Liquid Crystal Elastomer Actuators Research Articles

Topological defects as nucleation points of the nematic-isotropic phase transition in liquid crystal shells

The transition from a nematic to an isotropic state in a self-closing spherical liquid crystal shell with tangential alignment is a stimulating phenomenon to investigate, as the topology dictates that the shell exhibits local isotropic points at all temperatures in the nematic phase range, in the form of topological defects. The defects may thus be expected to act as nucleation points for the phase transition upon heating beyond the bulk nematic stability range. Here we study this peculiar transition, theoretically and experimentally, for shells with two different configurations of four +1/2 defects, finding that the defects act as the primary nucleation points if they are co-localized in each other's vicinity. If the defects are instead spread out across the shell, they again act as nucleation points, albeit not necessarily the primary ones. Beyond adding to our understanding of how the orientational order-disorder transition can take place in the shell geometry, our results have practical relevance for, e.g., the use of curved liquid crystals in sensing applications or for liquid crystal elastomer actuators in shell shape, undergoing a shape change as a result of the nematic-isotropic transition. Published by the American Physical Society 2024

Programmable, Self-Healable, and Photochromic Liquid Crystal Elastomers Based on Dynamic Hindered Urea Bonds for Biomimetic Flowers.

Recently, researchers have been exploring the use of dynamic covalent bonds (DCBs) in the construction of exchangeable liquid crystal elastomers (LCEs) for biomimetic actuators and devices. However, a significant challenge remains in achieving LCEs with both excellent dynamic properties and superior mechanical strength and stability. In this study, a diacrylate-functionalized monomer containing dynamic hindered urea bonds (DA-HUB) is employed to prepare exchangeable LCEs through a self-catalytic Michael addition reaction. By incorporating DA-HUB, the LCE system benefits from DCBs and hydrogen bonding, leading to materials with high mechanical strength and a range of dynamic properties such as programmability, self-healing, and recyclability. Leveraging these characteristics, bilayer LCE actuators with controlled reversible thermal deformation and outstanding dimensional stability are successfully fabricated using a simple welding method. Moreover, a biomimetic triangular plum, inspired by the blooming of flowers, is created to showcase reversible color and shape changes triggered by light and heat. This innovative approach opens new possibilities for the development of biomimetic and smart actuators and devices with multiple functionalities.

Liquid Crystal Elastomer Actuators Enhanced by Tapered Optical Fibers for Controllable Bending Directions and Amplitudes

AbstractLiquid crystal elastomer (LCE) photoactuators offer a versatile platform for executing complicated tasks through precisely controlled bending directions and amplitudes. Optical waveguides assist photoactuators to perform tasks unimpededly. While most existing optical waveguide actuators are limited to unidirectional bending, limiting their versatility. Here, a LCE actuator integrated with a tapered optical fiber with controllable bending directions and amplitudes is presented. The LCE actuator displays bending motions induced by uneven shrinkage due to the off‐center fiber. The bending directions are parallel to the previous uniaxial alignment of the LCE film, which can be adjusted to an arbitrary angle with the tapered optical fiber. The LCE actuators exhibit adaptable bending directions at 0°, 45°, and 90°, representing the possible bending motions. The bending amplitudes are regulated by infrared laser power. A hook‐like LCE actuator with 0° bending direction carried the handle of a basket. In addition, a 90° bending direction LCE actuator displays the shrinkage behavior, showing a special bending motion control. Moreover, a bionic hand, comprising five LCE actuators with various bending directions, exhibited diverse gestures with precise control. The controllable bending motions in optical waveguide actuators expand the promising applications in complicated scenarios, such as catheters or other medical devices.

