Variable Stiffness Fiber with Self-Healing Capability

Magnetic steering continuum robot for transluminal procedures with programmable shape and functionalities

Millimeter-scale soft continuum robots offer safety and adaptability in transluminal procedures due to their passive compliance, but this feature necessitates interactions with surrounding lumina, leading to potential medical risks and restricted mobility. Here, we introduce a millimeter-scale continuum robot, enabling apical extension while maintaining structural stability. Utilizing phase transition components, the robot executes cycles of tip-based elongation, steered accurately through programmable magnetic fields. Each motion cycle features a solid-like backbone for stability, and a liquid-like component for advancement, thereby enabling autonomous shaping without reliance on environmental interactions. Together with clinical imaging technologies, we demonstrate the capability of navigating through tortuous and fragile lumina to transport microsurgical tools. Once it reaches larger anatomical spaces such as stomach, it can morph into functional 3D structures that serve as surgical tools or sensing units, overcoming the constraints of initially narrow pathways. By leveraging this design paradigm, we anticipate enhanced safety, multi-functionality, and cooperative capabilities among millimeter-scale continuum robots, opening new avenues for transluminal robotic surgery.

Fast-Response Variable-Stiffness Magnetic Catheters for Minimally Invasive Surgery

In minimally invasive surgery, such as cardiac ablation, magnetically steered catheters made of variable-stiffness materials can enable higher dexterity and higher force application to human tissue. However, the long transition time between soft and rigid states leads to a significant increase in procedure duration. Here, a fast-response, multisegmented catheter is described for minimally invasive surgery made of variable-stiffness thread (FRVST) that encapsulates a helical cooling channel. The rapid stiffness change in the FRVST, composed of a nontoxic shape memory polymer, is achieved by an active cooling system that pumps water through the helical channel. The FRVST displays a 66 times stiffness change and a 26 times transition enhancement compare with the noncooled version. The catheter allows for selective bending of each segment up to 127° in air and up to 76° in water under an 80 mT external magnetic field. The inner working channel can be used for cooling an ablation tip during a procedure and for information exchange via the deployment of wires or surgical tools.

Variable Stiffness Fibers Enabled Universal and Programmable Re-Foldability Strategy for Modular Soft Robotics

Origami is a rich source of inspiration for creating soft actuators with complex deformations. However, implementing the re-foldability of origami on soft actuators remains a significant challenge. Herein, a universal and programmable re-foldability strategy is reported to integrate multiple origami patterns into a single soft origami actuator, thereby enabling multimode morphing capability. This strategy can selectively activate and deactivate origami creases through variable stiffness fibers. The utilization of these fibers enables the programmability of crease pattern quantity and types within a single actuator, which expands the morphing modes and deformation ranges without increasing their physical size and chamber number. The universality of this approach is demonstrated by developing a series of re-foldable soft origami actuators. Moreover, these soft origami actuators are utilized to construct a bidirectional crawling robot and a multimode soft gripper capable of adapting to object size, grasping orientation, and placing orientation. This work represents a significant step forward in the design of multifunctional soft actuators and holds great potential for the advancement of agile and versatile soft robots.

Phase Transition Liquid Metal Enabled Emerging Biomedical Technologies and Applications

Phase change materials that can absorb or release large amounts of heat during phase transition, play a critical role in many important processes, including heat dissipation, thermal energy storage, and solar energy utilization. In general, phase change materials are usually encapsulated in passive modules to provide assurance for energy management. The shape and mechanical changes of these materials are greatly ignored. An emerging class of phase change materials, liquid metals (LMs) have attracted significant interest beyond thermal management, including in transformable robots, flexible electronics, soft actuators, and biomedicine. Interestingly, the melting point of LM is highly tunable around body temperature, allowing it to experience considerable stiffness change when interacting with human organisms during solid-liquid change, which brings about novel phenomena, applied technologies, and therapeutic methods, such as mechanical destruction of tumors, neural electrode implantation technique, and embolization therapy. This review focuses on the technology, regulation, and application of the phase change process along with diverse changes of LM to facilitate emerging biomedical applications based on the influences of mechanical stiffness change and versatile regulation strategies. Typical applications will also be categorized and summarized. Lastly, the advantages and challenges of using the unique and reversible process for biomedicine will be discussed.

A Soft-Bodied Aerial Robot for Collision Resilience and Contact-Reactive Perching

Current aerial robots demonstrate limited interaction capabilities in unstructured environments when compared with their biological counterparts. Some examples include their inability to tolerate collisions and to successfully land or perch on objects of unknown shapes, sizes, and texture. Efforts to include compliance have introduced designs that incorporate external mechanical impact protection at the cost of reduced agility and flight time due to the added weight. In this work, we propose and develop a lightweight, inflatable, soft-bodied aerial robot (SoBAR) that can pneumatically vary its body stiffness to achieve intrinsic collision resilience. Unlike the conventional rigid aerial robots, SoBAR successfully demonstrates its ability to repeatedly endure and recover from collisions in various directions, not only limited to in-plane ones. Furthermore, we exploit its capabilities to demonstrate perching where the three-dimensional collision resilience helps in improving the perching success rates. We also augment SoBAR with a novel hybrid fabric-based bistable (HFB) grasper that can utilize impact energies to perform contact-reactive grasping through rapid shape conforming abilities. We exhaustively study and offer insights into the collision resilience, impact absorption, and manipulation capabilities of SoBAR with the HFB grasper. Finally, we compare the performance of conventional aerial robots with the SoBAR through collision characterizations, grasping identifications, and experimental validations of collision resilience and perching in various scenarios and on differently shaped objects.

