Miniature coiled artificial muscle for wireless soft medical devices

MXene-Based Soft Humidity-Driven Actuator with High Sensitivity and Fast Response

Soft actuators possessing notable mechanical deformations, high sensitivity, and fast response speed play a crucial role in various applications, such as artificial muscles, soft robots, and intelligent devices. In this study, a smart humidity-driven actuator was successfully fabricated by utilizing MXene/cellulose nanofiber (CNF)/LiCl (MCL) through vacuum-assisted filtration with fast response speed and high sensitivity. Utilizing the excellent humidity responsiveness of MXene/CNF and the robust hygroscopicity of LiCl, the synergistic effect of these materials enhances the hygroscopic properties and response speed of the actuator. The MCL actuator demonstrates excellent actuation performance, fast deformation, and reliable cyclic stability. To illustrate the extensive potential of the soft actuator, a range of applications, from bionic devices to soft grippers and crawling actuators, are showcased. Remarkably, the crawling actuator demonstrates sustained crawling motion without necessitating a humidity switch, relying on the humidity gradient from water droplets, and exhibits spontaneous directional motions within a certain range, which makes it a promising prospect in the field of soft robotics.

Untethered soft actuators for soft standalone robotics

Soft actuators produce the mechanical force needed for the functional movements of soft robots, but they suffer from critical drawbacks since previously reported soft actuators often rely on electrical wires or pneumatic tubes for the power supply, which would limit the potential usage of soft robots in various practical applications. In this article, we review the new types of untethered soft actuators that represent breakthroughs and discuss the future perspective of soft actuators. We discuss the functional materials and innovative strategies that gave rise to untethered soft actuators and deliver our perspective on challenges and opportunities for future-generation soft actuators.

Crack-Resistant and Tissue-Like Artificial Muscles with Low Temperature Activation and High Power Density

Constructing soft robotics with safe human-machine interactions requires low-modulus, high-power-density artificial muscles that are sensitive to gentle stimuli. In addition, the ability to resist crack propagation during long-term actuation cycles is essential for a long service life. Herein, a material design is proposed to combine all these desirable attributes in a single artificial muscle platform. The design involves the molecular engineering of a liquid crystalline network with crystallizable segments and an ethylene glycol flexible spacer. A high degree of crystallinity can be afforded by utilizing aza-Michael chemistry to produce a low covalent crosslinking density, resulting in crack-insensitivity with a high fracture energy of 33 720 J m-2 and a high fatigue threshold of 2250 J m-2. Such crack-resistant artificial muscle with tissue-matched modulus of 0.7 MPa can generate a high power density of 450 W kg-1 at a low temperature of 40 °C. Notably, because of the presence of crystalline domains in the actuated state, no crack propagation is observed after 500 heating-cooling actuation cycles under a static load of 220 kPa. This study points to a pathway for the creation of artificial muscles merging seemingly disparate, but desirable properties, broadening their application potential in smart devices.

Adaptive nanotube networks enabling omnidirectionally deformable electro-driven liquid crystal elastomers towards artificial muscles

Artificial muscles that can convert electrical energy into mechanical energy promise broad scientific and technological applications. However, existing electro-driven artificial muscles have been plagued with problems that hinder their practical applications: large electro-mechanical attenuation during deformation, high-driving voltages, small actuation strain, and low power density. Here, we design and create novel electro-thermal-driven artificial muscles rationally composited by hierarchically structured carbon nanotube (HS-CNT) networks and liquid crystal elastomers (LCEs), which possess adaptive sandwiched nanotube networks with angulated-scissor-like microstructures, thus effectively addressing above problems. These HS-CNT/LCE artificial muscles demonstrate not only large strain (>40%), but also remarkable conductive robustness (R/R0 < 1.03 under actuation), excellent Joule heating efficiency (≈ 233 °C at 4 V), and high load-bearing capacity (R/R0 < 1.15 at 4000 times its weight loaded). In addition, our artificial muscles exhibit real-muscle-like morphing intelligence that enables preventing mechanical damage in response to excessively heavyweight loading. These high-performance artificial muscles uniquely combining omnidirectional stretchability, robust electrothermal actuation, low driving voltage, and powerful mechanical output would exert significant technological impacts on engineering applications such as soft robotics and wearable flexible electronics.