Photothermal porous monodomain liquid crystal elastomer actuator based on confined expansion method

Light-driven monodomain liquid crystal elastomers (mLCEs) have the advantages of remote operation and local control, but the response time will increase rapidly with the increase of thickness due to the limitation of the incident light depth. In order to achieve fast response, porous AuNPs-mLCEs actuators are proposed and fabricated by a confined expansion method. The average pore size of the prepared rod-shaped porous mLCEs is 200 μm, the porosity is 46.5%, and the order parameter of mesogens is 0.41. When the diameter is 3 mm, the response time is 1.2 s at a wavelength of 532 nm and a light intensity of 2.6 W•cm−2. When the diameter increases to 10 mm, the response time only increases to 2.2 s. The contraction of porous mLCEs actuator caused by the photothermal effect enables it can not only to lift an object 321.5 times heavier than itself by contraction, but also raise-up an object 5.2 times heavier than itself by bending. This method can be used to design and manufacture various 3D porous photothermal actuators with fast response.

Programming Motion into Materials Using Electricity-Driven Liquid Crystal Elastomer Actuators.

As thermally driven smart materials capable of large reversible deformations, liquid crystal elastomers (LCEs) have great potential for applications in bionic soft robots, artificial muscles, controllable actuators, and flexible sensors due to their ability to program controllable motion into materials. In this article, we introduce conductive LCE actuators using a liquid metal electrothermal layer and a polyethylene terephthalate substrate. Our LCE actuators can be stimulated at low currents from 2 to 4 A and produce a maximum work density of 9.4 . We illustrate the potential applications of this system by designing a palm-activated artificial muscle gripper, which can be used to grasp soft objects ranging from 5 to 55 mm in size, and even ring-shaped workpieces with precise external or internal support. Furthermore, inspired by the movement of fruit fly larvae, we designed a new soft robot capable of bioinspired crawling and turning by inducing anisotropic friction with an asymmetric design. Finally, we illustrate advanced motional control by designing an autonomously rotating wheel based on the asymmetric contraction of its spokes. To assist in the production of autonomously moving robots, we provide a thorough characterization of its motion dynamics.

Thermoplastic liquid crystal elastomer soft actuators

ABSTRACT This invited article is an account of a lecture on thermoplastic liquid crystal elastomers (LCEs) given at the 16th European Conference on Liquid Crystals (ECLC) in Rende, Italy. Currently most LCE actuators are permanently crosslinked thermoset. We avoid permanently crosslinked networks and instead obtain melt- and re-processable, recyclable thermoplastic LCE actuators by introducing H-bond interactions as physical crosslinks. Post-polymerisation of the LCE actuators is not required. This short proceedings article summarises the research carried out in our group on thermoplastic LCE actuators. The LCEs show both temperature and light-driven deformations and thermoplastic properties, affording sustainable soft actuators exhibiting melt-processable and recyclable functionalities compatible with large-scale industrial processing methods. It is envisioned that these polymers will serve as starting point for industrially-relevant smart materials that are applicable in our society for a myriad of applications including soft robotics, smart textiles, smart surface, artificial muscles, and deployable soft actuating devices.

Bioinspired Jumping Soft Actuators of the Liquid Crystal Elastomer Enabled by Photo-Mechanical Coupling.

Jumping, a fundamental survival behavior observed in organisms, serves as a vital mechanism for adapting to the surrounding environment and overcoming significant obstacles within a given terrain. Here, we present a light-controlled soft jumping actuator inspired by asphondylia, which employs a closed-loop structure and utilizes a liquid crystal elastomer (LCE). Photo-mechanical coupling highlights the significant influence of the light source on the actuator's jumping behavior. Manipulating the light intensity, the relative position of stimulus and light lock, and the concentration of disperse red 1 (DR1) allows precise control over both the maximum take-off velocity and jump height. Furthermore, tailoring the size of the LCE actuator offers a means of regulating jumping behavior. Upon exposure to 460 nm LED irradiation, our actuator achieves remarkable performance, with a maximum jumping height of 10 body length (BL) and take-off velocity of 62 BL/s. These actuators accumulate and rapidly release energy, enabling the effective transportation of microcargos across substantial distances. Our research yields valuable insights into the realm of soft robotics, underscoring the pivotal importance of photo-mechanical coupling in the field of soft robotics, thereby serving as a catalyst for inspiring continued exploration of agile and capable systems by prestoring elastic energy.

