Large-Stroke Electrochemical Carbon Nanotube/Graphene Hybrid Yarn Muscles

Reversible Electrochemical Swelling of Straight Carbon Nanotube Yarns for High-Performance Linear Actuation

Coiled artificial muscle yarns outperform their straight counterparts in contractile strokes. However, challenges persist in the fabrication complexity and the susceptibility of the coiled yarns to becoming stuck by surrounding objects during contraction and recovery. Additionally, torsional stability remains a concern. In this study, it is reported that straight carbon nanotube (CNT) yarns when driven by a low-voltage electrochemical approach, can achieve a contractile stroke that surpasses even NiTi shape memory alloy fibers. The key lies in the suitable match between a yarn consisting of randomly aligned CNTs and the reversible and substantial electrochemical swelling induced by solvated ions. Wrinkled structures are formed on the surface of the CNT yarn to adapt to the swelling process. This not only assures torsional stability but also enhances the surface area for improved electrode-electrolyte interaction during electrochemical actuation. Remarkably, the CNT artificial muscle yarn generates a contractile stroke of 8.8% and an isometric stress of 7.5 MPa under 2.5 V actuation voltages, demonstrating its potential for applications requiring low energy consumption while maintaining high operational efficiency. This study highlights the crucial impact of CNT orientation on the effectiveness of electrochemically-driven artificial muscles, signaling new possibilities in smart material and biomechanical system development.

Electrochemically-driven actuators: from materials to mechanisms and from performance to applications

Soft actuators, pivotal for converting external energy into mechanical motion, have become increasingly vital in a wide range of applications, from the subtle engineering of soft robotics to the demanding environments of aerospace exploration. Among these, electrochemically-driven actuators (EC actuators), are particularly distinguished by their operation through ion diffusion or intercalation-induced volume changes. These actuators feature notable advantages, including precise deformation control under electrical stimuli, freedom from Carnot efficiency limitations, and the ability to maintain their actuated state with minimal energy use, akin to the latching state in skeletal muscles. This review extensively examines EC actuators, emphasizing their classification based on diverse material types, driving mechanisms, actuator configurations, and potential applications. It aims to illuminate the complicated driving mechanisms of different categories, uncover their underlying connections, and reveal the interdependencies among materials, mechanisms, and performances. We conduct an in-depth analysis of both conventional and emerging EC actuator materials, casting a forward-looking lens on their trajectories and pinpointing areas ready for innovation and performance enhancement strategies. We also navigate through the challenges and opportunities within the field, including optimizing current materials, exploring new materials, and scaling up production processes. Overall, this review aims to provide a scientifically robust narrative that captures the current state of EC actuators and sets a trajectory for future innovation in this rapidly advancing field.

Knittable Electrochemical Yarn Muscle for Morphing Textiles

Morphing textiles, crafted using electrochemical artificial muscle yarns, boast features such as adaptive structural flexibility, programmable control, low operating voltage, and minimal thermal effect. However, the progression of these textiles is still impeded by the challenges in the continuous production of these yarn muscles and the necessity for proper structure designs that bypass operation in extensive electrolyte environments. Herein, a meters-long sheath-core structured carbon nanotube (CNT)/nylon composite yarn muscle is continuously prepared. The nylon core not only reduces the consumption of CNTs but also amplifies the surface area for interaction between the CNT yarn and the electrolyte, leading to an enhanced effective actuation volume. When driven electrochemically, the CNT@nylon yarn muscle demonstrates a maximum contractile stroke of 26.4%, a maximum contractile rate of 15.8% s-1, and a maximum power density of 0.37 W g-1, surpassing pure CNT yarn muscles by 1.59, 1.82, and 5.5 times, respectively. By knitting the electrochemical CNT@nylon artificial muscle yarns into a soft fabric that serves as both a soft scaffold and an electrolyte container, we achieved a morphing textile is achieved. This textile can perform programmable multiple motion modes in air such as contraction and sectional bending.

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.

