Self-Sustained Snapping Drives Autonomous Dancing and Motion in Free-Standing Wavy Rings

Emergent Locomotion in Self-Sustained, Mechanically Connected Soft Matter Ringsf

In nature, the interplay between individual organisms often leads to the emergence of complex belabours, of which sophistication has been refined through millions of years of evolution. Synthetic materials research has focused on mimicking the natural complexity, e.g., by harnessing non-equilibrium states to drive self-assembly processes. However, it is highly challenging to understand the interaction dynamics between non-equilibrium entities and to obtain collective behavior that can arise autonomously through interaction. In this study, thermally fueled, twisted rings exhibiting self-sustained movements are used as fundamental units and their interactive behaviors and emergent functions are investigated. The rings are fabricated from connected thermoresponsive liquid crystal elastomers (LCEs) strips that undergo zero-elastic-energy-mode, autonomous motions upon a heat gradient. Single-ring structures with various twisting numbers and nontrivial links, and connected knots where several LCE rings (N = 2,3,4,5) are studied and linked. The observations uncover that controlled locomotion of the structures can emerge when N ≥ 3. The locomotion can be programmed by controlling the handedness at the connection points between the individual rings. These findings illustrate how group activity emerges from individual responsive material components through mechanical coupling, offering a model for programming autonomous locomotion in soft matter constructs.

Aerial Track-Guided Autonomous Soft Ring Robot

Navigating in three-dimensional (3D) environments with precise motion control is challenging for soft robots due to their inherent flexibility. Inspired by aerial trams, here, an autonomous soft twisted ring robot is reported capable of navigating pre-defined tracks in 3D space under constant photothermal actuation, without requiring spatiotemporal control of actuation sources. Made of liquid crystal elastomers, the ring robot, suspended on thread-based tracks, self-flips around its centerline when exposed to constant infrared light. Curling the twisted ring around tracks converts its self-rotary motion into autonomous linear movement via screw theory. This mechanism enables the autonomous robot to adapt to tracks of various materials and micron-to-millimeter sizes, overcome obstacles like knots on tracks, transport loads over 12 times its weight, ascend and descend steep slopes up to 80°, and navigate complex paths, including circular, polygonal, and 3D spiral tracks, as well as loose threads with dynamically changing shapes.

Self-Sustained Liquid Crystal Elastomer Actuators with Geometric Zero-Elastic-Energy Modes

Recently, a novel and fascinating actuation mode of liquid crystal elastomers (LCEs), known as geometric zero-elastic-energy modes (ZEEMs), has drawn intensive research interest. Based on this actuation mechanism, LCE actuators exhibit untethered, autonomous movements under external stimulations, demonstrating significant potential for applications in intelligent soft robots, autonomous energy conversion systems, and smart optical tuning components. This perspective provides a timely summary of the current research on LCE actuators based on ZEEMs and highlights their future development trends and prospects, which will be of great interest to broad communities of researchers in fields of LCEs, biomimetic smart materials, soft robotics, and actuators.

An autonomous snapper featuring adaptive actuation and embodied intelligence

Developing artificial systems with autonomous motion is essential for creating devices that emulate nature's adaptive mechanisms. Here, we introduce a light-driven liquid crystalline network snapper that integrates both sensing and actuation capabilities, enabling adaptive responses to environmental conditions. Under constant light illumination, the snapper undergoes spontaneous snap-through transformation driven by the elastic instability embedded within the material. The snapper achieves out-of-equilibrium motion through continuous energy transfer with the environment, enabling it to sustain dynamic, reversible cycles of snapping without external control. We demonstrate the ability of the liquid crystalline network snapper to detect environmental changes-such as shifts in temperature, surface roughness, and color-demonstrating a form of embodied intelligence. This work offers a distinctive strategy for designing biomimetic devices that merge embodied intelligence with autonomous motion, opening pathways for advanced, adaptive systems for soft robotics.

