Robotic surfaces with reversible, spatiotemporal control for shape morphing and object manipulation

Shape-morphing bioelectronic devices

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

Shape and topology morphing of closed surfaces integrating origami and kirigami

A closed surface is generally more resistant to deformation and shape changes than an open surface. An empty closed box, for example, is stiffer and more stable than when it is open. The presence of an opening makes it less constrained, more deformable, and easier to morph, as demonstrated by several studies on open-surface morphing across patterns, materials, and scales. Here, we present a platform to morph closed surfaces with bistability that harnesses a balanced integration of origami and kirigami principles. By harmonizing panel rotation around creases nearly tangent to the closed surface and panel rotation around hinges nearly perpendicular to the closed surface, we show that origami-kirigami assemblages can shape-morph between a cube and a sphere, scale between spheres of dissimilar size, and change topology between a sphere and a torus, with programmed bistability. The framework offers a promising strategy for designing bistable reconfigurable structures and metamaterials with enclosed configurations.

Tunable photo-responsive liquid crystal elastomer fibers via disperse dyeing for smart textiles

Active fibers, responding autonomously to environmental changes, are the basis of the development of smart textiles. However, there are still challenges in achieving responsive specificity and self-resilience of these fibers, which restrict the implementation of precise and complex actuation behaviors. Herein, an efficient strategy with a combination of a two-step crosslinking and disperse dyeing method was proposed to integrate multiple independent and non-interfering photo-thermal conversion nanoparticles into liquid crystal elastomer fibers (LCEFs). Three dyed LCEFs that selectively respond to 532 nm, 808 nm, and 980 nm wavelengths of light have been achieved. Based on this, a Delta robot was constructed with the capability of identifying specific light. The dyed LCEFs were also successfully incorporated into functional textiles through different fabrication technologies, demonstrating an embroidered anti-counterfeit logo, a 2D to 3D transformable disc-woven bionic flower, and an adaptive breathing knitted fabric. This work may facilitate the development of untethered soft robots with tunable and complex actuation, as well as the advancement of novel smart fabrics.

Self-Oscillation in Active Wires with Asymmetric Willis-Type Viscosity

Wires designed to carry loads through uniaxial tensile forces are particularly useful in creating large-span, yet lightweight, structures of diverse shapes and functions. Incorporating self-oscillation capabilities within the wires could be a promising approach to enhance the autonomous dynamic functions of these structures. However, self-oscillation, which usually appears in biological organisms and active materials, requires complex feedback interactions between activity and elasticity, hindering their application in self-oscillating wires. Here, a simple self-oscillation strategy is suggested by extending Willis elasticity to Willis-type viscosity through an irreversible coupling between strain rate and body force. A class of active wires equipped with electro-magneto-mechanically coupled feedforward loops is designed to realize the irreversible coupling. Numerical experiments show that the active wire hosts biased limit-cycle self-oscillation, where oscillation amplitudes are amplified linearly along one direction. Using continuum models, the linear amplification is interpreted as the force equilibrium caused by asymmetric Willis viscosity. In the design, the oscillation mode shape can be tailored independently of frequency, and the active wire supports the standing propagating mode transition. The design and continuum model suggested can benefit the development of autonomous materials for large-span structures.

Exploiting photopolymerization to modulate liquid crystalline network actuation

Liquid Crystalline Networks (LCNs) are widely investigated to develop actuators, from soft robots to artificial muscles. Indeed, they can produce forces and movements in response to a plethora of external stimuli, showing kinetics up to the millisecond time-scale. One of the most explored preparation technique involves the photopolymerization of an aligned layer of reactive mesogens. Following this approach, side-chain polymers are widely described, while a detailed comparison of light-responsive LCNs with different architectures is not properly addressed. In this paper, two synthetic approaches are exploited leading to photoresponsive LCNs with different architectures. Mixed main-chain/side-chain LCNs are obtained in one-pot through a thiol-acrylate chain-transfer reaction, while main-chain LCNs are achieved by a two-step approach involving an aza-Michael addition followed by acrylate crosslinking. Comparison among the two materials highlighted the superior performances in terms of tension developed upon light-activation of the former one, showing muscle-like force production comparable to standard side-chain LCNs combined with the greater ability to contract from common main-chain LCNs.

Omnidirectional Bending Sensor with Bianisotropic Structure for Wearable Electronics

Bending sensors are critical to the advancement of wearable electronics and can be applied in the dynamic monitoring of flexible object morphology. However, current bending sensors are constrained by sensing range and precision, especially in full-range detection. The maximum sensing range of existing flexible bending sensors is 0-240°. This study introduces a bianisotropic responsive structure into the design of an all-textile bending sensor, thereby realizing 0-360° full-range omnidirectional bending sensing. First, the project elucidated the sensing mechanism of the piezoresistive bianisotropic structured bending sensor and identified critical factors through a numerical simulation method. Then, the bianisotropic structured bending sensors were produced through the stitch method and analyzed on their electromechanical performance. Further, the recognition model for both bending angle and direction parameters was developed via numerical calculation, achieving a high accuracy with an error rate of 2.82%. Last, according to the ergonomics of body joints, the sensors were customized and validated in body joint monitoring scenarios. This work significantly enhances the performance of flexible bending sensors in sensing range, accuracy, and comfort for the wearer. The versatility of this bending sensor positions it as a promising candidate to supplant traditional heavy equipment or rigid devices, particularly in wearable joint motion monitoring and soft robotics.

