Innervated, Self-Sensing Liquid Crystal Elastomer Actuators with Closed Loop Control

Responsive Industrial Polymers: A Marriage of Polyurethanes with Liquid Crystal Elastomers?

Responsive polymers have yet to significantly impact the marketplace. In this Perspective, we offer a glimpse of a possible future industrial-scale responsive polymer. We begin by briefly reviewing two different existing polymer materials, one with high volume, excellent processability, and commercial impact (polyurethanes), the other with stimuli responsive functional properties (liquid crystal elastomers). We explore the possibilities of combining the properties of these two disparate entities into a single material. We offer intriguing possibilities for a bulk polymer with both responsivity and processability that could compete in the market with the long-established residents and discuss some of the research roadblocks that need to be overcome to reach this lofty goal.

Shape-morphing bioelectronic devices

Shape-morphing bioelectronic devices, which can actively transform their geometric configurations in response to external stimuli (e.g., light, heat, electricity, and magnetic fields), have enabled many unique applications in different areas, ranging from human-machine interfaces to biomedical applications. These devices can not only realize in vivo deformations to execute specific tasks, form conformal contacts with target organs for real-time monitoring, and dynamically reshape their structures to adjust functional properties, but also assist users in daily activities through physical interactions. In this review, we provide a comprehensive overview of recent advances in shape-morphing bioelectronic devices, covering their fundamental working principles, representative deformation modes, and advanced manufacturing methodologies. Then, a broad range of practical applications of shape-morphing bioelectronics are summarized, including electromagnetic devices, optoelectronic devices, biological devices, biomedical devices, and haptic interfaces. Finally, we discuss key challenges and emerging opportunities in this rapidly evolving field, providing insights into future research directions and potential breakthroughs.

A framework for handweaving robotic textiles with liquid crystal elastomer fibers

Textile production methods present a rich set of strategies for developing materials with both form and function encoded at the fiber scale. Beyond simply acting as a static flexible barrier, the ability to incorporate environmentally responsible materials into fabric architectures significantly expands the textile design space by adding on-demand and programmable 3D structural morphing. To this end, liquid crystal elastomers (LCEs) are a promising candidate for enabling these reversible actuation behaviors in fabric-based constructs. Drawing on traditional textile manufacturing techniques and through a detailed exploration of the vast woven textile design space, we have demonstrated programmable and reversible curling, puffing, and in-plane shrinkage behaviors by embedding the functionality of LCE fibers into single and multi-layered woven structures. Predictable shifts in fabric structure directly influence the mechanical properties and the resulting form factor of the actuated textiles, which can in turn be effectively leveraged for the generation of multi-functional devices, enabling new directions for the engineering of flexible stimuli-responsive materials.

Biomimetic Soft Actuator with Deformation and Motion Driven by Near-Infrared Light

Restricted by the inherent low sensitivity of materials and complex integration technology, it is difficult for existing soft actuators (s-actuators) to simultaneously possess the advantages of flexibility, fast response, and simple manufacturing, which greatly limits their practical applications. Herein, a stretchable (ε = 200%) nanocomposite film capable of deformation and motion driven by near infrared light (NIR) was developed using multi-walled carbon nanotubes (MWCNTs) as the light absorption-photothermal conversion nanonetwork, and liquid crystal polymer (LCP) as an elastic matrix featured reversible phase transition. Furthermore, s-actuators with various deformation and motion modes have been realized employing MWCNT/LCP nanocomposite film. Based on the mechanism that photothermal-effect-regulated liquid crystal-isotropic phase transition in LCP can induce macroscopic deformation of nanocomposites, MWCNT/LCP s-actuators have completed a series of complex deformation and motion tasks such as opening the knot, "V"-shape reversible deformation (30 s per cycle), the "spring" rotating and unfolding, imitating a "caterpillar" walking in a straight line (the average speed is 13 s/mm), etc. This work provides an effective strategy for the intelligent development of s-actuators.

Photodriven Aquatic Locomotion in Liquid Crystalline Elastomer Composites with Tunable Wettability

Photodriven liquid crystalline elastomer (LCE) composites with thiol-functionalized Ti₃C₂Tx MXene nanosheets are introduced as a versatile material system for achieving controlled aquatic locomotion. By incorporating superhydrophobic or superhydrophilic coatings, these composites demonstrate distinct modalities at the air-water interface and underwater. The stimuli-responsive behavior of the LCE nanocomposites is enhanced through the homogeneous dispersion of MXene platelets within the LCE matrix, facilitated by thiol-functionalization. Superhydrophobic coatings increase buoyancy and reduce drag, enabling locomotion akin to water striders at the air-water interface. Conversely, superhydrophilic coatings submerse the composites to allow photomechanical actuation to drive underwater locomotion against gravity. By combining tunable wettability with robust photothermal performance, these MXene-LCE composites open new opportunities for designing and integrating stimuli-responsive materials in aquatic actuation systems.

Luminescent Liquid Crystalline Elastomer Promoted Self-Adaptive Smart Active Optical Waveguide with Ultra-Low Optical Loss

Currently, optical waveguides show extensive application in photonics and optoelectronic devices due to their high information capacity and transmission capabilities. However, developing self-adaptive, smart optical waveguide materials with ultra-low optical loss remains a significant challenge. To address this issue, luminescent liquid crystalline elastomers (LLCEs) with remarkable flexibility and minimal optical loss through one-pot synthetic method is synthesized, marking the first example of such an approach. The resultant organic optical waveguide materials (OOWMs) demonstrate exceptional mechanical performance and low optical loss, even under significant deformation. An optical loss coefficient of 0.0375 dB mm-1 has been achieved in LLCE-based OOWMs through synergistic Förster resonance energy transfer. Additionally, these flexible OOWMs can endure large deformations and be shaped into arbitrary forms within macro-scale dimensions. Notably, LLCE-based OOWMs demonstrate smart, self-adaptive behavior with ultra-low optical loss when exposed to heat or light. Consequently, these OOWMs can be used to fabricate photo switches of various shapes. This work provides a feasible approach to achieving integrated photonic systems with low optical loss for intelligent high-speed data transmission.

Force-Trainable Liquid Crystal Elastomer Enabled by Mechanophore-Induced Radical Polymerization

In nature, organisms adapt to environmental changes through training to learn new abilities, offering valuable insights for developing intelligent materials. However, replicating this "adaptive learning" in synthetic materials presents a significant challenge. This study introduces a feasible approach to train liquid crystal elastomers (LCEs) by integrating a mechanophore tetraarylsuccinonitrile into their main chain, addressing the challenge of enabling synthetic materials to exchange substances with their environment. Inspired by biological training, the LCEs can self-strengthen and acquire new functionalities through mechanical stress-induced radical polymerization. The research not only enhances the mechanical performance of LCEs, but also endows them with the ability to learn properties such as flexibility, light responsiveness, and fluorescence. These advancements are crucial for overcoming the limitations of current materials, paving the way for the creation of advanced intelligent soft materials with autonomous self-improvement, akin to the adaptive skills of living organisms.

