Simultaneous Surface Covalent Bonding and Radical Polymerization for Constructing Robust Soft Actuators with Fast Underwater Response

Although extensive attention is focused on layered hydrogel actuators, realizing both fast bending and recovery in short time along with multidirectional space deformation is still a challenge. Herein, novel thermoresponsive hydrogel/elastomer composite actuators are fabricated by simultaneous growth and interface bonding of an ultrathin poly(N-isopropylacrylamide) (PNIPAAm)/PAA hydrogel layer on the poly(dimethylsiloxane) (PDMS) sheet using the surface catalytically initiated radical polymerization method. The growing hydrogel layer shows a strong interface combination force of ∼142 N m^-1 against the PDMS sheet. The composite actuators show fast, reversible, and diverse responsive bending/recovery behavior in response to temperature change of the water bath. In a typical case, the actuator demonstrates both fast responsive bending from 0 to 500° (50 °C underwater) and fast deformation recovery from 500 to 0° (20 °C underwater) in less than 7 s. Such a fast responsive behavior can be attributed to the ultrathin thickness (μm) of the hydrogel layer. Based on this advantage, our actuators can generate directional movement as a soft robot on the ratchet surface upon applying alternating temperature field. Furthermore, novel asymmetric actuators can be easily developed by the selective growth of the hydrogel layer onto each side of the structured PDMS sheet, which can achieve a bidirectional bending and recovery behavior along with a spatial transportation of cargos.