Structure-induced Intelligence of Liquid Crystal Elastomers.

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

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.

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.

The development of smart actuators that integrate robust mechanical properties with rapid responsiveness remains a critical challenge for advancing artificial muscles. To address this, this study incorporates lignin—a renewable resource—into liquid crystalline elastomer (LCE) to fabricate a high‐performance photothermal‐responsive artificial muscle material. By leveraging the inherent aromatic conjugated structure of lignin as an efficient photothermal unit, we successfully integrated it with mesogens to construct a liquid crystalline network featuring a nanoscale microphase‐separated architecture. This network incorporates lignin‐based phenolic carbamate bonds and disulfide bonds concurrently, forming a dual dynamic crosslinking architecture that significantly enhances the mechanical properties of LCE, achieving a tensile strength of 21.5 MPa and an elongation at break exceeding 754%, thereby substantially surpassing conventional single‐network LCEs. It also confers outstanding photothermal response, enabling rapid, self‐actuated reversible deformation (33% strain). As a light‐driven artificial muscle, it achieves 40% reversible strain and 30.0 kJ/m 3 output power density. This work presents a novel strategy for developing high‐performance light‐driven LCEs through the integration of biomass lignin as both a reinforcing phase and an intrinsic photothermal converter. This approach not only enhances the material's performance but also offers promising pathways for the design and fabrication of next‐generation soft robots and artificial muscles.

The reliance of feedback mechanisms in conventional light-fueled self-oscillating systems on spatially distributed light and intricately designed structures impedes their application and development in micro-robots, miniature actuators, and other small-scale devices. This paper presents a straightforward rheostat feedback mechanism to create an electrically driven liquid crystal elastomer (LCE) self-oscillator which comprises an LCE fiber, a rheostat, a spring, and a mass. Based on the electrothermally responsive LCE model, we first derive the governing equation for the system’s dynamics and subsequently formulate the asymptotic equation. Numerical calculations reveal two motion phases, i.e., static and self-oscillating, and elucidate the mechanism behind self-oscillation. By employing the multi-scale method, we identify the Hopf bifurcation and establish the analytical solutions for amplitude and frequency. The influence of various system parameters on the amplitude and frequency of self-oscillation was analyzed, with numerical solutions being validated against analytical results to ensure consistency. The proposed rheostat feedback mechanism can be extended to cases with rheostats that have more general resistance properties and offers advantages such as simple design, adjustable dimensions, and rapid operation. The findings are expected to inspire broader design concepts for applications in soft robotics, sensors, and adaptive structures.

<p indent=0mm>Artificial muscle is a bio-inspired soft actuator that generates large output force and displacement with flexibility and compliance and can interact with humans and biological environments. Due to low load capacity, high hysteresis, and short life cycle of existing artificial muscles, a new kind of twisted and coiled artificial muscles based on polymer fibers, such as nylon, has been developed and called twisted and coiled polymer fiber (TCPF). This artificial muscle is capable of straining more than 40%, which has a load capacity of up to 100 times higher than that of a human muscle, has energy density of up to <sc>5.3 kW/kg,</sc> and it is a hot topic in current research. This study reviews the development of state-of-the-art TCPF, focusing on fabrication and actuation mechanism, new materials and structures, improvements on actuation strategy, and applications in soft robotics. Current states are summarized and discussed for future development directions.

Hofmeister effect-inspired Ti3C2Tx MXene-based robust, multifunctional hydrogels

Materials that dynamically respond to their environment have diverse applications in artificial muscles, soft robotics, and smart textiles. Inspired by biological systems, humidity- and water-responsive actuators that bend, twist, and contract have been previously demonstrated. However, more powerful artificial muscles with large strokes and high work densities are needed, especially those that can be made cost-effectively from eco-friendly materials. We here derive such muscles from naturally abundant lotus fibers. A coiled lotus fiber yarn muscle provides a large, reversible tensile stroke of 38% and a work capacity during contraction of 450 J/kg, which is 56 times higher than that of natural skeletal muscles and higher than that for any other reported natural fiber muscles. In addition, highly twisted lotus fiber yarn muscles provide a fully reversible torsional stroke of 200°/mm of muscle length and a peak rotation speed of 200 rpm, with a generated specific torque of 488 mN·m/kg for a 2.5 cm long muscle. Potential applications of these lotus fiber yarn muscles are demonstrated for a weight-lifting artificial limb and a smart textile.

Dielectric elastomers are soft, lightweight, and highly deformable electroactive polymers that undergo large deformations when subjected to mechanical and electrical loading. This unique property makes them promising candidates for applications in artificial muscles, soft robotics, biomedical actuators, and sensors. In this study, the large deformation behavior of a flat, circular dielectric elastomer membrane, clamped at the boundary, is investigated under the combined influence of lateral pressure and an applied electric potential across its thickness. The resulting model provides a framework for predicting deformation profiles and electromechanical stability limits in dielectric elastomer actuators.