High-Performance One-Body Electrochemical Torsional Artificial Muscles Built Using Carbon Nanotubes and Ion-Exchange Polymers

Electrochemical torsional artificial muscles have the potential to replace electric motors in the field of miniaturization. In particular, carbon nanotubes (CNTs) are some of the best materials for electrochemical torsional artificial muscles due to their remarkable mechanical strength and high electrical conductivity. However, previous studies on CNT torsional muscle utilize only half of the whole potential range for torsional actuation because the actuations in the positive and negative voltage ranges offset each other. Here, we used an ion-exchange polymer, poly(sodium 4-styrenesulfonate) (PSS), which leads to the participation of only positive ions in the actuation of CNT muscles so that the whole potential range can be used for torsional actuation. As a result, PSS-coated CNT muscle can provide 1.9 times higher torsional actuation compared to neat CNT torsional muscle. This PSS-coated CNT muscle not only provides high performance but also facilitates a one-body system for electrochemical torsional actuation. From these advantages, we implement a one-body torsional muscle for the realization of the forward motion of a model boat. This high performance and one-body structure for electrochemical torsional muscles can be used for further applications, such as soft robotics and implantable devices.

Dual-Ion Co-Regulation System Enabling High-Performance Electrochemical Artificial Yarn Muscles with Energy-Free Catch States

Artificial yarn muscles show great potential in applications requiring low-energy consumption while maintaining high performance. However, conventional designs have been limited by weak ion-yarn muscle interactions and inefficient "rocking-chair" ion migration. To address these limitations, we present an electrochemical artificial yarn muscle design driven by a dual-ion co-regulation system. By utilizing two reaction channels, this system shortens ion migration pathways, leading to faster and more efficient actuation. During the charging/discharging process, [Formula: see text] ions react with carbon nanotube yarn, while Li+ ions react with an Al foil. The intercalation reaction between [Formula: see text] and collapsed carbon nanotubes allows the yarn muscle to achieve an energy-free high-tension catch state. The dual-ion coordinated yarn muscles exhibit superior contractile stroke, maximum contractile rate, and maximum power densities, exceeding those of "rocking-chair" type ion migration yarn muscles. The dual-ion co-regulation system enhances the ion migration rate during actuation, resulting in improved performance. Moreover, the yarn muscles can withstand high levels of isometric stress, displaying a stress of 61 times that of skeletal muscles and 8 times that of "rocking-chair" type yarn muscles at higher frequencies. This technology holds significant potential for various applications, including prosthetics and robotics.

Moisture-Adaptive Contractile Biopolymer-Derived Fibers for Wound Healing Promotion

The advancement in thermosensitive active hydrogels has opened promising opportunities to dynamic full-thickness skin wound healing. However, conventional hydrogels lack breathability to avoid wound infection and cannot adapt to wounds with different shapes due to the isotropic contraction. Herein, a moisture-adaptive fiber that rapidly absorbs wound tissue fluid and produces a large lengthwise contractile force during the drying process is reported. The incorporation of hydroxyl-rich silica nanoparticles in the sodium alginate/gelatin composite fiber greatly improves the hydrophilicity, toughness, and axial contraction performance of the fiber. This fiber exhibits a dynamic contractile behavior as a function of humidity, generating ≈15% maximum contraction strain or ≈24 MPa maximum isometric contractile stress. The textile knitted by the fibers features excellent breathability and generates adaptive contraction in the target direction during the natural desorption of tissue fluid from the wounds. In vivo animal experiments further demonstrate the advantages of the textiles over traditional dressings in accelerating wound healing.

Tethering of twisted-fiber artificial muscles

Twisted-fiber artificial muscles, a new type of soft actuator, exhibit significant potential for use in applications related to lightweight smart devices and soft robotics. Fiber twisting generates internal torque and a spiral architecture, exhibiting rotation, contraction, or elongation as a result of fiber volume change. Untethering a twisted fiber often results in fiber untwisting and loss of stored torque energy. Preserving the torque in twisted fibers during actuation is necessary to realize a reversible and stable artificial muscle performance; this is a key issue that has not yet been systematically discussed and reviewed. This review summarizes the mechanisms for preserving the torque within twisted fibers and the potential applications of such systems. The potential challenges and future directions of research related to twisted-fiber artificial muscles are also discussed.

