Low-voltage electrically driven homeostatic hydrogel-based actuators for underwater soft robotics

We present the synthesis, fabrication and electro-mechanical characterization of a novel electro-responsive hydrogel based on Na-4-vinylbenzenesulfonate (Na-4-VBS) that can operate as a fast response bending actuator in a low voltage regime (0.2-5 V), in NaCl aqueous solutions. The bending speed can reach values up to 22°/s at 3 V and of 2.7°/s at 1 V. The responsive behavior of the benders was observed in physiological environments as well, such as phosphate buffer solution (PBS) and Dulbecco's Modified Eagle's Medium (DMEM) and exhibited similar performance. The material is a co-polymer comprising also hydroxyethyl methacrylate (HEMA) and acrylonitrile (AN), to confer high hydrophilicity to the structure and to enhance its elastic properties. According to the swelling and electro-mechanical testing results, the electrically driven deformation of the hydrogels was interpreted as a dynamic osmotic equilibrium effect taking place at the interface between the polymer and the surrounding medium, induced by the free ionic species migration throughout the polymer. This material constitutes a promising solution for the design and production of highly performing soft underwater actuators and biomimetic smart systems that can be controllably operated at the macro and mesoscale in fluids of biological interest, with minimal power consumption and below the standard potential of water electrolysis.