Microsensor-Internalized Fibers as Autonomously Controllable Soft Actuators

Despite their strengths in flexibility and miniaturization, the stable operation of soft actuators under ever-changing environmental and biological conditions is hindered by the lack of applicable methods using internal sensors to detect unintentional stimuli. Here, the integration of a microscale driving source and sensors in a single fiber via thermal drawing is presented as a strategy to scalably produce autonomously responsive, feedback-controllable soft actuators. The regulation of the input electrothermal stimuli via a closed loop control system that is based on completely coupled internal sensory components enables multimodal actuation of fiber-based actuators, which is further demonstrated through preservation of actuating conditions, actuation of selected devices in their bundles, and modulation of motion characteristics. The approach to manufacturing autonomously controllable soft actuators can expand applications of soft actuators in kaleidoscopic biomedical and bioengineering fields for transportation, robotics, and prosthetics.

Strengthening Liquid Crystal Elastomer Muscles

ConspectusLiquid crystal elastomer fibers (LCEFs) are reversible artificial muscles capable of stimuli-responsive functions, making them promising competitors for ideal soft actuators. These remarkable actuation properties depend strongly on their mechanical properties, such as elastic modulus and breaking stress. It is necessary to strengthen the LCEF muscles to meet the demands of advanced applications. However, despite the significant progress in LCEFs, there is currently no such Account systematically summarizing and analyzing the strategies adopted for enhancing their mechanical and actuation properties. The intuitive variations among the different enhancement strategies further call for investigations into how to choose the most suitable ones based on specific situations. In this Account, for the first time, we systematically summarize existing approaches to strengthening LCEF-based artificial muscles, contributing to the development of more robust and smarter fibrous artificial muscles.In the first section, we focus on the latest and most valuable progress on strengthening LCEF-based artificial muscles, highlighting the need for a comprehensive summary of the various approaches utilized. The mechanical properties of LCEFs can be tailored through molecular design, physical interactions, and fiber integration. The adjustment of hard/soft segment features, the introduction of additional microstructures, and the fiber integration provide opportunities to strengthen LCEF-based artificial muscles, which are discussed in the second section. Subsequently, we delve into the impact of various preparation methods on the performance of LCEFs, and LCEFs fabricated by different spinning and alignment techniques exhibited rather different mechanical and actuation properties. This has been adopted to engineer novel, stronger, and tailored fibrous artificial muscles, as described in the third section. Moreover, we show that the incorporation of rigid composite materials via coating and doping has emerged as a powerful strategy to strengthen LCEFs, such as core-shell structures. Such enhancements also introduce multifunctionality for LCE-based artificial muscles that can enrich the fiber structure and actuation mechanism, which are elucidated in the fourth section. Finally, we conclude this Account with a critical analysis of the challenges and prospects of LCE-based artificial muscles, hoping to pave the way for the construction of more powerful fibrous artificial muscles.

Time Code for multifunctional 3D printhead controls

Direct Ink Writing, an extrusion-based 3D printing technique, has attracted growing interest due to its ability to process a broad range of materials and integrate multifunctional printheads with features such as shape-changing nozzles, in-situ curing, material switching, and material mixing. Despite these advancements, incorporating auxiliary controls into Geometry Code (G-Code), the standard programming language for these printers, remains challenging. G-Code's line-by-line execution requires auxiliary control commands to interrupt the print path motion, causing defects in the printed structure. We propose a generalizable time-based synchronization approach called Time Code (T-Code), which decouples auxiliary control from G-Code, enabling uninterrupted print path enrichment. We demonstrate the method's effectiveness with both high-end and affordable 3D printers by fabricating functional gradients and parallelizing printhead auxiliary devices for mass customization. Our method reduces defects, enhances print speed, and minimizes the mechanical burden on 3D printers, enabling the rapid creation of complex multimaterial structures.

Spatially programmed alignment and actuation in printed liquid crystal elastomers

Liquid crystal elastomers (LCEs) exhibit reversible shape morphing behavior when cycled above their nematic-to-isotropic transition temperature. During extrusion-based 3D printing, LCE inks are subjected to coupled shear and extensional flows that can be harnessed to spatially control the alignment of their nematic director along prescribed print paths. Here, we combine experiment and modeling to elucidate the effects of ink composition, nozzle geometry, and printing parameters on director alignment. From rheological measurements, we quantify the dimensionless Weissenberg number (Wi) for the flow field each ink experiences as a function of printing conditions and demonstrate that Wi is a strong predictor of LCE alignment. We find that director alignment in LCE filaments printed through a tapered nozzle varies radially when Wi < 1, while it is uniform when Wi ≫ 1. Based on COMSOL simulations and in operando X-ray measurements, we show that LCE inks printed through nozzles with an internal hyperbolic geometry exhibit a more uniform director alignment for a given Wi compared to those through tapered nozzles. Concomitantly, the stiffness along the print direction and actuation strain of printed LCEs increases substantially under such conditions. By varying Wi during printing through adjusting the flow rate "on the fly", LCE architectures with uniform composition, yet locally encoded shape morphing transitions can be realized.

Large stroke radially oriented MXene composite fiber tensile artificial muscles

Actuation is normally dramatically enhanced by introducing so much yarn fiber twist that the fiber becomes fully coiled. In contrast, we found that usefully high muscle strokes and contractile work capacities can be obtained for non-twisted MXene (Ti3C2Tx) fibers comprising MXene nanosheets that are stacked in the fiber direction. The MXene fiber artificial muscles are called MFAMs. We obtained MFAMs that have high modulus in both the radial and axial directions by spinning a solution containing MXene nanosheets dispersed in an aqueous cellulose solution. We observed a highly reversible muscle contraction of 21.0% for a temperature increase from 25° to 125°C. The tensile actuation of MFAMs mainly results from reversible hydrogen bond orientation change during heating, which decreases intra-sheet spacing. The MFAMs exhibited fast, stable actuation to multiple temperature-generating stimuli, which increases their applications in smart textiles, robotic arms, and robotic grippers.

