Recent progress in structure-function integration of liquid crystal elastomers
Liquid crystal elastomers(LCEs)are soft matter-based active materials that can generate mechanical responses to external stimuli,such as heat,light,electric fields,and magnetic fields.The mechanical response and actuation mechanism of LCEs are based on the disruption of the ordered LC phases retained in the LCEs upon exposure to stimuli(heat or light).Owing to their strong response to stimuli,fast and large-amplitude deformation,and reversible shape-morphing capability,LCEs are cutting-edge research topics in functional polymers and soft materials.They are of particular interest in the growing areas of soft robots,artificial muscles,and intelligent and optical devices.The major advantages of LCEs for soft robotics and smart devices are programmable shape morphing and functionalities that are derived from the interplay between LC alignment and structural morphology.In recent years,the intelligence of LCEs has increasingly relied on the smartness and properties of the structures,such as the developable deformation of structural folding origami and sustainable rotation derived from the mechanical properties of Möbius strips(i.e.,the torsional strain redistributed away from the twist region).Consequently,structure-function integration has become a new category of research in advanced shape-morphing materials.Owing to the precise structural design of LCEs,the physical and mechanical characteristics of the structures can be used to construct complex deformations and actuating functions in soft actuators.Recent advances in the structure-function integration of liquid crystal elastomers are discussed in this review.The structure-function integration of LCEs relies on the synergistic effect of liquid-crystal alignments with geometric structural characteristics,which enhances the sensitivity,responsiveness,lightness,and integrated functionality of LCE materials.Intrinsic liquid-crystal mesogenic alignment-control technologies can program the magnitude or direction of the local actuation strain.Alignment arises from the application of an external force and includes mechanical alignment,surface alignment,field-assisted alignment,and shear alignment.Many types of geometric structures exist in the field of liquid crystal elastomer materials and actuators,and they exhibit rich functions and deformation methods.This review divides these materials into three major categories,based on their geometric properties and structural morphologies.First,conventional structural forms in biological systems are classified into common structures,including films,fibers,and tubules.Common structure-function integration is used to develop complex shape transformations,various locomotions,and self-sustainable autonomous concepts of LCEs.Second,novel artificially designed structural forms are classified as smart structures,consisting of origami,mechanical metamaterials,topology,bi-stable structures,and topography microstructures.Smart structure-function integration is applied to construct new and unconventional actuation,such as the auxeticity of metamaterials.Third,in integrated structures,the LCEs are regarded as actuation components.This integration emphasizes the modular assembly of multiple functional components and the design of soft robotics.Structure-function integration realizes in-situ deformation,locomotion in diverse media,snapping-through movements,multimodal motions,autonomous controls,and self-regulation.Despite these recent efforts,several challenges remain.Smart,bistable/multistable,and tensegrity structures embedded in soft robotics have emerged as promising research fields.Numerous geometric structures have not yet been studied,and an in-depth understanding of their novel physical and mechanical properties is required.Unlocking inconvenient functions through the regulation of the spatial form and alignment of LCEs is a crucial challenge in soft actuation fields,the resolution of which will expand the scope of LCE-based structure-function integration.We expect that various disciplines(e.g.,theoretical calculation,chemistry,material,engineering,machinery,and biomedicine)will undertake further collaborations to enrich the concepts,functions,and applications of liquid crystal elastomers.
liquid crystal elastomerstructure-function integrationalignmentsoft actuatorstimulus-responsive material
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