Research Progress in Biomimetic Actuators of Responsive Cross-linked Liquid Crystal Polymer

Cross-linked liquid crystal polymer (CLCP), as a combination of the order of liquid crystal orientation and the entropy elasticity of the polymer network, has exhibited unique properties such as actuation, soft elasticity, and birefringence. CLCP can mimic the behavior of organisms in a dynamically adjustable and reversible way to achieve actuation. When subjected to a specific external stimulus (such as light, heat, magnetism, or humidity), CLCP responds to the stimuli by presenting changes in LC arrangement at the micro level and reversible shape or size changes at the macro level. Because of the initial arrangement of the mesogen memorized through the network, the macroscopic deformation of CLCP is reversible. Materials with photothermal conversion effect, such as graphene, carbon nanotubes, and gold nanorods, can be combined with polymers to convert the absorbed light energy into heat energy so as to realize photothermal actuation. In addition, when photo-response moieties like azobenzene are appropriately added to CLCP, the reversible trans-cis isomerization of azobenzene under specific wavelengths of light will cause the change of LC director, thus bringing out its photodeformation performance. Moreover, there is also magnetic stimulation driven by magnetic fields and humidity stimulation based on CLCP reversible anisotropic contraction or expansion. In this review, the synthesis methods of CLCP (one-step cross-linking, two-step cross-linking, post-crosslinking, and dynamic covalent bond cross-linking) and molding technology (inkjet printing, microfluidic transportation, soft lithography, direct laser writing, and 4D printing) are described. Through reasonable structure and material design, CLCP can be fabricated into a variety of flexible actuators. Herein, this review mainly summarizes the recent research progress in the applications of intelligently responsive cross-linked liquid crystal polymer in the field of the biomimetic actuators, such as artificial muscles, and microfluidic transportation. The future development and challenges of intelligently responsive cross-linked liquid crystal polymer in actuator field are also discussed.