Embedded Physical Intelligence in Liquid Crystalline Polymer Actuators and Robots

Responsive materials possess the inherent capacity to autonomously sense and respond to various external stimuli, demonstrating physical intelligence. Among the diverse array of responsive materials, liquid crystalline polymers (LCPs) stand out for their remarkable reversible stimuli-responsive shape-morphing properties and their potential for creating soft robots. While numerous reviews have extensively detailed the progress in developing LCP-based actuators and robots, there exists a need for comprehensive summaries that elucidate the underlying principles governing actuation and how physical intelligence is embedded within these systems. This review provides a comprehensive overview of recent advancements in developing actuators and robots endowed with physical intelligence using LCPs. This review is structured around the stimulus conditions and categorizes the studies involving responsive LCPs based on the fundamental control and stimulation logic and approach. Specifically, three main categories are examined: systems that respond to changing stimuli, those operating under constant stimuli, and those equip with learning and logic control capabilities. Furthermore, the persisting challenges that need to be addressed are outlined and discuss the future avenues of research in this dynamic field.

Mesogen Organizations in Nematic Liquid Crystal Elastomers Under Different Deformation Conditions

Liquid crystal elastomers (LCEs) exhibit unique mechanical properties of soft elasticity and reversible shape-changing behaviors, and so serve as potentially transformative materials for various protective and actuation applications. This study contributes to filling a critical knowledge gap in the field by investigating the microscale mesogen organization of nematic LCEs with diverse macroscopic deformation. A polarized Fourier transform infrared light spectroscopy (FTIR) tester is utilized to examine the mesogen organizations, including both the nematic director and mesogen order parameter. Three types of material deformation are analyzed: uniaxial tension, simple shear, and bi-axial tension, which are all commonly encountered in practical designs of LCEs. By integrating customized loading fixtures into the FTIR tester, mesogen organizations are examined across varying magnitudes of strain levels for each deformation mode. Their relationships with macroscopic stress responses are revealed and compared with predictions from existing theories. Furthermore, this study reveals unique features of mesogen organizations that have not been previously reported, such as simultaneous evolutions of the mesogen order parameter and nematic director in simple shear and bi-axial loading conditions. Overall, the findings presented in this study offer significant new insights for future rational designs, modeling, and applications of LCE materials.

Wearable and Implantable Soft Robots

Soft robotics presents innovative solutions across different scales. The flexibility and mechanical characteristics of soft robots make them particularly appealing for wearable and implantable applications. The scale and level of invasiveness required for soft robots depend on the extent of human interaction. This review provides a comprehensive overview of wearable and implantable soft robots, including applications in rehabilitation, assistance, organ simulation, surgical tools, and therapy. We discuss challenges such as the complexity of fabrication processes, the integration of responsive materials, and the need for robust control strategies, while focusing on advances in materials, actuation and sensing mechanisms, and fabrication techniques. Finally, we discuss the future outlook, highlighting key challenges and proposing potential solutions.

Closed-Loop Recyclable and Totally Renewable Liquid Crystal Networks with Room-Temperature Programmability and Reconfigurable Functionalities

Dynamic covalent liquid crystal networks (DCv-LCNs) with straightforward (re)programmability, reprocessability, and recyclability facilitates the manufacture of sophisticated LCN actuators and intelligent robots. However, the DCv-LCNs are still limited to heat-assisted programming and polymer-to-polymer reprocessing/recycling, which inevitably lead to deterioration of the LCN structures and the actuation performances after repeated programming/processing treatments, owing to the thermal degradation of the polymer network and/or external agent interference. Here, a totally renewable azobenzene-based DCv-LCN with room-temperature programmability and polymer-to-monomers chemical recyclability is reported, which was synthesized by crosslinking the azobenzene-containing dibenzaldehyde monomer and the triamine monomer via the dynamic and dissociable imine bonds. Thanks to the water-activated dynamics of the imine bonds, the resultant DCv-LCN can be simply programmed, upon water-soaking at room temperature, to yield a UV/Vis light-driven actuator. Importantly, the reported DCv-LCN undergoes depolymerization in an acid-solvent medium at room temperature because of the acid-catalyzed hydrolysis of the imine bonds, giving rise to easy separation and recovery of both monomers in high purity, even with tolerance to additives. The recovered pure monomers can be used to regenerate totally new DCv-LCNs and actuators, and their functionalities can be reconfigured by removing old and introducing new additives, by implementing the closed-loop polymer-monomers-polymer recycling.

Embedding Biomimetic Vascular Networks via Coaxial Sacrificial Writing into Functional Tissue

Printing human tissues and organs replete with biomimetic vascular networks is of growing interest. While it is possible to embed perfusable channels within acellular and densely cellular matrices, they do not currently possess the biomimetic architectures found in native vessels. Here, coaxial sacrificial writing into functional tissues (co-SWIFT) is developed, an embedded bioprinting method capable of generating hierarchically branching, multilayered vascular networks within both granular hydrogel and densely cellular matrices. Coaxial printheads are designed with an extended core-shell configuration to facilitate robust core-core and shell-shell interconnections between printed branching vessels during embedded bioprinting. Using optimized core-shell ink combinations, biomimetic vessels composed of a smooth muscle cell-laden shell that surrounds perfusable lumens are coaxially printed into granular matrices composed of: 1) transparent alginate microparticles, 2) sacrificial microparticle-laden collagen, or 3) cardiac spheroids derived from human induced pluripotent stem cells. Biomimetic blood vessels that exhibit good barrier function are produced by seeding these interconnected lumens with a confluent layer of endothelial cells. Importantly, it is found that co-SWIFT cardiac tissues mature under perfusion, beat synchronously, and exhibit a cardio-effective drug response in vitro. This advance opens new avenues for the scalable biomanufacturing of vascularized organ-specific tissues for drug testing, disease modeling, and therapeutic use.

Coiled Carbon Nanotube Fibers Sheathed by a Reinforced Liquid Crystal Elastomer for Strong and Programmable Artificial Muscles

Fibers of liquid crystal elastomers (LCEs) as promising artificial muscle show ultralarge and reversible contractile strokes. However, the contractile force is limited by the poor mechanical properties of the LCE fibers. Herein, we report high-strength LCE fibers by introducing a secondary network into the single-network LCE. The double-network LCE (DNLCE) shows considerable improvements in tensile strength (313.9%) and maximum actuation stress (342.8%) compared to pristine LCE. To facilitate the controllability and application, a coiled artificial muscle fiber consisting of DNLCE-coated carbon nanotube (CNT) fiber is prepared. When electrothermally driven, the artificial muscle fiber outputs a high actuation performance and programmable actuation. Furthermore, by knitting the artificial muscle fibers into origami structures, an intelligent gripper and crawling inchworm robot have been demonstrated. These demonstrations provide promising application scenarios for advanced intelligent systems in the future.