Scalable functionalized liquid crystal elastomer fiber soft actuators with multi-stimulus responses and photoelectric conversion

Liquid crystal elastomer (LCE) fibers exhibit large deformation and reversibility, making them an ideal candidate for soft actuators. It is still challenging to develop a scalable strategy and endow fiber actuators with photoelectric functions to achieve tailorable photo-electro-thermal responsiveness and rapid large actuation deformation. Herein, we fabricated a multiresponsive actuator that consists of LCE long fibers obtained by continuous dry spinning and further coated it with polydopamine (PDA)-modified MXene ink. The designed PDA@MXene-integrated LCE fiber is used for shape-deformable and multi-trigger actuators that can be photo- and electro-thermally actuated. The proposed LCE fiber actuator combines an excellent photothermal and long-term electrically conductive PDA@MXene and a shape-morphing LCE fiber, enabling their robust mechanical flexibility, multiple fast responses (∼0.4 s), and stable and large actuation deformation (∼60%). As a proof-of-concept, we present near-infrared light-driven artificial muscle that can lift 1000 times the weight and an intelligent circuit switch with stable controllability and fast responsiveness (∼0.1 s). Importantly, an adaptive smart window system that integrates light-driven energy harvesting/conversion functions is ingeniously constructed by the integration of a propellable curtain woven by the designed fiber and solar cells. This work can provide insights into the development of advanced intelligent materials toward soft robotics, sustainable energy savings and beyond.

Tuning Stiffness with Granular Chain Structures for Versatile Soft Robots

Stiffness variation can greatly enhance soft robots' load capacity and compliance. Jamming methods are widely used where stiffness variation is realized by jamming of particles, layers, or fibers. It is still challenging to make the variable stiffness components lightweight and adaptive. Besides, the existing jamming mechanisms generally encounter deformation-induced softening, restricting their applications in cases where large deformation and high stiffness are both needed. Herein, a multifunctional granular chain assemblage is proposed, where particles are formed into chains with threads. The chain jamming can be classified into two types. Granular chain jamming (GCJ) utilizes typical particles such as spherical particles, which can achieve both high stiffness and great adaptability while keeping jamming components relatively lightweight, while by using cubic particles, a peculiar deformation-induced stiffening mechanism is found, which is termed as stretch-enhanced particle jamming (SPJ). The versatility of GCJ and SPJ mechanisms in soft robots is demonstrated through soft grippers, soft crawlers, or soft bending actuators, where great passive adaptability, high load capacity, joint-like bending, friction enhancement, or postponing buckling can be realized, respectively. This work thus offers a facile and low-cost strategy to fabricate versatile soft robots.

Magnetically induced stiffening for soft robotics

Soft robots are well-suited for human-centric applications, but the compliance that gives soft robots this advantage must also be paired with adequate stiffness modulation such that soft robots can achieve more rigidity when needed. For this reason, variable stiffening mechanisms are often a necessary component of soft robot design. Many techniques have been explored to introduce variable stiffness structures into soft robots, such as pneumatically-controlled jamming and thermally-controlled phase change materials. Despite fast response time, jamming methods often require a bulkier pneumatic pressure line which limits portability; and while portable via electronic control, thermally-induced methods require compatibility with high temperatures and often suffer from slow response time. In this paper, we present a magnetically-controlled stiffening approach that combines jamming-based stiffening principles with magnetorheological fluid to create a hybrid mechanical and materials approach. In doing so, we combine the advantages of fast response time from pneumatically-based jamming with the portability of thermally-induced phase change methods. We explore the influence of magnetic field strength on the stiffening of our magnetorheological jamming beam samples in two ways: by exploiting the increase in yield stress of magnetorheological fluid, and by additionally using the clamping force between permanent magnets to further stiffen the samples via a clutch effect. We introduce an analytical model to predict the stiffness of our samples as a function of the magnetic field. Finally, we demonstrate electronic control of the stiffness using electropermanent magnets. In this way, we present an important step towards a new electronically-driven stiffening mechanism for soft robots that interact safely in close contact with humans, such as in wearable devices.

Designable Electrical/Thermal Coordinated Dual-Regulation Based on Liquid Metal Shape Memory Polymer Foam for Smart Switch

Electronic components with tunable resistance, especially with synergistic regulation of thermal conductivity, play important roles in the fields of electronics, smart switch, soft robots, and so on. However, it is still a challenge to get the material with various resistance and thermal conductivity stably without lasting external force. Herein, a liquid metal shape memory polymer foam (LM-SMF) is developed by loading electrically and thermally conductive liquid metal (LM) on deformable foam skeleton. Based on thermal response shape memory effect, the foam skeleton can be reversibly pressed, the process of which enables LM to transfer between connected and disconnected states. As a result, obtained LM-SMF shows that the resistance stably changes from 0.8 Ω (conductor) to 200 MΩ (insulator), and the thermal conductivity difference is up to 4.71 times (0.108 to 0.509 W m^-1 K^-1 ), which indicates that LM-SMF can achieve the electrical and thermal dual-regulation. Moreover, LM-SMF can be used as a designable self-feedback/-warning integrated smart switch or tunable infrared stealth switch. This work proposes a novel strategy to get the material with electrical-thermal coordinated dual-regulation, which is possibly applied in intelligent heating system with real-time monitoring function, electrothermal sensor in the future.

Electro-mechano responsive elastomers with self-tunable conductivity and stiffness

Materials with programmable conductivity and stiffness offer new design opportunities for next-generation engineered systems in soft robotics and electronic devices. However, existing approaches fail to harness variable electrical and mechanical properties synergistically and lack the ability to self-respond to environmental changes. We report an electro-mechano responsive Field's metal hybrid elastomer exhibiting variable and tunable conductivity, strain sensitivity, and stiffness. By synergistically harnessing these properties, we demonstrate two applications with over an order of magnitude performance improvement compared to state-of-the-art, including a self-triggered multiaxis compliance compensator for robotic manipulators, and a resettable, highly compact, and fast current-limiting fuse with an adjustable fusing current. We envisage that the extraordinary electromechanical properties of our hybrid elastomer will bring substantial advancements in resilient robotic systems, intelligent instruments, and flexible electronics.

