Physically Intelligent Liquid Crystalline Polymers for Soft Robotics and Shape-Reconfigurable Devices

Abstract

Physically intelligent materials offer wirelessly powered maneuverability independent of onboard energy sources or sophisticated control algorithms, rendering them highly suitable for miniaturized robotic systems. Liquid crystalline polymers (LCPs) are capable of morphing their shape upon temperature changes via the order–disorder transition of anisotropic liquid crystalline molecules. Liquid crystals (LCs) formed by mesogenic units exhibit various mesophases, particularly the nematic phase characterized by long-range orientational order. The LC order originates from the interplay between short-range repulsive forces induced by the anisotropic geometry of mesogenic units and long-range intermolecular interactions (such as π–π stacking and dispersion forces) that stabilize the aligned state. LCPs are synthesized by covalently incorporating thermotropic LC molecules into a network structure. Upon heating, the LC molecules partially lose their orientational order, leading to anisotropic contraction along the molecular alignment direction. To achieve untethered shape control, LCPs are molecularly engineered or integrated with functional materials to respond to various external stimuli. These stimuli can be categorized into thermal (conduction, photothermal, and electrothermal), electric field, magnetic field, and light stimuli, each offering distinct pathways for actuation. Among these, light drives LCP morphing via two primary mechanisms: photochemical and photothermal. The photochemical effect is based on the photoisomerization of embedded photoswitches (e.g., azobenzene), resulting in macroscopic changes via the skin-bulk effect. Alternatively, the photothermal effect can be achieved by either embedding functional materials (e.g., photothermal particles or dyes) or by integrating them into a bilayer structure, so as to convert absorbed photons into heat to induce the order–disorder transition of the LCP. LCP systems have progressively evolved from performing stationary deformations into sophisticated dynamic locomotion, including autonomous movement in terrestrial, aquatic, and aerial environments. The realization of such higher-level LCP locomotors relies on a comprehensive understanding of robot–environment interactions and the integration of sophisticated engineering strategies. In this regard, this spotlight outlines the strategies for molecular engineering, geometric tailoring, and stimuli modulation to advance the performance of LCP-based actuators. We discuss the underlying principles of converting photon or thermal energy into mechanical energy in LCP-based soft robotics and shape-reconfigurable devices. Our recent advances in nature-inspired locomotion of LCPs are also highlighted, with an emphasis on engineering molecular geometry to enable diverse dynamic behaviors, including rolling, climbing, and jumping. Finally, we emphasize design strategies for LCP composites as functional systems, such as flexible electronics and locomotive power sources, to provide insights into practical applications.