Liquid Crystal Elastomers Research Articles

Direct‐Ink‐Written Cholesteric Liquid Crystal Elastomer with Programmable Mechanochromic Response

AbstractCholesteric liquid crystal elastomers (CLCEs) are unique anisotropic rubbers that can change their structural color in response to various stimuli such as heat, chemicals, electric fields, and mechanical stress. Methods such as anisotropic deswelling and surface alignment have been adopted to prepare CLCEs; however, they have limitations in creating spatially controlled CLCE geometries. In this work, a direct ink writing (DIW)‐based 3D‐printable CLCE that can be prepared by extruding viscous CLC ink is developed through a 3D printer nozzle, followed by photopolymerization. Interestingly, the helical axis is inclined to the printing direction by ≈32° due to a combination of the shear‐induced alignment causes during extrusion and the elongational force generated during deposition onto the substrate. This unusual helical axis distortion leads to both blue and red shifts of the reflection color depending on the direction of observation relative to the printing axis. Notably, the printed CLCE exhibits anisotropic mechanochromism upon stretching, because of the stretching‐direction‐dependent variations in the slant angle of the helical axis. This anisotropic mechanochromism can be harnessed to develop a unique CLCE‐based strain sensor displaying intricate color patterns upon stretching, with significant application potential in encryption, anticounterfeiting, and structural health monitoring.

Liquid Crystal Elastomer-Liquid Metal Composite: Ultrafast, Untethered, and Programmable Actuation by Induction Heating.

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive materials that have been intensively studied for applications including artificial muscles, shape morphing structures, and soft robotics due to their capability of large, programmable, and fully reversible actuation strains. To fully take advantage of LCEs, rapid, untethered, and programmable actuation methods are highly desirable. Here, a liquid crystal elastomer-liquid metal (LCE-LM) composite is reported, which enables ultrafast and programmable actuations by eddy current induction heating. The composite consists of LM sandwiched between two LCE layers printed via direct ink writing (DIW). When subjected to a high-frequency alternating magnetic field, the composite is actuated in milliseconds. By moving the magnetic field, the eddy current is spatially controlled for selective actuation. Additionally, sequential actuation is achievable by programming the LM thickness distribution in a sample. With these capabilities, the LCE-LM composite is further exploited for multimodal deformation of a pop-up structure, on-ground omnidirectional robotic motion, and in-water targeted object manipulation and crawling.

Programming actuation onset of a liquid crystalline elastomer via isomerization of network topology

Tuning actuation temperatures of liquid crystalline elastomers (LCEs) achieves control of their actuation onsets, which is generally accomplished in the synthesis step and cannot be altered afterward. Multiple actuation onsets in one LCE can be encoded if the post-synthesis regulation of actuation temperature can be spatiotemporally achieved. This would allow realizing a logical time-evolution of actuation, desired for future soft robots. Nevertheless, this task is challenging given the additional need to ensure mesogen alignment required for actuation. We achieved this goal with a topology isomerizable network (TIN) of LCE containing aromatic and aliphatic esters in the mesogenic and amorphous phases, respectively. These two ester bonds can be distinctly activated for transesterification. The homolytic bond exchange between aliphatic esters allows mechanically induced mesogen alignment without affecting the mesogenic phase. Most importantly, the heterolytic exchange between aromatic and aliphatic esters changes the actuation temperature under different conditions. Spatial control of the two mechanisms via a photo-latent catalyst unleashes the freedom in regulating actuation temperature distribution, yielding unusual controllability in actuation geometries and logical sequence. Our principle is generally applicable to common LCEs containing both aromatic and aliphatic esters.

A Light-Powered Liquid Crystal Elastomer Roller.

Achieving and controlling the desired movements of active machines is generally accomplished through precise control of artificial muscles in a distributed and serialized manner, which is a significant challenge. The emerging motion control strategy based on self-oscillation in active machines has unique advantages, including directly harvesting energy from constant ambient light, and it has no need for complex controllers. Inspired by the roller, we have innovatively developed a self-rolling roller that consists of a roller and a liquid crystal elastomer (LCE) fiber. By utilizing a well-established dynamic LCE model and subjecting it to constant illumination, we have investigated the dynamic behavior of the self-rolling roller. Based on numerical calculations, it has been discovered that the roller, when subjected to steady illumination, exhibits two distinct motion regimes: the static regime and the self-rolling regime. The self-rolling regime, characterized by continuous periodic rolling, is sustained by the interaction between light energy and damping dissipation. The continuous periodic rolling observed in the self-rolling regime is maintained through the interplay between the dissipation of damping and the absorption of light energy. In the static state, the rolling angle of the roller begins to decrease rapidly and then converges to zero. Detailed investigations have been conducted to determine the critical conditions required to initiate self-rolling, as well as the essential system parameters that influence its frequency and amplitude. The proposed self-rolling roller has superiorities in its simple structure, light weight, alternative to manual labor, and speediness. This advancement is expected to inspire greater design diversity in micromachines, soft robotics, energy harvesters, and similar areas.

