Liquid Crystal Elastomer with Integrated Soft Thermoelectrics for Shape Memory Actuation and Energy Harvesting

Kirigami-Inspired Light-Responsive Conical Spiral Actuators with Large Contraction Ratio Using Liquid Crystal Elastomer Fiber

Liquid crystal elastomers (LCEs) are among the key smart materials driving soft robotics and LCE fibers have garnered significant attention for their rapid response characteristics. A convenient and fast method for programming orientations of liquid crystal molecules is a focal issue in LCE applications. Inspired by the Kirigami technique, here, we propose a novel method for fabricating LCE fibers based on customizable cutting paths and secondary photo-cross-linking. While most existing LCE actuators exhibit contraction ratios of around 30 to 40%, our conical spiral actuator, fabricated from LCE-carbon nanotube (CNT) fiber using the proposed method, demonstrates a significantly higher contraction ratio, reaching up to 80%. The contraction ratio can be controlled by adjusting the cutting path parameters and we elucidate the mechanism linking liquid crystal orientation to the distribution of contraction ratio. Additionally, the conical spiral deformation of the actuator can be manipulated with light radiation, enabling versatile functionalities such as catching, twisting, and gripping. We hope that the novel LCE fiber fabrication method presented provides new insights for programming and preparing LCE fibers, offering a valuable reference for the application of smart soft materials.

Flexible pyroelectric energy harvesters from nanocomposites of liquid crystal elastomers/lead zirconate titanate nanoparticles

Pyroelectric materials that can generate electric charges when subjected to temperature changes are of interest for renewable energy. However, current flexible pyroelectric energy harvesters suffer from low output. Here, we present a nanocomposite of liquid crystalline elastomer (LCE) and pyroelectric lead zirconate titanate (PZT) nanoparticles and demonstrate a flexible heat harvesting device with high output. The overall pyroelectricity is enhanced by the secondary pyroelectricity generated from the thermal stress imposed on the LCE. Calculations and simulations corroborate with experiments, suggesting that the monodomain LCE/PZT with fixed boundaries offers the most enhancement. At a maximum heating rate of 0.20 kelvin per second, the fixed monodomain film (42.7 weight % PZT) shows an output current of 2.81 nanoamperes and a voltage of 6.23 volts, corresponding to a pyroelectric coefficient p of -4.01 nanocoulombs per square centimeter per kelvin, 49% higher than that of the widely used polyvinylidene fluoride. Our energy harvester can charge capacitors and power electronic devices such as light-emitting diodes.

A compliant metastructure design with reconfigurability up to six degrees of freedom

Compliant mechanisms with reconfigurable degrees of freedom are gaining attention in the development of kinesthetic haptic devices, robotic systems, and mechanical metamaterials. However, available devices exhibit limited programmability and form-customizability, restricting their versatility. To address this gap, we propose a metastructure concept featuring reconfigurable motional freedom and tunable stiffness, adaptable to various form factors and applications. These devices incorporate passive flexures and actively stiffness-changing rods to modify kinematic freedom. A rational design pipeline informs the flexures' topological arrangements, geometric parameters, and control signals based on targeted mobilities, enabling the creation of unitary joints with up to six degrees of freedom. Our demonstrative application examples include a wrist device that has an effective stiffness of 0.370 Nm/deg (unlocked state, 5% displacement) to 2.278 Nm/deg (locked state, 1% displacement) to enable dynamic joint mobility control, a haptic thimble device (2.27-52.815 Nmm-1 at 1% displacement) that mimics the sensation of touching physical materials ranging from soft gel to metal surfaces, and a wearable device composed of multiple joints tailored for the arm and hand to augment haptic experiences or facilitate muscle training. We believe the presented method can help democratize compliant metastructures development and expand their versatility for broader contexts.

