Liquid crystal elastomers (LCEs) have long held significant promise as materials for artificial muscles and smart actuators. Recent advancements in this field have introduced innovative LCE ...structures at various scales, resulting in novel properties and functionalities that further accentuate their actuation advantages, bolstering their potential as future soft actuation systems. The ongoing pursuit of enhanced performance and functionality in LCE actuators, essential for advancing them towards superior material-based machines and devices, is intricately linked to the understanding of the fundamental structure-property-function relationships. This review provides a perspective on these relationships across multiple structural levels, encompassing chemical structures, mesophase structures, and micro-to-macroscale programmed structures. It delves into the impact of various LCE structures on key actuation-related properties, actuation features, and functionalities. This review aspires to provide valuable insights into the design of high-performance LCE actuators, the development of exceptional actuation modes and behaviors, and the expansion of achievable functionality.
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A liquid crystal elastomer (LCE) can be regarded as an integration of mesogenic molecules into a polymer network. The LCE can generate large mechanical actuation when subjected to various external ...stimuli. Recently, it has been extensively explored to make artificial muscle and multifunctional devices. However, in the commonly adopted two-step crosslinking method for synthesizing monodomain LCEs, the LCE needs to be well-cross-linked in the first step before stretching, which increases the disorder of mesogenic molecules in the final state of the LCE and makes it very challenging to fabricate the LCE of complex shapes. In this article, we developed a new LCE with disulfide bonds, which can be reprogrammed from the polydomain state to the monodomain state either through heating or UV illumination, owing to the rearrangement of the polymer network induced by the metathesis reaction of disulfide bonds. In addition, the newly developed LCE can be easily reprocessed and self-healed by heating. Because of the excellent reprogrammability as well as reprocessability of the LCE, we further fabricated LCE-based active micropillar arrays through robust imprint lithography, which can be hardly achieved using the LCE prepared previously. Finally, we showed an excellent long-term durability of the newly developed LCE.
Liquid crystal elastomers (LCEs) are soft materials capable of large, reversible shape changes, which may find potential application as artificial muscles, soft robots, and dynamic functional ...architectures. Here, the design and additive manufacturing of LCE actuators (LCEAs) with spatially programed nematic order that exhibit large, reversible, and repeatable contraction with high specific work capacity are reported. First, a photopolymerizable, solvent‐free, main‐chain LCE ink is created via aza‐Michael addition with the appropriate viscoelastic properties for 3D printing. Next, high operating temperature direct ink writing of LCE inks is used to align their mesogen domains along the direction of the print path. To demonstrate the power of this additive manufacturing approach, shape‐morphing LCEA architectures are fabricated, which undergo reversible planar‐to‐3D and 3D‐to‐3D′ transformations on demand, that can lift significantly more weight than other LCEAs reported to date.
3D liquid‐crystal elastomer actuators with programed director alignment are designed and fabricated via highoperating‐temperature direct ink writing. The additive manufacturing method produces dynamic shape‐morphing architectures in arbitrary form factors that are capable of lifting heavy loads.
The programmable assembly of innervated LCE actuators (iLCEs) with prescribed contractile actuation, self‐sensing, and closed loop control via core–shell 3D printing is reported. This extrusion‐based ...direct ink writing method enables coaxial filamentary features composed of pure LM core surrounded by an LCE shell, whose director is aligned along the print path. Specifically, the thermal response of the iLCE fiber‐type actuators is programmed, measured, and modeled during Joule heating, including quantifying the concomitant changes in fiber length and resistance that arise during simultaneous heating and self‐sensing. Due to their reversible, high‐energy actuation and their resistive feedback, it is also demonstrated that iLCEs can be regulated with closed loop control even when perturbed with large bias loads. Finally, iLCE architectures capable of programmed, self‐sensing 3D shape change with closed loop control are fabricated.