Recent progress in fiber-based soft electronics enabled by liquid metal

Soft electronics can seamlessly integrate with the human skin which will greatly improve the quality of life in the fields of healthcare monitoring, disease treatment, virtual reality, and human-machine interfaces. Currently, the stretchability of most soft electronics is achieved by incorporating stretchable conductors with elastic substrates. Among stretchable conductors, liquid metals stand out for their metal-grade conductivity, liquid-grade deformability, and relatively low cost. However, the elastic substrates usually composed of silicone rubber, polyurethane, and hydrogels have poor air permeability, and long-term exposure can cause skin redness and irritation. The substrates composed of fibers usually have excellent air permeability due to their high porosity, making them ideal substrates for soft electronics in long-term applications. Fibers can be woven directly into various shapes, or formed into various shapes on the mold by spinning techniques such as electrospinning. Here, we provide an overview of fiber-based soft electronics enabled by liquid metals. An introduction to the spinning technology is provided. Typical applications and patterning strategies of liquid metal are presented. We review the latest progress in the design and fabrication of representative liquid metal fibers and their application in soft electronics such as conductors, sensors, and energy harvesting. Finally, we discuss the challenges of fiber-based soft electronics and provide an outlook on future prospects.

A Stretchable Fiber with Tunable Stiffness for Programmable Shape Change of Soft Robots

All soft robots require the same functionality, that is, controlling the shape of a structure made from soft materials. However, existing approaches for shape control of soft robots are primarily dominated by modular pneumatic actuators, which require multichambers and complex flow control components. Nature shows exciting examples of manipulation (shape change) in animals, such as worms, using a single-chambered soft body and programmable stiffness changes in the skin; controlling the spatial distribution of changes in stiffness enables achieving complex shape evolutions. However, such stiffness control requires a drastic membrane stiffness contrast between stiffened and nonstiffened states. Generally, this is extremely challenging to accomplish in stretchable materials. Inspired by longitudinal muscle fibers in the skin of worms, we developed a new concept for fabricating a hybrid fiber with tunable stiffness, that is, a fiber comprising both stiff and soft parts connected in a series. A substantial change in membrane stiffness was then observed by the locking/unlocking of the soft part. Our proposed hybrid fiber cyclically produced a membrane stiffness contrast of more than 100 × in less than 6 s using an input power of 3 W. A network of these hybrid fibers with tunable stiffness could manipulate a single-chambered soft body in multiple directions and transform it into a complex shape by selectively varying the stiffness at different locations.

Fabrication and Functionality Integration Technologies for Small-Scale Soft Robots

Small-scale soft robots are attracting increasing interest for visible and potential applications owing to their safety and tolerance resulting from their intrinsic soft bodies or compliant structures. However, it is not sufficient that the soft bodies merely provide support or system protection. More importantly, to meet the increasing demands of controllable operation and real-time feedback in unstructured/complicated scenarios, these robots are required to perform simplex and multimodal functionalities for sensing, communicating, and interacting with external environments during large or dynamic deformation with the risk of mismatch or delamination. Challenges are encountered during fabrication and integration, including the selection and fabrication of composite/materials and structures, integration of active/passive functional modules with robust interfaces, particularly with highly deformable soft/stretchable bodies. Here, methods and strategies of fabricating structural soft bodies and integrating them with functional modules for developing small-scale soft robots are investigated. Utilizing templating, 3D printing, transfer printing, and swelling, small-scale soft robots can be endowed with several perceptual capabilities corresponding to diverse stimulus, such as light, heat, magnetism, and force. The integration of sensing and functionalities effectively enhances the agility, adaptability, and universality of soft robots when applied in various fields, including smart manufacturing, medical surgery, biomimetics, and other interdisciplinary sciences.

Self-Reporting Joule Heating Modulated Stiffness of Polymeric Nanocomposites for Shape Reconfiguration

Shape reconfigurable devices, e.g., foldable phones, have emerged with the development of flexible electronics. But their rigid frames limit the feasible shapes for the devices. To achieve freely changeable shapes yet keep the rigidity of devices for user-friendly operations, stiffness-tunable materials are desired, especially under electrical control. However, current such systems are multilayer with at least a heater layer and a structural layer, leading to complex fabrication, high cost, and loss of reprocessability. Herein, we fabricate covalent adaptable networks-carbon nanotubes (CAN-CNT) composites to realize Joule heating controlled stiffness. The nanocomposites function as stiffness-tunable matrices, electric heaters, and softening sensors all by themselves. The self-reporting of softening is used to regulate the power control, and the sensing mechanism is investigated by simulating the CNT-polymer chain interactions at the nanoscale during the softening process. The nanocomposites not only have adjustable mechanical and thermodynamic properties but also are easy to fabricate at low cost and exhibit reprocessability and recyclability benefiting from the dynamic exchange reactions of CANs. Shape and stiffness control of flexible display systems are demonstrated with the nanocomposites as framing material, where freely reconfigurable shapes are realized to achieve convenient operation, wearing, or storage, fully exploiting their flexible potential.

A novel design of a passive variable stiffness soft robotic gripper

Inspired by the twisting and hanging phenomenon of vines, this paper proposes and designs a passive variable stiffness soft robotic gripper to grasp an object in a simple and robust manner using the principle of jamming. This method has the characteristics of high reliability and good stability, which can achieve soft grasping and rigid load-bearing of the object. Firstly, according to two key issues, the design model of the gripper is proposed, the principle of the proposed gripper is analyzed, and the relationship between the stiffness of the gripper and the stiffness of the object is revealed. Secondly, the model of the robotic gripper is created using a conventional motor drive method, and the grasping process and deformation causes of the gripper are analyzed by using the principle of instability effect and large deformation principle. Finally, the experimental prototype is developed and the feasibility of the design principle and the grasping deformation process of the gripper are verified by gripping experiments.