Network Topology Optimization for Alignment Programming of a Dynamic Liquid Crystalline Organo-Gel.

Liquid crystalline elastomers (LCEs) exhibit muscle-like actuation upon an external stimulus. To control this, various alignment programming strategies have been developed over the past decades. Among them, force-directed solvent evaporation, namely, that the alignment depends on the applied external force during solvent evaporation, is appreciated for its universality in material design and versatility in attainable actuations. Here, we investigate the influence of network topology on the alignment programming of a liquid crystalline (LC) organo-gel via varying feeding ratios of the monomers. As a result, distinct self-supporting actuations can be repeatedly introduced into a topology-optimized LC organo-gel. Beyond this, the bond exchange reaction of the embedded ester groups can be activated upon heating, which enables alignment manipulation based on dynamic network reconfiguration after drying. The availability of inviting two distinct programming strategies into one LCE network allows us to regulate the LCE alignment at both the gel and dried states, offering ample room to diversify actuation manners. Our design principle shall be adopted by other dynamic LCE systems owing to its maneuverability.

Programmable liquid crystal elastomers with dynamic covalent siloxane bonds based on a dual self-catalytic strategy

The exchange reaction of preparing the siloxane-based liquid crystal elastomers (Si-LCEs) always require the acid or base catalysts, which will increase the complexity of the operation and reduces the stability of the Si-LCE materials. In this paper, a novel and simple dual self-catalytic strategy for producing Si-LCEs with dynamic covalent siloxane bonds was proposed. A readily available compound-1,3-bis (aminopropyl) tetramethyldisiloxane (BATS) acts as chain extender and crosslinker for elastomers and provides dynamic covalent bonds (DCBs) as well. More importantly, amino groups of BATS play the role of dual self-catalyst, not only accelerating the thiol Michael addition reaction and aza Michael addition reaction simultaneously during producing Si-LCEs, but also promoting dynamic covalent siloxane bonds exchange reaction at an elevated temperature after the formation of the cross-linked network. With this ingenious design, the obtained fully cured polydomain Si-LCEs possess excellent self-healing and processing abilities enabled by DCBs and outstanding mechanical properties facilitated by hydrogen bond, which can be easily aligned, programmed and recycled. By exploiting these dynamic features, a series of multi-functional soft actuators were realized by simple welding, showing a local fast response and precise control by light and heat. Furthermore, soft robots fabricated with a bilayer structure of Si-LCEs can perform crawling and coiling actuation controlled by near-infrared (NIR) light facilely. These results manifest that the self-catalytic dynamic covalent siloxane bonds strategy is a facile and effective approach for preparing Si-LCEs and provides a new solution for the exploitation of multi-functional liquid crystal elastomers with fast response to various stimulus.

A Dual‐Responsive Liquid Crystal Elastomer for Multi‐Level Encryption and Transient Information Display

AbstractInformation security has gained increasing attention in the past decade, leading to the development of advanced materials for anti‐counterfeiting, encryption and instantaneous information display. However, it remains challenging to achieve high information security with simple encryption procedures and low‐energy stimuli. Herein, a series of strain/temperature‐responsive liquid crystal elastomers (LCEs) are developed to achieve dual‐modal, multi‐level information encryption and real‐time, rewritable transient information display. The as‐prepared polydomain LCEs can change from an opaque state to a transparent state under strain or temperature stimuli, with the transition strains or temperatures highly dependent on the concentration of long‐chain flexible spacers. Information encrypted by different LCE inks can be decrypted under specific strains or temperatures, leading to multi‐level protection of information security. Furthermore, with the combination of the phase transition of polydomain LCEs and the photothermal effect of multi‐walled carbon nanotubes (MWCNTs), we achieved a repeatable transient information display by using near‐infrared (NIR) light as a pen for writing. This study provides new insight into the development of advanced encryption materials with versatility and high security for broad applications.

A Dual-Responsive Liquid Crystal Elastomer for Multi-Level Encryption and Transient Information Display.