3D Soft Architectures for Stretchable Thermoelectric Wearables with Electrical Self-Healing and Damage Tolerance

Flexible thermoelectric devices (TEDs) exhibit adaptability to curved surfaces, holding significant potential for small-scale power generation and thermal management. However, they often compromise stretchability, energy conversion, or robustness, thus limiting their applications. Here, the implementation of 3D soft architectures, multifunctional composites, self-healing liquid metal conductors, and rigid semiconductors is introduced to overcome these challenges. These TEDs are extremely stretchable, functioning at strain levels as high as 230%. Their unique design, verified through multiphysics simulations, results in a considerably high power density of 115.4 µW cm⁻2 at a low-temperature gradient of 10 °C. This is achieved through 3D printing multifunctional elastomers and examining the effects of three distinct thermal insulation infill ratios (0%, 12%, and 100%) on thermoelectric energy conversion and structural integrity. The engineered structure is lighter and effectively maintains the temperature gradient across the thermoelectric semiconductors, thereby resulting in higher output voltage and improved heating and cooling performance. Furthermore, these thermoelectric generators show remarkable damage tolerance, remaining fully functional even after multiple punctures and 2000 stretching cycles at 50% strain. When integrated with a 3D-printed heatsink, they can power wearable sensors, charge batteries, and illuminate LEDs by scavenging body heat at room temperature, demonstrating their application as self-sustainable electronics.

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.

Programmable Chemical Reactions Enable Ultrastrong Soft Pneumatic Actuation

Soft pneumatic actuation is widely used in wearable devices, soft robots, artificial muscles, and surgery machines. However, generating high-pressure gases in a soft, controllable, and portable way remains a substantial challenge. Here, a class of programmable chemical reactions that can be used to controllably generate gases with a maximum pressure output of nearly 6 MPa is reported. It is proposed to realize the programmability of the chemical reaction process using thermoelectric material with programmable electric current and employing preprogrammed reversible chemical reactants. The programmable chemical reactions as soft pneumatic actuation can be operated independently as miniature gas sources (∼20-100 g) or combined with arbitrary physical structures to make self-contained machines, capable of generating unprecedented pressures of nearly 6 MPa or forces of about 18 kN in a controllable, portable, and silent manner. Striking demonstrations of breaking a brick, a marble, and concrete blocks, raising a sightseeing car, and successful applications in artificial muscles and soft assistive wearables illustrate tremendous application prospects of soft pneumatic actuation via programmable chemical reactions. The study establishes a new paradigm toward ultrastrong soft pneumatic actuation.

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

4D Printing of Reprogrammable Liquid Crystal Elastomers with Synergistic Photochromism and Photoactuation

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

Fast and Facile Liquid Metal Printing via Projection Lithography for Highly Stretchable Electronic Circuits

Soft electronic circuits are crucial for wearable electronics, biomedical technologies, and soft robotics, requiring soft conductive materials with high conductivity, high strain limit, and stable electrical performance under deformation. Liquid metals (LMs) have become attractive candidates with high conductivity and fluidic compliance, while effective manufacturing methods are demanded. Digital light processing (DLP)-based projection lithography is a high-resolution and high-throughput printing technique for primarily polymers and some metals. If LMs can be printed with DLP as well, the entire soft devices can be fabricated by one printer in a streamlined and highly efficient process. Herein, fast and facile DLP-based LM printing is achieved. Simply with 5-10 s of patterned ultraviolet (UV)-light exposure, a highly conductive and stretchable pattern can be printed using a photo-crosslinkable LM particle ink. The printed eutectic gallium indium traces feature high resolution (≈20 µm), conductivity (3 × 106 S m-1), stretchability (≈2500%), and excellent stability (consistent performance at different deformation). Various patterns are printed in diverse material systems for broad applications including stretchable displays, epidermal strain sensors, heaters, humidity sensors, conformal electrodes for electrography, and multi-layer actuators. The facile and scalable process, excellent performance, and diverse applications ensure its broad impact on soft electronic manufacturing.