Soft Polymer-Actuated Compliant Microgripper with Adaptive Vibration-Controlled Grasp and Release

Microgrippers that incorporate soft actuators are appropriate for micromanipulation or microsurgery owing to their ability to grasp objects without causing damage. However, developing a microgripper with a large gripping range that can produce a large force with high speed remains challenging in soft actuation mechanisms. Herein, we introduce a compliant microgripper driven by a soft dielectric elastomer actuator (DEA) called a spiral flexure cone DEA (SFCDEA). The submillimeter-scale SFCDEA exhibited a controllable linear displacement over a high bandwidth and the capability of lifting 100.9 g, which was 670 times higher than its mass. Subsequently, we developed a compliant microgripper based on the SFCDEA using smart composite microstructure technology to fabricate three-dimensional gripper linkages. We demonstrated that the microgripper was able to grasp various millimeter-scale objects with different shapes, sizes, and weights without a complex feedback control owing to its compliance. We proved the versatility of our gripper in robotic manipulation by demonstrating adaptive grasping and releasing of small objects using vibrations owing to its high bandwidth.

Viscoelastic dynamics of a soft strip subject to a large deformation

To produce sounds, we adjust the tension of our vocal folds to shape their properties and control the pitch. This efficient mechanism offers inspiration for designing reconfigurable materials and adaptable soft robots. However, understanding how flexible structures respond to a significant static strain is not straightforward. This complexity also limits the precision of medical imaging when applied to tensioned organs like muscles, tendons, ligaments and blood vessels among others. In this article, we experimentally and theoretically explore the dynamics of a soft strip subject to a substantial static extension, up to 180%. Our observations reveal a few intriguing effects, such as the resilience of certain vibrational modes to a static deformation. These observations are supported by a model based on the incremental displacement theory. This has promising practical implications for characterizing soft materials but also for scenarios where external actions can be used to tune properties.

Fiberbots: Robotic fibers for high-precision minimally invasive surgery

Precise manipulation of flexible surgical tools is crucial in minimally invasive surgical procedures, necessitating a miniature and flexible robotic probe that can precisely direct the surgical instruments. In this work, we developed a polymer-based robotic fiber with a thermal actuation mechanism by local heating along the sides of a single fiber. The fiber robot was fabricated by highly scalable fiber drawing technology using common low-cost materials. This low-profile (below 2 millimeters in diameter) robotic fiber exhibits remarkable motion precision (below 50 micrometers) and repeatability. We developed control algorithms coupling the robot with endoscopic instruments, demonstrating high-resolution in situ molecular and morphological tissue mapping. We assess its practicality and safety during in vivo laparoscopic surgery on a porcine model. High-precision motion of the fiber robot delivered endoscopically facilitates the effective use of cellular-level intraoperative tissue identification and ablation technologies, potentially enabling precise removal of cancer in challenging surgical sites.

Soft Fiber/Textile Actuators: From Design Strategies to Diverse Applications

Fiber/textile-based actuators have garnered considerable attention due to their distinctive attributes, encompassing higher degrees of freedom, intriguing deformations, and enhanced adaptability to complex structures. Recent studies highlight the development of advanced fibers and textiles, expanding the application scope of fiber/textile-based actuators across diverse emerging fields. Unlike sheet-like soft actuators, fibers/textiles with intricate structures exhibit versatile movements, such as contraction, coiling, bending, and folding, achieved through adjustable strain and stroke. In this review article, we provide a timely and comprehensive overview of fiber/textile actuators, including structures, fabrication methods, actuation principles, and applications. After discussing the hierarchical structure and deformation of the fiber/textile actuator, we discuss various spinning strategies, detailing the merits and drawbacks of each. Next, we present the actuation principles of fiber/fabric actuators, along with common external stimuli. In addition, we provide a summary of the emerging applications of fiber/textile actuators. Concluding with an assessment of existing challenges and future opportunities, this review aims to provide a valuable perspective on the enticing realm of fiber/textile-based actuators.

Effective On-Line Performance Modulation and Efficient Continuous Preparation of Ultra-Long Twisted and Coiled Polymer Artificial Muscles for Engineering Applications

Artificial muscle is a kind of thread-like actuator that can produce contractile strain, generate force, and output mechanical work under external stimulations to imitate the functions and achieve the performances of biological muscles. It can be used to actuate various bionic soft robots and has broad application prospects. The electrically controlled twisted and coiled polymer (TCP) artificial muscles, with the advantages of high power density, large stroke and low driving voltage, while also being electrolyte free, are the most practical. However, the relationship between the muscle performances and its preparation parameters is not very clear yet, and the complete procedure of designing and preparing TCP muscles according to actual needs has not been established. Besides, current preparation approaches are very time-consuming and cannot make ultra-long TCP muscles. These problems greatly limit wide applications of TCP artificial muscles. In this study, we studied and built the relationship between the actuating performances of TCP muscles and their preparation parameters, so that suitable TCP muscles can be easily designed and prepared according to actual requirements. Moreover, an efficient preparation method integrating one-step annealing technique has been developed to realize on-line performance modulation and continuous fabrication of ultra-long TCP muscles. By graphically assembling long muscles on heat-resist films, we designed and produced a series of fancy soft robots (butterfly, flower, starfish), which can perform various bionic movements and complete specific tasks. This work has achieved efficient on-demand preparation and large-scale assembly of ultra-long TCP muscles, laying solid foundations for their engineering applications in soft robot field.

