Multi-strand Fibers with Hierarchical Helical Structures Driven by Water or Moisture for Soft Actuators

Smart actuators that combine excellent mechanical properties and responsive actuating performance like biological muscles have attracted considerable attention. In this study, a water/humidity responsive actuator, consisting of multi-strand carboxyl methyl cellulose (CMC) fibers with helical structures, was prepared using wet-spinning and twisting methods. The results showed that owing to the multi-strand structure, the actuator consisted of one-, two-, three-, and four-strand helical fibers, thus achieving a combination of high strength (∼27 MPa), high toughness (>10.34 MJ/m³), and large load limit (>0.30 N), which enable the actuator to theoretically withstand a weight that is at least 20,000 times its weight. Meanwhile, owing to the excellent moisture-responsive ability of CMC, the actuator, with a 5 g load, could achieve untwisting motion. Additionally, its maximum speed was approximately 2158 ± 233 rpm/m under water stimulation, whereas the recovery speed could reach 804 ± 44 rpm/m. Moreover, this untwisting-recovery reversible process was cyclic, whereas the shape and the actuating speed of the actuator remained stable after more than 150 cycles. The actuator improved the load limit that the fiber could withstand when driving under stimulation, thereby enabling the actuator to lift or move heavy objects like human muscles when executing spontaneously under external stimuli. This result shows considerable potential applications in artificial muscles and biomimetic robots.