Engineering a conduction-consistent cardiac patch with rGO/PLCL electrospun nanofibrous membranes and human iPSC-derived cardiomyocytes

The healthy human heart has special directional arrangement of cardiomyocytes and a unique electrical conduction system, which is critical for the maintenance of effective contractions. The precise arrangement of cardiomyocytes (CMs) along with conduction consistency between CMs is essential for enhancing the physiological accuracy of in vitro cardiac model systems. Here, we prepared aligned electrospun rGO/PLCL membranes using electrospinning technology to mimic the natural heart structure. The physical, chemical and biocompatible properties of the membranes were rigorously tested. We next assembled human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) on electrospun rGO/PLCL membranes in order to construct a myocardial muscle patch. The conduction consistency of cardiomyocytes on the patches were carefully recorded. We found that cells cultivated on the electrospun rGO/PLCL fibers presented with an ordered and arranged structure, excellent mechanical properties, oxidation resistance and effective guidance. The addition of rGO was found to be beneficial for the maturation and synchronous electrical conductivity of hiPSC-CMs within the cardiac patch. This study verified the possibility of using conduction-consistent cardiac patches to enhance drug screening and disease modeling applications. Implementation of such a system could one day lead to in vivo cardiac repair applications.

Conductive Textiles for Signal Sensing and Technical Applications

Conductive textiles have found notable applications as electrodes and sensors capable of detecting biosignals like the electrocardiogram (ECG), electrogastrogram (EGG), electroencephalogram (EEG), and electromyogram (EMG), etc; other applications include electromagnetic shielding, supercapacitors, and soft robotics. There are several classes of materials that impart conductivity, including polymers, metals, and non-metals. The most significant materials are Polypyrrole (PPy), Polyaniline (PANI), Poly(3,4-ethylenedioxythiophene) (PEDOT), carbon, and metallic nanoparticles. The processes of making conductive textiles include various deposition methods, polymerization, coating, and printing. The parameters, such as conductivity and electromagnetic shielding, are prerequisites that set the benchmark for the performance of conductive textile materials. This review paper focuses on the raw materials that are used for conductive textiles, various approaches that impart conductivity, the fabrication of conductive materials, testing methods of electrical parameters, and key technical applications, challenges, and future potential.

Carbon nanotube-graphene hybrids for soft electronics, sensors, and actuators

Soft devices that are mechanically flexible and stretchable are considered as the building blocks for various applications ranging from wearable devices to robotics. Among the many candidate materials for constructing soft devices, carbon nanomaterials such as carbon nanotubes (CNTs) and graphene have been actively investigated owing to their outstanding characteristics, including their intrinsic flexibility, tunable conductivity, and potential for large-area processing. In particular, hybrids of CNTs and graphene can improve the performance of soft devices and provide them with novel capabilities. In this review, the advances in CNT-graphene hybrid-based soft electrodes, transistors, pressure and strain sensors, and actuators are discussed, highlighting the performance improvements of these devices originating from the synergistic effects of the hybrids of CNT and graphene. The integration of multidimensional heterogeneous carbon nanomaterials is expected to be a promising approach for accelerating the development of high-performance soft devices. Finally, current challenges and future opportunities are summarized, from the processing of hybrid materials to the system-level integration of multiple components.

Moisture-Sensitive Response and High-Reliable Cycle Recovery Effectiveness of Yarn-Based Actuators with Tether-Free, Multi-Hierarchical Hybrid Construction

Yarn-based muscle actuators are highly desired for applications in soft robotics, flexible sensors, and other related applications due to their actuation properties. Although the tethering avoiding release of inserted twist, the complex preparation process and harsh experimental conditions make tether-free structures yarn actuator with reliable cycle recovery effectiveness is needed. Herein, a tether-free, multi-hierarchical hybrid construction of a moisture-sensitive responsive yarn-based actuator with the viscose/PET ratio (VPR) = 0.9 exhibited a contraction stroke of 83.15%, a work capacity of 52.98 J·kg^-1, and an exerting force of 0.15 MPa. Additionally, the maximum cycle recovery rate of 99% is comparable to that of human skeletal muscles, confirming the advantages of a two-component hybrid structure. The underlying mechanism is discussed based on geometric characterization and energy conversion analysis between the actuation source and the spring frame. The mechanical manufacturing process makes it simple to expand the structurally stable yarn muscles into fabric muscles, opening up new opportunities to advance the usage of yarn-based actuators in smart textiles, medical materials, intelligent plants, and other versatile fields.