Information security has gained increasing attention in the past decade, leading to the development of advanced materials for anti-counterfeiting, encryption and instantaneous information display. However, it remains challenging to achieve high information security with simple encryption procedures and low-energy stimuli. Herein, a series of strain/temperature-responsive liquid crystal elastomers (LCEs) are developed to achieve dual-modal, multi-level information encryption and real-time, rewritable transient information display. The as-prepared polydomain LCEs can change from an opaque state to a transparent state under strain or temperature stimuli, with the transition strains or temperatures highly dependent on the concentration of long-chain flexible spacers. Information encrypted by different LCE inks can be decrypted under specific strains or temperatures, leading to multi-level protection of information security. Furthermore, with the combination of the phase transition of polydomain LCEs and the photothermal effect of multi-walled carbon nanotubes (MWCNTs), we achieved a repeatable transient information display by using near-infrared (NIR) light as a pen for writing. This study provides new insight into the development of advanced encryption materials with versatility and high security for broad applications.

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.

Reprogrammable 4D Printed Liquid Crystal Elastomer Photoactuators by Means of Light‐Reversible Perylene Diimide Radicals

AbstractReconfigurable soft actuators can be programmed to morph into different 3D shapes under the same stimulus exhibiting great potential for adaptive robotic functionalities. Liquid crystalline crosslinked materials programmed and controlled by light have demonstrated great potential in this area, however, their implementation is mainly based on azobenzene chromophores, using ultraviolet light that can potentially damage the device and its surroundings, especially if living cells are present. Here, an ink is presented, containing a green‐absorbing perylene diimide chromophore, to prepare light active liquid crystalline elastomer (LCE) actuators via direct ink writing. Green light irradiation of the LCE elements leads to photothermal actuation, but also to new absorption bands in the far‐red and near‐infrared, ascribed to the formation of radical species. Far‐red irradiation results in mechanical actuation and, advantageously, a recovery of the original absorption spectrum. This reversible transformation enables spatial reconfigurability of the actuator's response to far‐red light. The reconfigurable system gives access to complex deformation modes by simply exciting the element with homogeneous far‐red light, without the need for any structural modification of the actuator. This material strategy, using green and far‐red light, less harmful than ultraviolet, shows significant promise for future development of reconfigurable actuators for biomedical applications.

Switchable photothermal conversion efficiency for reprogrammable actuation

Reprogrammable soft matter brings flexibility to soft robots so that they can display various motions, which is flourishing in soft robotics. However, the reprogramming of photoresponsive materials used in soft robots is time-consuming using existing methods. In this study, we promote a strategy for rapid reprogramming via switchable photothermal conversion efficiency (PCE). The liquid crystalline elastomers doped with semiconductor bismuth compounds (Bi-LCE) used in this work exhibited large photothermal actuation with over 35% shrinkage in 5 s at high PCE state, which demonstrated little deformation at low PCE state. Furthermore, the material was capable of being reprogrammed up to 10 times, with only 20 min required for one PCE reversible switch. Based on this switchable PCE effect, the same Bi-LCE film displayed various shape changes through different programmable pattern. Additionally, a reprogrammable hollow tube made of PCE reprogrammable materials could tune the diameter, cross-section configuration, and surface morphology, which was crucial for microfluidics field. Reprogrammable materials provide endless possibilities for reusability and sustainability in robotics.

Controllable Deformations of Unconstrained Ideal Nematic Elastomers

We establish that, for ideal unconstrained uniaxial nematic elastomers described by a homogeneous isotropic strain-energy density function, the only smooth deformations that can be controlled by the application of surface tractions only and are universal in the sense that they are independent of the strain-energy density are those for which the deformation gradient is constant and the liquid crystal director is either aligned uniformly or oriented randomly in Cartesian coordinates. This result generalizes the classical Ericksen’s theorem for nonlinear homogeneous isotropic hyperelastic materials. While Ericksen’s theorem is directly applicable to liquid crystal elastomers in an isotropic phase where the director is oriented randomly, in a nematic phase, the constitutive strain-energy density must account also for the liquid crystal orientation which leads to significant differences in the analysis compared to the purely elastic counterpart.

