Thermal Gradient-Driven Heterogeneous Actuation of Liquid Crystal Elastomers for a Crawling Robot

Emerging soft robots based on liquid crystal elastomers (LCEs) exhibit remarkable capabilities for large reversible shape morphing, enabling them to adapt to complex environments and perform diverse tasks such as locomotion and camouflage. Despite extensive studies, current methods for locally controlled actuation of LCE-based soft robots often involve intricate structural design, complex programming of LCEs, incorporation of multiple materials, or complex actuation methods. Here, we present a simple and efficient approach to achieve multiple deformation modes within a simply programmed LCE structure by harnessing Joule heating-induced thermal gradients across the LCE volume. Oxidized liquid metal (LM) thin films, which exhibit increased resistance, enhanced viscosity, high thermal conductivity, and large deformability, are employed for Joule heating in this study. Using an LCE strip programmed via uniaxial stretching as an example, we perform systematic studies on the effect of essential parameters, including the actuation voltage, LCE dimensions, and the LM-to-LCE thickness ratio, on the deformation behaviors of LCEs induced by three-dimensional thermal gradients across the LCE volume. In addition, concurrently actuating two adjacent surfaces of the LCE strip yields previously inaccessible coupled bending behaviors. Finally, we demonstrate a crawling robot constructed from LM-coated LCE strips with adjustable bending capabilities, which enable multimode locomotion, including forward movement and turns, enhancing biomimetic functionality akin to leg movements observed in living organisms like reptiles. The reported strategy, which is both straightforward and versatile, promises scalability and holds potential for various applications in multifunctional intelligent systems including soft robotics and biomedical devices.