Recent Advances in 4D Printing of Advanced Materials and Structures for Functional Applications

4D printing has attracted tremendous worldwide attention during the past decade. This technology enables the shape, property, or functionality of printed structures to change with time in response to diverse external stimuli, making the original static structures alive. The revolutionary 4D-printing technology offers remarkable benefits in controlling geometric and functional reconfiguration, thereby showcasing immense potential across diverse fields, including biomedical engineering, electronics, robotics, and photonics. Here, a comprehensive review of the latest achievements in 4D printing using various types of materials and different additive manufacturing techniques is presented. The state-of-the-art strategies implemented in harnessing various 4D-printed structures are highlighted, which involve materials design, stimuli, functionalities, and applications. The machine learning approach explored for 4D printing is also discussed. Finally, the perspectives on the current challenges and future trends toward further development in 4D printing are summarized.

Bioinspired Liquid Metal Based Soft Humanoid Robots

The pursuit of constructing humanoid robots to replicate the anatomical structures and capabilities of human beings has been a long-standing significant undertaking and especially garnered tremendous attention in recent years. However, despite the progress made over recent decades, humanoid robots have predominantly been confined to those rigid metallic structures, which however starkly contrast with the inherent flexibility observed in biological systems. To better innovate this area, the present work systematically explores the value and potential of liquid metals and their derivatives in facilitating a crucial transition towards soft humanoid robots. Through a comprehensive interpretation of bionics, an overview of liquid metals' multifaceted roles as essential components in constructing advanced humanoid robots-functioning as soft actuators, sensors, power sources, logical devices, circuit systems, and even transformable skeletal structures-is presented. It is conceived that the integration of these components with flexible structures, facilitated by the unique properties of liquid metals, can create unexpected versatile functionalities and behaviors to better fulfill human needs. Finally, a revolution in humanoid robots is envisioned, transitioning from metallic frameworks to hybrid soft-rigid structures resembling that of biological tissues. This study is expected to provide fundamental guidance for the coming research, thereby advancing the area.

Liquid Crystal Elastomer Lattices with Thermally Programmable Deformation via Multi-Material 3D Printing

An integrated design, modeling, and multi-material 3D printing platform for fabricating liquid crystal elastomer (LCE) lattices in both homogeneous and heterogeneous layouts with spatially programmable nematic director order and local composition is reported. Depending on their compositional topology, these lattices exhibit different reversible shape-morphing transformations upon cycling above and below their respective nematic-to-isotropic transition temperatures. Further, it is shown that there is good agreement between their experimentally observed deformation response and model predictions for all LCE lattice designs evaluated. Lastly, an inverse design model is established and the ability to print LCE lattices with the predicted deformation behavior is demonstrated. This work opens new avenues for creating architected LCE lattices that may find potential application in energy-dissipating structures, microfluidic pumping, mechanical logic, and soft robotics.

Photothermal Actuation of Thick 3D-Printed Liquid Crystalline Elastomer Nanocomposites

Liquid crystalline elastomers (LCEs) are stimuli-responsive materials that transduce an input energy into a mechanical response. LCE composites prepared with photothermal agents, such as nanoinclusions, are a means to realize wireless, remote, and local control of deformation with light. Amongst photothermal agents, gold nanorods (AuNRs) are highly efficient converters when the irradiation wavelength matches the longitudinal surface plasmon resonance (LSPR) of the AuNRs. However, AuNR aggregation broadens the LSPR which also reduces photothermal efficiency. Here, the surface chemistry of AuNRs is engineered via a well-controlled two-step ligand exchange with a monofunctional poly(ethylene glycol) (PEG) thiol that greatly improves the dispersion of AuNRs in LCEs. Accordingly, LCE-AuNR nanocomposites with very low PEG-AuNR content (0.01 wt%) prepared by 3D printing are shown to be highly efficient photothermal actuators with rapid response (>60% strain s-1) upon irradiation with near-infrared (NIR; 808 nm) light. Because of the excellent dispersion of PEG-AuNR within the LCE, unabsorbed NIR light transmits through the nanocomposites and can actuate a series of samples. Further, the dispersion also allows for the optical deformation of millimeter-thick 3D printed structures without sacrificing actuation speed. The realization of well-dispersed nanoinclusions to maximize the stimulus-response of LCEs can benefit functional implementation in soft robotics or medical devices.

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.

4D Printing of Shape-Morphing Liquid Crystal Elastomers

In nature, biological systems can sense environmental changes and alter their performance parameters in real time to adapt to environmental changes. Inspired by these, scientists have developed a range of novel shape-morphing materials. Shape-morphing materials are a kind of "intelligent" materials that exhibit responses to external stimuli in a predetermined way and then display a preset function. Liquid crystal elastomer (LCE) is a typical representative example of shape-morphing materials. The emergence of 4D printing technology can effectively simplify the preparation process of shape-morphing LCEs, by changing the printing material compositions and printing conditions, enabling precise control and macroscopic design of the shape-morphing modes. At the same time, the layer-by-layer stacking method can also endow the shape-morphing LCEs with complex, hierarchical orientation structures, which gives researchers a great degree of design freedom. 4D printing has greatly expanded the application scope of shape-morphing LCEs as soft intelligent materials. This review systematically reports the recent progress of 3D/4D printing of shape-morphing LCEs, discusses various 4D printing technologies, synthesis methods and actuation modes of 3D/4D printed LCEs, and summarizes the opportunities and challenges of 3D/4D printing technologies in preparing shape-morphing LCEs.

Multiscale Heterogeneous Polymer Composites for High Stiffness 4D Printed Electrically Controllable Multifunctional Structures

4D printing is an emerging field where 3D printing techniques are used to pattern stimuli-responsive materials to create morphing structures, with time serving as the fourth dimension. However, current materials utilized for 4D printing are typically soft, exhibiting an elastic modulus (E) range of 10-4 to 10 MPa during shape change. This restricts the scalability, actuation stress, and load-bearing capabilities of the resulting structures. To overcome these limitations, multiscale heterogeneous polymer composites are introduced as a novel category of stiff, thermally responsive 4D printed materials. These inks exhibit an E that is four orders of magnitude greater than that of existing 4D printed materials and offer tunable electrical conductivities for simultaneous Joule heating actuation and self-sensing capabilities. Utilizing electrically controllable bilayers as building blocks, a flat geometry is designed and printed that morphs into a 3D self-standing lifting robot, setting new records for weight-normalized load lifted and actuation stress when compared to other 3D printed actuators. Furthermore, the ink palette is employed to create and print planar lattice structures that transform into various self-supporting complex 3D shapes. These contributions are integrated into a 4D printed electrically controlled multigait crawling robotic lattice structure that can carry 144 times its own weight.

