From Anisotropic Molecules and Particles to Small-Scale\nActuators and Robots: An Account of Polymerized Liquid Crystals

ConspectusUntethered small-scale (milli-, micro-, and\n nano-) soft robots\npromise minimally invasive and targeted medical procedures in tiny,\nflooded, and confined environments like inside the human body. Despite\nsuch potentials, small-scale robots have not yet found their way to\nreal-world applications. This can be mainly attributed to the fundamental\nand technical challenges in the fabrication, powering, navigation,\nimaging, and closed-loop control of robots at submillimiter scales.\nPertinent to this Account, the selection of building block materials\nof small-scale robots also poses a challenge that is directly related\nto their fabrication and function.Early work in microrobotics\nfocused on the mechanism of locomotion\nin fluids with low Reynolds number (<i>Re</i> ≪ 1),\nwhich was mainly inspired by the motility of cells and microorganisms.\nLooking closely at the motile cells and microorganisms, one can find\nboth order and anisotropy within their microstructure, driving out-of-equilibrium\nasymmetric deformations of their soft bodies and appendages like cilia\nand flagella, resulting in locomotion and function in environments\nwith low <i>Re</i> number. Microroboticists aim to mimic\nmicroorganisms’ locomotion and function in developing mobile\nsmall-scale robots. It is known that soft, ordered, and anisotropic\nmicrostructures of microorganisms are examples of liquid crystalline\nsystems. With this in mind, we believe that liquid crystals are underutilized\nin the design of small-scale robots, even though they have remarkable\nsimilarities to biological materials and constructs.In this\nAccount, we have shed light on the role liquid crystals\nhave played and can play in the design of small-scale robots. For\nthis, we have first elaborated on the fundamentals of liquid crystals,\nwhich include a discussion of the various types of liquid crystals\nand their characteristics, their mesophase behavior, and their anisotropic\nproperties. Then, we have discussed the applicability of anisotropic\nelastic networks of liquid crystals in the design of actuators which\nmust satisfy all four programming pillars, including elasticity, alignment,\nresponsiveness, and initial geometry. We have highlighted landmark\nreports where anisotropic elastic networks of liquid crystals, such\nas liquid crystal elastomers (LCEs), networks (LCNs), and hydrogels,\nare utilized as structural materials in the design of soft, small-scale\nactuators and robots. We point out the prevalence of the nematic phase\nand thermotropic liquid crystals utilized in these constructs over\nother mesophases and liquid crystal types as part of our discussion\non the pros and cons of liquid crystals for microrobotics research.\nFinally, paths forward for the widespread applicability of liquid\ncrystal microrobotics are envisaged. Specifically, the potential of\nsoft robots constructed from elastic networks of chromonic and micellar\nlyotropic liquid crystals provides a substantial, yet daunting, opportunity\nfor research. Furthermore, miniaturizing these constructs through\ninnovative, combinatorial alignment–fabrication strategies\non the microscale could realize liquid crystal soft robots suitable\nfor biomedical applications, unlike those made from thermotropic liquid\ncrystals. Additionally, programming alignment in alternative mesophases,\nsuch as smectic, may portray new research avenues in this emerging\ntechnology.