Locally Addressable Energy Efficient Actuation of Magnetic Soft Actuator Array Systems

Advances in magnetoresponsive composites and (electro-)magnetic actuators have led to development of magnetic soft machines (MSMs) as building blocks for small-scale robotic devices. Near-field MSMs offer energy efficiency and compactness by bringing the field source and effectors in close proximity. Current challenges of near-field MSM are limited programmability of effector motion, dimensionality, ability to perform collaborative tasks, and structural flexibility. Herein, a new class of near-field MSMs is demonstrated that combines microscale thickness flexible planar coils with magnetoresponsive polymer effectors. Ultrathin manufacturing and magnetic programming of effectors is used to tailor their response to the nonhomogeneous near-field distribution on the coil surface. The MSMs are demonstrated to lift, tilt, pull, or grasp in close proximity to each other. These ultrathin (80 µm) and lightweight (100 gm-2 ) MSMs can operate at high frequency (25 Hz) and low energy consumption (0.5 W), required for the use of MSMs in portable electronics.

Pangolin-inspired untethered magnetic robot for on-demand biomedical heating applications

Untethered magnetic miniature soft robots capable of accessing hard-to-reach regions can enable safe, disruptive, and minimally invasive medical procedures. However, the soft body limits the integration of non-magnetic external stimuli sources on the robot, thereby restricting the functionalities of such robots. One such functionality is localised heat generation, which requires solid metallic materials for increased efficiency. Yet, using these materials compromises the compliance and safety of using soft robots. To overcome these competing requirements, we propose a pangolin-inspired bi-layered soft robot design. We show that the reported design achieves heating > 70 °C at large distances > 5 cm within a short period of time <30 s, allowing users to realise on-demand localised heating in tandem with shape-morphing capabilities. We demonstrate advanced robotic functionalities, such as selective cargo release, in situ demagnetisation, hyperthermia and mitigation of bleeding, on tissue phantoms and ex vivo tissues.

Photocured Liquid-Crystalline Polymer Electrolytes with 3D Ion Transport Pathways for Electromechanical Actuators

Self-assembly of ionic molecules into hierarchical ordered structures is a promising route to new types of solid electrolytes with enhanced ion transport. Herein, we report a liquid-crystalline polymer electrolyte membrane that contains three-dimensionally (3D) interconnected ionic pathways. To build this membrane, we used wedge-shaped amphiphilic molecules that have two ionic heads and a lipophilic tail. These molecules were combined with a low content of ionic liquid (5.6 wt %) to form a hexagonal columnar phase, where the self-assembled lipophilic cylinders were surrounded by the ionic shell. Photopolymerization of this phase produced flexible nanostructured films with 3D ionic pathways, which can serve as an electrolyte layer in soft robotic actuators. Ionic transport in the 3D pathways leads to shape memory capability as well as durable bending actuation with a voltage-controllable blocking force. Furthermore, we find a significant enhancement of actuation for the nanostructured electrolyte compared with the corresponding amorphous electrolyte.

Artificial neuromuscular fibers by multilayered coaxial integration with dynamic adaption

Integrating sense in a thin artificial muscle fiber for environmental adaption and actuation path tracing, as a snail tentacle does, is highly needed but still challenging because of the interfacing mismatch between the fiber's actuation and sensing components. Here, we report an artificial neuromuscular fiber by wrapping a carbon nanotube (CNT) fiber core in sequence with an elastomer layer, a nanofiber network, and an MXene/CNT thin sheath, achieving the ingenious sense-judge-act intelligent system in an elastic fiber. The CNT/elastomer components provide actuation, and the sheath enables touch/stretch perception and hysteresis-free cyclic actuation tracing due to its strain-dependent resistance. As a whole, the coaxial structure builds a dielectric capacitor that enables sensitive touchless perception. The key to seamless integration is to use a nanofiber interface that allows the sensing layer to adaptively trace but not restrict actuation. This work provides promising solutions for closed-loop control for future intelligent soft robots.

Hybrid Compliant Musculoskeletal System for Fast Actuation in Robots

A nature-inspired musculoskeletal system is designed and developed to examine the principle of nonlinear elastic energy storage-release for robotic applications. The musculoskeletal system architecture consists of elastically rigid segments and hyperelastic soft materials to emulate rigid-soft interactions in limbless vertebrates. The objectives are to (i) improve the energy efficiency of actuation beyond that of current pure soft actuators while (ii) producing a high range of motion similar to that of soft robots but with structural stability. This paper proposes a musculoskeletal design that takes advantage of structural segmentation to increase the system's degrees of freedom, which enhances the range of motion. Our findings show that rigid-soft interactions provide a remarkable increase in energy storage and release and, thus, an increase in the undulation speed. The energy efficiency achieved is approximately 68% for bending the musculoskeletal system from the straight configuration, compared to 2.5-30% efficiency in purely soft actuators. The hybrid compliance of the musculoskeletal system under investigation shows promise for alleviating the need for actuators at each joint in a robot.