Soft Materials and Devices Enabling Sensorimotor Functions in Soft Robots

Sensorimotor functions, the seamless integration of sensing, decision-making, and actuation, are fundamental for robots to interact with their environments. Inspired by biological systems, the incorporation of soft materials and devices into robotics holds significant promise for enhancing these functions. However, current robotics systems often lack the autonomy and intelligence observed in nature due to limited sensorimotor integration, particularly in flexible sensing and actuation. As the field progresses toward soft, flexible, and stretchable materials, developing such materials and devices becomes increasingly critical for advanced robotics. Despite rapid advancements individually in soft materials and flexible devices, their combined applications to enable sensorimotor capabilities in robots are emerging. This review addresses this emerging field by providing a comprehensive overview of soft materials and devices that enable sensorimotor functions in robots. We delve into the latest development in soft sensing technologies, actuation mechanism, structural designs, and fabrication techniques. Additionally, we explore strategies for sensorimotor control, the integration of artificial intelligence (AI), and practical application across various domains such as healthcare, augmented and virtual reality, and exploration. By drawing parallels with biological systems, this review aims to guide future research and development in soft robots, ultimately enhancing the autonomy and adaptability of robots in unstructured environments.

Self-flickering bioinspired actuator with autonomous motion and structural color switching

Color-tunable actuators with motion and color-changing functions have attracted considerable attention in recent years, yet it remains a challenge to achieve the autonomous regulation of motion and color. Inspired by Apatura ilia butterfly with dynamic structural color and Pelargonium carnosum plant with moisture responsive bilayer structure, an automatic color-tunable actuator is developed by integrating photonic crystals layer and hygroscopic layer. Taking advantage of the asymmetric hygroscopicity between two layers and the angle-dependent structural color of photonic crystals, this actuator can continuously self-flicker in humid environment by visual switching in structural color due to automated cyclic motion. The actuator is assembled into the self-flapping biomimetic butterfly with switchable color and the self-reporting information array with dynamic visual display, demonstrating its autoregulatory motion and color. This work provides a new strategy for developing automatic color-tunable actuator and suggests its potential in the intelligent robot and optical display.

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-steerable locomotion using zero-elastic-energy modes

Driving synthetic materials out of equilibrium via dissipative mechanisms paves the way towards autonomous, self-sustained robotic motions. However, obtaining agile movement in diverse environments with dynamic steerability remains a challenge. Here we report a light-fuelled soft liquid crystal elastomer torus with self-sustained out-of-equilibrium movement. Under constant light excitation, the torus undergoes spontaneous rotation arising from the formation of zero-elastic-energy modes. By exploiting dynamic friction or drag, the zero-elastic-energy-mode-based locomotion direction can be optically controlled in various dry and fluid environments. We demonstrate the ability of the liquid crystal elastomer torus to laterally and vertically swim in the Stokes regime. The torus navigation can be extended to three-dimensional space with full steerability of the swimming direction. These results demonstrate the possibilities enabled by prestrained topological structures towards robotic functions of out-of-equilibrium soft matter.

Self-Oscillation of Liquid Crystal Elastomer Fiber-Slide System Driven by Self-Flickering Light Source

Self-oscillation, a control approach inspired by biological systems, demonstrates an autonomous, continuous, and regular response to constant external environmental stimuli. Until now, most self-oscillation systems have relied on a static external environment that continuously supplies energy, while materials typically absorb ambient energy only intermittently. In this article, we propose an innovative self-oscillation of liquid crystal elastomer (LCE) fiber-slide system driven by a self-flickering light source, which can efficiently regulate the energy input in sync with the self-oscillating behavior under constant voltage. This system primarily consists of a photo-responsive LCE fiber, a slider that includes a conductive segment and an insulating segment, a light source, and a conductive track. Using the dynamic LCE model, we derive the governing equation for the motion of the LCE fiber-slider system. Numerical simulations show that the LCE fiber-slide system under constant voltage exhibits two distinct motion phases, namely the stationary phase and the self-oscillation phase. The self-oscillation occurs due to the photo-induced contraction of the LCE fiber when the light source is activated. We also investigate the critical conditions required to initiate self-oscillation, and examine key system parameters influencing its frequency and amplitude. Unlike the continuous energy release from the static environmental field in most self-oscillation systems, our LCE fiber-slide self-oscillation system is driven by a self-flickering light source, which dynamically adjusts the energy input under a constant voltage to synchronize with the self-oscillating behavior. Our design features advantages such as spontaneous periodic lighting, a simple structure, energy efficiency, and ease of operation. It shows significant promise for dynamic circuit systems, monitoring devices, and optical applications.