Self‐sensing, innervated liquid crystal elastomers in both fiber and square spiral motifs (shown) are produced by core–shell 3D printing. The printed core–shell filaments consist of a liquid metal core surrounded by a liquid crystal elastomer shell. When Joule heated above their nematic‐to‐isotropic transition temperature, these self‐sensing architectures exhibit actuation and large work output, which can be regulated via closed loop control.
Endowing artificial advanced materials and systems with biomimetic self‐regulatory intelligence is of paramount significance for the development of somatosensory soft robotics and adaptive ...optoelectronics. Herein, a bioinspired phototropic MXene‐reinforced soft tubular actuator is reported that exhibits omnidirectional self‐orienting ability and is capable of quickly sensing, continuously tracking, and adaptively interacting with incident light in all zenithal and azimuthal angles of 3D space. The novelty of the soft tubular actuator lies in three aspects: 1) the new polymerizable MXene nanomonomer shows high compatibility with liquid crystal elastomer (LCE) matrices and can be in situ photopolymerized into the polymer networks, thus enhancing the mechanical and photoactuation properties; 2) the distinct hollow and radially symmetrical structure facilitates the actuator with fast photoresponsiveness and phototropic performance through retarding the heat conduction along the radial direction; 3) the MXene‐LCE soft tubular actuator simultaneously integrates sensing, actuation, and built‐in feedback loop, thus leading to a high light‐tracking accuracy and adaptive phototropism like a hollow stem of plants in nature. As a proof‐of‐concept demonstration, an adaptive photovoltaic system with solar energy harvesting maximization is illustrated. This work can provide insights into the development of artificial intelligent materials toward adaptive optoelectronics, intelligent soft robotics, and beyond.
Bioinspired MXene‐reinforced soft tubular actuators with omnidirectional light‐tracking and adaptive phototropism are demonstrated through in situ photopolymerization of a judiciously designed photopolymerizable MXene nanomonomer into reversible shape‐morphing crosslinked main‐chain liquid crystal elastomers. Like the hollow stem of plants, the resulting actuators show a high light‐tracking accuracy and are capable of quickly sensing, continuously tracking, and adaptively interacting with the incident light in all zenithal and azimuthal angles of 3D space.
Liquid crystal elastomers (LCEs), a class of soft materials capable of a large and reversible change in the shape under the trigger of external stimuli, can be fabricated into diverse architectures ...with complicated deformation modes through four-dimensional (4D) printing. However, the printable LCE ink is only in the form of monomeric precursors and the deformation mode is limited to contraction/extension deformation. Herein, we report a novel approach to break through these limitations. We achieved 4D printing of a single-component liquid crystal polymer ink in its isotropy state through direct ink writing (DIW) technology. The drawing force imposed by the movement of nozzle in the extruded printing process was able to align the mesogen units along the specific printing path. An orientation gradient perpendicular to the printing direction was obtained due to the existence of a temperature gradient between the two sides of printed samples and could be further fixed by post-photo-cross-linking treatment through the dimerizable groups in the LCE, realizing a new actuation mode in the field of extrusion-based printing of LCE actuators. The printed film was able to change reversibly from a strip to a tightly hollow cylinder and could reversibly lift up an object with roughly 600 times its own weight. The orientation gradient can be patterned through liquid-assistant printing or programmed structure design to integrate both bending and contraction actuation modes on the same printed sample, leading to complex deformation and two-dimensional (2D) planar porous structure to three-dimensional (3D) porous cylinder transition. This study opens up a new prospect to directly print a wide variety of LCE actuators with versatile actuation modes.