Reprogrammable Flexible Piezoelectric Actuator Arrays with a High Degree of Freedom for Shape Morphing and Locomotion

The high degree of freedom (DoF) shape morphing widely exists in biology for mimicry, camouflage, and locomotion. Currently, a lot of bionic soft/flexible actuators and robots with shape-morphing functions have been developed to realize conformity, grasp, and movement. Among these solutions, two-dimensional responsive materials and structures that can shape morph into different three-dimensional configurations are valuable for creating reversible high DoF shape morphing. However, most existing methods are predetermined through the fabrication process and cannot reprogram their shape, facing limitations on multifunction. Besides, the achievable geometries are very limited due to the device's low integrated level of actuator elements. Here, we develop a polyvinylidene fluoride flexible piezoelectric actuator array based on a row/column addressing (RCA) scheme for reprogrammable high DoF shape morphing and locomotion. The specially designed row/column electrodes form a 6 × 6 array, which contains 36 actuator elements. By developing a high-voltage RCA control system, we can individually control all the elements in the array, leading to a highly reprogrammable array with various sophisticated high DoF shape morphing. We also demonstrate that the array is capable of propelling a robotic fish with various locomotions. This research provides a new method and approach for biomimetic robotics with better mimicry, aero/hydrodynamic efficiency, and maneuverability, as well as haptic display and object manipulation.

Organic Ink Multi-Material 3D Printing of Sustainable Soft Systems

Drawing inspiration from nature, soft materials are at the core of a transformation toward adaptive and responsive engineered systems, capable of conquering demanding terrain and safe when interacting with biological life. Despite recent advances in 3D printing of soft materials, researchers are still far from being able to print complex soft systems where a multitude of different components need to work together symbiotically. Closing this gap necessitates a platform that unites diverse materials into one synergetic process. Here, a multi-material printing system is presented, combining gelatin-based hydrogels with a new biodegradable support material. This organic ink maintains up to 60° overhang and is printable over gaps to structurally support the main biogel body, while triggered dissolution enables its selective removal and the formation of internal cavities. Therefore, the creation of vascular networks, tunable scaffolds, and embedded sensors within a single printing process becomes feasible. Furthermore, a perforation-resistant, joint-like vacuum actuator (VAc) is designed and 3D printed, capable of bending to angles up to 60° at fast response times down to 0.23 s. Combining these approaches in an efficient, streamlined fabrication process with biodegradable materials will unlock new sustainability dimensions for complex and durable soft systems.

Magnetic kirigami dome metasheet with high deformability and stiffness for adaptive dynamic shape-shifting and multimodal manipulation

Soft shape-shifting materials offer enhanced adaptability in shape-governed properties and functionalities. However, once morphed, they struggle to reprogram their shapes and simultaneously bear loads for fulfilling multifunctionalities. Here, we report a dynamic spatiotemporal shape-shifting kirigami dome metasheet with high deformability and stiffness that responds rapidly to dynamically changing magnetic fields. The magnetic kirigami dome exhibits over twice higher doming height and 1.5 times larger bending curvature, as well as sevenfold enhanced structural stiffness compared to its continuous counterpart without cuts. The metasheet achieves omnidirectional doming and multimodal translational and rotational wave-like shape-shifting, quickly responding to changing magnetic fields within 2 milliseconds. Using the dynamic shape-shifting and adaptive interactions with objects, we demonstrate its applications in voxelated dynamic displays and remote magnetic multimodal directional and rotary manipulation of nonmagnetic objects without grasping. It shows high-load transportation ability of over 40 times its own weight, as well as versatility in handling objects of different materials (liquid and solid), sizes, shapes, and weights.

Dynamically reconfigurable shape-morphing and tactile display via hydraulically coupled mergeable and splittable PVC gel actuator

Shape-morphing displays alter their surface geometry to convey information through three-dimensional shapes. However, rapid transformation into seamless shapes with multimodal tactile sensations poses challenges. Here, we introduce a versatile soft shape-morphing and tactile display, using a novel actuator that combines a PVC gel composite, dielectric liquid, and an electrode array. Proposed device facilitates on-demand liquid flow control through electrohydraulic actuation. Liquid channels within the device can be dynamically reconfigured using localized electrostatic zipping, enabling swift shape morphing and reconfiguration into diverse seamless 3D shapes. Our device achieves a large deformation and high output force, in a slim and lightweight framework. It also offers various haptic feedback, including dynamic tactile patterns and vibrations for localizable surface textures on the morphed shape. Additionally, its potential in robotics was demonstrated through high-speed object manipulation, leveraging liquid flow-induced inertia. In summary, our innovative soft shape-morphing tactile display could open new ways that we interact with technology, offering a more immersive and intuitive experience.

