Photothermal‐Responsive Lignin‐Based Liquid Crystalline Elastomers: Constructing High‐Performance Artificial Muscles via a Dual Dynamic Network

A method for fabricating long, soft, and reversibly actuatable liquid crystal elastomer (LCE) fibers by using direct ink write (DIW) printing was developed. Here, the LCE was produced based on a two-stage thermal-photo curingreaction between a difunctional acrylate monomer and thiol. The LCE ink, mixed with nanoclay to increase the viscosity, was extruded through a nozzle onto a rotating mandrel to obtain a long fiber. After printing, the fiber was first thermally cured on the mandrel, then mechanically stretched, and photocured to achieve liquid crystal chain alignment for stress-free reversible activation. Upon optimizing the ink viscosity and DIW printing parameters, long fibers (up to 1.5 m long from the laboratory) were obtained. The resulting fiber had a modulus of 2 MPa, 51% actuation strain, and a failure strain of well over 100%. The potential of these fibers for applications was demonstrated. The LCE fibers were knit, sewn, and woven to form a variety of smart textiles. The fiber was also used to mimic bicep muscles with both large activation force and activation strain. By incorporating further intelligent characteristics, such as conductivity and biosensing into a single fiber, the LCE fibers could be potentially used for smart clothing, soft robotics, and biomedical devices.

Human muscles, including skeletal, smooth, and cardiac muscles, are able to perform diverse deformations and execute complex biofunctions stimulated by nerve signals. Similarly, liquid crystal elastomer (LCE), which can respond to external stimuli with large and reversible deformations, demonstrates superior advantages to mimic nature muscles to fabricate artificial muscles. Till now, LCE has been utilized to simulate deformations and related functions of skeletal and smooth muscles. However, limited by the existing fabrication strategy, employing LCE to mimic the motion of cardiac muscles and further realizing the structure‐determined pumping functions, is still an open challenge. Learning from the specific spatial arrangements and synergistic actuation of cardiac muscle fibers within human heart, a simple and general strategy to construct artificial cardiac muscles with LCE fibers is proposed. In this work, LCE fibers with similar modulus and actuation behavior to muscle fibers are fabricated and spatially arranged in biological architectures as cardiac muscle fibers. As a result, artificial cardiac muscles are constructed and are able to perform simultaneous contraction and torsion motions, realizing heart‐pumping functions. This general strategy should be also applicable for other smart materials to conduct challenging tasks.

Synthesis-Processing-Property Relationships in Thermomechanics of Liquid Crystal Elastomers

Morphing of stiffness-heterogeneous liquid crystal elastomers via mechanical training and locally controlled photopolymerization

Liquid crystal elastomers (LCEs) are shape-morphing materials whose large and reversible shape transformations are caused by the coupling between the mobile anisotropic properties of liquid crystal (LC) units and the rubber elastic of polymer networks. Their shape-changing behaviors under certain stimuli are largely directed by the LC orientation; therefore, various strategies have been developed to spatially modulate the LC alignments. However, most of these methods are limited as they require complex fabrication technologies or have intrinsic limitations in applicability. To address this issue, programmable complex shape changes in some LCE types, such as polysiloxane side-chain LCEs, thiol-acrylate main-chain LCEs, etc., were achieved by using a mechanical alignment programming process coupled with two-step crosslinking. Here, we report a polysiloxane main-chain LCE with programmable 2- and 3D shape-changing abilities that were created by mechanically programming the polydomain LCE with two crosslinking steps. The resulting LCEs exhibited a reversible thermal-induced shape transformation between the initial and programmed shapes due to the two-way memory between the first and second network structures. Our findings expand on the applications of LCE materials in actuators, soft robotics, and smart structures where arbitrary and easily programmed shape morphing is needed.

