Toward a New Generation of Electrically Controllable Hygromorphic Soft Actuators

Abstract

An innovative processing strategy for fabricating soft structures that possess electric- and humidity-driven active/passive actuation capabilities along with touch- and humidity-sensing properties is reported. The intrinsically multifunctional material comprises an active thin layer of poly(3,4-ethylenedioxythiophene):poly­(styrene sulfonate) in a double-layered structure with a silicone elastomer and provides an opportunity toward developing a new class of smart structures for soft robotics. The emergence of responsive polymers is of fundamental interest, and their ability to reversibly reply to a given stimulus, such as heat, electric voltage, or light, has practical applications in several fields, including soft robotics, active sensing, and actuation.1 Notably, conjugated polymers (CP), such as polypyrrole (PPy), polyaniline, and poly(3,4-ethylenedioxythiophene) (PEDOT), have shown great potential with respect to actuation and significant efforts have been focused in particular on the realization of “dry” CP actuators that can function in air.[2] Recently, Okuzaki et al.3 proposed a new class of CP actuators that function in ambient air based on the cooperation between the electrical conductivity and hygroscopic nature of conductive polymers. These authors discovered that electrochemically synthesized PPy films exhibit a significant reversible volume expansion in air resulting from the absorption and desorption of water vapor present in ambient air.4 Furthermore, a volume contraction was observed under the application of an electric current, which was attributed to the desorption of water vapor as a result of Joule heating.5 A similar electrically induced isotropic dimensional change was subsequently observed in ≈20 μm thick poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films prepared using the solution-casting method.6 The ubiquitous presence of humidity in ambient air and its variation makes the development of humidity-responsive actuators both appealing and of importance. By chance, the capability to convert simple environmental stimuli, such as humidity, into mechanical reversible motion is regularly observed in living systems, particularly plants.7 These systems are capable of converting the sorption and desorption of water into driving forces for movement. A well-known example is the release of ripe seeds from pine cones, which open due to a bending movement of their scales during drying in ambient air and close in wet conditions.8 Similarly, seeds from wild wheat are propelled into soil after being released, which is solely due to the daily change in humidity that induces a curvature of the awns depending on the moisture level.9 Interestingly, many actuation systems in plants have a common structural feature: the humidity-responsive actuation results from the coupling of two differently structured tissue layers with different elongations along a specific direction. These layers are interfacially bound to each other to form a laminated, double-layered composite structure.10 Although the aforementioned actuators developed by Okuzaki's group undergo isotropic dimensional changes, anisotropic motions have not been thoroughly explored. Inspired by the differential swelling or shrinking of natural double-layered structures, one approach for achieving anisotropic motion in artificial actuators is to use a bilayer system in which the active layer is composed of a humidity-responsive material and the other layer is a passive material that is inert to humidity, that provides mechanical strength and that converts the isotropic volume change into a bending motion. In this work, we present a double-layered, anisotropic humidity-driven actuator based on an ultrathin active layer of PEDOT:PSS and a passive layer composed of poly(dimethylsiloxane) (PDMS), with intrinsic sensing capability, and able to be controlled both by joule effect or directly by environmental humidity variations. PDMS elastomer was selecte