Liquid crystal elastomers (LCEs), owing to their intrinsic anisotropic property and capability of generating programmable complex morphologies under heat, have been widely used for applications ranging from soft robotics, photonic devices, cell culture, to tissue engineering. To fulfill the applications under various circumstances, high actuation efficiency, high mechanical strength, large heat and electrical conductivity, or responses to multiple stimuli are required. Therefore, design and fabrication of LCE composites are a promising strategy to enhanced physical properties and offer additional stimuli responses to the LCEs such as light, electric, and magnetic fields. In this review, we focus on recent advances in LCE composites, where LCEs are defined as anisotropic elastomeric materials in a broader context. Classic LCE composites with metallic nanoparticles, magnetic particles, liquid metal, carbon nanotubes, graphene and its derivative, and carbon black, and LCE composites from cellulose nanocrystals within the polymer network where cellulose can provide the unique liquid crystal anisotropy will be discussed. We conclude with the challenges and future research opportunities. 

Multi-functional liquid crystal elastomer composites

Many inspirations for soft robotics are from the natural world, such as octopuses, snakes, and caterpillars. Here, we report a caterpillar-inspired, energy-efficient crawling robot with multiple crawling modes, enabled by joule heating of a patterned soft heater consisting of silver nanowire networks in a liquid crystal elastomer (LCE)–based thermal bimorph actuator. With patterned and distributed heaters and programmable heating, different temperature and hence curvature distribution along the body of the robot are achieved, enabling bidirectional locomotion as a result of the friction competition between the front and rear end with the ground. The thermal bimorph behavior is studied to predict and optimize the local curvature of the robot under thermal stimuli. The bidirectional actuation modes with the crawling speeds are investigated. The capability of passing through obstacles with limited spacing are demonstrated. The strategy of distributed and programmable heating and actuation with thermal responsive materials offers unprecedented capabilities for smart and multifunctional soft robots.

https://www.science.org/doi/pdf/10.1126/sciadv.adf8014?download=true

Caterpillar-inspired soft crawling robot with distributed programmable thermal actuation

Mechanostructures: rational mechanical design, fabrication, performance evaluation, and industrial application of advanced structures

Untethered twist fibers do not require end-anchoring structures to hold their twist orientation and offer simple designs and convenient operation. The reversible responsiveness of these fibers allows them to generate torque and rotational deformation continuously upon the application of external stimuli. The fibers therefore have potential in rotating microengines. In practical applications, high torque and rotational deformation are desirable to meet work capacity requirements. However, the simultaneous endowment of reversible responsiveness and high rotational performance to untethered twist fibers remains a challenge. In this study, a liquid crystal elastomer twist fiber (LCETF) is designed and developed with a fixed twisting alignment of mesogens to provide untethered and reversible responsiveness. Outstanding rotational performance can be achieved when the mesogenic orientation is disrupted through heat triggering. Owing to the significant intrinsic contractile ratio of the LCE material, the rotational deformation of the LCETF can reach 243.6° mm-1 . More importantly, the specific torque can reach 10.1 N m kg-1 , which exceeds previously reported values. In addition, the LCETF can be exploited in a rotating microengine to convert heat into electricity with an induction voltage as high as 9.4 V. This work broadens the applications of LCEs for energy harvesters, micromachines, and soft robots. 

Liquid Crystal Elastomer Twist Fibers toward Rotating Microengines

Leveraging liquid crystal elastomers (LCEs) to realize scalable fabrication of high-performing fibrous artificial muscles is of particular interest because these active soft materials can provide large, reversible, programmable deformations upon environmental stimuli. High-performing fibrous LCEs require the used processing technology to enable shaping LCEs into micro-scale fine fibers as thin as possible while achieving macroscopic LC orientation, which however remains a daunting challenge. Here, we report a bioinspired spinning technology that allows for continuous, high-speed production (fabrication speed up to 8400 m/h) of thin and aligned LCE microfibers combined with rapid deformation (actuation strain rate up to 810% s−1), powerful actuation (actuation stress up to 5.3 MPa), high response frequency (50 Hz), and long cycle life (250000 cycles without obvious fatigue). Inspired by liquid crystalline spinning of spiders that take advantage of multiple drawdowns to thin and align their dragline silks, we employed internal drawdown via tapered-wall-induced-shearing and external drawdown via mechanical stretching to shape LCEs into long, thin, aligned microfibers with the desirable actuation performances, which few processing technologies can achieve. This bioinspired processing technology capable of scalable production of high-performing fibrous LCEs would benefit the development of smart fabrics, intelligent wearable devices, humanoid robotics, and other areas. This article is protected by copyright. All rights reserved 

Bioinspired Liquid Crystalline Spinning Enables Scalable Fabrication of High‐Performing Fibrous Artificial Muscles

Liquid crystal elastomer (LCE) fibers are capable of large and reversible deformations, making them an ideal artificial muscle. However, limited to stimulating source and structural design, current LCE fibers have not yet achieved both large contraction ratio and fast contraction rate to perform the intense motion. In this work, electrothermal-responsive liquid metal (LM) containing LCE (LM-LCE) fibers is reported. By introducing flexible liquid metal, LM-LCE fibers retain deformability with a large contraction ratio similar to that of pure LCE fibers and are endowed with electrical responsiveness. Applying precisely controlled electrical stimulation, the contraction ratio and rate of LM-LCE fibers can be programmed by adjusting voltage value and pulse time. Under electrical stimulation at 1.25 V cm-1 , 0.1 s, LM-LCE fibers can produce over 40% contraction ratio at an ultrafast contraction rate of up to 280% s-1 . Furthermore, LM-LCE fibers mimic human triceps muscle and can conduct precise ball shooting. LM-LCE fibers with excellent contraction ratio and rate extend their functionality as artificial muscles to perform intense movements and are expected to enrich the challenging applications of soft robots.

Ultrafast, High-Contractile Electrothermal-Driven Liquid Crystal Elastomer Fibers towards Artificial Muscles.

Bioinspired Construction of Artificial Cardiac Muscles Based on Liquid Crystal Elastomer Fibers

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Ultrafast, High-Contractile Electrothermal-Driven Liquid Crystal Elastomer Fibers towards Artificial Muscles.

Bioinspired Construction of Artificial Cardiac Muscles Based on Liquid Crystal Elastomer Fibers