Dynamic diselenide bond‐enabled liquid crystal elastomer‐based two‐way shape memory aerogels with weldability and closed‐loop recyclability

AbstractTwo‐way shape memory polymeric aerogels (2W‐SMPAs), with the ability to undergo reversible shape deformation in response to external stimuli, have extensive application in diverse fields such as actuators, sensors, robotics, and other relevant domains. In this study, we introduce a novel approach for fabricating a 2W‐SMPA material based on liquid crystal elastomers (LCEs) incorporating dynamic diselenide bonds. The aerogel exhibits liquid crystal phases, excellent compressibility and shape stability, and the mesogens are uniaxial‐oriented along the stretching direction. By capitalizing on the dynamic diselenide bonds, the LCE‐based aerogel demonstrated remarkable reprogrammability, weldability, and recyclability through thermal reorganization. The shape‐programmed aerogel sample exhibits reversible shrinking deformation during the heating and cooling cycles, ultimately achieving a maximum shrinkage ratio of 26.1%. Moreover, the LCE‐based aerogel's porous structure and monodomain orientation effectively enable the adsorption of the photothermal dye DR1 and facilitated the reversible photothermal‐induced shape deformation when exposed to 520 nm light irradiation. These findings reveal the potential application of this innovative LCE‐based aerogel material, enabled by dynamic diselenide bonds, in various areas including control devices, soft actuators, and other diverse fields.

A theoretical model for power generation via liquid crystal elastomers

Motivated by the need for new materials and green energy production and conversion processes, a class of mathematical models for liquid crystal elastomers (LCEs) integrated within a theoretical charge pump electrical circuit is considered. The charge pump harnesses the chemical and mechanical properties of LCEs transitioning from the nematic to isotropic phase when illuminated or heated to generate higher voltage from a lower voltage supplied by a battery. For the material constitutive model, purely elastic and neoclassical-type strain energy densities applicable to a wide range of monodomain nematic elastomers are combined, while elastic and photothermal responses are decoupled to make the investigation analytically tractable. By varying the model parameters of the elastic and neoclassical terms, it is found that LCEs are more effective than rubber when used as dielectric material within a charge pump capacitor.

Light-powered self-propelled trolley with a liquid crystal elastomer pendulum motor

Conventional liquid crystal elastomer (LCE)-based robots require complex controllers and bulky power supplies, limiting their applications in areas such as microrobots and soft robots. In this paper, a light-powered self-propelled trolley with a LCE pendulum motor is proposed, and the dynamics of its self-propelled motion is investigated theoretically. Under steady illumination, the periodic radial contraction and relaxation of LCE can trigger the trolley to move forward through the self-rotation of the LCE pendulum. The coupling of the optically-induced deformation with its motion facilitates the energy input from ambient illumination to compensate for the damping dissipation so as to maintain the sustained motion. Through theoretical modeling and numerical calculations, three typical motion states of the LCE pendulum are identified, namely static state, self-oscillation state and self-rotation state. Among them, the LCE pendulum can propel the trolley when it is self-rotating. The influences of different physical parameters on the system motion are studied quantitatively. The results show that the period of the simple pendulum and the trolley speed can be controlled by adjusting the illumination range, optical intensity, damping coefficient of pendulum and damping coefficient of trolley. The light-powered self-propelled trolley constructed in current paper does not require complex controllers or self-contained power supplies, and possesses the characteristics of simple structure, light weight and energy efficient, which is of latent applied value in microrobots, soft robots and other fields.

A light-powered liquid crystal elastomer semi-rotary motor

Self-excited oscillators are characterized by active energy harvesting from the stable external environment, autonomy and portability, which allow them to act as engines to drive other working devices. Taking inspiration from the pump drill, a semi-rotary motor consisting of a motor rotor and three liquid crystal elastomer (LCE) fibers is constructed in this paper, which can realize self-rotation under stable illumination. Considering the existing twisted wire model and the dynamic LCE model, a nonlinear dynamics model for the semi-rotary motor under stable illumination is established. Numerical calculations witness the existence of two motion modes for the semi-rotary motor under stable illumination, namely the static mode and the self-rotation mode. Under stable illumination, the light-driven contraction of twisted segments in LCE fibers for the winding state can trigger the self-rotation and maintain the continuous periodic motion through the interrelationship between optical energy and damping dissipation. The critical conditions required to trigger the self-rotation are also investigated in detail, as well as the crucial system parameters affecting the self-rotation frequency and amplitude. The proposed self-rotating semi-rotary motor has a simple structure, customizable dimensions and high speed, prompting its potential use in soft robots, energy harvesters, and micro-machines.