A Porous Multi‐Stimuli‐Responsive Liquid Crystal Elastomer Actuator Enabled by Mof Loading

AbstractA porous actuator is prepared using a liquid crystal elastomer (LCE) loaded with metal‐organic framework (MOF) nanoparticles (MIL‐88A). While the swellable MOF additive endows the LCE actuator with nanoscale pores, after its removal by chemical etching, macroporous LCE actuator is obtained. This LCE‐MOF actuator displays several interesting features. 1) Selective etching allows the actuator in water to curl in one direction and then reverse the curling direction owing to differential water absorption of the nano‐ and macroporous layers. 2) The stretching‐induced alignment of mesogens in the actuator is little affected by the presence of MOF and substantially retained after water uptake, so that an actuation deformation can be generated either by water absorption or thermally induced order‐disorder phase transition. 3) Assisted by the UV irradiation used for polymer cross‐linking, magnetic iron oxide FeO appears to be formed after chemical etching, which allows the actuator to gain the ability to move on water surface guided by a magnet. 4) The in situ formation of magnetic iron oxide in the macroporous actuator also provides an enhanced photothermal effect, making light‐driven locomotion of the actuator more effective. The use of active porogen like MOF opens a new way to explore porous LCE actuators.

Multimodal and Multistimuli 4D-Printed Magnetic Composite Liquid Crystal Elastomer Actuators.

Liquid crystal elastomer (LCE)-based soft actuators are being studied for their significant shape-changing abilities when they are exposed to heat or light. Nevertheless, their relatively slow response compared with soft magnetic materials limits their application possibilities. Integration of magnetic responsiveness with LCEs has been previously attempted; however, the LCE response is typically jeopardized in high volumes of magnetic microparticles (MMPs). Here, a multistimuli, magnetically active LCE (MLCE), capable of producing programmable and multimodal actuation, is presented. The MLCE, composed of MMPs within an LCE matrix, is generated through extrusion-based 4D printing that enables digital control of mesogen orientation even at a 1:1 (LCE:MMPs) weight ratio, a challenging task to accomplish with other methods. The printed actuators can significantly deform when thermally actuated as well as exhibit fast response to magnetic fields. When combining thermal and magnetic stimuli, modes of actuation inaccessible with only one input are achieved. For instance, the actuator is reconfigured into various states by using the heat-mediated LCE response, followed by subsequent magnetic addressing. The multistimuli capabilities of the MLCE composite expand its applicability where common LCE actuators face limitations in speed and precision. To illustrate, a beam-steering device developed by using these materials is presented.

Liquid Crystal Elastomer Artificial Tendrils with Asymmetric Core-Sheath Structure Showing Evolutionary Biomimetic Locomotion.

The sophisticated and complex haptonastic movements in response to environmental-stimuli of living organisms have always fascinated scientists. However, how to fundamentally mimic the sophisticated hierarchical architectures of living organisms to provide the artificial counterparts with similar or even beyond-natural functions based on the underlying mechanism remains a major scientific challenge. Here, liquid crystal elastomer (LCE) artificial tendrils showing evolutionary biomimetic locomotion are developed following the structure-function principle that is used in nature to grow climbing plants. These elaborately designed tendril-like LCE actuators possess an asymmetric core-sheath architecture which shows a higher-to-lower transition in the degree of LC orientation from the sheath-to-core layer across the semi-ellipse cross-section. Upon heating and cooling, the LCE artificial tendril can undergo reversible tendril-like shape-morphing behaviors, such as helical coiling/winding, and perversion. The fundamental mechanism of the helical shape-morphing of the artificial tendril is revealed by using theoretical models and finite element simulations. Besides, the incorporation of metal-ligand coordination into the LCE network provides the artificial tendril with reconfigurable shape-morphing performances such as helical transitions and rotational deformations. Finally, the abilities of helical and rotational deformations are integrated into a new reprogrammed flagellum-like architecture to perform evolutionary locomotion mimicking the haptonastic movements of the natural flagellum.