Energy-efficient dynamic 3D metasurfaces via spatiotemporal jamming interleaved assemblies for tactile interfaces

Inspired by the natural shape-morphing abilities of biological organisms, we introduce a strategy for creating energy-efficient dynamic 3D metasurfaces through spatiotemporal jamming of interleaved assemblies. Our approach, diverging from traditional shape-morphing techniques reliant on continuous energy inputs, utilizes strategically jammed, paper-based interleaved assemblies. By rapidly altering their stiffness at various spatial points and temporal phases during the relaxation of the soft substrate through jamming, we enable the formation of refreshable, intricate 3D shapes with a desirable load-bearing capability. This process, which does not require ongoing energy consumption, ensures energy-efficient and lasting shape displays. Our theoretical model, linking buckling deformation to residual pre-strain, underpins the inverse design process for an array of interleaved assemblies, facilitating the creation of diverse 3D configurations. This metasurface holds notable potential for tactile displays, particularly for the visually impaired, heralding possibilities in visual impaired education, haptic feedback, and virtual/augmented reality applications.

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

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

Adaptive hierarchical origami-based metastructures

Shape-morphing capabilities are crucial for enabling multifunctionality in both biological and artificial systems. Various strategies for shape morphing have been proposed for applications in metamaterials and robotics. However, few of these approaches have achieved the ability to seamlessly transform into a multitude of volumetric shapes post-fabrication using a relatively simple actuation and control mechanism. Taking inspiration from thick origami and hierarchies in nature, we present a hierarchical construction method based on polyhedrons to create an extensive library of compact origami metastructures. We show that a single hierarchical origami structure can autonomously adapt to over 103 versatile architectural configurations, achieved with the utilization of fewer than 3 actuation degrees of freedom and employing simple transition kinematics. We uncover the fundamental principles governing theses shape transformation through theoretical models. Furthermore, we also demonstrate the wide-ranging potential applications of these transformable hierarchical structures. These include their uses as untethered and autonomous robotic transformers capable of various gait-shifting and multidirectional locomotion, as well as rapidly self-deployable and self-reconfigurable architecture, exemplifying its scalability up to the meter scale. Lastly, we introduce the concept of multitask reconfigurable and deployable space robots and habitats, showcasing the adaptability and versatility of these metastructures.

Electrochemically actuated microelectrodes for minimally invasive peripheral nerve interfaces

Electrode arrays that interface with peripheral nerves are used in the diagnosis and treatment of neurological disorders; however, they require complex placement surgeries that carry a high risk of nerve injury. Here we leverage recent advances in soft robotic actuators and flexible electronics to develop highly conformable nerve cuffs that combine electrochemically driven conducting-polymer-based soft actuators with low-impedance microelectrodes. Driven with applied voltages as small as a few hundreds of millivolts, these cuffs allow active grasping or wrapping around delicate nerves. We validate this technology using in vivo rat models, showing that the cuffs form and maintain a self-closing and reliable bioelectronic interface with the sciatic nerve of rats without the use of surgical sutures or glues. This seamless integration of soft electrochemical actuators with neurotechnology offers a path towards minimally invasive intraoperative monitoring of nerve activity and high-quality bioelectronic interfaces.

A mechanically robust and facile shape morphing using tensile-induced buckling

Inspired by the adaptive mechanisms observed in biological organisms, shape-morphing soft structures have emerged as promising platforms for many applications. In this study, we present a shape-morphing strategy to overcome existing limitations of the intricate fabrication process and the lack of mechanical robustness against mechanical perturbations. Our method uses tensile-induced buckling, achieved by attaching restraining strips to a stretchable substrate. When the substrate is stretched, the stiffness mismatch between the restraining strips and the substrate, and the Poisson's effect on the substrate cause the restraining strips to buckle, thereby transforming initially flat shapes into intricate three-dimensional (3D) configurations. Guided by an inverse design method, we demonstrate the capability to achieve complicated and diverse 3D shapes. Leveraging shape morphing, we further develop soft grippers exhibiting outstanding universality, high grasping efficiencies, and exceptional durability. Our proposed shape-morphing strategy is scalable and material-independent, holding notable potential for applications in soft robotics, haptics, and biomedical devices.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

SonoTransformers: Transformable acoustically activated wireless microscale machines

Shape transformation, a key mechanism for organismal survival and adaptation, has gained importance in developing synthetic shape-shifting systems with diverse applications ranging from robotics to bioengineering. However, designing and controlling microscale shape-shifting materials remains a fundamental challenge in various actuation modalities. As materials and structures are scaled down to the microscale, they often exhibit size-dependent characteristics, and the underlying physical mechanisms can be significantly affected or rendered ineffective. Additionally, surface forces such as van der Waals forces and electrostatic forces become dominant at the microscale, resulting in stiction and adhesion between small structures, making them fracture and more difficult to deform. Furthermore, despite various actuation approaches, acoustics have received limited attention despite their potential advantages. Here, we introduce "SonoTransformer," the acoustically activated micromachine that delivers shape transformability using preprogrammed soft hinges with different stiffnesses. When exposed to an acoustic field, these hinges concentrate sound energy through intensified oscillation and provide the necessary force and torque for the transformation of the entire micromachine within milliseconds. We have created machine designs to predetermine the folding state, enabling precise programming and customization of the acoustic transformation. Additionally, we have shown selective shape transformable microrobots by adjusting acoustic power, realizing high degrees of control and functional versatility. Our findings open new research avenues in acoustics, physics, and soft matter, offering new design paradigms and development opportunities in robotics, metamaterials, adaptive optics, flexible electronics, and microtechnology.