Integrated Design and Fabrication of Pneumatic Soft Robot Actuators in a Single Casting Step

Bio-inspired soft robots have already shown the ability to handle uncertainty and adapt to unstructured environments. However, their availability is partially restricted by time-consuming, costly, and highly supervised design-fabrication processes, often based on resource-intensive iterative workflows. Here, we propose an integrated approach targeting the design and fabrication of pneumatic soft actuators in a single casting step. Molds and sacrificial water-soluble hollow cores are printed using fused filament fabrication. A heated water circuit accelerates the dissolution of the core's material and guarantees its complete removal from the actuator walls, while the actuator's mechanical operability is defined through finite element analysis. This enables the fabrication of actuators with non-uniform cross-sections under minimal supervision, thereby reducing the number of iterations necessary during the design and fabrication processes. Three actuators capable of bending and linear motion were designed, fabricated, integrated, and demonstrated as 3 different bio-inspired soft robots, an earthworm-inspired robot, a 4-legged robot, and a robotic gripper. We demonstrate the availability, versatility, and effectiveness of the proposed methods, contributing to accelerating the design and fabrication of soft robots. This study represents a step toward increasing the accessibility of soft robots to people at a lower cost.

Recent Progress in 3D Printing of Polymer Materials as Soft Actuators and Robots

With inspiration from natural systems, various soft actuators and robots have been explored in recent years with versatile applications in biomedical and engineering fields. Soft active materials with rich stimulus-responsive characteristics have been an ideal candidate to devise these soft machines by using different manufacturing technologies. Among these technologies, three-dimensional (3D) printing shows advantages in fabricating constructs with multiple materials and sophisticated architectures. In this Review, we aim to provide an overview of recent progress on 3D printing of soft materials, robotics performances, and representative applications. Typical 3D printing techniques are briefly introduced, followed by state-of-the-art advances in 3D printing of hydrogels, shape memory polymers, liquid crystalline elastomers, and their hybrids as soft actuators and robots. From the perspective of material properties, the commonly used printing techniques and action-generation principles for typical printed constructs are discussed. Actuation performances, locomotive behaviors, and representative applications of printed soft materials are summarized. The relationship between printing structures and action performances of soft actuators and robots is also briefly discussed. Finally, the advantages and limitations of each soft material are compared, and the remaining challenges and future directions in this field are prospected.

Modeling of a light-fueled self-paddling boat with a liquid crystal elastomer-based motor

Active materials possess unique properties of being able to respond autonomously to external stimuli, yet realizing and regulating the motion behavior of active machines remains a major challenge. Conventional control approaches, including sensor control and external device control, are both complex and difficult to implement. In contrast, active materials-based self-oscillators offer distinct properties such as periodic motion and ease of regulation. Inspired by paddle boats, we have proposed a conceptual light-fueled self-paddling boat with a photothermally responsive liquid crystal elastomer (LCE)-based motor that operates under steady illumination and incorporates an LCE fiber. Based on the well-established dynamic LCE model and rotation dynamics, the dynamic equations for governing the self-paddling of the LCE-steered boat are derived, and the driving torque of the LCE-based motor and the paddling velocity of the LCE-steered boat are formulated successively. The numerical results show that two motion modes of the boat under steady illumination: the static mode and the self-paddling mode. The self-paddling regime arises from the competition between the light-fueled driving torque and the frictional torque. Moreover, the critical conditions required to trigger the self-paddling are quantitatively examined as well as the significant system parameters affecting the driving torque, angular velocity, and paddling velocity. The proposed conceptual light-fueled self-paddling LCE-steered boat exhibits benefits including customizable size and being untethered and ambient powered, which provides valuable insights into the design and application of micromachines, soft robotics, energy harvesters, and beyond.

Liquid Metal-Polymer Conductor-Based Conformal Cyborg Devices

Gallium-based liquid metal (LM) exhibits exceptional properties such as high conductivity and biocompatibility, rendering it highly valuable for the development of conformal bioelectronics. When combined with polymers, liquid metal-polymer conductors (MPC) offer a versatile platform for fabricating conformal cyborg devices, enabling functions such as sensing, restoration, and augmentation within the human body. This review focuses on the synthesis, fabrication, and application of MPC-based cyborg devices. The synthesis of functional materials based on LM and the fabrication techniques for MPC-based devices are elucidated. The review provides a comprehensive overview of MPC-based cyborg devices, encompassing their applications in sensing diverse signals, therapeutic interventions, and augmentation. The objective of this review is to serve as a valuable resource that bridges the gap between the fabrication of MPC-based conformal devices and their potential biomedical applications.