Stepwise Artificial Yarn Muscles with Energy-Free Catch States Driven by Aluminum-Ion Insertion

Present artificial muscles have been suffering from poor actuation step precision and the need of energy input to maintain actuated states due to weak interactions between guest and host materials or the unstable structural changes. Herein, these challenges are addressed by deploying a mechanism of reversible faradaic insertion and extraction reactions between tetrachloroaluminate ions and collapsed carbon nanotubes. This mechanism allows tetrachloroaluminate ions as a strong but dynamic "locker" to achieve an energy-free high-tension catch state and programmable stepwise actuation in the yarn muscle. When powered off, the muscle nearly 100% maintained any achieved contractile strokes even under loads up to 96,000 times the muscle weight. The actuation mechanism allowed the programmable control of stroke steps down to 1% during reversible actuation. The isometric stress generated by the yarn muscle (14.6 MPa in maximum, 40 times that of skeletal muscles) was also energy freely lockable and step controllable with high precision. Importantly, when fully charged, the muscle stored energy with a high capacity of 102 mAh g^-1, allowing the muscle as a battery to power secondary muscles or other devices.

Oscillating light engine realized by photothermal solvent evaporation

Continuous mechanical work output can be generated by using combustion engines and electric motors, as well as actuators, through on/off control via external stimuli. Solar energy has been used to generate electricity and heat in human daily life; however, the direct conversion of solar energy to continuous mechanical work has not been realized. In this work, a solar engine is developed using an oscillating actuator, which is realized through an alternating volume decrease of each side of a polypropylene/carbon black polymer film induced by photothermal-derived solvent evaporation. The anisotropic solvent evaporation and fast gradient diffusion in the polymer film sustains oscillating bending actuation under the illumination of divergent light. This light-driven oscillator shows excellent oscillation performance, excellent loading capability, and high energy conversion efficiency, and it can never stop with solvent supply. The oscillator can cyclically lift up a load and output work, exhibiting a maximum specific work of 30.9 × 10⁻⁵ J g⁻¹ and a maximum specific power of 15.4 × 10⁻⁵ W g⁻¹ under infrared light. This work can inspire the development of autonomous devices and provide a design strategy for solar engines.

Developing an oscillating actuator that can directly convert solar energy into mechanical energy is highly desirable. Here, authors report a solvent-assisted light-driven oscillator by porous film that achieves excellent oscillating actuation performance and can even oscillate by carrying a load under light irradiation.

High-Power Hydro-Actuators Fabricated from Biomimetic Carbon Nanotube Coiled Yarns with Fast Electrothermal Recovery

Bioinspired yarn/fiber structured hydro-actuators have recently attracted significant attention. However, most water-driven mechanical actuators are unsatisfactory because of the slow recovery process and low full-time power density. A rapidly recoverable high-power hydro-actuator is reported by designing biomimetic carbon nanotube (CNT) yarns. The hydrophilic CNT (HCNT) coiled yarn was prepared by storing pre-twist into CNT sheets and subsequent electrochemical oxidation (ECO) treatment. The resulting yarn demonstrated structural stability even when one end was cut off without the possible loss of pre-stored twists. The HCNT coiled yarn actuators provided maximal contractile work of 863 J/kg at 11.8 MPa stress when driven by water. Moreover, the recovery time of electrically heated yarns at a direct current voltage of 5 V was 95% shorter than that of neat yarns without electric heating. Finally, the electrothermally recoverable hydro-actuators showed a high actuation frequency (0.17 Hz) and full-time power density (143.8 W/kg).