Additive Manufacturing Provides Infinite Possibilities for Self-Sensing Technology

Integrating sensors and other functional parts in one device can enable a new generation of integrated intelligent devices that can perform self-sensing and monitoring autonomously. Applications include buildings that detect and repair damage, robots that monitor conditions and perform real-time correction and reconstruction, aircraft capable of real-time perception of the internal and external environment, and medical devices and prosthetics with a realistic sense of touch. Although integrating sensors and other functional parts into self-sensing intelligent devices has become increasingly common, additive manufacturing has only been marginally explored. This review focuses on additive manufacturing integrated design, printing equipment, and printable materials and stuctures. The importance of the material, structure, and function of integrated manufacturing are highlighted. The study summarizes current challenges to be addressed and provides suggestions for future development directions.

Knotted Artificial Muscles for Bio-Mimetic Actuation under Deepwater

Muscles featuring high frequency and high stroke linear actuation are essential for animals to achieve superior maneuverability, agility, and environmental adaptability. Artificial muscles are yet to match their biological counterparts, due to inferior actuation speed, magnitude, mode, or adaptability. Inspired by the hierarchical structure of natural muscles, artificial muscles are created that are powerful, responsive, robust, and adaptable. The artificial muscles consist of knots braided from 3D printed liquid crystal elastomer fibers and thin heating threads. The unique hierarchical, braided knot structure offers amplified linear stroke, force rate, and damage-tolerance, as verified by both numerical simulations and experiments. In particular, the square knotted artificial muscle shows reliable cycles of actuation at 1Hz in 3000m depth underwater. Potential application is demonstrated by propelling a model boat. Looking ahead, the knotted artificial muscles can empower novel biomedical devices and soft robots to explore various environments, from inside human body to the mysterious deep sea.

Beyond Color Boundaries: Pioneering Developments in Cholesteric Liquid Crystal Photonic Actuators

Creatures in nature make extensive use of structural color adaptive camouflage to survive. Cholesteric liquid crystals, with nanostructures similar to those of natural organisms, can be combined with actuators to produce bright structural colors in response to a wide range of stimuli. Structural colors modulated by nano-helical structures can continuously and selectively reflect specific wavelengths of light, breaking the limit of colors recognizable by the human eye. In this review, the current state of research on cholesteric liquid crystal photonic actuators and their technological applications is presented. First, the basic concepts of cholesteric liquid crystals and their nanostructural modulation are outlined. Then, the cholesteric liquid crystal photonic actuators responding to different stimuli (mechanical, thermal, electrical, light, humidity, magnetic, pneumatic) are presented. This review describes the practical applications of cholesteric liquid crystal photonic actuators and summarizes the prospects for the development of these advanced structures as well as the challenges and their promising applications.

Multiscale 3D printing via active nozzle size and shape control

Three-dimensional (3D) printers extruding filaments through a fixed nozzle encounter a conflict between high resolution, requiring small diameters, and high speed, requiring large diameters. This limitation is especially pronounced in multiscale architectures featuring both bulk and intricate elements. Here, we introduce adaptive nozzle 3D printing (AN3DP), a technique enabling dynamic alteration of nozzle diameter and cross-sectional shape during printing. The AN3DP nozzle consists of eight independently controllable, tendon-driven pins arrayed around a flexible, pressure-resistant membrane. The design incorporates a tapered angle optimized for extruding shear-thinning inks and a pointed tip suitable for constrained-space printing, such as conformal and embedded printing. AN3DP's efficacy is demonstrated through the fabrication of components with continuous gradients, eliminating the need for discretization, and achieving enhanced density and contour precision compared to traditional 3D printing methods. This platform substantially expands the scope of extrusion-based 3D printers, thus facilitating diverse applications, including bioprinting cell-laden and hierarchical implants with bone-like microarchitecture.

Fast-Crosslinking Enabled Self-Roughed Polydimethylsiloxane Transparent Superhydrophobic Coating and Its Application in Anti-Liquid-Interference Electrothermal Device

Polydimethylsiloxane (PDMS)-based transparent and superhydrophobic coatings have important applications, such as anti-icing, corrosion resistance, self-cleaning, etc. However, their applications are limited by the inevitable introduction of nanoparticles/high-temperature/segmented PDMS to facilitate a raspy surface. In this study, a self-roughed, neat PDMS superhydrophobic coating with high transparency is developed via a one-step spray-coating technique. PDMS suspensions with various droplet sizes are synthesized and used as building blocks for raspy surface formation by controlled curing on the warm substrate. The optimal coating exhibits a large water contact angle of 155.4° and transparency (T550 = 82.3%). Meanwhile, the employed spray-coating technique is applicable to modify a plethora of substrates. For proof-of-concept demonstrations, the use of the PDMS hydrophobic coating for anti-liquid-interference electrothermal devices and further transparent observation window for long-term operation in a sub-zero environment is shown successful. The proposed facile synthesis method of hydrophobic PDMS coating is expected to have great potential for a broad range of applications in the large-scale fabrication of fluorine-free, eco-friendly superhydrophobic surfaces.

Fiber Actuators Based on Reversible Thermal Responsive Liquid Crystal Elastomer

Soft actuators inspired by the movement of organisms have attracted extensive attention in the fields of soft robotics, electronic skin, artificial intelligence, and healthcare due to their excellent adaptability and operational safety. Liquid crystal elastomer fiber actuators (LCEFAs) are considered as one of the most promising soft actuators since they can provide reversible linear motion and are easily integrated or woven into complex structures to perform pre-programmed movements such as stretching, rotating, bending, and expanding. The research on LCEFAs mainly focuses on controllable preparation, structural design, and functional applications. This review, for the first time, provides a comprehensive and systematic review of recent advances in this important field by focusing on reversible thermal response LCEFAs. First, the thermal driving mechanism, and direct and indirect heating strategies of LCEFAs are systematically summarized and analyzed. Then, the fabrication methods and functional applications of LCEFAs are summarized and discussed. Finally, the challenges and technical difficulties that may hinder the performance improvement and large-scale production of LCEFAs are proposed, and the development opportunities of LCEFAs are prospected.