Electroactive Bi-Functional Liquid Crystal Elastomer Actuators

Liquid crystal elastomers (LCEs) with promising applications in the field of actuators and soft robotics are reported. However, most of them are activated by external heating or light illumination. The examples of electroactive LCEs are still limited; moreover, they are monofunctional with one type of deformation (bending or contraction). Here, the study reports on trilayer electroactive LCE (eLCE) by intimate combination of LCE and ionic electroactive polymer device (i-EAD). This eLCE is bi-functional and can perform either bending or contractile deformations by the control of the low-voltage stimulation. By applying a voltage of ±2 V at 0.1 Hz, the redox behavior and associated ionic motion provide a bending strain difference of 0.80%. Besides, by applying a voltage of ±6 V at 10 Hz, the ionic current-induced Joule heating triggers the muscle-like linear contraction with 20% strain for eLCE without load. With load, eLCE can lift a weight of 270 times of eLCE-actuator weight, while keeping 20% strain and affording 5.38 kJ·m-3 work capacity. This approach of combining two smart polymer technologies (LCE and i-EAD) in a single device is promising for the development of smart materials with multiple degrees of freedom in soft robotics, electronic devices, and sensors.

Room-Temperature Liquid Metals for Flexible Electronic Devices

Room-temperature gallium-based liquid metals (RT-GaLMs) have garnered significant interest recently owing to their extraordinary combination of fluidity, conductivity, stretchability, self-healing performance, and biocompatibility. They are ideal materials for the manufacture of flexible electronics. By changing the composition and oxidation of RT-GaLMs, physicochemical characteristics of the liquid metal can be adjusted, especially the regulation of rheological, wetting, and adhesion properties. This review highlights the advancements in the liquid metals used in flexible electronics. Meanwhile related characteristics of RT-GaLMs and underlying principles governing their processing and applications for flexible electronics are elucidated. Finally, the diverse applications of RT-GaLMs in self-healing circuits, flexible sensors, energy harvesting devices, and epidermal electronics, are explored. Additionally, the challenges hindering the progress of RT-GaLMs are discussed, while proposing future research directions and potential applications in this emerging field. By presenting a concise and critical analysis, this paper contributes to the advancement of RT-GaLMs as an advanced material applicable for the new generation of flexible electronics.

Fabrication and Photothermal Actuation Performances of Electrospun Carbon Nanotube/Liquid Crystal Elastomer Blend Yarn Actuators

Liquid crystal elastomers (LCEs) are a kind of polymer network that combines the entropic elasticity of polymer networks and the mesogenic unit by means of mild cross-linking. LCEs have been extensively investigated in various fields, including artificial muscles, actuators, and microrobots. However, LCEs are characterized by the poor mechanical properties of the light polymers themselves. In this study, we propose to prepare a carbon nanotube/liquid crystal elastomer (CNT/LCE) composite yarn by electrospinning technology and a two-step cross-linking strategy. The CNT/LCE composite yarn exhibits a reversible shrinkage ratio of nearly 70%, a tensile strength of 16.45 MPa, and a relatively sensitive response speed of ∼3 s, enabling a fast response by photothermal actuation. The research disclosed in this article may provide new insights for the development of artificial muscles and next-generation smart robots.

Near-Infrared Light-Driven MXene/Liquid Crystal Elastomer Bimorph Membranes for Closed-Loop Controlled Self-Sensing Bionic Robots

