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

Connecting the Dots: Sintering of Liquid Metal Particles for Soft and Stretchable Conductors

This review focuses on the sintering of liquid metal particles (LMPs). Here, sintering means the partial merging or connecting of particles (or droplets) to form a network of percolated and, thus, conductive electrical pathways. LMPs are attractive materials because they can be suspended in a carrier fluid to create printable inks or distributed in an elastomer to create soft, stretchable composites. However, films and traces of LMPs are not typically conductive as fabricated due to the native oxide that forms on the surface of the particles. In the case of composites, polymers can also get between particles, making sintering more challenging. Sintering can be done via a variety of ways, such as mechanical, thermal, and chemical processing. This review discusses the mechanisms to sinter these particles, patterning techniques that use sintering, unique properties of sintered LMPs, and their practical applications in fields such as stretchable electronics, soft robotics, and active materials.

A linearly programmable strategy for polymer elastomer mechanics

The mechanical properties of polymer elastomer materials, such as strength and ductility, play important roles in a wide range of applications, including the carrying of major equipment and the construction of infrastructure. However, owing to the widespread disordered physicochemical bonding and unpredictable internal phase separation phenomenon, traditional materials show a complex nonlinear correlation between the material structure and its performance, which makes it difficult to accurately adapt to the performance requirements of various specific application scenarios. In view of the above challenges, this paper innovatively proposes a strategy to achieve linear programmability in the mechanical properties of polymer elastomer materials. Instead of increasing the entropy value of the material, which may be brought about by the traditional physical composite method, this strategy adopts a unique path of introducing special dynamic chain segments (AlPUs). This innovative design leads to a highly ordered microscopic hydrogen bonding arrangement within the elastomer, which effectively reduces the free volume within the material, thus bringing the mechanical response of the material closer to the ideal state. Furthermore, by fine-tuning the content of material components, we are able to achieve linear control of key mechanical indexes, such as tensile strength and elongation at break, which is a significant advantage in terms of precision, range of adjustment, and versatility. The successful implementation of this work opens up a new way toward logical, fine and intelligent design and preparation of polymer materials, providing a solid materials science foundation and unlimited possibilities to promote technological innovation and development in the field of future major equipment and infrastructure.

Thermal Gradient-Driven Heterogeneous Actuation of Liquid Crystal Elastomers for a Crawling Robot

Emerging soft robots based on liquid crystal elastomers (LCEs) exhibit remarkable capabilities for large reversible shape morphing, enabling them to adapt to complex environments and perform diverse tasks such as locomotion and camouflage. Despite extensive studies, current methods for locally controlled actuation of LCE-based soft robots often involve intricate structural design, complex programming of LCEs, incorporation of multiple materials, or complex actuation methods. Here, we present a simple and efficient approach to achieve multiple deformation modes within a simply programmed LCE structure by harnessing Joule heating-induced thermal gradients across the LCE volume. Oxidized liquid metal (LM) thin films, which exhibit increased resistance, enhanced viscosity, high thermal conductivity, and large deformability, are employed for Joule heating in this study. Using an LCE strip programmed via uniaxial stretching as an example, we perform systematic studies on the effect of essential parameters, including the actuation voltage, LCE dimensions, and the LM-to-LCE thickness ratio, on the deformation behaviors of LCEs induced by three-dimensional thermal gradients across the LCE volume. In addition, concurrently actuating two adjacent surfaces of the LCE strip yields previously inaccessible coupled bending behaviors. Finally, we demonstrate a crawling robot constructed from LM-coated LCE strips with adjustable bending capabilities, which enable multimode locomotion, including forward movement and turns, enhancing biomimetic functionality akin to leg movements observed in living organisms like reptiles. The reported strategy, which is both straightforward and versatile, promises scalability and holds potential for various applications in multifunctional intelligent systems including soft robotics and biomedical devices.

Wireless Flexible Actuator Photoelectric Synergistically Driven for Environment Adaptability Crawling Robots

Wirelessly driven flexible actuators are crucial to the development of flexible robotic crawling. However, great challenges still remain for the crawling of flexible actuators in complex environments. Herein, we reported a wireless flexible actuator synergistically driven by wireless power transmission (WPT) technology and near-infrared (NIR) light, which consists of a poly(dimethylsiloxane)-graphene oxide (PDMS-GO) composite layer, eutectic gallium-indium alloy (EGaIn), a PDMS layer, and a polyimide (PI) layer. By optimizing the parameters of EGaIn and the concentration of the PDMS-GO composite film, the actuator has excellent bending ability and blocking force under different conditions driven by photoelectronic synergy. In addition, we fabricated a flexible crawling robot with high environmental adaptability by adding crawling structures at both ends of the actuator, which causes a discrepancy in friction between the front and rear feet. The flexible crawling robot has high stability, large deformation, and excellent crawling ability for wirelessly crawling on a plane, slope, and plane with different roughnesses. This work provides an idea for the application of wireless robots in complex environments.