Ligand-Directed Shape Reconfiguration in Inorganic Materials

Polymer elastomers with reversible shape-changing capability have led to significant development of artificial muscles, functional devices, and soft robots. By contrast, reversible shape transformation of inorganic nanoparticles is notoriously challenging due to their relatively rigid lattice structure. Here, the authors demonstrate the synthesis of shape-changing nanoparticles via an asymmetrical surface functionalization process. Various ligands are investigated, revealing the essential role of steric hindrance from the functional groups. By controlling the unbalanced structural hindrance on the surface, the as-prepared clay nanoparticles can transform their shape in a fast, facile, and reversible manner. In addition, such flexible morphology-controlled mechanism provides a platform for developing self-propelled shape-shifting nanocollectors. Owing to the ion-exchanging capability of clay, these self-propelled nanoswimmers (NS) are able to autonomously adsorb rare earth elements with ultralow concentration, indicating the feasibility of using naturally occurring materials for self-powered nanomachine.

Multi-modal deformation and temperature sensing for context-sensitive machines

Owing to the remarkable properties of the somatosensory system, human skin compactly perceives myriad forms of physical stimuli with high precision. Machines, conversely, are often equipped with sensory suites constituted of dozens of unique sensors, each made for detecting limited stimuli. Emerging high degree-of-freedom human-robot interfaces and soft robot applications are delimited by the lack of simple, cohesive, and information-dense sensing technologies. Stepping toward biological levels of proprioception, we present a sensing technology capable of decoding omnidirectional bending, compression, stretch, binary changes in temperature, and combinations thereof. This multi-modal deformation and temperature sensor harnesses chromaticity and intensity of light as it travels through patterned elastomer doped with functional dyes. Deformations and temperature shifts augment the light chromaticity and intensity, resulting in a one-to-one mapping between stimulus modes that are sequentially combined and the sensor output. We study the working principle of the sensor via a comprehensive opto-thermo-mechanical assay, and find that the information density provided by a single sensing element permits deciphering rich and diverse human-robot and robot-environmental interactions.

Embedded shape morphing for morphologically adaptive robots

Shape-morphing robots can change their morphology to fulfill different tasks in varying environments, but existing shape-morphing capability is not embedded in a robot's body, requiring bulky supporting equipment. Here, we report an embedded shape-morphing scheme with the shape actuation, sensing, and locking, all embedded in a robot's body. We showcase this embedded scheme using three morphing robotic systems: 1) self-sensing shape-morphing grippers that can adapt to objects for adaptive grasping; 2) a quadrupedal robot that can morph its body shape for different terrestrial locomotion modes (walk, crawl, or horizontal climb); 3) an untethered robot that can morph its limbs' shape for amphibious locomotion. We also create a library of embedded morphing modules to demonstrate the versatile programmable shapes (e.g., torsion, 3D bending, surface morphing, etc.). Our embedded morphing scheme offers a promising avenue for robots to reconfigure their morphology in an embedded manner that can adapt to different environments on demand.

Phase-transforming mechanical metamaterials with dynamically controllable shape-locking performance

Active mechanical metamaterials with customizable structures and deformations, active reversible deformation, dynamically controllable shape-locking performance and stretchability are highly suitable for applications in soft robotics and flexible electronics, yet it is challenging to integrate them due to their mutual conflicts. Here, we introduce a class of phase-transforming mechanical metamaterials (PMMs) that integrate the above properties. Periodically arranging basic actuating units according to the designed pattern configuration and positional relationship, PMMs can customize complex and diverse structures and deformations. Liquid-vapor phase transformation provides active reversible large deformation while a silicone matrix offers stretchability. The contained carbonyl iron powder endows PMMs with dynamically controllable shape-locking performance, thereby achieving magnetically assisted shape locking and energy storing in different working modes. We build a theoretical model and finite element simulation to guide the design process of PMMs, so as to develop a variety of PMMs with different functions suitable for different applications, such as a programmed PMM, reconfigurable antenna, soft lens, soft mechanical memory, biomimetic hand, biomimetic flytrap and self-contained soft gripper. PMMs are applicable to achieve various 2D deformations and 2D-to-3D deformations, and integrate multiple properties, including customizable structures and deformations, active reversible deformation, rapid reversible shape locking, adjustable energy storing and stretchability, which could open a new application avenue in soft robotics and flexible electronics.

Multistable sheets with rewritable patterns for switchable shape-morphing

Flat sheets patterned with folds, cuts or swelling regions can deform into complex three-dimensional shapes under external stimuli1-24. However, current strategies require prepatterning and lack intrinsic shape selection5-24. Moreover, they either rely on permanent deformations6,12-14,17,18, preventing corrections or erasure of a shape, or sustained stimulation5,7-11,25, thus yielding shapes that are unstable. Here we show that shape-morphing strategies based on mechanical multistability can overcome these limitations. We focus on undulating metasheets that store memories of mechanical stimuli in patterns of self-stabilizing scars. After removing external stimuli, scars persist and force the sheet to switch to sharply selected curved, curled and twisted shapes. These stable shapes can be erased by appropriate forcing, allowing rewritable patterns and repeated and robust actuation. Our strategy is material agnostic, extendable to other undulation patterns and instabilities, and scale-free, allowing applications from miniature to architectural scales.