Programmable, Self-Healable, and Photochromic Liquid Crystal Elastomers Based on Dynamic Hindered Urea Bonds for Biomimetic Flowers

Recently, researchers have been exploring the use of dynamic covalent bonds (DCBs) in the construction of exchangeable liquid crystal elastomers (LCEs) for biomimetic actuators and devices. However, a significant challenge remains in achieving LCEs with both excellent dynamic properties and superior mechanical strength and stability. In this study, a diacrylate-functionalized monomer containing dynamic hindered urea bonds (DA-HUB) is employed to prepare exchangeable LCEs through a self-catalytic Michael addition reaction. By incorporating DA-HUB, the LCE system benefits from DCBs and hydrogen bonding, leading to materials with high mechanical strength and a range of dynamic properties such as programmability, self-healing, and recyclability. Leveraging these characteristics, bilayer LCE actuators with controlled reversible thermal deformation and outstanding dimensional stability are successfully fabricated using a simple welding method. Moreover, a biomimetic triangular plum, inspired by the blooming of flowers, is created to showcase reversible color and shape changes triggered by light and heat. This innovative approach opens new possibilities for the development of biomimetic and smart actuators and devices with multiple functionalities.

Reversible Snapping of Constrained Anisotropic Hydrogels Upon Light Stimulations

Creatures, such as Venus flytrap and hummingbirds, capable of rapid predation through snap-through transition, provide paradigms for the design of soft actuators and robots with fast actions. However, these artificial "snappers" usually need contact stimulations to trigger the flipping. Reported here is a constrained anisotropic poly(N-isopropylacrylamide) hydrogel showing fast snapping upon light stimulation. This hydrogel is prepared by flow-induced orientation of nanosheets (NSs) within a rectangular tube. The precursor containing gold nanoparticles is immediately exposed to UV light for photopolymerization to fix the ordered structure of NSs. Two ends of the slender gel are clamped to form a buckle with bistability nature, which snaps to the other side upon laser irradiation. Systematic experiments are conducted to investigate the influences of power intensity and irradiation angle of the laser, as well as thickness and buckle height of the gel, on the snapping behaviors. The fast snapping is further used to kick a plastic bead and control the switch state. Furthermore, synergetic or oscillated snapping of the gel with two buckles of opposite directions is realized by inclined irradiation of a laser or horizontal irradiation with two lasers, respectively. Such light-steered snapping of hydrogels should merit designing soft robots, energy harvests, etc.

A Graphene-Mica-Based Photo-Thermal Actuator for Small-Scale Soft Robots

Small-scale soft robots demonstrate intricate life-like behavior and allow navigation through arduous terrains and confined spaces. However, the primary challenges in soft robotics are 1) creating actuators capable of quick, reversible 22D-to-3D shape morphing with adjustable stiffness, 2) improving actuation force and robustness for wider applications, and 3) designing holistic systems for untethered manipulation and flexible multimodality in practical scenarios. Here, mechanically compliant paper-like robots are presented with multiple functionalities. The robots are based on photothermally activated polymer bimorph actuators that incorporate graphene for the photo-thermal conversion of energy and muscovite mica, with its high Young's modulus, providing the required stiffness. Conversion of light into heat leads to thermal expansion and bending of the stress-mismatched structures. The actuators are designed on the basis of a modified Timoshenko model, and numerical simulations are employed to evaluate their actuation performance. The membranes can be utilized for light-driven programmable shape-morphing. Localized control allows the implementation of active hinges at arbitrary positions within the membrane. Integrated into small-scale soft robots in mass production, the membrane facilitates locomotion, rolling, and flipping of the robots. Further, grasping and kicking mechanisms are demonstrated, highlighting the potential of such actuators for future applications.