Tough Bonding of Liquid Metal-Elastomer Composites for Multifunctional Adhesives

Liquid metal (LM) composites, which consist of LM droplets dispersed in highly deformable elastomers, have recently gained interest as a multifunctional material for soft robotics and electronics. The incorporation of LM into elastic solids allows for unique combinations of material properties such as high stretchability with thermal and electrical conductivity comparable to metals. However, it is currently a challenge to incorporate LM composites into integrated systems consisting of diverse materials and components due to a lack of adhesion control. Here, a chemical anchoring methodology to increase adhesion of LM composites to diverse substrates is presented. The fracture energy increases up to 100× relative to untreated surfaces, with values reaching up to 7800 J m^-2 . Furthermore, the fracture energy, tensile modulus, and thermal conductivity can be tuned together by controlling the microstructure of LM composites. Finally, the bonding technique is used to integrate LM composites with functional electronic components without encapsulation or clamping, allowing for extreme deformations while maintaining exceptional thermal and electrical conductivity. These findings can accelerate the adoption of LM composites into complex soft robotic and electronic systems where strong, reliable bonding between diverse materials and components is required.

Anti-Freezing, Non-Drying, Localized Stiffening, and Shape-Morphing Organohydrogels

Artificial shape-morphing hydrogels are emerging toward various applications, spanning from electronic skins to healthcare. However, the low freezing and drying tolerance of hydrogels hinder their practical applications in challenging environments, such as subzero temperatures and arid conditions. Herein, we report on a shape-morphing system of tough organohydrogels enabled by the spatially encoded rigid structures and its applications in conformal packaging of “island–bridge” stretchable electronics. To validate this method, programmable shape morphing of Fe (III) ion-stiffened Ca-alginate/polyacrylamide (PAAm) tough organohydrogels down to −50 °C, with long-term preservation of their 3D shapes at arid or even vacuum conditions, was successfully demonstrated, respectively. To further illustrate the potency of this approach, the as-made organohydrogels were employed as a material for the conformal packaging of non-stretchable rigid electronic components and highly stretchable liquid metal (galinstan) conductors, forming a so-called “island–bridge” stretchable circuit. The conformal packaging well addresses the mechanical mismatch between components with different elastic moduli. As such, the as-made stretchable shape-morphing device exhibits a remarkably high mechanical durability that can withstand strains as high as 1000% and possesses long-term stability required for applications under challenging conditions.

Reconfigurable Fiber Triboelectric Nanogenerator for Self-Powered Defect Detection

With the extensive applications of portable, wearable, and stretchable electronics, the fiber triboelectric nanogenerator (TENG) has been developed particularly and rapidly. However, variable stiffness or even switchable stiffness for the fiber TENG is also urgently needed in some specific service conditions. Here, the functional, reconfigurable fiber TENG is presented for harvesting mechanical energy and self-powered sensors. It is mainly composed of soft tubes with filled low-melting-point alloy (LMPA), conductive wire, and electrically heated wire. Under an input frequency of 3 Hz, this fiber TENG produces a maximum peak power density of 348.5 μW/m. Due to its excellent reconfigurable characteristics, it can be switched back and forth in many different application situations. It can be intelligently used not only as a self-powered tactile and mechanical sensor but also as a self-powered splint for postdisaster relief work. Besides, the cracking detection of a gear and a lead screw is also realized using this fiber TENG. This work strongly promotes the application of variable stiffness LMPAs in the TENG, especially for the reconfigurable fiber TENG. It also promotes the potential self-powered applications of the TENG in the fields of sensors and detection, such as mechanical flaw detection and self-powered tactile detection.

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.

Shape morphing mechanical metamaterials through reversible plasticity

Biological organisms such as the octopus can reconfigure their shape and properties to perform diverse tasks. However, soft machines struggle to achieve complex configurations, morph into shape to support loads, and go between multiple states reversibly. Here, we introduce a multifunctional shape-morphing material with reversible and rapid polymorphic reconfigurability. We couple elastomeric kirigami with an unconventional reversible plasticity mechanism in metal alloys to rapidly (<0.1 seconds) morph flat sheets into complex, load-bearing shapes, with reversibility and self-healing through phase change. This kirigami composite overcomes trade-offs in deformability and load-bearing capacity and eliminates power requirements to sustain reconfigured shapes. We demonstrate this material through integration with onboard control, motors, and power to create a soft robotic morphing drone, which autonomously transforms from a ground to air vehicle and an underwater morphing machine, which can be reversibly deployed to collect cargo.

Design, modeling, and testing of a soft actuator with variable stiffness using granular jamming

Soft robots combine the load-bearing capability of rigid material with the resilience, shape-shifting capabilities of soft materials. This paper presents a novel soft actuator with stiffness variation using particulate jamming technology. We design a hybrid composite structure consisting of driving layer and jamming layer. The driving layer with the arc air chamber aim to achieve large bending deformation. A membrane containing particles is integrated with driving layer to module its stiffness. The influence factors of stiffness variation were analyzed from energy of point of view. The dependence of granular attributes on the stiffness of the actuator was studied. Furthermore, we illustrated influence of stiffness changes on the kinematic and dynamic performance of the soft actuator. The experimental results showed these performance indexes are twofold. On the one hand, the structural parameters have significant effect on the bending angle, but on the other hand they have little effect on the end force. We found that flow resistance inside air chamber results in bending morphology variation. The dynamic response subjected to a square-wave air pressure was analyzed to exhibit the actuator’s transient and steady vibration behavior. The actuator with greater stiffness has faster responsiveness, but smaller range of motion. These conclusions are helpful to adjust the stiffness behavior and to improve motion performance.

Biohybrid Variable-Stiffness Soft Actuators that Self-Create Bone

Inspired by the dynamic process of initial bone development, in which a soft tissue turns into a solid load-bearing structure, the fabrication, optimization, and characterization of bioinduced variable-stiffness actuators that can morph in various shapes and change their properties from soft to rigid are hereby presented. Bilayer devices are prepared by combining the electromechanically active properties of polypyrrole with the compliant behavior of alginate gels that are uniquely functionalized with cell-derived plasma membrane nanofragments (PMNFs), previously shown to mineralize within 2 days, which promotes the mineralization in the gel layer to achieve the soft to stiff change by growing their own bone. The mineralized actuator shows an evident frozen state compared to the movement before mineralization. Next, patterned devices show programmed directional and fixated morphing. These variable-stiffness devices can wrap around and, after the PMNF-induced mineralization in and on the gel layer, adhere and integrate onto bone tissue. The developed biohybrid variable-stiffness actuators can be used in soft (micro-)robotics and as potential tools for bone repair or bone tissue engineering.