Wireless Miniature Magnetic Phase-Change Soft Actuators

Wireless miniature soft actuators are promising for various potential high-impact applications in medical, robotic grippers, and artificial muscles. However, these miniature soft actuators are currently constrained by a small output force and low work capacity. To address such challenges, a miniature magnetic phase-change soft composite actuator is reported. This soft actuator exhibits an expanding deformation and enables up to a 70 N output force and 175.2 J g-1 work capacity under remote magnetic radio frequency heating, which are 106 -107 times that of traditional magnetic soft actuators. To demonstrate its capabilities, a wireless soft robotic device is first designed that can withstand 0.24 m s-1 fluid flows in an artery phantom. By integrating it with a thermally-responsive shape-memory polymer and bistable metamaterial sleeve, a wireless reversible bistable stent is designed toward future potential angioplasty applications. Moreover, it can additionally locomote inside and jump out of granular media. At last, the phase-change actuator can realize programmable bending deformations when a specifically designed magnetization profile is encoded, enhancing its shape-programming capability. Such a miniature soft actuator provides an approach to enhance the mechanical output and versatility of magnetic soft robots and devices, extending their medical and other potential applications.

Solid-Liquid State Transformable Magnetorheological Millirobot

Magnetically actuated soft millirobots (magneto-robot) capable of accomplishing on-demand tasks in a remote-control manner using noninvasive magnetic fields are of great interest in biomedical settings. However, the solid magneto-robots are usually restricted by the limited deformability due to the predesigned shape, while the liquid magneto-robots are capable of in situ shape reconfiguration but limited by the low stiffness and geometric instability due to the fluidity. Herein, we propose a magneto-active solid-liquid state transformable millirobot (named MRF-Robot) made from a magnetorheological fluid (MRF). The MRF-Robot can transform freely and rapidly between the Newtonian fluid in the liquid state upon a weak magnetic field (∼0 mT) and the Bingham plasticity in the solid state upon a strong magnetic field (∼100 mT). The MRF-Robot in the liquid state can realize diverse behaviors of large deformation, smooth navigation, in situ splitting, merging, and gradient pulling actuated by a weak magnetic field with a high gradient. The MRF-Robot in the solid state is distinguished for the controllable locomotion with reconfigured shapes and versatile object manipulations (including pull, push, and rotate the objects) driven by a strong magnetic field with a high gradient. Moreover, the MRF-Robot could continuously maneuver to accomplish diverse tasks in the comprehensive scenes and achieve liquid-drug delivery, thrombus clearance, and fluid-flow blockage in the phantom vascular model under magnetic actuation.

Biorobotics: An Overview of Recent Innovations in Artificial Muscles

In this overview of recent developments in the field of biorobotics we cover the developments in materials such as the use of polyester fabric being used as artificial skin and the start of whole new ways to actuate artificial muscles as a whole. In this, we discuss all of the relevant innovations from the fields of nano and microtechnology, as well as in the field of soft robotics to summarize what has been over the last 4 years and what could be improved for artificial muscles in the future. The goal of this paper will be to gain a better understanding of where the current field of biorobotics is at and what its current trends in manufacturing and its techniques are within the last several years.

Overcoming the Force Limitations of Magnetic Robotic Surgery: Magnetic Pulse Actuated Collisions for Tissue-Penetrating-Needle for Tetherless Interventions

The field of magnetic robotics aims to obviate physical connections between the actuators and end-effectors. Such tetherless control may enable new ultra-minimally invasive surgical manipulations in clinical settings. While wireless actuation offers advantages in medical applications, the challenge of providing sufficient force to magnetic needles for tissue penetration remains a barrier to practical application. Applying sufficient force for tissue penetration is required for tasks such as biopsy, suturing, cutting, drug delivery, and accessing deep seated regions of complex structures in organs such as the eye. To expand the force landscape for such magnetic surgical tools, an impact-force based suture needle capable of penetrating in vitro and ex vivo samples with 3-DOF planar motion is proposed. Using custom-built 14G and 25G needles, we demonstrate generation of 410 mN penetration force, a 22.7-fold force increase with more than 20 times smaller volume compared to similar magnetically guided needles. With the MPACT-Needle, in vitro suturing of a gauze mesh onto an agar gel is demonstrated. In addition, we have reduced the tip size to 25G, which is a typical needle size for interventions in the eye, to demonstrate ex vivo penetration in a rabbit eye, mimicking procedures such as corneal injections and transscleral drug delivery.