Modeling the dynamic response of a light-powered self-rotating liquid crystal elastomer-based system

Self-oscillating systems possess the capability to convert ambient energy directly into mechanical work. Consequently, the development of novel self-oscillating systems holds significant potential for applications in energy harvesting, engines, and actuators, making them highly worthwhile to be designed and implemented. Inspired by the rotation of a skipping rope, we creatively developed a self-rotating liquid crystal elastomer (LCE)-based system composed of a LCE fiber and a mass, which triggers self-rotation under steady illumination. In this work, a nonlinear dynamic model is proposed to study the dynamic behaviors of optically-responsive LCE-based systems. Numerical calculations show that the self-rotating LCE-based system can undergo a supercritical Hopf bifurcation between the static state and the self-rotation state, which is maintained by the energy input of ambient illumination to compensate the damping dissipation. In addition, the Hopf bifurcation conditions, as well as the important system parameters affecting the frequency of self-rotation, are studied in detail. Especially for the cases of unilateral asymmetrical lighting, the system parameters have different effects on the clockwise and counterclockwise rotation. Different from the existing abundant self-oscillating system, the self-rotating LCE-based system has the advantages of simple structure, customizable size and multimode rotation, and is expected to provide more diversified design ideas for soft robotics, energy harvesters, micro-machines, etc.

Lifting, Loading, and Buckling in Conical Shells.

Liquid crystal elastomer films that morph into cones are strikingly capable lifters. Thus motivated, we combine theory, numerics, and experiments to reexamine the load-bearing capacity of conical shells. We show that a cone squashed between frictionless surfaces buckles at a smaller load, even in scaling, than the classical Seide-Koiter result. Such buckling begins in a region of greatly amplified azimuthal compression generated in an outer boundary layer with oscillatory bend. Experimentally and numerically, buckling then grows subcritically over the full cone. We derive a new thin-limit formula for the critical load, ∝t^{5/2}, and validate it numerically. We also investigate deep postbuckling, finding further instabilities producing intricate states with multiple Pogorelov-type curved ridges arranged in concentric circles or Archimedean spirals. Finally, we investigate the forces exerted by such states, which limit lifting performance in active cones.

Three-Dimensional Printing of Shape Memory Liquid Crystalline Thermoplastic Elastomeric Composites Using Fused Filament Fabrication.

Liquid crystalline elastomers (LCEs) are stimuli-responsive materials utilised in shape memory applications. The processability of these materials via advanced manufacturing is being paid increasing attention to advance their volume production on an industrial scale. Fused filament fabrication (FFF) is an extrusion-based additive manufacturing (AM) technique that offers the potential to address this. The critical challenge, however, is the rheological characteristics of LCEs that need to be tuned to achieve a facile processability through the extrusion-based method. In this work, new filaments of liquid crystalline thermoplastic elastomer (LCTPE) and its composites with lignin were made by the ternary system of LCE, thermoplastic polyurethane (TPU), and lignin. The results showed that TPU improves the melt flow index of the LCTPE system to approximately 10.01 g/10 min, while adding lignin further enhances the value of this index for the composites up to 21.82 g/10 min. The microstructural analysis indicated that the effective distribution of lignin and reduced domain size of the LCEs in the ternary blend contribute to the enhanced flowability of this filament through 3D printing. Samples of 3D-printed LCTPE and LCTPE/lignin composites maintained their shape memory characteristics via thermomechanical activation. Full shape recovery of the new LCTPE matrix and its composites with lignin was achieved in 39 s and 32 s at 130 °C, followed by 28 s and 24 s at 160 °C, respectively. The successful fabrication of LCTPE and LCTPE/lignin composite samples through 3D printing demonstrates a potential procedure for processing these shape memory materials using the FFF technique, and lignin offers a sustainable and cost-effective material solution that enhances the properties of this composite material.

Shape-Tunable 4D Printing of LCEs via Cooling Rate Modulation: Stimulus-Free Locking of Actuated State at Room Temperature.

Liquid crystal elastomers (LCEs) have garnered considerable attention in the field of four-dimensional (4D) printing due to their large, reversible, and anisotropic shape-morphing capabilities. By utilizing direct ink writing, intricate LCE structures with programmable shape morphing can be achieved. However, the maintenance of the actuated state for LCEs requires continuous and substantial external stimuli, presenting challenges for practical applications, particularly under ambient conditions. This study reports a straightforward and effective physical approach to lock the actuated state of LCEs through rapid cooling while preserving their reversible performance. Rapid cooling significantly reduces the mobility of the lightly cross-linked network in LCEs, resulting in a notably slow recovery of mesogen alignment. As a result, the locked LCE structures retain their actuated state even at room temperature. Moreover, we demonstrate the ability to achieve tunable shapes between the original and actuated states by modulating the cooling rate, i.e., varying the temperature and type of cooling medium. The proposed method opens up new possibilities to achieve stable and tunable shape locking of soft devices for engineering applications.