Thermoset Shape Memory Polymer Variable Stiffness 4D Robotic Catheters

Variable stiffness catheters are typically composed of an encapsulated core. The core is usually composed of a low melting point alloy (LMPA) or a thermoplastic polymer (TP). In both cases, there is a need to encapsulate the core with an elastic material. This imposes a limit to the volume of variable stiffness (VS) material and limits miniaturization. This paper proposes a new approach that relies on the use of thermosetting materials. The variable stiffness catheter (VSC) proposed in this work eliminates the necessity for an encapsulation layer and is made of a unique biocompatible thermoset polymer with an embedded heating system. This significantly reduces the final diameter, improves manufacturability, and increases safety in the event of complications. The device can be scaled to sub-millimeter dimensions, while maintaining a high stiffness change. In addition, integration into a magnetic actuation system allows for precise actuation of one or multiple tools.

Transient Electrically Driven Stiffness-Changing Materials from Liquid Metal Polymer Composites

Stiffness-changing materials (SCMs) have received lots of interests due to their reversible transition between their soft and rigid states for modern applications. However, the irreversible stiffness transition, slow response, and sustained external stimuli strictly hinder the broad utilizations of SCMs. Here, this work reports electrically driven SCMs based on supercooled liquid metals (LMs). A small voltage (5 V) can successfully initiate the stable and reversible stiffness change of the SCMs in electrolyte solution. Surprisingly, the LM-based SCMs (LM-SCMs) exhibited a significant change in 1000 times difference of moduli (65 kPa versus 79 MPa). Moreover, such a stiffness transition of the LM-SCM was ultrarapidly completed in a few seconds (<30 s). Importantly, after transient stimulation of LM nucleation, the rigidity of the LM-SCM could be maintained when the external stimulus (voltage) was removed, highly different from previously reported SCMs that require sustained energy to maintain their mechanical states. Based on the unique features of LM-SCMs, advanced robotics like smart valves and mechanical paws in seawater were successfully fabricated.

Solution-based fabrication of mechanically transformative materials for implantable applications

Implantable probes and needles represent multifunctional biomedical platforms by integrating sensing, stimulation, and drug delivery capabilities. Conventional rigid probes often result in inflammatory responses due to large mechanical mismatch with soft biological tissues, whereas soft probes with improved long-term performances are difficult to be inserted deep into the compliant biological tissues. An emerging class of mechanically transformative materials addresses the challenge by embedding a phase-change material of gallium within an elastomeric matrix. These materials exhibit high stiffness under ambient conditions to enable facile insertion and compliant mechanical properties after implantations. The widespread implementation of mechanically transformative materials is primarily hindered by the lack of facile fabrication techniques for delicate gallium features. In this study, we introduce a solution-based approach for scalable fabrication of gallium-based mechanically transformative materials, which exhibit bistable mechanical properties with large modulations in the modulus by five orders of magnitude. In a solution-based coating process, gallium features are created based on a patterned copper film and then encapsulated with elastomers to form mechanically transformative materials. The height profile of the gallium feature is controlled by the two-dimensional design of the copper pattern, which provides access to delicate and complex three-dimensional features as exemplified by mechanically transformative indwelling needles with sharp tips. The practical suitability is demonstrated by the in vivo implementation of the indwelling needles for long-term chemotherapy. The excellent biocompatibility enables applications of mechanically transformative biomedical devices in chronic implantable systems.

Textiles in soft robots: Current progress and future trends

Soft robotics have substantial benefits of safety, adaptability, and cost efficiency compared to conventional rigid robotics. Textiles have applications in soft robotics either as an auxiliary material to reinforce the conventional soft material or as an active soft material. Textiles of various types and configurations have been fabricated into key components of soft robotics in adaptable formats. Despite significant advancements, the efficiency and characteristics of textile actuators in practical applications remain unsatisfactory. To address these issues, novel structural and material designs as well as new textile technologies have been introduced. Herein, we aim at giving an insight into the current state of the art in textile technology for soft robotic manufacturing. We firstly discuss the fundamental actuation mechanisms for soft robotics. We then provide a critical review on the recently developed functional textiles as reinforcements, sensors, and actuators in soft robotics. Finally, the future trends and current strategies that can be employed in textile-based actuator manufacturing process have been explored to address the critical challenges in soft robotics.

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.

Materials with Electroprogrammable Stiffness

Stiffness is a mechanical property of vital importance to any material system and is typically considered a static quantity. Recent work, however, has shown that novel materials with programmable stiffness can enhance the performance and simplify the design of engineered systems, such as morphing wings, robotic grippers, and wearable exoskeletons. For many of these applications, the ability to program stiffness with electrical activation is advantageous because of the natural compatibility with electrical sensing, control, and power networks ubiquitous in autonomous machines and robots. The numerous applications for materials with electrically driven stiffness modulation has driven a rapid increase in the number of publications in this field. Here, a comprehensive review of the available materials that realize electroprogrammable stiffness is provided, showing that all current approaches can be categorized as using electrostatics or electrically activated phase changes, and summarizing the advantages, limitations, and applications of these materials. Finally, a perspective identifies state-of-the-art trends and an outlook of future opportunities for the development and use of materials with electroprogrammable stiffness.