In Situ Synthesis of Liquid Metal Conductive Fibers toward Smart Cloth

To meet the diverse needs of humans, smart cloth has become a potential research hotspot to replace traditional cloth. However, it is challenging to manufacture a flexible fabric with multiple functions. Here, we introduce a smart cloth based on liquid metal (LM) conductive fibers. Ga2O3 nanoparticles are obtained through ultrasonic pretreatment. Furthermore, a coordination bond is formed between thiol groups on the surface of protein fibers and Ga2O3 through a scraping method, allowing Ga2O3 particles to be grafted onto the surface of protein fibers in situ. Finally, LM conductive fibers are encapsulated using a photocuring adhesive. In addition, a wearable smart cloth integrated with multiple sensors has been developed based on LM conductive fibers. Users can not only monitor their movement trajectory and the surrounding environment in real time but also have their data supervised by family members through a client, achieving remote and continuous monitoring. The development of this wearable smart cloth provides strong support for future wearable, flexible electronic devices.

Liquid Crystal Orientation and Shape Optimization for the Active Response of Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are responsive materials that can undergo large reversible deformations upon exposure to external stimuli, such as electrical and thermal fields. Controlling the alignment of their liquid crystals mesogens to achieve desired shape changes unlocks a new design paradigm that is unavailable when using traditional materials. While experimental measurements can provide valuable insights into their behavior, computational analysis is essential to exploit their full potential. Accurate simulation is not, however, the end goal; rather, it is the means to achieve their optimal design. Such design optimization problems are best solved with algorithms that require gradients, i.e., sensitivities, of the cost and constraint functions with respect to the design parameters, to efficiently traverse the design space. In this work, a nonlinear LCE model and adjoint sensitivity analysis are implemented in a scalable and flexible finite element-based open source framework and integrated into a gradient-based design optimization tool. To display the versatility of the computational framework, LCE design problems that optimize both the material, i.e., liquid crystal orientation, and structural shape to reach a target actuated shapes or maximize energy absorption are solved. Multiple parameterizations, customized to address fabrication limitations, are investigated in both 2D and 3D. The case studies are followed by a discussion on the simulation and design optimization hurdles, as well as potential avenues for improving the robustness of similar computational frameworks for applications of interest.

Advancing interactive systems with liquid crystal network-based adaptive electronics

Achieving adaptive behavior in artificial systems, analogous to living organisms, has been a long-standing goal in electronics and materials science. Efforts to integrate adaptive capabilities into synthetic electronics traditionally involved a typical architecture comprising of sensors, an external controller, and actuators constructed from multiple materials. However, challenges arise when attempting to unite these three components into a single entity capable of independently coping with dynamic environments. Here, we unveil an adaptive electronic unit based on a liquid crystal polymer that seamlessly incorporates sensing, signal processing, and actuating functionalities. The polymer forms a film that undergoes anisotropic deformations when exposed to a minor heat pulse generated by human touch. We integrate this property into an electric circuit to facilitate switching. We showcase the concept by creating an interactive system that features distributed information processing including feedback loops and enabling cascading signal transmission across multiple adaptive units. This system responds progressively, in a multi-layered cascade to a dynamic change in its environment. The incorporation of adaptive capabilities into a single piece of responsive material holds immense potential for expediting progress in next-generation flexible electronics, soft robotics, and swarm intelligence.

3D printing with a 3D printed digital material filament for programming functional gradients

Additive manufacturing, or 3D printing attracts growing attention as a promising method for creating functionally graded materials. Fused deposition modeling (FDM) is widely available, but due to its simple process, creating spatial gradation of diverse properties using FDM is challenging. Here, we present a 3D printed digital material filament that is structured towards 3D printing of functional gradients, utilizing only a readily available FDM printer and filaments. The DM filament consists of multiple base materials combined with specific concentrations and distributions, which are FDM printed. When the DM filament is supplied to the same printer, its constituent materials are homogeneously blended during extrusion, resulting in the desired properties in the final structure. This enables spatial programming of material properties in extreme variations, including mechanical strength, electrical conductivity, and color, which are otherwise impossible to achieve with traditional FDMs. Our approach can be readily adopted to any standard FDM printer, enabling low-cost production of functional gradients.

Novel Biomimetic "Spider Web" Robust, Super-Contractile Liquid Crystal Elastomer Active Yarn Soft Actuator

In nature, spider web is an interwoven network with high stability and elasticity from silk threads secreted by spider. Inspired by the structure of spider webs, light-driven liquid crystal elastomer (LCE) active yarn is designed with super-contractile and robust weavability. Herein, a novel biomimetic gold nanorods (AuNRs) @LCE yarn soft actuator with hierarchical structure is fabricated by a facile electrospinning and subsequent photocrosslinking strategies. Meanwhile, the inherent mechanism and actuation performances of the as-prepared yarn actuator with interleaving network are systematically analyzed. Results demonstrate that thanks to the unique "like-spider webs" structure between fibers, high molecular orientation within the LCE microfibers and good flexibility, they can generate super actuation strain (≈81%) and stable actuation performances. Importantly, benefit from the robust covalent bonding at the organic-inorganic interface, photopolymerizable AuNRs molecules are uniformly introduced into the polymer backbone of electrospun LCE yarn to achieve tailorable shape-morphing under different light intensity stimulation. As a proof-of-concept illustration, light-driven artificial muscles, micro swimmers, and hemostatic bandages are successfully constructed. The research disclosed herein can offer new insights into continuous production and development of LCE-derived yarn actuator that are of paramount significance for many applications from smart fabrics to flexible wearable devices.

Adaptive nanotube networks enabling omnidirectionally deformable electro-driven liquid crystal elastomers towards artificial muscles

Artificial muscles that can convert electrical energy into mechanical energy promise broad scientific and technological applications. However, existing electro-driven artificial muscles have been plagued with problems that hinder their practical applications: large electro-mechanical attenuation during deformation, high-driving voltages, small actuation strain, and low power density. Here, we design and create novel electro-thermal-driven artificial muscles rationally composited by hierarchically structured carbon nanotube (HS-CNT) networks and liquid crystal elastomers (LCEs), which possess adaptive sandwiched nanotube networks with angulated-scissor-like microstructures, thus effectively addressing above problems. These HS-CNT/LCE artificial muscles demonstrate not only large strain (>40%), but also remarkable conductive robustness (R/R0 < 1.03 under actuation), excellent Joule heating efficiency (≈ 233 °C at 4 V), and high load-bearing capacity (R/R0 < 1.15 at 4000 times its weight loaded). In addition, our artificial muscles exhibit real-muscle-like morphing intelligence that enables preventing mechanical damage in response to excessively heavyweight loading. These high-performance artificial muscles uniquely combining omnidirectional stretchability, robust electrothermal actuation, low driving voltage, and powerful mechanical output would exert significant technological impacts on engineering applications such as soft robotics and wearable flexible electronics.