A multifunctional soft robotic shape display with high-speed actuation, sensing, and control

Shape displays which actively manipulate surface geometry are an expanding robotics domain with applications to haptics, manufacturing, aerodynamics, and more. However, existing displays often lack high-fidelity shape morphing, high-speed deformation, and embedded state sensing, limiting their potential uses. Here, we demonstrate a multifunctional soft shape display driven by a 10 × 10 array of scalable cellular units which combine high-speed electrohydraulic soft actuation, magnetic-based sensing, and control circuitry. We report high-performance reversible shape morphing up to 50 Hz, sensing of surface deformations with 0.1 mm sensitivity and external forces with 50 mN sensitivity in each cell, which we demonstrate across a multitude of applications including user interaction, image display, sensing of object mass, and dynamic manipulation of solids and liquids. This work showcases the rich multifunctionality and high-performance capabilities that arise from tightly-integrating large numbers of electrohydraulic actuators, soft sensors, and controllers at a previously undemonstrated scale in soft robotics.

Nonlinear Topological Mechanics in Elliptically Geared Isostatic Metamaterials

Despite the extensive studies of topological systems, the experimental characterizations of strongly nonlinear topological phases have been lagging. To address this shortcoming, we design and build elliptically geared isostatic metamaterials. Their nonlinear topological transitions can be realized by collective soliton motions, which stem from the transition of nonlinear Berry phase. Endowed by the intrinsic nonlinear topological mechanics, surface polar elasticity and dislocation-bound zero modes can be created or annihilated as the topological polarization reverses orientation. Our approach integrates topological physics with strongly nonlinear mechanics and promises multiphase structures at the micro- and macroscales.

Passively addressed robotic morphing surface (PARMS) based on machine learning

Reconfigurable morphing surfaces provide new opportunities for advanced human-machine interfaces and bio-inspired robotics. Morphing into arbitrary surfaces on demand requires a device with a sufficiently large number of actuators and an inverse control strategy. Developing compact, efficient control interfaces and algorithms is vital for broader adoption. In this work, we describe a passively addressed robotic morphing surface (PARMS) composed of matrix-arranged ionic actuators. To reduce the complexity of the physical control interface, we introduce passive matrix addressing. Matrix addressing allows the control of N2 independent actuators using only 2N control inputs, which is substantially lower than traditional direct addressing (N2 control inputs). Using machine learning with finite element simulations for training, our control algorithm enables real-time, high-precision forward and inverse control, allowing PARMS to dynamically morph into arbitrary achievable predefined surfaces on demand. These innovations may enable the future implementation of PARMS in wearables, haptics, and augmented reality/virtual reality.

Bio-Mimic, Fast-Moving, and Flippable Soft Piezoelectric Robots

Cheetahs achieve high-speed movement and unique athletic gaits through the contraction and expansion of their limbs during the gallop. However, few soft robots can mimic their gaits and achieve the same speed of movement. Inspired by the motion gait of cheetahs, here the resonance of double spiral structure for amplified motion performance and environmental adaptability in a soft-bodied hopping micro-robot is exploited. The 0.058 g, 10 mm long tethered soft robot is capable of achieving a maximum motion speed of 42.8 body lengths per second (BL/s) and a maximum average turning speed of 482° s-1 . In addition, this robot can maintain high speed movement even after flipping. The soft robot's ability to move over complex terrain, climb hills, and carry heavy loads as well as temperature sensors is demonstrated. This research opens a new structural design for soft robots: a double spiral configuration that efficiently translates the deformation of soft actuators into swift motion of the robot with high environmental adaptability.

Robotic Materials Transformable Between Elasticity and Plasticity

Robotic materials, with coupled sensing, actuation, computation, and communication, have attracted increasing attention because they are able to not only tune their conventional passive mechanical property via geometrical transformation or material phase change but also become adaptive and even intelligent to suit varying environments. However, the mechanical behavior of most robotic materials is either reversible (elastic) or irreversible (plastic), but not transformable between them. Here, a robotic material whose behavior is transformable between elastic and plastic is developed, based upon an extended neutrally stable tensegrity structure. The transformation does not depend on conventional phase transition and is fast. By integrating with sensors, the elasticity-plasticity transformable (EPT) material is able to self-sense deformation and decides whether to undergo transformation or not. This work expands the capability of the mechanical property modulation of robotic materials.

Programming 3D curved mesosurfaces using microlattice designs

Cellular microstructures form naturally in many living organisms (e.g., flowers and leaves) to provide vital functions in synthesis, transport of nutrients, and regulation of growth. Although heterogeneous cellular microstructures are believed to play pivotal roles in their three-dimensional (3D) shape formation, programming 3D curved mesosurfaces with cellular designs remains elusive in man-made systems. We report a rational microlattice design that allows transformation of 2D films into programmable 3D curved mesosurfaces through mechanically guided assembly. Analytical modeling and a machine learning-based computational approach serve as the basis for shape programming and determine the heterogeneous 2D microlattice patterns required for target 3D curved surfaces. About 30 geometries are presented, including both regular and biological mesosurfaces. Demonstrations include a conformable cardiac electronic device, a stingray-like dual mode actuator, and a 3D electronic cell scaffold.