A Submillimeter Continuous Variable Stiffness Catheter for Compliance Control

Minimally invasive robotic surgery often requires functional tools that can change their compliance to adapt to the environment and surgical needs. This paper proposes a submillimeter continuous variable stiffness catheter equipped with a phase-change alloy that has a high stiffness variation in its different states, allowing for rapid compliance control. Variable stiffness is achieved through a variable phase boundary in the alloy due to a controlled radial temperature gradient. This catheter can be safely navigated in its soft state and rigidified to the required stiffness during operation to apply a desired force at the tip. The maximal contact force that the catheter applies to tissue can be continuously modified by a factor of 400 (≈20 mN-8 N). The catheter is equipped with a magnet and a micro-gripper to perform a fully robotic ophthalmic minimally invasive surgery on an eye phantom by means of an electromagnetic navigation system.

Soft and Stretchable Liquid Metal Composites with Shape Memory and Healable Conductivity

Shape memory composites are fascinating materials with the ability to preserve deformed shapes that recover when triggered by certain external stimuli. Although elastomers are not inherently shape memory materials, the inclusion of phase-change materials within the elastomer can impart shape memory properties. When this filler changes the phase from liquid to solid, the effective modulus of the polymer increases significantly, enabling stiffness tuning. Using gallium, a metal with a low melting point (29.8 °C), it is possible to create elastomeric materials with metallic conductivity and shape memory properties. This concept has been used previously in core-shell (gallium-elastomer) fibers and foams, but here, we show that it can also be implemented in elastomeric films containing microchannels. Such microchannels are appealing because it is possible to control the geometry of the filler and create metallically conductive circuits. Stretching the solidified metal fractures the fillers; however, they can heal by body heat to restore conductivity. Such conductive, shape memory sheets with healable conductivity may find applications in stretchable electronics and soft robotics.

Flexible Manipulator with Low-Melting-Point Alloy Actuation and Variable Stiffness

Flexible manipulators offer significant advantages over traditional rigid manipulators in minimally invasive surgery, because they can flexibly navigate around obstacles and pass cramped or tortuous paths. However, due to the inherent low stiffness, the ability to control/obtain higher stiffness when required remains to be further explored. In this article, we propose a flexible manipulator that exploits the phase transformation property of low-melting-point alloy to hydraulically drive and change the stiffness by heating and cooling. A prototype was fabricated, and experiments were conducted to evaluate the motion characteristics, stiffness performance, and rigid-flexible transition efficiency. The experimental results demonstrate that the proposed manipulator can freely adjust heading direction in the three-dimensional space. The experimental results also indicate that it took 9.2-10.3 s for the manipulator to transform from a rigid state to a flexible state and 15.4 s to transform from a flexible state to a rigid state. The lateral stiffness and flexural stiffness of the manipulator were 95.54 and 372.1 Ncm² in the rigid state and 7.26 and 0.78 Ncm² in the flexible state. The gain of the lateral stiffness and flexural stiffness was 13.15 and 477.05, respectively. In the rigid state, the ultimate force without shape deformation was more than 0.98 N in the straight condition (0°) and 1.36 N in the bending condition (90°). By assembling flexible surgical tools, the manipulator can enrich the diagnosis or treatment functions, which demonstrated the potential clinical value of the proposed manipulator.

Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface

Soft robotics inspired by the movement of living organisms, with excellent adaptability and accuracy for accomplishing tasks, are highly desirable for efficient operations and safe interactions with human. With the emerging wearable electronics, higher tactility and skin affinity are pursued for safe and user-friendly human-robot interactions. Fabrics interlocked by fibers perform traditional static functions such as warming, protection, and fashion. Recently, dynamic fibers and fabrics are favorable to deliver active stimulus responses such as sensing and actuating abilities for soft-robots and wearables. First, the responsive mechanisms of fiber/fabric actuators and their performances under various external stimuli are reviewed. Fiber/yarn-based artificial muscles for soft-robots manipulation and assistance in human motion are discussed, as well as smart clothes for improving human perception. Second, the geometric designs, fabrications, mechanisms, and functions of fibers/fabrics for sensing and energy harvesting from the human body and environments are summarized. Effective integration between the electronic components with garments, human skin, and living organisms is illustrated, presenting multifunctional platforms with self-powered potential for human-robot interactions and biomedicine. Lastly, the relationships between robotic/wearable fibers/fabrics and the external stimuli, together with the challenges and possible routes for revolutionizing the robotic fibers/fabrics and wearables in this new era are proposed.

Phase change mediated mechanically transformative dynamic gel for intelligent control of versatile devices

Traditional devices, including conventional rigid electronics and machines, as well as emerging wearable electronics and soft robotics, almost all have a single mechanical state for particular service purposes. Nonetheless, dynamic materials with interchangeable mechanical states, which enable more diverse and versatile applications, are urgently necessary for intelligent and adaptive application cases in the future electronic and robot fields. Here, we report a gel-like material composed of a crosslinking polymer network impregnated with a phase changing molten liquid, which undergoes an exceptional stiffness transition in response to a thermal stimulus. Vice versa, the material switches from a soft gel state to a rigid solid state with a dramatic stiffness change of 10⁵ times (601 MPa versus 4.5 kPa) benefiting from the liquid-solid phase change of the crystalline polymer once cooled. Such reversibility of the phase and mechanical transition upon thermal stimuli enables the dynamic gel mechanical transformation, demonstrating potential applications in an adhesive thermal interface gasket (TIG) to facilitate thermal transport, a high-temperature warning sensor and an intelligent gripper. Overall, this dynamic gel with a tunable stiffness change paves a new way to design and fabricate adaptive smart materials toward intelligent control of versatile devices.