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

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.

4D printing of liquid crystal elastomers (LCEs) via direct ink writing has opened up great opportunities to create stimuli-responsive actuations for applications such as soft robotics. However, most 4D-printed LCEs are limited to thermal actuation and fixed shape morphing, posing a challenge for achieving multiple programmable functionalities and reprogrammability. Here, a 4D-printable photochromic titanium-based nanocrystal (TiNC)/LCE composite ink is developed, which enables the reprogrammable photochromism and photoactuation of a single 4D-printed architecture. The printed TiNC/LCE composite exhibits reversible color-switching between white and black in response to ultraviolet (UV) irradiation and oxygen exposure. Upon near-infrared (NIR) irradiation, the UV-irradiated region can undergo photothermal actuation, allowing for robust grasping and weightlifting. By precisely controlling the structural design and the light irradiation, the single 4D-printed TiNC/LCE object can be globally or locally programmed, erased, and reprogrammed to achieve desirable photocontrollable color patterns and 3D structure constructions, such as barcode patterns and origami- and kirigami-inspired structures. This work provides a novel concept for designing and engineering adaptive structures with unique and tunable multifunctionalities, which have potential applications in biomimetic soft robotics, smart construction engineering, camouflage, multilevel information storage, etc.

The ability for materials to adapt their shape and mechanical properties to the local environment is useful in a variety of applications, from soft robots to deployable structures. In this work, we integrate liquid crystal elastomers (LCEs) with multistable structures to allow autonomous reconfiguration in response to local changes in temperature. LCEs are incorporated in a kirigami-inspired system in which squares are connected at their vertices by small hinges composed of LCE-silicone bilayers. These bend and soften as the temperature increases above room temperature. By choosing geometric parameters for the hinges such that bifurcation points in the stability exist, a transition from mono- or tristability to bistability can be triggered by a sufficient increase in temperature, forcing rearrangements of the structure as minima in the energy landscape are removed. We demonstrate temperature-induced propagation of transition waves, enabling local structural changes to autonomously propagate and affect other parts of the structure. These effects could be harnessed in applications in interface control, reconfigurable structures, and soft robotics.

Inspired by the environment-adaptive behaviors of water striders, we 3D-printed a light-driven liquid crystal elastomer (LCE) swimming robot, OptiLCE Strider, capable of multimodal locomotion and adaptive reconfiguration at the air-water interface. Utilizing carbon nanotubes (CNTs) as photothermal fillers and dynamic disulfide bonds for shape reconfigurability, the robot exhibits three distinct propulsion modes: Marangoni-effect-driven continuous motion under low light intensity (1.3-7.2 mm s- 1), steam-wave-induced pulsatile locomotion under high light intensity (12.5-16.8mms- 1), and flapping propulsion enabled by reversible LCE deformation (4.6-6.9 mm s- 1). The dynamic disulfide bonds enable exceptional structural reconfigurability and environmental adaptability for the LCE robot to execute complex tasks, including maze navigation, cargo capture/transport, programmable rotation, and light-powered jumping (escape from grounded or obstructed states via actuation energy storage/release, with jumping height/distance 6×/3.3× the robot length). The qualitative phase map guides locomotion mode selection, while energetic cost analysis reveals a clear force-efficiency trade off among the three modes, guiding application specific selection. This study highlights the potential of dynamic LCE-based robots for intelligent systems in liquid interface environments, paving the way for versatile applications in soft robotics and biomimetic engineering.

The use of photopolymers for 3D printing facilitates high-precision fabrication of geometrically complex structures, offering exceptional dimensional accuracy and rapid curing capabilities that position it as a cornerstone of modern additive manufacturing. However, conventional photopolymers form permanently crosslinked networks, which are resistant to recycling. This inherent limitation generates persistent waste streams that are fundamentally incompatible with circular economy principles. Consequently, the development of recyclable photopolymers that maintain printability while enabling closed-loop material recovery represents a critical Frontier for sustainable manufacturing. Advancing these materials will remain essential for reconciling technological progress with environmental stewardship in the foreseeable future. This review examines recent breakthroughs in recyclable photopolymer systems for 3D printing. First, mainstream photopolymerization techniques compatible with recyclable materials are outlined, followed by an elucidation of core design strategies incorporating chemical depolymerization, thermo-mechanical reprocessing, and noncovalent interactions. Fundamental recycling mechanisms are detailed alongside performance modulation methodologies. Furthermore, emerging applications in soft robotics, wearable devices, and bioelectronic devices where recyclable photopolymers enable multifunctional devices are highlighted. Finally, persistent challenges regarding network durability and recycling efficiency are addressed, and future research directions toward truly sustainable 3D printing are proposed.