Intelligent Liquid Crystal Elastomer Actuators with High Mechanical Strength, Self‐Sensing, and Automatic Control

AbstractSelf‐sensing liquid crystal elastomer (LCE) actuators with integration of sensing and actuating have attracted significant attention in health monitoring and physical rehabilitation therapy. However, the development of LCE actuators equipped with high mechanical strength, self‐sensing capabilities, and automatic control simultaneously is still a significant challenge. Here, intelligent self‐sensing LCE actuators with high mechanical strength and automatic control are demonstrated by doping carbon‐materials composite of graphite and carbon black. The proposed actuators, driven by the electrothermal effect, demonstrate a significant increase in the ratio of lifted object mass/self‐weight ratio up to 1168 times, which is then tested in activities such as football kicking and arm swinging of a doll. The self‐sensing LCE actuators possess a 30% actuation strain and have real‐time sensing capability during Joule heating. They can be programmed by control circuits based on conditional logic judgments of the target. In addition, a smart glove equipped with self‐sensing LCE actuators, which have high mechanical strength and an automatic control system for assisting finger movement, is also demonstrated. The proposed LCE actuator shows great potential for applications in assisting finger movement, aiding in joint and knee recovery, and other smart physical rehabilitation therapy applications.

Programmable and Self‐Healable Liquid Crystal Elastomer Actuators Based on Halogen Bonding

AbstractShape‐changing polymeric materials have gained significant attention in the field of bioinspired soft robotics. However, challenges remain in versatilizing the shape‐morphing process to suit different tasks and environments, and in designing systems that combine reversible actuation and self‐healing ability. Here, we report halogen‐bonded liquid crystal elastomers (LCEs) that can be arbitrarily shape‐programmed and that self‐heal under mild thermal or photothermal stimulation. We incorporate halogen‐bond‐donating diiodotetrafluorobenzene molecules as dynamic supramolecular crosslinks into the LCEs and show that these relatively weak crosslinks are pertinent for their mechanical programming and self‐healing. Utilizing the halogen‐bonded LCEs, we demonstrate proof‐of‐concept soft robotic motions such as crawling and rolling with programmed velocities. Our results showcase halogen bonding as a promising, yet unexplored tool for the preparation of smart supramolecular constructs for the development of advanced soft actuators.

Programmable and Self-Healable Liquid Crystal Elastomer Actuators Based on Halogen Bonding.

Shape-changing polymeric materials have gained significant attention in the field of bioinspired soft robotics. However, challenges remain in versatilizing the shape-morphing process to suit different tasks and environments, and in designing systems that combine reversible actuation and self-healing ability. Here, we report halogen-bonded liquid crystal elastomers (LCEs) that can be arbitrarily shape-programmed and that self-heal under mild thermal or photothermal stimulation. We incorporate halogen-bond-donating diiodotetrafluorobenzene molecules as dynamic supramolecular crosslinks into the LCEs and show that these relatively weak crosslinks are pertinent for their mechanical programming and self-healing. Utilizing the halogen-bonded LCEs, we demonstrate proof-of-concept soft robotic motions such as crawling and rolling with programmed velocities. Our results showcase halogen bonding as a promising, yet unexplored tool for the preparation of smart supramolecular constructs for the development of advanced soft actuators.

Fully (Re)configurable Interactive Material through a Switchable Photothermal Charge Transfer Complex Gated by a Supramolecular Liquid Crystal Elastomer Actuator.

Charge transfer complexes (CTCs) based on self-assembled donor and acceptor molecules allow light absorption of significantly redshifted wavelengths to either the donor or acceptor. In this work, we demonstrate a CTC embedded in a hydrogen-bonded liquid crystal elastomer (LCE), which in itself is fully reformable and reprocessable. The LCE host acts as a gate, directing the self-assembly of the CTC. When hydrogen bonding is present, the CTC behaves as a near-infrared (NIR) dye allowing photothermal actuation of the LCE. The CTC can be disassembled in specific regions of the LCE film by disrupting the hydrogen bond interactions, allowing selective NIR heating and localized actuation of the films. The metastable non-CTC state may persist for weeks or can be recovered on demand by heat treatment. Besides the CTC variability, the capability of completely reforming the shape, color, and actuation mode of the LCE provides an interactive material with unprecedented application versatility.

