Arbitrarily and repeatedly programmable multi-layer soft actuators via “stress-caching”

Multi-layered strategy is prevalent in actuator fabrication since only a mismatch of geometric changing between layer(s) under the external stimulus is required, allowing to avoid meticulous molecular synthesis. Nonetheless, their actuation manners are usually limited to simple bending, of which the direction as well as amplitude are not (re)programmable after synthesis. Here, an arbitrarily and repeatedly programmable multi-layered actuator via “stress-caching”, where a shape memory polymer (SMP) is sandwiched with two coated hydrogel layers, is presented. Via varying the swelling/de-swelling degree between the top and bottom hydrogel layers, the designed internal stress, determining the bending direction and angle, was stored within the actuator due to the constraint of the rigid SMP layer. Upon heating, the release of the cached stress induced a “snap-like” fast actuation and the resultant geometry can be designed, preserved or recovered on demand. Additionally, the flame-retardant hydrogel layers safeguarded the actuator to work at high temperatures (even directly exposed to flame), ensuring potential applications in a fire-rescuing scenario as demonstrated. Thus, our stress-caching approach shall diversify the designs of actuator since other myriad smart materials with distinct responsiveness and functionality can be straightforwardly employed.