Intelligent electroactive material systems with self-adaptive mechanical memory and sequential logic

By synthesizing the requisite functionalities of intelligence in an integrated material system, it may become possible to animate otherwise inanimate matter. A significant challenge in this vision is to continually sense, process, and memorize information in a decentralized way. Here, we introduce an approach that enables all such functionalities in a soft mechanical material system. By integrating nonvolatile memory with continuous processing, we develop a sequential logic-based material design framework. Soft, conductive networks interconnect with embedded electroactive actuators to enable self-adaptive behavior that facilitates autonomous toggling and counting. The design principles are scaled in processing complexity and memory capacity to develop a model 8-bit mechanical material that can solve linear algebraic equations based on analog mechanical inputs. The resulting material system operates continually to monitor the current mechanical configuration and to autonomously search for solutions within a desired error. The methods created in this work are a foundation for future synthetic general intelligence that can empower materials to autonomously react to diverse stimuli in their environment.

Rotini-like MXene@LCE Actuator with Diverse and Programmable Actuation Based on Dual-mode Synergy

Liquid crystalline elastomer (LCE) exhibits muscle-like actuation upon order-disturbed stimulus, offering ample room for designing soft robotic systems. Multimodal LCE is demonstrated to unleash the potential to perform multitasks. However, each actuation mode is typically isolated. In contrast, coordination between different actuation modes based on an MXene-doped LCE is realized, whose actuation can be triggered either by directly heating/cooling or using near-infrared light due to the photo-thermal effect of MXene. As such, the two activation modes (heat and light) not only can work individually to offer stable actuation under different conditions but also can collaborate synergistically to generate more intelligent motions, such as achieving the brake and turn of an autonomous rolling. The principle therefore can diversify the design principles for multifunctional soft actuators and robotics.

Shape programming of liquid crystal elastomers

Liquid crystal elastomers (LCEs) are shape-morphing materials that demonstrate reversible actuation when exposed to external stimuli, such as light or heat. The actuation's complexity depends heavily on the instilled liquid crystal alignment, programmed into the material using various shape-programming processes. As an unavoidable part of LCE synthesis, these also introduce geometrical and output restrictions that dictate the final applicability. Considering LCE's future implementation in real-life applications, it is reasonable to explore these limiting factors. This review offers a brief overview of current shape-programming methods in relation to the challenges of employing LCEs as soft, shape-memory components in future devices.

Melt electrowriting enabled 3D liquid crystal elastomer structures for cross-scale actuators and temperature field sensors

Liquid crystal elastomers (LCEs) have garnered attention for their remarkable reversible strains under various stimuli. Early studies on LCEs mainly focused on basic dimensional changes in macrostructures or quasi-three-dimensional (3D) microstructures. However, fabricating complex 3D microstructures and cross-scale LCE-based structures has remained challenging. In this study, we report a compatible method named melt electrowriting (MEW) to fabricate LCE-based microfiber actuators and various 3D actuators on the micrometer to centimeter scales. By controlling printing parameters, these actuators were fabricated with high resolutions (4.5 to 60 μm), actuation strains (10 to 55%), and a maximum work density of 160 J/kg. In addition, through the integration of a deep learning-based model, we demonstrated the application of LCE materials in temperature field sensing. Large-scale, real-time, LCE grid-based spatial temperature field sensors have been designed, exhibiting a low response time of less than 42 ms and a high precision of 94.79%.

Journey from human hands to robot hands: biological inspiration of anthropomorphic robotic manipulators

The development of robotic hands that can replicate the complex movements and dexterity of the human hand has been a longstanding challenge for scientists and engineers. A human hand is capable of not only delicate operation but also crushing with power. For performing tasks alongside and in place of humans, an anthropomorphic manipulator design is considered the most advanced implementation, because it is able to follow humans' examples and use tools designed for people. In this article, we explore the journey from human hands to robot hands, tracing the historical advancements and current state-of-the-art in hand manipulator development. We begin by investigating the anatomy and function of the human hand, highlighting the bone-tendon-muscle structure, skin properties, and motion mechanisms. We then delve into the field of robotic hand development, focusing on highly anthropomorphic designs. Finally, we identify the requirements and directions for achieving the next level of robotic hand technology.

Multiscale Heterogeneous Polymer Composites for High Stiffness 4D Printed Electrically Controllable Multifunctional Structures

4D printing is an emerging field where 3D printing techniques are used to pattern stimuli-responsive materials to create morphing structures, with time serving as the fourth dimension. However, current materials utilized for 4D printing are typically soft, exhibiting an elastic modulus (E) range of 10-4 to 10 MPa during shape change. This restricts the scalability, actuation stress, and load-bearing capabilities of the resulting structures. To overcome these limitations, multiscale heterogeneous polymer composites are introduced as a novel category of stiff, thermally responsive 4D printed materials. These inks exhibit an E that is four orders of magnitude greater than that of existing 4D printed materials and offer tunable electrical conductivities for simultaneous Joule heating actuation and self-sensing capabilities. Utilizing electrically controllable bilayers as building blocks, a flat geometry that morphs into a 3D self-standing lifting robot is designed and printed, setting new records for weight-normalized load lifted and actuation stress when compared to other 3D printed actuators. Furthermore, this ink palette is employed to create and print planar lattice structures that transform into various self-supporting complex 3D shapes. Finally these inks are integrated into a 4D printed electrically controlled multigait crawling robotic lattice structure that can carry 144 times its own weight.

Three-dimensional blueprinting of molecular patterns in liquid crystalline polymers

Exploiting the interplay of anisotropic diamagnetic susceptibility of liquid crystalline monomers and site selective photopolymerization enables the fabrication of 3D freeforms with highly refined microstructures. Utilizing chain transfer agents in the mesogenic inks presents a pathway for broadly tuning the mechanical properties of liquid crystalline polymers and their response to stimuli. In particular, the combination of 1,4-benzenedimethanethiol and tetrabromomethane is shown to enable voxelated blueprinting of molecular order, while allowing for a modulation of the crosslink density and the mechanical properties. The formulation of these monomers allows for the resolution of the voxels to approach the limits set by the coherence lengths defined by the anchoring from surfaces. These compositions demonstrate the expected thermotropic responses while allowing for their functionalization with photochromic switches to elicit photomechanical responses. Actuation strains are shown to outstrip that accomplished with prior systems that did not access chain transfer agents to modulate the structure of the macromolecular network. Test cases of this system are shown to create freeform actuators that exploit the refined director patterns during high-resolution printing. These include topological defects, hierarchically-structured light responsive grippers, and biomimetic flyers whose flight dynamics can be actively modulated via irradiation with light.

