Chemo-mechanical instability of light-induced humidity responsive bilayered actuators

Recently discovered light-induced bilayered actuators comprising a light-responsive actuating layer supported by a passive layer are versatile in miniaturized robotics applications, owing to their simple, compact construction and wireless, self-contained mode of actuation. However, the chemo-mechanics and quantitative description of their actuation mechanisms are not sufficiently understood. Here, based on a chemo-mechanics model, a novel instability phenomenon leading to extraordinarily large magnitudes of the bending actuation of bilayered actuators is found and experimentally proven. At specific ratios of the elastic moduli and thicknesses of the active and passive layers, and activation volume of the actuation mechanism, the actuation of the active layer will be put into a positive feedback mode where the actuation-induced bending of the cantilever structure triggers a compressive stress in a surface region of the active layer which enhances further contractive actuation of the latter by means of light-induced water de-intercalation. The beneficial instability is observed and analyzed for two active material systems that exhibit such a light-induced water de-intercalation mechanism, namely, cobalt-oxides/hydroxides (C-O-H) and nickel hydroxide/oxyhydroxide (N-H-O). Experimental results agree well with predictions of the chemo-mechanics model, thus verifying its applicability to design high-performing actuation systems.