Additive Manufacturing of Liquid Crystal Elastomer Actuators Based on Knitting Technology.

Liquid crystal elastomer (LCE) exhibits large and reversible deformability originating from the alignment of liquid crystal mesogens. Additive manufacturing provides high controllability in the alignment and shaping process of LCE actuators. However, it still remains a challenge to customize LCE actuators with both diverse 3D deformability and recyclability. In this study, a new strategy is developed to exploit knitting technique to additively manufacture LCE actuators. The obtained LCE actuators are fabric-structured with designed geometry and deformability. By accurately adjusting the parameters of the knitting patterns as modules, diverse geometry is pixel-wise designed, and complex 3D deformations including bending, twisting, and folding are quantitatively controlled. In addition, the fabric-structured LCE actuators can be threaded, stitched, and reknitted to achieve advanced geometry, integrated multi-functions and efficient recyclability. This approach allows the fabrication of versatile LCE actuators with potential applications in smart textiles and soft robots.

Modeling Based on a Two-Step Parameter Identification Strategy for Liquid Crystal Elastomer Actuator Considering Dynamic Phase Transition Process.

Liquid crystal elastomer (LCE) is a promising candidate for actuation in light-driven soft robot applications. Due to the fact that LCE has complex hysteretic nonlinearities, which are highly dependent on the environment, modeling of actuators made of LCE is a very challenging issue. In this article, a model is proposed to describe the deformation of the LCE actuator accurately and analytically by considering the dynamic phase transition process of LCE molecules. First, an overview of the physical process of LCE's deformation is presented, and the schematic of the LCE actuator, as well as the modeling scheme are then introduced. Next, a thermodynamic analysis of the system's free energy is performed to establish the model for the LCE actuator, which gives the relationship between the system's deformation and the temperature. Here, to describe the complex hysteretic nonlinearity in the model, the dynamic process of the phase transition of LCE molecules is exploited. To effectively identify the model parameters, a two-step parameter identification strategy based on the differential evolution algorithm and nonlinear least-squares method is utilized. Finally, experimental results verify the validity of the proposed model. This modeling provides an approach to describe LCE's deformation with high accuracy and can fully reflect the physical nature of LCE's deformation, especially hysteresis. It can be utilized as a basis for accurate control over LCE actuators in photoresponsive soft robot applications.

Wireless Power Transfer to Electrothermal Liquid Crystal Elastomer Actuators.

Wireless actuation of electrically driven soft actuators is of paramount importance for the development of bioinspired soft robotics without physical connection or on-board batteries. Here, we demonstrate untethered electrothermal liquid crystal elastomer (LCE) actuators based on emerging wireless power transfer (WPT) technology. We first design and fabricate electrothermal LCE-based soft actuators that consist of an active LCE layer, a conductive liquid metal-filled polyacrylic acid (LM-PA) layer, and a passive polyimide layer. LM can function not only as an electrothermal transducer to endow resulting soft actuators with electrothermal responsiveness but also as an embedded sensor to track the resistance changes. Various shape-morphing and locomotive modes such as directional bending, chiral helical deformation, and inchworm-inspired crawling can be facilely obtained through appropriately controlling the molecular alignment direction of monodomain LCEs, and the reversible shape-deformation behaviors of resulting soft actuators can be monitored in real-time through resistance changes. Interestingly, untethered electrothermal LCE-based soft actuators have been achieved by designing a closed conductive LM circuit within the actuators and combining it with inductive-coupling WPT technology. When the resulting soft actuator approaches a commercially available wireless power supply system, an induced electromotive force can be generated within the closed LM circuit, which results in Joule heating and wireless actuation. As proof-of-concept illustrations, wirelessly driven soft actuators that can exhibit programmable shape-morphing behaviors are demonstrated. The research disclosed herein can provide insights into the development of bioinspired somatosensory soft actuators, battery-free wireless soft robots, and beyond.