Programmable shape-shifting materials can take different physical forms to achieve multifunctionality in a dynamic and controllable manner. Although morphing a shape from 2D to 3D via programmed ...inhomogeneous local deformations has been demonstrated in various ways, the inverse problem—finding how to program a sheet in order for it to take an arbitrary desired 3D shape—is much harder yet critical to realize specific functions. Here, we address this inverse problem in thin liquid crystal elastomer (LCE) sheets, where the shape is preprogrammed by precise and local control of the molecular orientation of the liquid crystal monomers. We show how blueprints for arbitrary surface geometries can be generated using approximate numerical methods and how local extrinsic curvatures can be generated to assist in properly converting these geometries into shapes. Backed by faithfully alignable and rapidly lockable LCE chemistry, we precisely embed our designs in LCE sheets using advanced top-down microfabrication techniques. We thus successfully produce flat sheets that, upon thermal activation, take an arbitrary desired shape, such as a face. The general design principles presented here for creating an arbitrary 3D shape will allow for exploration of unmet needs in flexible electronics, metamaterials, aerospace and medical devices, and more.
•A novel light-powered oscillatory self-spinning button spinner is originally proposed.•Two motion regimes namely the static regime and the self-spinning regime are presented.•The triggering ...conditions, frequency and amplitude of self-spinning can be modulated.•Results aid the design and control of self-spinning button spinner.
Self-oscillation can maintain own continuous motion by means of energy conversion from constant external stimuli to mechanical work, making them highly suitable for being applied in soft robotics, motors, military industry and so on. With the inspiration provided by the spinning of button spinner toy, we creatively develop a self-spinning button spinner, containing a button rotor and a pair of liquid crystal elastomer (LCE) fibers under steady illumination. Based on well-established twisting thread model and dynamic LCE model, a nonlinear dynamic model of self-spinning button spinner under steady illumination is proposed. Numerical calculation reveals that two motion regimes are involved for the button spinner on exposure to steady illumination, which are distinguished as the static regime and the self-spinning regime. The self-spinning of button spinner originates from the contraction of twisted segments of LCE fibers in illumination at winding state, and its continuous periodic motion is sustained through the interrelation between light energy and damping dissipation. In addition, the critical conditions necessary for triggering the self-spinning, as well as the vital system parameters affecting its frequency and amplitude, are investigated in detail. Different from the existing abundant self-oscillating systems, this self-spinning button spinner has superiority in simple structure, customizable size, and rapid speed, and it is anticipated to provide more diverse design ideas for soft robotics, energy harvesters, micromachines and so on.
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Motion in plants often relies on dynamic helical systems as seen in coiling tendrils, spasmoneme springs, and the opening of chiral seedpods. Developing nanotechnology that would allow ...molecular‐level phenomena to drive such movements in artificial systems remains a scientific challenge. Herein, we describe a soft device that uses nanoscale information to mimic seedpod opening. The system exploits a fundamental mechanism of stimuli‐responsive deformation in plants, namely that inflexible elements with specific orientations are integrated into a stimuli‐responsive matrix. The device is operated by isomerization of a light‐responsive molecular switch that drives the twisting of strips of liquid‐crystal elastomers. The strips twist in opposite directions and work against each other until the pod pops open from stress. This mechanism allows the photoisomerization of molecular switches to stimulate rapid shape changes at the macroscale and thus to maximize actuation power.
Like a seedpod: A soft device that mimics seedpod opening is described. Isomerization of a light‐responsive molecular switch drives the twisting of liquid‐crystal elastomer strips. As these strips twist in opposite directions, the pod eventually pops open from stress.
Cuttlefish can modify their body shape and both their pigmentary and structural colors for protection. This adaptability has inspired the development of appearance-changing polymers such as ...structural color actuators, although in most cases, the original shape has been confined to being flat, and pigmented structural color actuators have not yet been reported. Here, we have successfully created a pigmented structural color actuator using a cholesteric liquid crystal elastomer with a lower actuation temperature where both actuation and coloration (structural and pigmental) are tunable with temperature and NIR light. The shape, structural color, and absorption of the NIR-absorbing dye pigment of the actuator all change with temperature. Light can be used to trigger local in-plane bending actuation in flat films and local shape changes in a variety of 3D-shaped objects. A cuttlefish mimic that can sense light and respond by locally changing its appearance was also made to demonstrate the potential of pigmented structural color actuators for signaling and camouflage in soft robotics.
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