More recently, soft actuators have evoked great interest in the next generation of soft robots. Despite significant progress, the majority of current soft actuators suffer from the lack of real-time sensory feedback and self-control functions, prohibiting their effective sensing and multitasking functions. Therefore, in this work, a near-infrared-driven bimorph membrane, with self-sensing and feedback loop control functions, is produced by layer by layer (LBL) assembling MXene/PDDA (PM) onto liquid crystal elastomer (LCE) film. The versatile integration strategy successfully prevents the separation issues that arise from moduli mismatch between the sensing and the actuating layers, ultimately resulting in a stable and tightly bonded interface adhesion. As a result, the resultant membrane exhibited excellent mechanical toughness (tensile strengths equal to 16.3 MPa (||)), strong actuation properties (actuation stress equal to 1.56 MPa), and stable self-sensing (gauge factor equal to 4.72) capabilities. When applying the near-infrared (NIR) laser control, the system can perform grasping, traction, and crawling movements. Furthermore, the wing actuation and the closed-loop controlled motion are demonstrated in combination with the insect microcontroller unit (MCU) models. The remote precision control and the self-sensing capabilities of the soft actuator pave a way for complex and precise task modulation in the future.

Toward a Unified Naming Scheme for Thermo-Active Soft Actuators: A Review of Materials, Working Principles, and Applications

Soft robotics is a rapidly growing field that spans the fields of chemistry, materials science, and engineering. Due to the diverse background of the field, there have been contrasting naming schemes such as "intelligent," "smart," and "adaptive" materials, which add vagueness to the broad innovation among literature. Therefore, a clear, functional, and descriptive naming scheme is proposed in which a previously vague name-Soft Material for Soft Actuators-can remain clear and concise-Phase-Change Elastomers for Artificial Muscles. By synthesizing the working principle, material, and application into a naming scheme, the searchability of soft robotics can be enhanced and applied to other fields. The field of thermo-active soft actuators spans multiple domains and requires added clarity. Thermo-active actuators have potential for a variety of applications spanning virtual reality haptics to assistive devices. This review offers a comprehensive guide to selecting the type of thermo-active actuator when one has an application in mind. In addition, it discusses future directions and improvements that are necessary for implementation.

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.

Thermoelectrics and thermocells for fire warning applications

Historically, fire disasters have killed numerous human lives, and caused tremendous property loss. Fire warning systems play a vital role in predicting fire risks, and are strongly desired to effectively prevent the disaster occurrence and significantly reduce the loss. Among the developed fire warning systems, thermoelectrics (TEs) and thermocells (TECs)-based fire warning materials are extremely important and indispensable in future research, owing to their unique capability of direct conversion between heat and electricity. Here, we present this review of the recent progress of TEs and TECs in fire warning field. Firstly, a brief introduction of existing fire warning systems is provided, including the mechanisms and features of various types. Then, the mechanisms of electronic TE (eTE), ionic TE (iTE) and TEC are elucidated. Next, the basic principles for the material preparation and device fabrication are discussed in their dimension sequence. Subsequently, some important advances or examples of TE fire warnings are highlighted in details. Finally, the challenges and prospects are outlooked.

Electrostatically Powered Multimode Liquid Crystalline Elastomer Actuators

Soft actuators based on liquid crystalline elastomers (LCEs) are captivating significant interest because of their unique properties combining the programmable liquid crystalline molecular order and elasticity of polymeric materials. For practical applications, the ability to perform multimodal shape changes in a single LCE actuator at a subsecond level is a bottleneck. Here, we fabricate a monodomain LCE powered by electrostatic force, which enables fast multidirectional bending, oscillation, rotation, and complex actuation with a high degree of freedom. By tuning the dielectric constant and resistivity in LCE gels, a complete cycle of oscillation and rotation only takes 0.1 s. In addition, monodomain actuators exhibit anisotropic actuation behaviors that promise a more complex deployment in a potential electromechanical system. The presented study will pave the way for electrostatically controllable isothermal manipulation for a fast and multimode soft actuator.

Evolution of Thermoelectric Generators: From Application to Hybridization

Considerable thermal energy is emitted into the environment from human activities and equipment operation in the course of daily production. Accordingly, the use of thermoelectric generators (TEGs) can attract wide interest, and it shows high potential in reducing energy waste and increasing energy recovery rates. Notably, TEGs have aroused rising attention and been significantly boosted over the past few years, as the energy crisis has worsened. The reason for their progress is that thermoelectric generators can be easily attached to the surface of a heat source, converting heat energy directly into electricity in a stable and continuous manner. In this review, applications in wearable devices, and everyday life are reviewed according to the type of structure of TEGs. Meanwhile, the latest progress of TEGs' hybridization with triboelectric nanogenerator (TENG), piezoelectric nanogenerator (PENG), and photovoltaic effect is introduced. Moreover, prospects and suggestions for subsequent research work are proposed. This review suggests that hybridization of energy harvesting, and flexible high-temperature thermoelectric generators are the future trends.