Multimaterial Printing of Liquid Crystal Elastomers with Integrated Stretchable Electronics.

Liquid crystal elastomers (LCEs) have grown in popularity in recent years as a stimuli-responsive material for soft actuators and shape reconfigurable structures. To make these material systems electrically responsive, they must be integrated with soft conductive materials that match the compliance and deformability of the LCE. This study introduces a design and manufacturing methodology for combining direct ink write (DIW) 3D printing of soft, stretchable conductive inks with DIW-based "4D printing" of LCE to create fully integrated, electrically responsive, shape programmable matter. The conductive ink is composed of a soft thermoplastic elastomer, a liquid metal alloy (eutectic gallium indium, EGaIn), and silver flakes, exhibiting both high stretchability and conductivity (order of 105 S m-1). Empirical tuning of the LCE printing parameters gives rise to a smooth surface (<10 μm) for patterning the conductive ink with controlled trace dimensions. This multimaterial printing method is used to create shape reconfigurable LCE devices with on-demand circuit patterning that could otherwise not be easily fabricated through traditional means, such as an LCE bending actuator able to blink a Morse code signal and an LCE crawler with an on/off photoresistor controller. In contrast to existing fabrication methodologies, the inclusion of the conductive ink allows for stable power delivery to surface mount devices and Joule heating traces in a highly dynamic LCE system. This digital fabrication approach can be leveraged to push LCE actuators closer to becoming functional devices, such as shape programmable antennas and actuators with integrated sensing.

The Interplay between Different Stimuli in a 4D Printed Photo-, Thermal-, and Water-Responsive Liquid Crystal Elastomer Actuator.

Multi-stimuli responsivity in 3D-printed objects is receiving much attention. However, the simultaneous interplay between different environmental stimuli is largely unexplored. In this work, we demonstrate direct ink writing of an oligomeric ink containing an azobenzene photo-switch with an accessible hydrogen bond allowing triple responsivity to light, heat, and water. The resulting printed liquid crystal elastomer performs multiple actuations, the specific response depending on the environmental conditions. Bilayer films formed by printing on a static substrate can rapidly change shape, bending almost 80 degrees if irradiated in air or undergoing a shrinkage of about 50 % of its length when heated. The bilayer film assumes dramatically different shapes in water depending on combined environmental temperature and lighting conditions.

Toward Fully Printed Soft Actuators: UV‐Assisted Printing of Liquid Crystal Elastomers and Biphasic Liquid Metal Conductors

AbstractDevelopment of soft and compliant actuators has attracted tremendous attention due to their use in soft robotics, wearables, haptics, and assistive devices. Despite decades of progress, the goal of entirely digitally‐printed actuators has yet to be fully demonstrated. Digital printing permits rapid customization of the actuator's geometry, size, and deformation profile, and is a step toward mass customization of user‐specific wearables and soft robotic systems. Herein, a set of materials and methods are demonstrated for rapid fabrication of 3D‐printed Liquid Crystal Elastomer actuators electrically stimulated via a printed joule heater composed of a Liquid Metal (LM)‐filled elastomer composite. Unlike other Ag‐based inks, this LM‐elastomer composite is sinter‐free, enabling room‐temperature printing, and is stretchable, allowing cyclic actuation without electrical or mechanical failure of the conductor. By optimizing printing parameters, and improving the photo‐polymerization setup, a printed actuator that bends to an angle of 320° is demonstrated, with lower power consumption than previous LCE actuators. We also demonstrate a customized UV‐polymerization setup that permits photo‐curing of the LCE actuator in ≈90s, i.e., &gt;500x faster compared to previous works. The rapid photo‐polymerization enables progress toward 3D‐printing of multi‐layer actuators and is a step toward mass customization of fully digitally‐printed robotic and wearable devices.