Tethering of twisted-fiber artificial muscles

Strengthening Liquid Crystal Elastomer Muscles

ConspectusLiquid crystal elastomer fibers (LCEFs) are reversible artificial muscles capable of stimuli-responsive functions, making them promising competitors for ideal soft actuators. These remarkable actuation properties depend strongly on their mechanical properties, such as elastic modulus and breaking stress. It is necessary to strengthen the LCEF muscles to meet the demands of advanced applications. However, despite the significant progress in LCEFs, there is currently no such Account systematically summarizing and analyzing the strategies adopted for enhancing their mechanical and actuation properties. The intuitive variations among the different enhancement strategies further call for investigations into how to choose the most suitable ones based on specific situations. In this Account, for the first time, we systematically summarize existing approaches to strengthening LCEF-based artificial muscles, contributing to the development of more robust and smarter fibrous artificial muscles.In the first section, we focus on the latest and most valuable progress on strengthening LCEF-based artificial muscles, highlighting the need for a comprehensive summary of the various approaches utilized. The mechanical properties of LCEFs can be tailored through molecular design, physical interactions, and fiber integration. The adjustment of hard/soft segment features, the introduction of additional microstructures, and the fiber integration provide opportunities to strengthen LCEF-based artificial muscles, which are discussed in the second section. Subsequently, we delve into the impact of various preparation methods on the performance of LCEFs, and LCEFs fabricated by different spinning and alignment techniques exhibited rather different mechanical and actuation properties. This has been adopted to engineer novel, stronger, and tailored fibrous artificial muscles, as described in the third section. Moreover, we show that the incorporation of rigid composite materials via coating and doping has emerged as a powerful strategy to strengthen LCEFs, such as core-shell structures. Such enhancements also introduce multifunctionality for LCE-based artificial muscles that can enrich the fiber structure and actuation mechanism, which are elucidated in the fourth section. Finally, we conclude this Account with a critical analysis of the challenges and prospects of LCE-based artificial muscles, hoping to pave the way for the construction of more powerful fibrous artificial muscles.

High-Modulus Homochiral Torsional Oxide Ceramic Artificial Muscles

Fiber-based artificial muscles are soft actuators used to mimic the movement of human muscles. However, using high modulus oxide ceramics to fabricate artificial muscles with high energy and power is a challenge as they are prone to brittle fracture during torsion. Here, a ceramic metallization strategy is reported that solves the problem of low torsion and low ductility of alumina (Al2O3) ceramics by chemical plating a thin copper layer on alumina filaments. These filaments with a high modulus of ≈180 GPa can be twisted into chiral coiled artificial muscles, exhibiting a unique electric thermal actuation mechanism. This tough and robust alumina artificial muscle can carry objects equivalent to 0.28 million times its weight and provide high actuation stress of up to 483.5 MPa. In addition, it exhibits 18 times higher contraction power and 240 times higher energy density than human muscles, as well as a high energy conversion efficiency of up to 7.59%, which far exceeds most reported actuated carbon and polymer artificial muscles. This work has achieved large-scale manufacturing of high-modulus oxide ceramic muscles for the first time.

Electrothermally-Driven Carbon Nanotube Fiber Artificial Muscle With Endomysium-Inspired Sheath and Multifilament Core

Electrothermally-driven carbon nanotube (CNT) fiber artificial muscles have attracted considerable interest in the fields of soft robotics, sensors, and intelligent control, owing to their excellent flexibility, abundant guest material sources, lightweight properties, rapid response, and ease of control. Nevertheless, their practical application has been impeded by the limited contractile stroke and contractile stress under high load conditions. Drawing inspiration from the physiological structure of muscle fiber, this study reports an endomysium-inspired-sheath artificial muscle (ESM). The ESMs are characterized by polydimethylsiloxane (PDMS) sheaths that mimick the endomysium, and multifilament cores made from CNT fibers imitating the myofibrils. The ESMs demonstrate excellent actuation performance, achieving maximum contractile stress of 40.1 MPa, the highest reported value among electrothermally-driven CNT fiber artificial muscles. The ESMs also demonstrate large contractile stroke and high work capacity under heavy loads. Furthermore, a crawling robot with the capacity of carrying loads, driven by a single ESM, is developed to demonstrate the practical application potential of ESMs.