Magneto-Photochemically Responsive Liquid Crystal Elastomer for Underwater Actuation

The quest for small-scale, remotely controlled soft robots has led to the exploration of magnetic and optical fields for inducing shape morphing in soft materials. Magnetic stimulus excels when navigation in confined or optically opaque environments is required. Optical stimulus, in turn, boasts superior spatial precision and individual control over multiple objects. Herein, we bring these two methodologies together and present a monolithic liquid crystal elastomer (LCE) system that synergistically combines magnetic and photochemical actuation schemes. The resultant composite material showcases versatile possibilities for underwater actuation, and we demonstrate robotic functionalities where the optical and magnetic response can be leveraged in different tasks (object gripping and object translocation, respectively) or where light can be used as a control signal to tune the magnetically induced actuation. Combining these two remote actuation methods offers powerful, dual-mode control in wireless, small-scale robotics, especially in submersed environments due to their isothermal nature.

Programmable Wrinkled MXene-Based Soft Actuators with Moisture- and Light-Responsive Deformation and Water-Surface Locomotion Capabilities

Soft actuators are limited by single-mode driving technology, which poses challenges in dealing with complex and multidimensional movements. In this study, a multiresponsive soft actuator was fabricated by integrating a microwrinkling structure into an MXene-based film, enabling programmable motions. To achieve this, we introduced n-hexane into the film preparation process and utilized its rapid volatilization to accelerate the shrinkage difference between the film and the substrate. This resulted in anisotropic folding and excellent mechanical properties. Due to the remarkable moisture absorption and excellent toughness of MXene-based films, it exhibits rapid actuation in response to moisture gradients and light stimuli with large bending deformation, fast actuation speed, as well as excellent stability and durability. The anisotropic expansion and mechanical properties of the film enable it to have capability of three-dimensional shape-programmable configuration control. Furthermore, taking advantage of the exceptional photothermal properties of MXene-based films, we developed light-driven actuators that utilize the Marangoni effect for propulsion on the water surface, enabling programmable navigational control. Such a soft actuator has a broad applications prospect in the fields of biomimetic botanical models, terrestrial crawlers, and aquatic surface transport devices.

Light-induced wrinkling in annulus anisotropic liquid crystal elastomer films

Liquid crystal elastomer (LCE) films deform under external stimuli showing significant potential in smart structures and flexible sensors. Due to the small out-of-plane stiffness of LCE films, wrinkling instability poses a critical challenge for the application of LCE films. Different from previous studies, we utilize the anisotropy constitutive model to investigate light-induced wrinkling instability in annulus LCE films with an internal hole. The degree of anisotropy in light-sensitive LCE is determined by two parameters: the anisotropic ratio (ratio of the elastic modulus) and the spontaneous strain ratio. Liquid crystal directors align radially, causing in-plane radial shrinkage and expansion in the other directions under light radiation. Using linear perturbation analysis, we study the critical conditions and wrinkle numbers for the onset of wrinkling instability. Our findings reveal that annulus LCE films with anisotropy exhibit a higher number of wrinkles upon instability. We present the phase diagrams of flat or wrinkled states illustrating the condition under which the film remains flat or transitions into a wrinkled state. This behavior depends on anisotropic parameters, light intensity, contraction coefficient, hole size, and film thickness. By adjusting the combination of these parameters, one can control the wrinkling instability of LCE films. Furthermore, we show that the critical time for the onset of wrinkling decreases as the light intensity and contraction coefficient increase. The findings from this work can provide references for the design of smart structures based on light-sensitive LCE films.

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.

Light-powered self-scrolling liquid crystal elastomer crane

Traditional liquid crystal elastomer (LCE)-based machines are constrained by the need for complex controllers and large power supplies, which limits their applicability in microrobots and other small-scale machines. In this paper, we propose a light-powered self-scrolling LCE crane, which is capable of self-scrolling to lift weights under steady light. Based on a dynamic LCE model, we derive the lateral curvature of the LCE crane and the driving moment for steady scrolling. By numerically solving the equilibrium equations, we found that the driving moment for the self-scrolling is originated from the uneven distribution of the LCE rod in the horizontal direction caused by light. The angular velocity of the self-scrolling depends on five system parameters: heat flux, coefficient of heat transfer, support spacing, weight mass, and scrolling friction coefficient. Through experimental comparative analysis, the results are consistent with the numerical simulation. The light-powered self-scrolling LCE crane device proposed in this paper features a simple structure, consistent horizontal illumination, and a compact light irradiation area. It advances the understanding of self-sustaining structures utilizing active materials and offers valuable insight into the potential applications of light-responsive LCEs in self-driven devices, medical instruments, robotics, sensors, and the energy sector.

