autonomy.