Self-folding soft-robotic chains with reconfigurable shapes and functionalities

Magnetic continuum soft robots can actively steer their tip under an external magnetic field, enabling them to effectively navigate in complex in vivo environments and perform minimally invasive interventions. However, the geometries and functionalities of these robotic tools are limited by the inner diameter of the supporting catheter as well as the natural orifices and access ports of the human body. Here, we present a class of magnetic soft-robotic chains (MaSoChains) that can self-fold into large assemblies with stable configurations using a combination of elastic and magnetic energies. By pushing and pulling the MaSoChain relative to its catheter sheath, repeated assembly and disassembly with programmable shapes and functions are achieved. MaSoChains are compatible with state-of-the-art magnetic navigation technologies and provide many desirable features and functions that are difficult to realize through existing surgical tools. This strategy can be further customized and implemented for a wide spectrum of tools for minimally invasive interventions.

3D printing programmable liquid crystal elastomer soft pneumatic actuators

Soft pneumatic actuators (SPAs) rely on anisotropic mechanical properties to generate specific motions after inflation. To achieve mechanical anisotropy, additional stiff materials or heterogeneous structures are typically introduced in isotropic base materials. However, the inherent limitations of these strategies may lead to potential interfacial problems or inefficient material usage. Herein, we develop a new strategy for fabricating SPAs based on an aligned liquid crystal elastomer (LCE) by a modified 3D printing technology. A rotating substrate enables the one-step fabrication of tubular LCE-SPAs with designed alignments in three dimensions. The alignment can be precisely programmed through printing, resulting in intrinsic mechanical anisotropy of the LCE. With a specially designed alignment, LCE-SPAs can achieve basic motions-contraction, elongation, bending, and twisting-and accomplish diverse tasks, e.g., grabbing objects and mixing water. This study provides a new perspective for the design and fabrication of SPAs.

Self-vectoring electromagnetic soft robots with high operational dimensionality

Soft robots capable of flexible deformations and agile locomotion similar to biological systems are highly desirable for promising applications, including safe human-robot interactions and biomedical engineering. Their achievable degree of freedom and motional deftness are limited by the actuation modes and controllable dimensions of constituent soft actuators. Here, we report self-vectoring electromagnetic soft robots (SESRs) to offer new operational dimensionality via actively and instantly adjusting and synthesizing the interior electromagnetic vectors (EVs) in every flux actuator sub-domain of the robots. As a result, we can achieve high-dimensional operation with fewer actuators and control signals than other actuation methods. We also demonstrate complex and rapid 3D shape morphing, bioinspired multimodal locomotion, as well as fast switches among different locomotion modes all in passive magnetic fields. The intrinsic fast (re)programmability of SESRs, along with the active and selective actuation through self-vectoring control, significantly increases the operational dimensionality and possibilities for soft robots.

Soft shape-programmable surfaces by fast electromagnetic actuation of liquid metal networks

Low modulus materials that can shape-morph into different three-dimensional (3D) configurations in response to external stimuli have wide-ranging applications in flexible/stretchable electronics, surgical instruments, soft machines and soft robotics. This paper reports a shape-programmable system that exploits liquid metal microfluidic networks embedded in an elastomer matrix, with electromagnetic forms of actuation, to achieve a unique set of properties. Specifically, this materials structure is capable of fast, continuous morphing into a diverse set of continuous, complex 3D surfaces starting from a two-dimensional (2D) planar configuration, with fully reversible operation. Computational, multi-physics modeling methods and advanced 3D imaging techniques enable rapid, real-time transformations between target shapes. The liquid-solid phase transition of the liquid metal allows for shape fixation and reprogramming on demand. An unusual vibration insensitive, dynamic 3D display screen serves as an application example of this type of morphable surface.

On the origin of elasticity and heat conduction anisotropy of liquid crystal elastomers at gigahertz frequencies

Liquid crystal elastomers that offer exceptional load-deformation response at low frequencies often require consideration of the mechanical anisotropy only along the two symmetry directions. However, emerging applications operating at high frequencies require all five true elastic constants. Here, we utilize Brillouin light spectroscopy to obtain the engineering moduli and probe the strain dependence of the elasticity anisotropy at gigahertz frequencies. The Young's modulus anisotropy, E_||/E_⊥~2.6, is unexpectedly lower than that measured by tensile testing, suggesting disparity between the local mesogenic orientation and the larger scale orientation of the network strands. Unprecedented is the robustness of E_||/E_⊥ to uniaxial load that it does not comply with continuously transformable director orientation observed in the tensile testing. Likewise, the heat conductivity is directional, κ_||/κ_⊥~3.0 with κ_⊥ = 0.16 Wm^-1K^-1. Conceptually, this work reveals the different length scales involved in the thermoelastic anisotropy and provides insights for programming liquid crystal elastomers on-demand for high-frequency applications.