Nanomaterials and their applications on bio-inspired wearable electronics

Wearable electronics featuring conformal attachment, sensitive perception and intellectual signal processing have made significant progress in recent years. However, when compared with living organisms, artificial sensory devices showed undeniable bulky shape, poor adaptability, and large energy consumption. To make up for the deficiencies, biological examples provide inspirations of novel designs and practical applications. In the field of biomimetics, nanomaterials from nanoparticles to layered two-dimensional materials are actively involved due to their outstanding physicochemical properties and nanoscale configurability. This review focuses on nanomaterials related to wearable electronics through bioinspired approaches on three different levels, interfacial packaging, sensory structure, and signal processing, which comprehensively guided recent progress of wearable devices in leveraging both nanomaterial superiorities and biorealistic functionalities. In addition, opinions on potential development trend are proposed aiming at implementing bioinspired electronics in multifunctional portable sensors, health monitoring, and intelligent prosthetics.

Programmable Contractile Actuations of Twisted Spider Dragline Silk Yarns

The contraction behavior of spider dragline silk upon water exposure has drawn particular interest in developing humidity-responsive smart materials. We report herein that the spider dragline silk yarns with moderate twists can generate much improved lengthwise contraction of 60% or an isometric stress of 11 MPa when wetted by water. Upon the removal of the absorbed water, the dried and contracted spider silk yarns showed programmable contractile actuations. These yarns can be plastically stretched to any specified lengths between the fully contracted state and the state before supercontraction and return to the fully contracted state when wetted. Moreover, the generated isometric stress of these yarns is also programmable, depending on the stretching ratio. The mechanism of the programmable reversible contraction is based on the plastic mechanical property of the dried and contracted spider silk yarns, which can be explained by the variation of the hydrogen bonds and the secondary structures of the proteins in spider dragline silk. Humidity alarm switches, smart doors, and wound healing devices based on the programmable contractile actuations of the spider silk yarns were demonstrated, which provide application scenarios for the supercontraction of spider dragline silk.

Strong and Robust Electrochemical Artificial Muscles by Ionic-Liquid-in-Nanofiber-Sheathed Carbon Nanotube Yarns

To address the lack of a suitable electrolyte that supports the stable operation of the electrochemical yarn muscles in air, an ionic-liquid-in-nanofibers sheathed carbon nanotube (CNT) yarn muscle is prepared. The nanofibers serve as a separator to avoid the short-circuiting of the yarns and a reservoir for ionic liquid. The ionic-liquid-in-nanofiber-sheathed yarn muscles are strong, providing an isometric stress of 10.8 MPa (about 31 times the skeletal muscles). The yarn muscles are highly robust, which can reversibly contract stably at such conditions as being knotted, wide-range humidity (30 to 90 RH%) and temperature (25 to 70 °C), and long-term cycling and storage in air. By utilizing the accumulated isometric stress, the yarn muscles achieve a high contraction rate of 36.3% s^-1 . The yarn muscles are tightly bundled to lift heavy weights and grasp objects. These unique features can make the strong and robust yarn muscles as a desirable actuation component for robotic devices.

Unipolar stroke, electroosmotic pump carbon nanotube yarn muscles

Success in making artificial muscles that are faster and more powerful and that provide larger strokes would expand their applications. Electrochemical carbon nanotube yarn muscles are of special interest because of their relatively high energy conversion efficiencies. However, they are bipolar, meaning that they do not monotonically expand or contract over the available potential range. This limits muscle stroke and work capacity. Here, we describe unipolar stroke carbon nanotube yarn muscles in which muscle stroke changes between extreme potentials are additive and muscle stroke substantially increases with increasing potential scan rate. The normal decrease in stroke with increasing scan rate is overwhelmed by a notable increase in effective ion size. Enhanced muscle strokes, contractile work-per-cycle, contractile power densities, and energy conversion efficiencies are obtained for unipolar muscles.

