Contractile and torsional dual-responsive artificial muscles actuated by electric heating and water droplet

Abstract Artificial muscle is a kind of soft actuators that can mimic biological muscles to realize contraction, torsion, and other action modes. Since the artificial muscles based on stimuli-responsive materials can drive the robots to achieve bionic motions under different external stimuli, they are expected to be used in various scenarios. Due to high requirements for the component fibrous materials, the artificial muscles capable of responding to two or more forms of stimuli and integrating multiple functions are rarely reported. Although some valuable attempts (host-guest, sheath-core, hybrid spinning methods) have been made to realize multi-responsive and functional artificial muscles, very complicated and demanding preparation processes are usually needed. Here, we used a direct and effective method to develop dual-responsive artificial muscles without excessive requirements on materials: by plying the viscose and silver-coated nylon coiled yarns (in response to water and electric heating, respectively) into double-helix structure, or tying them into single-strand serial structure, the contractile and torsional muscles can be fabricated readily and conveniently. Under concerted or alternate stimulation of water and electric heating, the dual-responsive muscles can exhibit significant performance improvements (in contractile stroke, output force, durability, etc) and achieve new actuation mode (bidirectional torsion) through the interaction and cooperation of the two component yarns. The moisture detectors based on the dual-responsive muscles were also demonstrated. This work provides a feasible way to prepare dual-/multi-responsive muscles using ordinary stimulus-responsive materials. Besides, the multiple stimuli and the interactions between the muscle components can be fully utilized to improve the muscle performances and extend new functions. These methods and concepts will facilitate the development of multi-responsive and functional artificial muscles, and promote their wide applications.