Michell's-Instability-Mediated Fast Reconfiguration of Hydrogel-Based Ring Actuators

Michell's instability, a classic type of mechanical instability, has received tremendous interest from different disciplines. It occurs when the pre-torsion in an elastic ring surpasses a critical value; the elastic ring spontaneously undergoes a transition from a flat or warped shape to a figure-of-eight configuration. However, such instability has rarely been exploited in designing soft actuators. Here, by employing a stimulus-triggered Michell's instability, hydrogel-based ring actuators capable of fast shape morphing are developed. Upon heating or light irradiation, the hydrogel ring transforms rapidly from the saddle-shape into the figure-of-eight configuration. Rigorous experiments and simulations reveal the underlying mechanism and attribute it to the stimuli-induced variations in bending and torsional stiffnesses of the gel string with anisotropic structure and response, which reduces the critical twist for Michell's instability of the ring actuator. Moreover, effects of light intensity, environmental temperature, ring size, and pre-torsion on reconfiguration are also investigated. The fast action of the ring actuators in aqueous conditions has been demonstrated through various tasks, such as screwing a bottle cap, kicking a ball, and triggering synergetic deformations. The design principle of soft actuators by harnessing Michell's instability should merit the development of other soft machines with fast action and large-amplitude reconfiguration.