A dynamically reprogrammable surface with self-evolving shape morphing

Dynamic shape-morphing soft materials systems are ubiquitous in living organisms; they are also of rapidly increasing relevance to emerging technologies in soft machines^1-3, flexible electronics^4,5 and smart medicines⁶. Soft matter equipped with responsive components can switch between designed shapes or structures, but cannot support the types of dynamic morphing capabilities needed to reproduce natural, continuous processes of interest for many applications^7-24. Challenges lie in the development of schemes to reprogram target shapes after fabrication, especially when complexities associated with the operating physics and disturbances from the environment can stop the use of deterministic theoretical models to guide inverse design and control strategies^25-30. Here we present a mechanical metasurface constructed from a matrix of filamentary metal traces, driven by reprogrammable, distributed Lorentz forces that follow from the passage of electrical currents in the presence of a static magnetic field. The resulting system demonstrates complex, dynamic morphing capabilities with response times within 0.1 second. Implementing an in situ stereo-imaging feedback strategy with a digitally controlled actuation scheme guided by an optimization algorithm yields surfaces that can follow a self-evolving inverse design to morph into a wide range of three-dimensional target shapes with high precision, including an ability to morph against extrinsic or intrinsic perturbations. These concepts support a data-driven approach to the design of dynamic soft matter, with many unique characteristics.

Electrically driven liquid crystal network actuators

Soft actuators based on liquid crystal networks (LCNs) have aroused great scientific interest for use as stimuli-controlled shape-changing and moving components for robotic devices due to their fast, large, programmable and solvent-free actuation responses. Recently, various LCN actuators have been implemented in soft robotics using stimulus sources such as heat, light, humidity and chemical reactions. Among them, electrically driven LCN actuators allow easy modulation and programming of the input electrical signals (amplitude, phase, and frequency) as well as stimulation throughout the volume, rendering them promising actuators for practical applications. Herein, the progress of electrically driven LCN actuators regarding their construction, actuation mechanisms, actuation performance, actuation programmability and the design strategies for intelligent systems is elucidated. We also discuss new robotic functions and advanced actuation control. Finally, an outlook is provided, highlighting the research challenges faced with this type of actuator.

Design and printing of proprioceptive three-dimensional architected robotic metamaterials

Advances in additive manufacturing techniques have enabled the creation of stimuli-responsive materials with designed three-dimensional (3D) architectures. Unlike biological systems in which functions such as sensing, actuation, and control are closely integrated, few architected materials have comparable system complexity. We report a design and manufacturing route to create a class of robotic metamaterials capable of motion with multiple degrees of freedom, amplification of strain in a prescribed direction in response to an electric field (and vice versa), and thus, programmed motions with self-sensing and feedback control. These robotic metamaterials consist of networks of piezoelectric, conductive, and structural elements interwoven into a designed 3D lattice. The resulting architected materials function as proprioceptive microrobots that actively sense and move.

Shape-programmable, deformation-locking, and self-sensing artificial muscle based on liquid crystal elastomer and low-melting point alloy

An artificial muscle capable of shape programmability, deformation-locking capacity without needing continuous external energy, and self-sensing capability is highly desirable yet challenging in applications of reconfigurable antenna, deployable space structures, etc. Inspired by coupled behavior of the muscles, bones, and nerve system of mammals, a multifunctional artificial muscle based on polydopamine-coated liquid crystal elastomer (LCE) and low-melting point alloy (LMPA) in the form of a concentric tube/rod is proposed. Thereinto, the outer LCE is used for reversible contraction and recovery (i.e., muscle function); the inner LMPA in the resolidification state is adopted for deformation locking, and that in the melt state is adopted for angle variation monitoring by detecting resistance change (i.e., bones and nerve functions, respectively). The proposed artificial muscle demonstrates multiple performances, including controllable bending angle, position, and direction; deformation locking for supporting heavy objects; and real-time monitoring of angle variation, which also provides a straightforward and effective approach for designing soft devices.

Digital synthesis of free-form multimaterial structures for realization of arbitrary programmed mechanical responses

SignificanceCreating structures to realize function-oriented mechanical responses is desired for many applications. Yet, the use of a single material phase and heuristics-based designs may fail to attain specific target behaviors. Here, through a deterministic algorithmic procedure, multiple materials with dissimilar properties are intelligently synthesized into composite structures to achieve arbitrary prescribed responses. Created structures possess unconventional geometry and seamless integration of multiple materials. Despite geometric complexity and varied material phases, these structures are fabricated by multimaterial manufacturing, and tested to demonstrate that wide-ranging nonlinear responses are physically and accurately realized. Upon heteroassembly, resulting structures provide architectures that exhibit highly complex yet navigable responses. The proposed strategy can benefit the design of function-oriented nonlinear mechanical devices, such as actuators and energy absorbers.

Liquid Crystal Elastomer Twist Fibers toward Rotating Microengines

Untethered twist fibers do not require end-anchoring structures to hold their twist orientation and offer simple designs and convenient operation. The reversible responsiveness of these fibers allows them to generate torque and rotational deformation continuously upon the application of external stimuli. The fibers therefore have potential in rotating microengines. In practical applications, high torque and rotational deformation are desirable to meet work capacity requirements. However, the simultaneous endowment of reversible responsiveness and high rotational performance to untethered twist fibers remains a challenge. In this study, a liquid crystal elastomer twist fiber (LCETF) is designed and developed with a fixed twisting alignment of mesogens to provide untethered and reversible responsiveness. Outstanding rotational performance can be achieved when the mesogenic orientation is disrupted through heat triggering. Owing to the significant intrinsic contractile ratio of the LCE material, the rotational deformation of the LCETF can reach 243.6° mm^-1 . More importantly, the specific torque can reach 10.1 N m kg^-1 , which exceeds previously reported values. In addition, the LCETF can be exploited in a rotating microengine to convert heat into electricity with an induction voltage as high as 9.4 V. This work broadens the applications of LCEs for energy harvesters, micromachines, and soft robots.