Liquid crystal elastomers (LCEs) are active soft matter-based materials with strong stimulus responsiveness and reversible, large-shape morphing capabilities. LCEs have demonstrated broad and growing applications in soft robotics, wearable devices, artificial muscles, and optical machines. The actuation intelligence and advanced functionality of LCEs depend on the smartness and properties of structures. In this review, we discuss recent advances in structure-induced intelligence of LCEs, specifically the integration of structural properties with the alignment and processing of LCEs. The structural design principles for three categories consisting of common structures (film, fiber, and tubule), smart structures (origami, kirigami, mechanical metamaterial, topology, and topography), and complex structures (monolithic and integrated) are presented. Various alignment controls of LCEs, including mechanical, surface, field-assisted, and shear alignment, are capable of inducing structural properties. The coupling and collaboration mechanisms of the LCE structures and the generated functions are discussed. The review concludes with perspectives on current challenges and emerging opportunities.

Liquid crystal elastomers (LCEs) are a class of smart materials that combine the anisotropic properties of liquid crystals with the elasticity of polymers, enabling reversible metamorphosis (i.e., shape transformation) in response to external stimuli. This reversible metamorphosis makes them ideal for many applications including soft robotics, artificial muscles, sensors, actuators, responsive coatings, etc. Prior studies have designed LCE surfaces with superhydrophobicity (i.e., extreme repellency to high surface tension liquid like water), but the combination of reversible metamorphosis of LCEs with superomniphobicity (i.e., extreme repellency to both high and low surface tension liquids) is unexplored. In this work, we developed LCE-based superomniphobic surfaces with reversible metamorphosis by laser texturing followed by low surface energy surface modification. Our LCE-based superomniphobic surfaces display extreme repellence to both aqueous and organic liquids as well as reversible metamorphosis due to nematic-isotropic transition of LCE. Utilizing these properties, we demonstrated loss-less manipulation of aqueous and organic liquid droplets, enabling merging, mixing, chemical reaction and microfluidic gating. We envision that our LCE-based superomniphobic surfaces with reversible metamorphosis will pave the way towards a wide range of applications including microfluidic reactors, lab-on-chip technologies, adaptive liquid-handling devices, controlled drug delivery systems etc.

Applying thin electrically conductive coatings to Liquid Crystalline Elastomers (LCEs) is an effective way of functionalizing two-way shape memory polymers with the ability to respond to electrical currents. However, achieving robust adhesion between a given electrically conductive coating and the surface of LCEs can be challenging. This can limit the functionality, performance, and potential applications of these materials. This work describes a facile method to develop electrically responsive Liquid Crystalline Elastomer polymeric artificial muscles with strain-sensing, self-actuation-sensing, and joule-heating features. In this work, the effect of treating LCEs with polydopamine (PDA) prior to functionalizing the LCE with an electrically conductive gold-sputtered coating was explored. The findings confirmed that the PDA treatment considerably improved the adhesion of the gold sputter coating to the LCEs, thereby leading to the fabrication of multi-functional strain-sensing, electrically conductive, and electro-responsive LCEs.

Fiber actuators underpin soft robots, artificial muscles, and smart textiles. A persistent bottleneck is the fabrication of monodomain liquid crystal elastomer (LCE) microfibers with narrow size distributions while preserving axial alignment. This work establishes a melt-centrifugal spinning (MCS) route with two-step UV fixation that separates flow-induced alignment from network crosslinking. High-speed rotation creates a long extensional jet; an obliquely incident, on-the-fly UV dose at touchdown locks the director, and a post-cure consolidates the network. The obtained LCE microfiber can achieve large reversible contraction (L/L0 = 0.56), lift a weight, and trigger the tweezers. The method produces a new approach for the fabrication of device-ready LCE actuators, establishes a general design principle for diameter control via curing sequence, and opens a practical path toward artificial muscles and flexible micro robotics.

HighlightsLiquid crystal elastomers enable programmable shape changes in soft robots, useful in artificial muscles, adaptive grippers, and robots that move or transform shape.Various alignment approaches, including mechanical techniques, electric and magnetic fields, and surface patterning, effectively control the direction and type of deformation.Heating, light exposure, and external fields trigger reversible actuation in liquid crystal elastomers, each offering unique benefits for specific robotic tasks.Challenges like manufacturing scalability, mechanical durability, and safety remain key obstacles to the practical commercialization of these soft robotic actuators.Future research will aim to overcome these limitations by improving materials, fabrication methods, and integration strategies, advancing intelligent robotic applications.