Multimodal Autonomous Locomotion of Liquid Crystal Elastomer Soft Robot

Self-oscillation phenomena observed in nature serve as extraordinary inspiration for designing synthetic autonomous moving systems. Converting self-oscillation into designable self-sustained locomotion can lead to a new generation of soft robots that require minimal/no external control. However, such locomotion is typically constrained to a single mode dictated by the constant surrounding environment. In this study, a liquid crystal elastomer (LCE) robot capable of achieving self-sustained multimodal locomotion, with the specific motion mode being controlled via substrate adhesion or remote light stimulation is presented. Specifically, the LCE is mechanically trained to undergo repeated snapping actions to ensure its self-sustained rolling motion in a constant gradient thermal field atop a hotplate. By further fine-tuning the substrate adhesion, the LCE robot exhibits reversible transitions between rolling and jumping modes. In addition, the rolling motion can be manipulated in real time through light stimulation to perform other diverse motions including turning, decelerating, stopping, backing up, and steering around complex obstacles. The principle of introducing an on-demand gate control offers a new venue for designing future autonomous soft robots.

Sidewinder-Inspired Self-Adjusting, Lateral-Rolling Soft Robots for Autonomous Terrain Exploration

Helical structures of liquid crystal elastomers (LCEs) hold promise in soft robotics for self-regulated rolling motions. The understanding of their motion paths and potentials for terrain exploration remains limited. This study introduces a self-adjusting, lateral-rolling soft robot inspired by sidewinder snakes. The spring-like LCE helical filaments (HFs) autonomously respond to thermal cues, demonstrating dynamic and sustainable locomotion with adaptive rolling along non-linear paths. By fine-tuning the diameter, pitch, and modulus of the LCE HFs, and the environmental temperature, the movements of the LCE HFs, allowing for exploration of diverse terrains over a 600 cm2 area within a few minutes, can be programmed. LCE HFs are showcased to navigate through over nine obstacles, including maze escaping, terrain exploration, target hunting, and successfully surmounting staircases through adaptable rolling.

Self-sustainable autonomous soft actuators

Self-sustainable autonomous locomotion is a non-equilibrium phenomenon and an advanced intelligence of soft-bodied organisms that exhibit the abilities of perception, feedback, decision-making, and self-sustainment. However, artificial self-sustaining architectures are often derived from algorithms and onboard modules of soft robots, resulting in complex fabrication, limited mobility, and low sensitivity. Self-sustainable autonomous soft actuators have emerged as naturally evolving systems that do not require human intervention. With shape-morphing materials integrating in their structural design, soft actuators can direct autonomous responses to complex environmental changes and achieve robust self-sustaining motions under sustained stimulation. This perspective article discusses the recent advances in self-sustainable autonomous soft actuators. Specifically, shape-morphing materials, motion characteristics, built-in negative feedback loops, and constant stimulus response patterns used in autonomous systems are summarized. Artificial self-sustaining autonomous concepts, modes, and deformation-induced functional applications of soft actuators are described. The current challenges and future opportunities for self-sustainable actuation systems are also discussed.

Defected twisted ring topology for autonomous periodic flip-spin-orbit soft robot

Periodic spin-orbit motion is ubiquitous in nature, observed from electrons orbiting nuclei to spinning planets orbiting the Sun. Achieving autonomous periodic orbiting motions, along circular and noncircular paths, in soft mobile robotics is crucial for adaptive and intelligent exploration of unknown environments-a grand challenge yet to be accomplished. Here, we report leveraging a closed-loop twisted ring topology with a defect for an autonomous soft robot capable of achieving periodic spin-orbiting motions with programmed circular and re-programmed irregular-shaped trajectories. Constructed by bonding a twisted liquid crystal elastomer ribbon into a closed-loop ring topology, the robot exhibits three coupled periodic self-motions in response to constant temperature or constant light sources: inside-out flipping, self-spinning around the ring center, and self-orbiting around a point outside the ring. The coupled spinning and orbiting motions share the same direction and period. The spinning or orbiting direction depends on the twisting chirality, while the orbital radius and period are determined by the twisted ring geometry and thermal actuation. The flip-spin and orbiting motions arise from the twisted ring topology and a bonding site defect that breaks the force symmetry, respectively. By utilizing the twisting-encoded autonomous flip-spin-orbit motions, we showcase the robot's potential for intelligently mapping the geometric boundaries of unknown confined spaces, including convex shapes like circles, squares, triangles, and pentagons and concaves shapes with multi-robots, as well as health monitoring of unknown confined spaces with boundary damages.

