Fast Hydrogel Micro-Actuators Driven by Electric Fields

Electrically-driven hydrogels are crosslinked, charged polymer networks that can deform in an electric field, prompted by osmotic pressure changes. To date, such actuators generally suffer from sluggish response time, with equilibrium actuation times ranging from minutes to hours in aqueous electrolytes, and lack sophisticated design, due to established manufacturing protocols. Herein, these limitations are overcome through the fabrication of polyelectrolyte hydrogel microstructures via two-photon polymerization (2PP). This approach allows for the realization of microscale electrically-driven actuators exhibiting fast actuation (∼200 ms equilibrium time). The work highlights three photoresist formulations for poly(anionic) and poly(cationic) hydrogel networks and their fabrication via 2PP to produce micro-electro-actuators with sub-micron features. The electrically-driven actuation performance is investigated by varying the hydrogel composition, actuator geometry, along with electric field strength and direction, and local environment (pH and electrolyte concentration) during actuation. It was determined that micro-cantilevers of 80 × 20 × 10 µm³ reached equilibrium bending of up to 44.9 ± 7.8°, in ∼ 200 ms, in response to electric fields of 6 V mm^-1. This pioneering work marks the first integration of 2PP with electrically actuated gelatin-based hydrogels, showcasing micro-electro-actuators with rapid and programmable 4D motion.