Thermal-Responsive Liquid Crystal Elastomer Foam-based Compressible and Omnidirectional Gripper

Liquid crystal elastomers (LCEs) are considered to be a promising material for the fabrication of soft grippers because of their large and reversible deformations, an LCE gripper with suitable compressibility and omnidirectionality has not yet been developed. To overcome these obstacles, this study utilizes salt template method to fabricate a rod-like LCE foam as gripper. The thickness of the compressible foam can be reduced by up to 77%, temporarily maintaining the deformation and enabling the gripper to pass through slits. The foam was aligned along the long axis and the length of the foam exhibits reversible thermal responsiveness and contract up to 57% along its alignment. Additionally, when the foam approaches a heat source, the generated temperature gradient results in a contraction gradient owing to the low thermal conductivity of the LCE foam. This in turn causes the foam to reversibly bend with a bending angle up to 93° and follow the movement of a heat source omnidirectionally. The developed gripper successfully grasps, moves, and releases hot objects in a cold and safe place, demonstrating its potential for emergency disposal. Thus, LCE foams can be considered suitable materials for novel gripper design and construction.

Multimaterial Printing of Liquid Crystal Elastomers with Integrated Stretchable Electronics

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

Toughening Thermoelectric Materials: From Mechanisms to Applications

With the tendency of thermoelectric semiconductor devices towards miniaturization, integration, and flexibility, there is an urgent need to develop high-performance thermoelectric materials. Compared with the continuously enhanced thermoelectric properties of thermoelectric materials, the understanding of toughening mechanisms lags behind. Recent advances in thermoelectric materials with novel crystal structures show intrinsic ductility. In addition, some promising toughening strategies provide new opportunities for further improving the mechanical strength and ductility of thermoelectric materials. The synergistic mechanisms between microstructure-mechanical performances are expected to show a large set of potential applications in flexible thermoelectric devices. This review explores enlightening research into recent intrinsically ductile thermoelectric materials and promising toughening strategies of thermoelectric materials to elucidate their applications in the field of flexible thermoelectric devices.

Advanced supramolecular design for direct ink writing of soft materials

The exciting advancements in 3D-printing of soft materials are changing the landscape of materials development and fabrication. Among various 3D-printers that are designed for soft materials fabrication, the direct ink writing (DIW) system is particularly attractive for chemists and materials scientists due to the mild fabrication conditions, compatibility with a wide range of organic and inorganic materials, and the ease of multi-materials 3D-printing. Inks for DIW need to possess suitable viscoelastic properties to allow for smooth extrusion and be self-supportive after printing, but molecularly facilitating 3D printability to functional materials remains nontrivial. While supramolecular binding motifs have been increasingly used for 3D-printing, these inks are largely optimized empirically for DIW. Hence, this review aims to establish a clear connection between the molecular understanding of the supramolecularly bound motifs and their viscoelastic properties at bulk. Herein, extrudable (but not self-supportive) and 3D-printable (self-supportive) polymeric materials that utilize noncovalent interactions, including hydrogen bonding, host-guest inclusion, metal-ligand coordination, micro-crystallization, and van der Waals interaction, have been discussed in detail. In particular, the rheological distinctions between extrudable and 3D-printable inks have been discussed from a supramolecular design perspective. Examples shown in this review also highlight the exciting macroscale functions amplified from the molecular design. Challenges associated with the hierarchical control and characterization of supramolecularly designed DIW inks are also outlined. The perspective of utilizing supramolecular binding motifs in soft materials DIW printing has been discussed. This review serves to connect researchers across disciplines to develop innovative solutions that connect top-down 3D-printing and bottom-up supramolecular design to accelerate the development of 3D-print soft materials for a sustainable future.

