Reversible Electrochemically Triggered Delamination Blistering of Hydrogel Films on Micropatterned Electrodes

Stimuli responsive elastic instabilities provide opportunities for controlling the structures and properties of polymer surfaces, offering a range of potential applications. Here, a surface actuator based on a temperature and electrically responsive poly(N-isopropyl acrylamide-co-sodium acrylate) hydrogel that undergoes a two-step delamination and buckling instability triggered using micropatterned electrodes is described. The electrically actuated structures entail large out-of-plane displacements that take place on time-scales of less than 1 s, in response to modest triggering voltages (-3-6 V). Alongside these experimental observations, finite element simulations are conducted to better understand the two-step nature of the instability. In the first step, hydrogel films undergo delamination and formation of blisters, facilitated by electrochemical reduction of the thiol groups anchoring the film to the electrodes. Subsequently, at larger reducing potentials, the electrolytic current is sufficient to nucleate a gas bubble between the electrode and the gel, causing the delaminated region to adopt a straight-sided blister shape. Finally, thermally induced deswelling of the gel allows the film to be returned to its flat state and readhered to the electrode, thereby allowing for repeated actuation.