Soft Fiber/Textile Actuators: From Design Strategies to Diverse Applications

Fiber/textile-based actuators have garnered considerable attention due to their distinctive attributes, encompassing higher degrees of freedom, intriguing deformations, and enhanced adaptability to complex structures. Recent studies highlight the development of advanced fibers and textiles, expanding the application scope of fiber/textile-based actuators across diverse emerging fields. Unlike sheet-like soft actuators, fibers/textiles with intricate structures exhibit versatile movements, such as contraction, coiling, bending, and folding, achieved through adjustable strain and stroke. In this review article, we provide a timely and comprehensive overview of fiber/textile actuators, including structures, fabrication methods, actuation principles, and applications. After discussing the hierarchical structure and deformation of the fiber/textile actuator, we discuss various spinning strategies, detailing the merits and drawbacks of each. Next, we present the actuation principles of fiber/fabric actuators, along with common external stimuli. In addition, we provide a summary of the emerging applications of fiber/textile actuators. Concluding with an assessment of existing challenges and future opportunities, this review aims to provide a valuable perspective on the enticing realm of fiber/textile-based actuators.

Liquid Metal Actuators: A Comparative Analysis of Surface Tension Controlled Actuation

Liquid metals, with their unique combination of electrical and mechanical properties, offer great opportunities for actuation based on surface tension modulation. Thanks to the scaling laws of surface tension, which can be electrochemically controlled at low voltages, liquid metal actuators stand out from other soft actuators for their remarkable characteristics such as high contractile strain rates and higher work densities at smaller length scales. This review summarizes the principles of liquid metal actuators and discusses their performance as well as theoretical pathways toward higher performances. The objective is to provide a comparative analysis of the ongoing development of liquid metal actuators. The design principles of the liquid metal actuators are analyzed, including low-level elemental principles (kinematics and electrochemistry), mid-level structural principles (reversibility, integrity, and scalability), and high-level functionalities. A wide range of practical use cases of liquid metal actuators from robotic locomotion and object manipulation to logic and computation is reviewed. From an energy perspective, strategies are compared for coupling the liquid metal actuators with an energy source toward fully untethered robots. The review concludes by offering a roadmap of future research directions of liquid metal actuators.

Tunable Folding Assembly Strategy for Soft Pneumatic Actuators

With intrinsic compliance, soft pneumatic actuators are widely utilized in delicate tasks. However, complex fabrication approaches and limited tunability are still problems. Here, we propose a tunable folding assembly strategy to design and fabricate soft pneumatic actuators called FASPAs (folding assembly soft pneumatic actuators). A FASPA consists only of a folded silicone tube constrained by rubber bands. By designing local stiffness and folding manner, the FASPA can be designed to achieve four configurations, pure bending, discontinuous-curvature bending, helix, and discontinuous-curvature helix. Analytical models are developed to predict the deformation and the tip trajectory of different configurations. Meanwhile, experiments are performed to verify the models. The stiffness, load capacity, output force, and step response are measured, and fatigue tests are performed. Further, grippers with single, double, and triple fingers are assembled by utilizing different types of FASPAs. As such, objects with different shapes, sizes, and weights can be easily grasped. The folding assembly strategy is a promising method to design and fabricate soft robots with complex configurations to complete tough tasks in harsh environments.

Tunable, Textile-Based Joint Impedance Module for Soft Robotic Applications

The design of soft actuators is often focused on achieving target trajectories or delivering specific forces and torques, rather than controlling the impedance of the actuator. This article outlines a new soft, tunable pneumatic impedance module based on an antagonistic actuator setup of textile-based pneumatic actuators intended to deliver bidirectional torques about a joint. Through mechanical programming of the actuators (select tuning of geometric parameters), the baseline torque to angle relationship of the module can be tuned. A high bandwidth fluidic controller that can rapidly modulate the pressure at up to 8 Hz in each antagonistic actuator was also developed to enable tunable impedance modulation. This high bandwidth was achieved through the characterization and modeling of the proportional valves used, derivation of a fluidic model, and derivation of control equations. The resulting impedance module was capable of modulating its stiffness from 0 to 100 Nm/rad, at velocities up to 120°/s and emulating asymmetric and nonlinear stiffness profiles, typical in wearable robotic applications.

Flexible Capacitive Sensors Based on Liquid Crystal Elastomer

The disordered transformation of the ordered aligned polar liquid crystal molecules in liquid crystal elastomers (LCEs) under the influence of an external field imbues them with the unique property of thermally reversible shape memory, making them highly valuable for various applications, particularly in actuators. In this study, we examined the high dielectric constant exhibited by the orientation polarization of polar liquid crystal molecules in RM257-LCE films, which holds significant potential for developing flexible capacitive sensors. By manipulating the flexibility of the molecular chain network and introducing hydrogen bonds and metal ions into the main chain, we were able to enhance the relative dielectric constant of LCEs to an impressive value of 62 (at 100 Hz), which is approximately 23 times higher than for polydimethylsiloxane (PDMS). This elevated dielectric constant displays a noteworthy positive temperature coefficient within a specific temperature range, starting from room temperature and extending to the clearing point. Using this property, we fabricated highly sensitive capacitive, flexible temperature sensors. Moreover, we successfully engineered a flexible pressure sensor with an excellent pressure-sensing range of 0-2 MPa by combining the porous structure of the prepared LCEs with mushroom electrodes. Additionally, the sensor showcases a remarkable capacitance recovery time of 0.8 s at 90 °C. These outstanding features collectively contribute to the excellent pressure-sensing characteristics of our sensor. The findings of this study offer valuable insights and serve as a reference for the design of innovative flexible sensors, enabling advancements in sensor technology.

Liquid Crystalline Elastomer for Separate or Collective Sensing and Actuation Functions

A porous liquid crystalline elastomer actuator filled with an ionic liquid (PLCE-IL) is shown to exhibit the functions of two classes of materials: electrically responsive, deformable materials for sensing and soft active materials for stimuli-triggered actuation. On one hand, upon the order-disorder phase transition of aligned mesogens, PLCE-IL behaves like a typical actuator capable of reversible shape change and can be used to assemble light-fuelled soft robot. On the other hand, at temperatures below the phase transition, PLCE-IL is an elastomer that can sustain and sense large deformations of various modes as well as environmental condition changes by reporting the corresponding electrical resistance variation. The two distinguished functions can also be used collectively with PLCE-IL integrated in one device. This intelligent feature is demonstrated with an artificial arm. When the arm is manually powered to fold and unfold, the PLCE-IL strip serves as a deformation sensor; while when the manual power is not available, the role of the PLCE-IL strip is switched to an actuator that enables light-driven folding and unfolding of the arm. This study shows that electrically responsive LCEs are a potential materials platform that offers possibilities for merging deformable electronic and actuation applications.