Liquid Metal-Doped Conductive Hydrogel for Construction of Multifunctional Sensors

Interest in wearable and stretchable multifunctional sensors has grown rapidly in recent years. The sensing elements must accurately detect external stimuli to expand their applicability as sensors. However, the sensor's self-healing and adhesion to a target object have been major challenges in developing such practical and versatile devices. In this study, we prepared a hydrogel (LM-SA-PAA) composed of liquid metal (LM), sodium alginate (SA), and poly(acrylic acid) (PAA) with ultrastretchable, excellent self-healing, self-adhesive, and high-sensitivity sensing capabilities that enable the conformal contact between the sensor and skin even during dynamic movements. The excellent self-healing performance of the hydrogel stems from its double cross-linked networks, including physical and chemical cross-linked networks. The physical cross-link formed by the ionic interaction between the carboxyl groups of PAA and gallium ions provide the hydrogel with reversible autonomous repair properties, whereas the covalent bond provides the hydrogel with a stable and strong chemical network. Alginate forms a microgel shell around LM nanoparticles via the coordination of its carboxyl groups with Ga ions. In addition to offering exceptional colloidal stability, the alginate shell has sufficient polar groups, ensuring that the hydrogel adheres to diverse substrates. Based on the efficient electrical pathway provided by the LM, the hydrogel exhibited strain sensitivity and enabled the detection of various human motions and electrocardiographic monitoring. The preparation method is simple and versatile and can be used for the low-cost fabrication of multifunctional sensors, which have broad application prospects in human-machine interface compatibility and medical monitoring.

3D Printing of Liquid Metal Embedded Elastomers for Soft Thermal and Electrical Materials

Liquid metal embedded elastomers (LMEEs) are composed of a soft polymer matrix embedded with droplets of metal alloys that are liquid at room temperature. These soft matter composites exhibit exceptional combinations of elastic, electrical, and thermal properties that make them uniquely suited for applications in flexible electronics, soft robotics, and thermal management. However, the fabrication of LMEE structures has primarily relied on rudimentary techniques that limit patterning to simple planar geometries. Here, we introduce an approach for direct ink write (DIW) printing of a printable LMEE ink to create three-dimensional shapes with various designs. We use eutectic gallium-indium (EGaIn) as the liquid metal, which reacts with oxygen to form an electrically insulating oxide skin that acts as a surfactant and stabilizes the droplets for 3D printing. To rupture the oxide skin and achieve electrical conductivity, we encase the LMEE in a viscoelastic polymer and apply acoustic shock. For printed composites with a 80% LM volume fraction, this activation method allows for a volumetric electrical conductivity of 5 × 10⁴ S cm^-1 (80% LM volume)─significantly higher than what had been previously reported with mechanically sintered EGaIn-silicone composites. Moreover, we demonstrate the ability to print 3D LMEE interfaces that provide enhanced charge transfer for a triboelectric nanogenerator (TENG) and improved thermal conductivity within a thermoelectric device (TED). The 3D printed LMEE can be integrated with a highly soft TED that is wearable and capable of providing cooling/heating to the skin through electrical stimulation.

A Method for 3D Printing and Rapid Prototyping of Fieldable Untethered Soft Robots

Because they are made of elastically deformable and compliant materials, soft robots can passively change shape and conform to their environment, providing potential advantages over traditional robotics approaches. However, existing manufacturing workflows are often labor intensive and limited in their ability to create highly integrated three-dimensional (3D) heterogeneous material systems. In this study, we address this with a streamlined workflow to produce field-deployable soft robots based on 3D printing with digital light processing (DLP) of silicone-like soft materials. DLP-based 3D printing is used to create soft actuators (2.2 g) capable of exerting up to 0.5 Newtons of force that are integrated into a bioinspired untethered soft robot. The robot walks underwater at speeds comparable with its biological analog, the brittle star. Using a model-free planning algorithm and feedback, the robot follows remote commands to move to desired positions. Moreover, we show that the robot is able to perform untethered locomotion outside of a laboratory and in a natural aquatic environment. Our results represent progress in soft robot manufacturing autonomy for a 3D printed untethered soft robot.