A Photoactuator Based on Stiffness-Variable Carbon Nanotube Nanocomposite Yarn

Actuators based on carbon nanotube (CNT) yarn have attracted extensive attention due to their great properties and potential applications such as artificial muscles, sensors, intelligent robots, and so on. However, the CNT yarn actuators with one-dimensional structure were often only used to drive through electrochemical, thermal, or electrical stimulation, which limits the applications of CNT yarn actuators. In addition, the slow response speed, low output stress, uncontrollable driving deformation, and self-recovery without an external stimulus are also great challenges. Here, we propose a photoactuator with large output stress, fast response speed, large and reversible driving deformation, and good reusability based on stiffness-variable CNT nanocomposite yarn (CNT-NCY). Such a CNT-NCY photoactuator can achieve torsional and contractive actuation under irradiation of near-infrared (NIR) light; it is important that the actuation is reversible and controllable. The maximum rotation rate of the CNT-NCY photoactuator during the torsional actuation is about 45 rpm, and the contractive deformation can reach more than 9%. This CNT-NCY photoactuator can create more than 12 MPa output stress, which is 40 times higher than that of the human skeletal muscle. The driving mechanism of this CNT-NCY photoactuator has been analyzed, and its potential application has also been demonstrated.

Graphene-Based Fibers: Recent Advances in Preparation and Application

Graphene-based fibers (GBFs) are macroscopic 1D assemblies formed by using microscopic 2D graphene sheets as building blocks. Their unique structure exhibits the same merits as graphene such as low weight, high specific surface area, excellent mechanical/electrical properties, and ease of functionalization. Furthermore, the fibrous nature of GBFs is intrinsically compatible with existing textile technologies, making them suitable for applications in flexible and wearable electronics. Recently, novel synthetic methods have endowed GBFs with new structures and functions, further improving their mechanical and electrical properties. These improvements have rapidly bridged the gaps between laboratory demonstrations and real-life applications in fiber-shaped batteries, supercapacitors, and electrochemical sensors. Recent advances in the fabrication, optimization, and application of GBFs are systematically reviewed and a perspective on their future development is given.

Carbon Nanotube Yarn for Fiber-Shaped Electrical Sensors, Actuators, and Energy Storage for Smart Systems

Smart systems are those that display autonomous or collaborative functionalities, and include the ability to sense multiple inputs, to respond with appropriate operations, and to control a given situation. In certain circumstances, it is also of great interest to retain flexible, stretchable, portable, wearable, and/or implantable attributes in smart electronic systems. Among the promising candidate smart materials, carbon nanotubes (CNTs) exhibit excellent electrical and mechanical properties, and structurally fabricated CNT-based fibers and yarns with coil and twist further introduce flexible and stretchable properties. A number of notable studies have demonstrated various functions of CNT yarns, including sensors, actuators, and energy storage. In particular, CNT yarns can operate as flexible electronic sensors and electrodes to monitor strain, temperature, ionic concentration, and the concentration of target biomolecules. Moreover, a twisted CNT yarn enables strong torsional actuation, and coiled CNT yarns generate large tensile strokes as an artificial muscle. Furthermore, the reversible actuation of CNT yarns can be used as an energy harvester and, when combined with a CNT supercapacitor, has promoted the next-generation of energy storage systems. Here, progressive advances of CNT yarns in electrical sensing, actuation, and energy storage are reported, and the future challenges in smart electronic systems considered.

Carbon nanotube and graphene fiber artificial muscles

Actuator materials capable of producing a rotational or tensile motion are rare and, yet, rotary systems are extensively utilized in mechanical systems like electric motors, pumps, turbines and compressors. Rotating elements of such machines can be rather complex and, therefore, difficult to miniaturize. Rotating action at the microscale, or even nanoscale, would benefit from the direct generation of torsion from an actuator material. Herein we discuss the advantages of using carbon nanotube (CNT) yarns and/or graphene (G) fibers as novel artificial muscles that have the ability to be driven by the electrochemical charging of helically wound multiwall carbon nanotubes or graphene fibers as well as elements in the ambient environment such as moisture to generate such rotational action. The torsional strain, torque, speed and lifetime have been evaluated under various electrochemical conditions to provide insight into the actuation mechanism and performance. Here the most recent advances in artificial muscles based on sheath-run artificial muscles (SRAMs) are reviewed. Finally, the rotating motion of the CNT yarn actuator and the humidity-responsive twisted graphene fibers have been coupled to a mixer for use in a prototype microfluidic system, moisture management and a humidity switch respectively.