Bending behavior of biomimetic scale covered beam with tunable stiffness scales

Biomimetic scales provide a convenient template to tailor the bending stiffness of the underlying slender substrate due to their mutual sliding after engagement. Scale stiffness can therefore directly impact the substrate behavior, opening a potential avenue for substrate stiffness tunability. Here, we have developed a biomimetic beam, which is covered by tunable stiffness scales. Scale tunability is achieved by specially designed plate like scales consisting of layers of low melting point alloy (LMPA) phase change materials fully enclosed inside a soft polymer. These composite scales can transition between stiff and soft states by straddling the temperatures across LMPA melting points thereby drastically altering stiffness. We experimentally analyze the bending behavior of biomimetic beams covered with tunable stiffness scales of two architectures—one with single enclosure of LMPA and one with two enclosures of different melting point LMPAs. These architectures provide a continuous stiffness change of the underlying substrate post engagement, controlled by the operating temperature. We characterize this response using three-point bending experiments at various temperature profiles. Our results demonstrate for the first time, the pronounced and reversible tunability in the bending behavior of biomimetic scale covered beam, which are strongly dependent on the scale material and architecture. Particularly, it is shown that the bending stiffness of the biomimetic scale covered beam can be actively and reversibly tuned by a factor of up to 7. The developed biomimetic beam has applications in soft robotic grippers, smart segmented armors, deployable structures and soft swimming robots.

Roboticizing fabric by integrating functional fibers

Fabrics are ubiquitous materials that have conventionally been passive assemblies of interlacing, inactive fibers. However, the recent emergence of active fibers with actuation, sensing, and structural capabilities provides the opportunity to impart robotic function into fabric substrates. Here we present an implementation of robotic fabrics by integrating functional fibers into conventional fabrics using typical textile manufacturing techniques. We introduce a set of actuating and variable-stiffness fibers, as well as printable in-fabric sensors, which allows for robotic closed-loop control of everyday fabrics while remaining lightweight and maintaining breathability. Finally, we demonstrate the utility of robotic fabrics through their application to an active wearable tourniquet, a transforming and load-bearing deployable structure, and an untethered, self-stowing airfoil.

Liquid metal nanocomposites

Liquid metal (LM) has attracted tremendous interest over the past decade for its enabling combination of high electrical and thermal conductivity and low mechanical compliance and viscosity. Efforts to harness LM in electronics, robotics, and biomedical applications have largely involved methods to encapsulate the liquid so that it can support functionality without leaking or smearing. In recent years, there has been increasing interest in LM "nanocomposites" in which either liquid metal is mixed with metallic nanoparticles or nanoscale droplets of liquid metal are suspended within a soft polymer matrix. Both of these material systems represent an important step towards utilizing liquid metal for breakthrough applications. In this minireview, we present a brief overview of recent progress over the past few years in methods to synthesize LM nanomaterials and utilize them as transducers for sensing, actuation, and energy harvesting. In particular, we focus on techniques for stable synthesis of LM nanodroplets, suspension of nanodroplets within various matrix materials, and methods for incorporating metallic nanoparticles within an LM matrix.

Tendon-Driven Jamming Mechanism for Configurable Variable Stiffness

Stiffness transition of a soft continuum body is an essential feature for dexterous interaction with an unstructured environment. Softness ensures safe interaction, whereas rigidness generates high force for movement or manipulation. Vacuum-based granular jamming is a widely used technique for on-line stiffness transition because of its high reconfigurability and intuitive driving method. However, vacuum driving method produces limited force levels, and the heavy weight and bulky size are unfavorable for portable applications. In this work, we propose a tendon-driven jamming mechanism for configurable variable stiffness. Compared with a vacuum system, an electric motor-tendon drive system has the benefits of force, bandwidth, size, and weight, but has different force characteristics for distribution, directionality, and transmissibility. In this study, a long snake-like shape is chosen instead of a lump shape for compatibility with tendon-drive characteristics. The snake-like shape is likely to cause buckling under the tendon force as the length increases, making the system extremely unstable. Implanting skeletal disk nodes in the structure is our solution to the buckling phenomenon by maintaining the tendon path in the desired position and for distributing the force evenly, thereby achieving stable stiffness transition capabilities for long free-curved shapes. As a proof of concept, a soft wearable device for wrist support is presented using the proposed variable stiffness mechanism. The weight of the device is 184 g, including the actuators, and it can support 2 kgf. Furthermore, the stiffness transition is completed within 2 s, thus achieving quick responses.

Phase Changing Materials-Based Variable-Stiffness Tensegrity Structures

Soft robots leverage deformable bodies to achieve different types of locomotion, improve transportability, and safely navigate cluttered environments. In this context, variable-stiffness structures provide soft robots with additional properties, such as the ability to increase forces transmitted to the environment, to lock into different body configurations, and to reduce the number of actuators required for morphological change. Tensegrity structures have been recently proposed as a biologically inspired design principle for soft robots. However, the few examples of tensegrity structures with variable stiffness displayed relatively small stiffness change (i.e., by a factor of 3) or resorted to multiple and bulky actuators. In this article, we describe a novel design approach to variable-stiffness tensegrity structures (VSTSs) that relies on the use of variable-stiffness cables (VSCs). As an example, we describe the design and implementation of a three-strut tensegrity structure with VSCs made of low melting point alloys. The resulting VSTS displays unprecedented stiffness changes by a factor of 28 in compression and 13 in bending. We show the capabilities of the proposed VSTS in three validation scenarios with different tensegrity architectures: (1) a beam with tunable load-bearing capability, (2) a structure that can self-deploy and lock its shape in both deployed and undeployed states, and (3) a joint with underactuated shape deformations.

Complex multiphase organohydrogels with programmable mechanics toward adaptive soft-matter machines

Many biological organisms can tune their mechanical properties to adapt to environments in multistable modes, but the current synthetic materials, with bistable states, have a limited ability to alter mechanical stiffness. Here, we constructed programmable organohydrogels with multistable mechanical states by an on-demand modular assembly of noneutectic phase transition components inside microrganogel inclusions. The resultant multiphase organohydrogel exhibits precisely controllable thermo-induced stepwise switching (i.e., triple, quadruple, and quintuple switching) mechanics and a self-healing property. The organohydrogel was introduced into the design of soft-matter machines, yielding a soft gripper with adaptive grasping through stiffness matching with various objects under pneumatic-thermal hybrid actuation. Meanwhile, a programmable adhesion of octopus-inspired robotic tentacles on a wide range of surface morphologies was realized. These results demonstrated the applicability of these organohydrogels in lifelike soft robotics in unconstructed and human body environments.