Azobenzene-Functionalized Semicrystalline Liquid Crystal Elastomer Springs for Underwater Soft Robotic Actuators

As actuated devices become smaller and more complex, there is a need for smart materials and structures that directly function as complete mechanical units without an external power supply. The strategy uses light-powered, twisted, and coiled azobenzene-functionalized semicrystalline liquid crystal elastomer (AC-LCE) springs. This twisting and coiling, which has previously been used for only thermally, electrochemically, or absorption-powered muscles, maximizes uniaxial and radial actuation. The specially designed photochemical muscles can undergo about 60% tensile stroke and provide 15 kJ m-3 of work capacity in response to light, thus providing about three times and two times higher performance, respectively, than previous azobenzene actuators. Since this actuation is photochemical, driven by ultraviolet (UV) light and reversed by visible light, isothermal actuation can occur in a range of environmental conditions, including underwater. In addition, photoisomerization of the AC-LCEs enables unique latch-like actuation, eliminating the need for continuous energy application to maintain the stroke. Also, as the light-powered muscles processed to be either homochiral or heterochiral, the direction of actuation can be reversed. The presented approach highlights the novel capabilities of photochemical actuator materials that can be manipulated in untethered, isothermal, and wet environmental conditions, thus suggesting various potential applications, including underwater soft robotics.

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

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

Mechanically Twisting-Induced Top-Down Chirality Transfer for Tunable Full-Color Circularly Polarized Luminescent Fibers

Circularly polarized luminescence (CPL) materials with rich optical information are highly attractive for optical display, information storage, and encryption. Although previous investigations have shown that external force fields can induce CPL activity in nonchiral systems, the unique role of macroscopic external forces in inducing CPL has not been demonstrated at the level of molecule or molecular aggregate. Here, a canonical example of CPL generation by mechanical twisting in an achiral system consisting of a polymer matrix with embedded fluorescent molecules is presented. By carefully adjusting the twisting parameters in time and space, in conjunction with circular dichroism (CD), CPL, and 2D wide-angle X-ray scattering (2D WAXS) studies, a twisting-induced top-down chiral transfer mechanism derived from the molecular-level asymmetric rearrangement of fluorescent units is elucidated within polymers under external torsional forces. This top-down chiral transfer provides a simple, scalable, and versatile mechanical twisting strategy for the fabrication of CPL materials, allowing for fabricating full-color and handedness-tunable CPL fibers, where the macroscopic twist direction determines the CPL handedness. Moreover, the weavability of CPL fibers greatly extend their applications in anti-counterfeit encryption, as demonstrated by using embroidery techniques to design multilevel encryption patterns.

Mimosa-Inspired Body Temperature-Responsive Shape Memory Polymer Networks: High Energy Densities and Multi-Recyclability

Inspired by the Mimosa plant, this study herein develops a unique dynamic shape memory polymer (SMP) network capable of transitioning from hard to pliable with heat, featuring reversible actuation, self-healing, recyclability, and degradability. This material is adept at simulating the functionalities of artificial muscles for a variety of tasks, with a remarkable specific energy density of 1.8 J g-1-≈46 times higher than that of human skeletal muscle. As an intelligent manipulator, it demonstrates remarkable proficiency in identifying and handling items at high temperatures. Its suitable rate of shape recovery around human body temperature indicates its promising utility as an implant material for addressing acute obstructions. The dynamic covalent bonding within the network structure not only provides excellent resistance to solvents but also bestows remarkable abilities for self-healing, reprocessing, and degradation. These attributes significantly boost its practicality and environmental sustainability. Anticipated to promote advancements in the sectors of biomedical devices, soft robotics, and smart actuators, this SMP network represents a forward leap in simulating artificial muscles, marking a stride toward the future of adaptive and sustainable technology.

High-performance electrically responsive artificial muscle materials for soft robot actuation

Traditional robotic devices are often bulky and rigid, making it difficult for them to adapt to the soft and complex shapes of the human body. In stark contrast, soft robots, as a burgeoning class of robotic technology, showcase exceptional flexibility and adaptability, positioning them as compelling contenders for a diverse array of applications. High-performance electrically responsive artificial muscle materials (ERAMMs), as key driving components of soft robots, can achieve efficient motion and deformation, as well as more flexible and precise robot control, attracting widespread attention. This paper reviews the latest advancements in high-performance ERAMMs and their applications in the field of soft robot actuation, using ionic polymer-metal composites and dielectric elastomers as typical cases. Firstly, the definition, characteristics, and electro-driven working principles of high-performance ERAMMs are introduced. Then, the material design and synthesis, fabrication processes and optimization, as well as characterization and testing methods of the ERAMMs are summarized. Furthermore, various applications of two typical ERAMMs in the field of soft robot actuation are discussed in detail. Finally, the challenges and future directions in current research are analyzed and anticipated. This review paper aims to provide researchers with a reference for understanding the latest research progress in high-performance ERAMMs and to guide the development and application of soft robots. STATEMENT OF SIGNIFICANCE.

