Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel.

Electrostatic repulsion, long used for attenuating surface friction, is not typically employed for the design of bulk structural materials. We recently developed a hydrogel with a layered structure consisting of cofacially oriented electrolyte nanosheets. Because this unusual geometry imparts a large anisotropic electrostatic repulsion to the hydrogel interior, the hydrogel resisted compression orthogonal to the sheets but readily deformed along parallel shear. Building on this concept, here we show a hydrogel actuator that operates by modulating its anisotropic electrostatics in response to changes of electrostatic permittivity associated with a lower critical solution temperature transition. In the absence of substantial water uptake and release, the distance between the nanosheets rapidly expands and contracts on heating and cooling, respectively, so that the hydrogel lengthens and shortens significantly, even in air. An L-shaped hydrogel with an oblique nanosheet configuration can thus act as a unidirectionally proceeding actuator that operates without the need for external physical biases.