Ultrastretchable Elastic Shape Memory Fibers with Electrical Conductivity

Herein, elastomeric fibers that have shape memory properties due to the presence of a gallium core that can undergo phase transition from solid to liquid in response to mild heating are described. The gallium is injected into the core of a hollow fiber formed by melt processing. This approach provides a straightforward method to create shape memory properties from any hollow elastic fiber. Solidifying the core changes the effective fiber modulus from 4 to 1253 MPa. This increase in stiffness can preserve the fiber in a deformed shape. The elastic energy stored in the polymer shell during deformation drives the fiber to relax back to its original geometry upon melting the solid gallium core, allowing for shape memory. Although waxes are used previously for this purpose, the use of gallium is compelling because of its metallic electrical and thermal conductivity. In addition, the use of a rigid metallic core provides perfect fixity of the shape memory fiber. Notably, the use of gallium-with a melting point above room temperature but below body temperature-allows the user to melt and deform local regions of the fiber by hand and thereby tune the effective modulus and shape of the fiber.

Mechanically transformative electronics, sensors, and implantable devices

Traditionally, electronics have been designed with static form factors to serve designated purposes. This approach has been an optimal direction for maintaining the overall device performance and reliability for targeted applications. However, electronics capable of changing their shape, flexibility, and stretchability will enable versatile and accommodating systems for more diverse applications. Here, we report design concepts, materials, physics, and manufacturing strategies that enable these reconfigurable electronic systems based on temperature-triggered tuning of mechanical characteristics of device platforms. We applied this technology to create personal electronics with variable stiffness and stretchability, a pressure sensor with tunable bandwidth and sensitivity, and a neural probe that softens upon integration with brain tissue. Together, these types of transformative electronics will substantially broaden the use of electronics for wearable and implantable applications.

A soft continuum robot, with a large variable-stiffness range, based on jamming

Inspired by the physiological structure of the hand capable of realizing the continuous change in finger stiffness when grasping objects of different masses, a self-locking soft continuum robot with a large variable-stiffness range based on particle jamming and fibre jamming is proposed in this paper to meet the requirements of it in practical application. In this paper, a variable stiffness range is derived due to the good fluidity and rigidity of the spherical particles and the low elasticity and high toughness of the fibres. Then, an analysis model is established to deduce its self-locking condition, and the deflection angle of self-locking under the influence of external force is about 0.17 rad. The maximum stiffness of the experimental prototype can reach 1223.58 N m^-1 due to the limitation of the experimental materials, despite the fact that the theoretical stiffness can be increased infinitely after self-locking. To explain the adaptability of the robot, the adaptive conditions of the soft continuum robot with variable stiffness are deduced. A new evaluation index, the adaptive intensity of the soft continuum robot, is introduced and the adaptability experiments are carried out. In adaptability experiments, the maximum bending angle of the continuum robot reaches 108°. Finally, the adaptability of the soft continuum robot to different geometries is discussed.

Magnetically Addressable Shape-Memory and Stiffening in a Composite Elastomer

With a specific stimulus, shape-memory materials can assume a temporary shape and subsequently recover their original shape, a functionality that renders them relevant for applications in fields such as biomedicine, aerospace, and wearable electronics. Shape-memory in polymers and composites is usually achieved by exploiting a thermal transition to program a temporary shape and subsequently recover the original shape. This may be problematic for heat-sensitive environments, and when rapid and uniform heating is required. In this work, a soft magnetic shape-memory composite is produced by encasing liquid droplets of magneto-rheological fluid into a poly(dimethylsiloxane) matrix. Under the influence of a magnetic field, this material undergoes an exceptional stiffening transition, with an almost 30-fold increase in shear modulus. Exploiting this transition, fast and fully reversible magnetic shape-memory is demonstrated in three ways, by embossing, by simple shear, and by unconstrained 3D deformation. Using advanced synchrotron X-ray tomography techniques, the internal structure of the material is revealed, which can be correlated with the composite stiffening and shape-memory mechanism. This material concept, based on a simple emulsion process, can be extended to different fluids and elastomers, and can be manufactured with a wide range of methods.

Conductive Fabric Heaters for Heat-Activated Soft Actuators

We examine electrically conductive fabrics as conductive heaters for heat-activated soft actuators. We have explored various fabric designs optimized for material properties, heat distribution and actuation/de-actuation characteristics of the soft actuators. We implemented this approach in the silicone/ethanol composite actuators, in which ethanol undergoes a thermally-induced phase change, leading to high actuation stress and strain. Various types of conductive fabrics were tested, and we developed a stretchable kirigami-based fabric design. We demonstrate a fabric heater that is capable of cyclic heating of the actuator to the required 80 °C. The fabric with the special kirigami design can withstand temperatures of up to 195 °C, can consume up to 30 W of power, and allows the actuator to reach >30% linear strain. This technology may be used in various systems involving thermally-induced actuation.

E-BiInSn Enhanced Rigidity Alterable Artificial Bandage

surgery, orthopedic cast is often implemented to stabilize and fix anatomical structures like broken bones. Plaster could harden after mixed with water, thus it is commonly utilized with cotton bandage to form a solid structure to encase a limb or other body parts. As plaster is heavy and impervious, cast could easily result in itching, rashes, allergic contact dermatitis or other cutaneous complications. In this paper we present a novel implementation for surgical fixation with low melting point alloy (LMPA) stuffed in silicone tubes, which is dubbed "LMPA enhanced bandage The alloy is heated by an enameled copper wire to alter the stiffness. When the alloy is in solid state, the bandage could withstand high load without significant deformation, while if heated to its melting point, the entire bandage would soften. We present several conceptual experiments to evaluate the mechanical performance and body fixation of the proposed bandage. Phase change process and temperature variation were recorded by an infrared camera. Preliminary results showed that the present fixation bandage design owns sufficient mechanical strength and necessary thermal response performance to meet the requirement of clinical applications.