Fiber Actuators Based on Reversible Thermal Responsive Liquid Crystal Elastomer

Soft actuators inspired by the movement of organisms have attracted extensive attention in the fields of soft robotics, electronic skin, artificial intelligence, and healthcare due to their excellent adaptability and operational safety. Liquid crystal elastomer fiber actuators (LCEFAs) are considered as one of the most promising soft actuators since they can provide reversible linear motion and are easily integrated or woven into complex structures to perform pre-programmed movements such as stretching, rotating, bending, and expanding. The research on LCEFAs mainly focuses on controllable preparation, structural design, and functional applications. This review, for the first time, provides a comprehensive and systematic review of recent advances in this important field by focusing on reversible thermal response LCEFAs. First, the thermal driving mechanism, and direct and indirect heating strategies of LCEFAs are systematically summarized and analyzed. Then, the fabrication methods and functional applications of LCEFAs are summarized and discussed. Finally, the challenges and technical difficulties that may hinder the performance improvement and large-scale production of LCEFAs are proposed, and the development opportunities of LCEFAs are prospected.

High Performance Proprioceptive Fiber Actuators Based on Ag Nanoparticles-Incorporated Hybrid Twisted and Coiled System

Thermally driven fiber actuators are emerging as promising tools for a range of robotic applications, encompassing soft and wearable robots, muscle function restoration, assistive systems, and physical augmentation. Yet, to realize their full potential in practical applications, several challenges, such as a high operational temperature, incorporation of intrinsic self-sensing capabilities for closed-loop feedback control, and reliance on bulky, intricate actuation systems, must be addressed. Here, an Ag nanoparticles-based twisted and coiled fiber actuator that achieves a high contractile actuation of ≈36% is reported at a considerably low operational temperature of ≈83 °C based on a synergistic effect of constituent fiber elements with low glass transition temperatures. The fiber actuator can monitor its contractile actuation in real-time based on the piezoresistive properties inherent to its Ag-based conductive region, demonstrating its proprioceptive sensing capability. By exploiting this capability, the proprioceptive fiber actuator adeptly maintains its intended contractile behavior, even when faced with unplanned external disturbances. To demonstrate the capabilities of the fiber actuator, this study integrates it into a closed-loop feedback-controlled bionic arm as an artificial muscle, offering fresh perspectives on the future development of intelligent wearable devices and soft robotic systems.

Porous Conductive Textiles for Wearable Electronics

Over the years, researchers have made significant strides in the development of novel flexible/stretchable and conductive materials, enabling the creation of cutting-edge electronic devices for wearable applications. Among these, porous conductive textiles (PCTs) have emerged as an ideal material platform for wearable electronics, owing to their light weight, flexibility, permeability, and wearing comfort. This Review aims to present a comprehensive overview of the progress and state of the art of utilizing PCTs for the design and fabrication of a wide variety of wearable electronic devices and their integrated wearable systems. To begin with, we elucidate how PCTs revolutionize the form factors of wearable electronics. We then discuss the preparation strategies of PCTs, in terms of the raw materials, fabrication processes, and key properties. Afterward, we provide detailed illustrations of how PCTs are used as basic building blocks to design and fabricate a wide variety of intrinsically flexible or stretchable devices, including sensors, actuators, therapeutic devices, energy-harvesting and storage devices, and displays. We further describe the techniques and strategies for wearable electronic systems either by hybridizing conventional off-the-shelf rigid electronic components with PCTs or by integrating multiple fibrous devices made of PCTs. Subsequently, we highlight some important wearable application scenarios in healthcare, sports and training, converging technologies, and professional specialists. At the end of the Review, we discuss the challenges and perspectives on future research directions and give overall conclusions. As the demand for more personalized and interconnected devices continues to grow, PCT-based wearables hold immense potential to redefine the landscape of wearable technology and reshape the way we live, work, and play.

Fast and Strong Carbon Nanotube Yarn Artificial Muscles by Electro-osmotic Pump

Previous electrochemically powered yarn muscles cannot be usefully operated between extreme negative and extreme positive potentials, since generated stresses during anion injection and cation injection partially cancel because they are in the same direction. We here report an ionomer-infiltrated hybrid carbon nanotube (CNT) yarn muscle that shows unipolar stress behavior in the sense that stress generation between extreme potentials is additive, resulting in an enhanced stress generation. Moreover, the stress generated by this muscle unexpectedly increases with the potential scan rate, which contradicts the fact that scan-rate-induced stress decreases for neat CNT muscles. It is revealed by the electro-osmotic pump effect that the effective ion size injected into the muscle increases with an increase in the scan rate. We demonstrate an electrochemically powered gel-elastomer-yarn muscle adhesive that generates and delivers muscle-contraction-mimicking stimulation to a target tissue.

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.