Laser-Printed All-Carbon Responsive Material and Soft Robot

Responsive materials and actuators are the basis for the development of various leading-edge technologies but have so far mostly been designed based on polymers, incurring key limitations related to sensitivity and environmental tolerance. This work reports a new responsive material, laser-printed carbon film (LPCF), produced via direct laser transformation of a liquid organic precursor and consists of graphitic and amorphous carbons. The high activity of amorphous carbon combined with the dual-gradient structure enables the LPCF to have a actuation speed of 9400° s-1 in response to the stimulus of organic vapor. LPCF exhibits a conductivity of 950 S m-1 and excellent resistance to various extreme environmental conditions, which are unachievable for polymer-based materials. Additionally, an LPCF-based all-carbon soft robot that can mimic the complex continuous backward somersaulting motions without manual intervention is constructed. The locomotion velocity of the robot reaches a value of 1.19 BL s-1, which is almost one to two orders of magnitude faster than that of reported soft robots. This work not only offers a new paradigm for highly responsive materials but also provides a great design and engineering example for the next generation of biomimetic robots with life-like performance.

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.

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.

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.

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.

Soft Robots with Plant-Inspired Gravitropism Based on Fluidic Liquid Metal

Plants can autonomously adjust their growth direction based on the gravitropic response to maximize energy acquisition, despite lacking nerves and muscles. Endowing soft robots with gravitropism may facilitate the development of self-regulating systems free of electronics, but remains elusive. Herein, acceleration-regulated soft actuators are described that can respond to the gravitational field by leveraging the unique fluidity of liquid metal in its self-limiting oxide skin. The soft actuator is obtained by magnetic printing of the fluidic liquid metal heater circuit on a thermoresponsive liquid crystal elastomer. The Joule heat of the liquid metal circuit with gravity-regulated resistance can be programmed by changing the actuator's pose to induce the flow of liquid metal. The actuator can autonomously adjust its bending degree by the dynamic interaction between its thermomechanical response and gravity. A gravity-interactive soft gripper is also created with controllable grasping and releasing by rotating the actuator. Moreover, it is demonstrated that self-regulated oscillation motion can be achieved by interfacing the actuator with a monostable tape spring, allowing the electronics-free control of a bionic walker. This work paves the avenue for the development of liquid metal-based reconfigurable electronics and electronics-free soft robots that can perceive gravity or acceleration.

Reprogramming Photoresponsive Liquid Crystalline Elastomer via Force-Directed Evaporation

Incorporating photothermal agents into thermoresponsive liquid crystalline elastomers (LCEs) offers remote and spatio-temporal control in actuation. Typically, both the light responsiveness and actuation behaviors are fixed since the agent doping and mesogen alignment are conducted before network formation. Here, we report an approach that enables programming photoresponsive LCEs after synthesis via force-directed evaporation. Different photothermal agents can be doped or removed by swelling the fully cross-linked LCEs in a specific solution, achieving the introduction and erasing of the photoresponsiveness. Moreover, the network swelling deletes the registered alignment, which allows for redefining the molecular order via re-evaporating the solvent with force imposed. This "one stone, two birds" strategy paves the way to simultaneously program/reprogram the actuation mode and responsiveness of LCEs, even in a spatio-selective manner to achieve complex actuations. Our approach is expandable to three-dimensional (3D) printed LCEs to access geometrically sophisticated shape-changing.

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.

Principles and methods of liquid metal actuators

As a promising material, liquid metals (LMs) have gained considerable interest in the field of soft robotics due to their ability to move as designed routines or change their shape dramatically under external stimuli. Inspired by the science fiction film Terminator, tremendous efforts have been devoted to liquid robots with high compliance and intelligence. How to manipulate LM droplets is crucial to achieving this goal. Accordingly, this review is dedicated to presenting the principles driving LMs and summarizing the potential methods to develop LM actuators of high maneuverability. Moreover, the recent progress of LM robots based on these methods is overviewed. The challenges and prospects of implementing autonomous robots have been proposed.