A Shift from Efficiency to Adaptability: Recent Progress in Biomimetic Interactive Soft Robotics in Wet Environments

Research field of soft robotics develops exponentially since it opens up many imaginations, such as human-interactive robot, wearable robots, and transformable robots in unpredictable environments. Wet environments such as sea and in vivo represent dynamic and unstructured environments that adaptive soft robots can reach their potentials. Recent progresses in soft hybridized robotics performing tasks underwater herald a diversity of interactive soft robotics in wet environments. Here, the development of soft robots in wet environments is reviewed. The authors recapitulate biomimetic inspirations, recent advances in soft matter materials, representative fabrication techniques, system integration, and exemplary functions for underwater soft robots. The authors consider the key challenges the field faces in engineering material, software, and hardware that can bring highly intelligent soft robots into real world.

Solvent-Assisted 4D Programming and Reprogramming of Liquid Crystalline Organogels

Encoding molecular ordering during liquid crystalline network (LCN) formation endows preprogrammed but fixed shape morphing in response to external stimuli. The incorporation of dynamic covalent bonds enables shape reprogramming but also permanently alters the network structures. Here, an entropic approach that can program complex shapes via directed solvent evaporation from an isotropic LCN organogel is discoursed. Different shapes can be erased and reprogrammed from the same LCN on demand depending on the modes of deformation of the organogel during solvent evaporation. The ability to decouple network synthesis and molecular alignment relaxes the requirements to LCN chemistry and alignment methods, allowing for the realization of a variety of origami/kirigami structures and 4D shape morphing of LCNs printed from the digital light processing technique with unattainable spatial and temporal controls.

Bionic Polyurethane with a Reversible Core-Sheath for Real-Time On-Demand Performance Adjustment and Fluorescence Self-Reflection

Smart materials that can respond to external stimuli have attracted considerable scientific interest and achieved fruitful results with the advancement of research. However, materials with adjustable performance and which could be intervened on-demand through stimulation are still rarely mentioned. Furthermore, most of these materials published so far usually require high temperature or the assistance of catalysts to change the structure and adjust their performance, and the process is always irreversible. Herein, we proposed an anthracene-functionalized novel polyurethane with adjustable performance and fluorescence self-reflection inspired by shellfish. Anthracene was used as a dynamic group to make the polymer chain structure topologically isomerize after UV exposure, finally constructing a reversible core-sheath in a homogeneous polymer. Moreover, this process is catalyst-free and has strong spatiotemporal controllability. The appearance of the reversible core-sheath structure could achieve the performance adjustment of materials, and the strength can be increased easily in real time and on-demand by UV light exposure. Through selective irradiation, spatial control stiffening of this material can also be realized. In addition, the performance can also be self-reflected through the fluorescence to realize the performance that is visualizable. This work dramatically simplifies the requirements and conditions for material performance adjustment while expanding the versatility and applications in intelligent materials such as artificial muscles, variably flexible electronic devices, heterogeneous materials, 4D printing, and what may be discovered in the future.

Reprogrammable soft actuation and shape-shifting via tensile jamming

The emerging generation of robots composed of soft materials strives to match biological motor adaptation skills via shape-shifting. Soft robots often harness volumetric expansion directed by strain limiters to deform in complex ways. Traditionally, strain limiters have been inert materials embedded within a system to prescribe a single deformation. Under changing task demands, a fixed deformation mode limits adaptability. Recent technologies for on-demand reprogrammable deformation of soft bodies, including thermally activated variable stiffness materials and jamming systems, presently suffer from long actuation times or introduce unwanted bending stiffness. We present fibers that switch tensile stiffness via jamming of segmented elastic fibrils. When jammed, tensile stiffness increases more than 20× in less than 0.1 s, but bending stiffness increases only 2×. When adhered to an inflating body, jamming fibers locally limit surface tensile strains, unlocking myriad programmable deformations. The proposed jamming technology is scalable, enabling adaptive behaviors in emerging robotic materials that interact with unstructured environments.

Toward Application of Liquid Crystalline Elastomer for Smart Robotics: State of the Art and Challenges

Liquid crystalline elastomers (LCEs) are lightly crosslinked polymers that combine liquid crystalline order and rubber elasticity. Owing to their unique anisotropic behavior and reversible shape responses to external stimulation (temperature, light, etc.), LCEs have emerged as preferred candidates for actuators, artificial muscles, sensors, smart robots, or other intelligent devices. Herein, we discuss the basic action, control mechanisms, phase transitions, and the structure-property correlation of LCEs; this review provides a comprehensive overview of LCEs for applications in actuators and other smart devices. Furthermore, the synthesis and processing of liquid crystal elastomer are briefly discussed, and the current challenges and future opportunities are prospected. With all recent progress pertaining to material design, sophisticated manipulation, and advanced applications presented, a vision for the application of LCEs in the next generation smart robots or automatic action systems is outlined.