ABSTRACTA novel liquid crystal elastomer (LCE) synthesized by melt polymerization, which exhibits the capacity of shape memory, is reported here for the first time. The method of synthesize the shape memory LCE has been explored. A facile two‐step method to synthesize these anisotropic materials to realize reversible shape change behavior is reported. The first reaction is the addition of nematic liquid crystal molecules to form a kind of liquid crystal polymer. Subsequently, the polymer is crosslinked to trap the order of the liquid crystal into a crosslinked LCE. The LCE exhibits liquid crystalline behavior which has shape memory with excellent fixity and recovery. Its shape memory and actuating properties also have been studied. When reheating the LCE to 165 °C, the shape will recover. The main chains and crosslinked bonds of the LCE contain ester groups, which are sensitive to alkaline and acidic condition. It turns out that the LCE is intact under acidic condition, but it can be degraded under alkaline condition. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 389–394

Liquid crystal elastomers (LCEs) are promising building blocks for soft robots, given their large, programmable, reversible, and stimuli‐responsive shape change. Enhancing LCEs’ stiffness and toughness has been a longstanding desire previously explored by reinforcing them with fillers, crystalline microdomains, and interpenetrating polymer networks. While promising, these methods adversely affect molecular order and thermal strain. Here, a significant enhancement of the stiffness of LCEs is reported by loading them with low molecular weight liquid crystals (LMWLCs) without sacrificing thermal strain and molecular order. While pristine LCEs rapidly transition to a soft elastic plateau when strained from poly‐ to monodomain, LC‐loaded samples (LC‐LCEs) first experience a pronounced linear elasticity, followed by a soft elastic plateau at higher stresses. Further thermomechanical and X‐ray analysis confirm the emergence of an additional mesophase in polydomain LC‐LCEs, which evolves to short‐range smectic (cybotactic) during the poly‐ to monodomain transition. Monodomain LC‐LCEs show between 6.5‐ and 9.0‐fold stiffness enhancement with improved molecular order and thermal strain. Their work densities are more than double that of pristine LCEs, with active thermal stroke of up to 25% under loads of over 2000 times their weight. Such remarkable behaviors are attributed to the interplay between post‐polymerization phase separation of LCs and their strain‐enhanced smectic ordering. The results suggest that LMWLC inclusion can be a simple yet robust method to significantly improve the mechanical properties of LCEs.

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.

This paper summarises the properties of archiral and chiral liquid crystalline (LC) elastomers, the latter of which have found interest due to their piezoelectric properties. In addition the formation of new LC elastomers by a radical polymerization of acrylate groups covalently linked to LC polymers was investigated. This was done concerning the progress of the crosslinking reaction, concerning the influence of the crosslinking on the LC phases and concerning the network properties (swelling ratio and elastic modulus) of the resulting elastomers. It turns out that the networks prepared in this way are very soft and their crosslinking density is low. This can be explained assuming that the radical polymerization of the acrylates leads only to short oligomers, due to steric restrictions. Photo crosslinking initiated by a photoinitiator in the LC phase creates a memory of the orientation before crosslinking.

Asymmetric bending enables rapid and efficient crawling motions in a soft robotic system. However, achieving asymmetric motion in soft robots with temperature‐responsive liquid crystal elastomers (LCEs) necessitates complex Joule heating configurations or precise temperature regulation. Herein, a simple electroless plating method is presented for patterning asymmetric temperature gradients on paper substrates, thereby facilitating the development of Joule heating, caterpillar‐inspired soft robots. Cellulose‐based paper substrates provide distinct advantages due to their porous structure, which enables facile electrode deposition via solution‐based processes and offers high mechanical deformability. Leveraging these material properties, Cu electrodes are asymmetrically deposited onto paper substrates by varying electrode widths. This variation induces differences in electrical resistance, generating significant temperature gradients across the substrate. Furthermore, it is demonstrated that temperature gradients could be modulated by adjusting the extent of Cu enhancement in the electrode patterns. it is fabricated energy‐efficient soft robots capable of directional crawling at a low actuation voltage of 0.5 V by integrating the paper substrate with LCEs in a bilayer architecture. The simplicity of the electrode patterning process, combined with the eco‐friendly nature of paper‐based actuators, presents a promising approach for the scalable and sustainable fabrication of soft robots with complex locomotion capabilities.