Physically intelligent autonomous soft robotic maze escaper

Autonomous maze navigation is appealing yet challenging in soft robotics for exploring priori unknown unstructured environments, as it often requires human-like brain that integrates onboard power, sensors, and control for computational intelligence. Here, we report harnessing both geometric and materials intelligence in liquid crystal elastomer-based self-rolling robots for autonomous escaping from complex multichannel mazes without the need for human-like brain. The soft robot powered by environmental thermal energy has asymmetric geometry with hybrid twisted and helical shapes on two ends. Such geometric asymmetry enables built-in active and sustained self-turning capabilities, unlike its symmetric counterparts in either twisted or helical shapes that only demonstrate transient self-turning through untwisting. Combining self-snapping for motion reflection, it shows unique curved zigzag paths to avoid entrapment in its counterparts, which allows for successful self-escaping from various challenging mazes, including mazes on granular terrains, mazes with narrow gaps, and even mazes with in situ changing layouts.

Self-Oscillating Liquid Crystal Elastomer Helical Spring Oscillator with Combined Tension and Torsion

Self-oscillation is the autonomous maintenance of continuous periodic motion through energy absorption from non-periodic external stimuli, making it particularly attractive for fabricating soft robots, energy-absorbing devices, mass transport devices, and so on. Inspired by the self-oscillating system that presents high degrees of freedom and diverse complex oscillatory motions, we created a self-oscillating helical spring oscillator with combined tension and torsion under steady illumination, among which a mass block and a liquid crystal elastomer (LCE) helical spring made with LCE wire are included. Considering the well-established helical spring model and the dynamic LCE model, a nonlinear dynamic model of the LCE helical spring oscillator under steady illumination is proposed. From numerical calculation, the helical spring oscillator upon exposure to steady illumination possesses two motion regimes, which are the static regime and the self-tension-torsion regime. Contraction of the LCE wire under illumination is necessary to generate the self-tension-torsion of the helical spring oscillator, with its continuous periodic motion being maintained by the mutual balance between light energy input and damping dissipation. Additionally, the critical conditions for triggering the self-tension-torsion, as well as the vital system parameters affecting its frequencies and amplitudes of the translation and the rotation, were investigated in detail. This self-tension-torsion helical spring oscillator is unique in its customizable mechanical properties via its structural design, small material strain but large structural displacement, and ease of manufacture. We envision a future of novel designs for soft robotics, energy harvesters, active machinery, and so on.

A modular strategy for distributed, embodied control of electronics-free soft robots

Robots typically interact with their environments via feedback loops consisting of electronic sensors, microcontrollers, and actuators, which can be bulky and complex. Researchers have sought new strategies for achieving autonomous sensing and control in next-generation soft robots. We describe here an electronics-free approach for autonomous control of soft robots, whose compositional and structural features embody the sensing, control, and actuation feedback loop of their soft bodies. Specifically, we design multiple modular control units that are regulated by responsive materials such as liquid crystal elastomers. These modules enable the robot to sense and respond to different external stimuli (light, heat, and solvents), causing autonomous changes to the robot's trajectory. By combining multiple types of control modules, complex responses can be achieved, such as logical evaluations that require multiple events to occur in the environment before an action is performed. This framework for embodied control offers a new strategy toward autonomous soft robots that operate in uncertain or dynamic environments.

Autonomous, untethered gait-like synchronization of lobed loops made from liquid crystal elastomer fibers via spontaneous snap-through

The transition from one equilibrium state to another via rapid snap-through can store elastic energy and release it as kinetic energy for rapid motion as seen in Venus flytrap and hummingbird to catch insects mid-flight. They are explored in soft robotics for repeated and autonomous motions. In this study, we synthesize curved liquid crystal elastomer (LCE) fibers as the building blocks that can undergo buckling instability upon heated on a hot surface, leading to autonomous snap-through and rolling behaviors. When they are connected into lobed loops, where each fiber is geometrically constrained by the neighboring ones, they demonstrate autonomous, self-regulated, and repeated synchronization with a frequency of ~1.8 Hz. By adding a rigid bead on the fiber, we can fine-tune the actuation direction and speed (up to ~2.4 mm/s). Last, we demonstrate various gait-like locomotion patterns using the loops as the robot's legs.