Knitted Carbon-Nanotube-Sheath/Spandex-Core Elastomeric Yarns for Artificial Muscles and Strain Sensing

An Overview of the Recent Progress in the Synthesis and Applications of Carbon Nanotubes

Carbon nanotubes (CNTs) are known as nano-architectured allotropes of carbon, having graphene sheets that are wrapped forming a cylindrical shape. Rolling of graphene sheets in different ways makes CNTs either metals or narrow-band semiconductors. Over the years, researchers have devoted much attention to understanding the intriguing properties CNTs. They exhibit some unusual properties like a high degree of stiffness, a large length-to-diameter ratio, and exceptional resilience, and for this reason, they are used in a variety of applications. These properties can be manipulated by controlling the diameter, chirality, wall nature, and length of CNTs which are in turn, synthesis procedure-dependent. In this review article, various synthesis methods for the production of CNTs are thoroughly elaborated. Several characterization methods are also described in the paper. The applications of CNTs in various technologically important fields are discussed in detail. Finally, future prospects of CNTs are outlined in view of their commercial applications.

The Development of Highly Flexible Stretch Sensors for a Robotic Hand

Demand for highly compliant mechanical sensors for use in the fields of robotics and wearable electronics has been constantly rising in recent times. Carbon based materials, and especially, carbon nanotubes, have been widely studied as a candidate piezoresistive sensing medium in these devices due to their favorable structural morphology. In this paper three different carbon based materials, namely carbon black, graphene nano-platelets, and multi-walled carbon nanotubes, were utilized as large stretch sensors capable of measuring stretches over 250%. These stretch sensors can be used in robotic hands/arms to determine the angular position of joints. Analysis was also carried out to understand the effect of the morphologies of the carbon particles on the electromechanical response of the sensors. Sensors with gauge factors ranging from one to 1.75 for strain up to 200% were obtained. Among these sensors, the stretch sensors with carbon black/silicone composite were found to have the highest gauge factor while demonstrating acceptable hysteresis in most robotic hand applications. The highly flexible stretch sensors demonstrated in this work show high levels of compliance and conformance making them ideal candidates as sensors for soft robotics.

High performance strain sensor based on buckypaper for full-range detection of human motions

A high-performance strain sensor based on buckypaper has been fabricated and studied. The sensor with an ultrahigh gauge factor of 20 216 can detect a maximum and a minimum strain range of 75% and 0.1%, respectively. During stretching, the strain sensor achieves a high stability and reproducibility of 10 000 cycles, and a fast response time of less than 87 ms. On the other hand, the sensor shows an excellent sensing performance upon pressure. The pressure range, pressure sensitivity and loading-unloading cycles are 0-1.68 MPa, 89.7 kPa-1 and 3000 cycles, respectively. A concept of the optimal value is utilized to evaluate the strain and pressure performances of the sensor. The optimal values of the sensor upon tensile strain and pressure are calculated to be 3.07 × 108 and 1.35 × 107, respectively, which are much higher than those of most strain and pressure sensors reported in the literature. Precise detection of full-range human motions, acoustic vibrations and even pulse waves at a small scale has been successfully demonstrated by the buckypaper-based sensor. Owning to its advantages including ultrahigh sensitivity, wide detection range and good stability, the buckypaper-based sensor suggests a great potential for applications in wearable sensors, electronic skins, micro/nano electromechanical systems, vibration sensing devices and other strain sensing devices.

Highly Stretchable and Biocompatible Strain Sensors Based on Mussel-Inspired Super-Adhesive Self-Healing Hydrogels for Human Motion Monitoring

Integrating multifunctionality such as adhesiveness, stretchability, and self-healing ability on a single hydrogel has been a challenge and is a highly desired development for various applications including electronic skin, wound dressings, and wearable devices. In this study, a novel hydrogel was synthesized by incorporating polydopamine-coated talc (PDA-talc) nanoflakes into a polyacrylamide (PAM) hydrogel inspired by the natural mussel adhesive mechanism. Dopamine molecules were intercalated into talc and oxidized, which enhanced the dispersion of talc and preserved catechol groups in the hydrogel. The resulting dopamine-talc-PAM (DTPAM) hydrogel showed a remarkable stretchability, with over 1000% extension and a recovery rate over 99%. It also displayed strong adhesiveness to various substrates, including human skin, and the adhesion strength surpassed that of commercial double-sided tape and glue sticks, even as the hydrogel dehydrated over time. Moreover, the DTPAM hydrogel could rapidly self-heal and regain its mechanical properties without needing any external stimuli. It showed excellent biocompatibility and improved cell affinity to human fibroblasts compared to the PAM hydrogel. When used as a strain sensor, the DTPAM hydrogel showed high sensitivity, with a gauge factor of 0.693 at 1000% strain, and was capable of monitoring various human motions such as the bending of a finger, knee, or elbow and taking a deep breath. Therefore, this hydrogel displays favorable attributes and is highly suitable for use in human-friendly biological devices.

Conductive Cotton Fabrics for Motion Sensing and Heating Applications

Conductive cotton fabric was prepared by coating single-wall carbon nanotubes (CNTs) on a knitted cotton fabric surface through a “dip-and-dry” method. The combination of CNTs and cotton fabric was analyzed using scanning electron microscopy (SEM) and Raman scattering spectroscopy. The CNTs coating improved the mechanical properties of the fabric and imparted conductivity to the fabric. The electromechanical performance of the CNT-cotton fabric (CCF) was evaluated. Strain sensors made from the CCF exhibited a large workable strain range (0~100%), fast response and great stability. Furthermore, CCF-based strain sensors was used to monitor the real-time human motions, such as standing, walking, running, squatting and bending of finger and elbow. The CCF also exhibited strong electric heating effect. The flexible strain sensors and electric heaters made from CCF have potential applications in wearable electronic devices and cold weather conditions.

Self-plied and twist-stable carbon nanotube yarn artificial muscles driven by organic solvent adsorption

Artificial yarn/fiber muscles have recently attracted considerable interest for various applications. These muscles can provide large-stroke tensile and torsional actuations, resulting from inserted twists. However, tensional tethering of twisted muscles is generally needed to avoid muscle snarling and untwisting. In this paper a carbon nanotube (CNT) yarn muscle that is tethering-free and twist-stable is reported. The yarn muscle is prepared by allowing the self-plying of a coiled CNT yarn. When driven by acetone adsorption, this muscle shows decoupled actuations, which provide fast and reversible ∼13.3% contraction strain against a constant stress corresponding to ∼38 000 times the muscle weight but almost zero torsional strokes. The cycling test shows that the self-plied muscle has very good structural stability and actuation reversibility. Applied joule heating can help increase the desorption of acetone and increase the operation frequency of the self-plied muscle. Furthermore, by controlling the coupling between the joule heating and acetone adsorption/desorption, tensile actuations from negative to positive have been achieved. This twist-stable feature could considerably facilitate the practical applications of such muscle.

Programmable and functional electrothermal bimorph actuators based on large-area anisotropic carbon nanotube paper

Electro-active polymer (EAP) actuators, such as electronic, ionic and electrothermal (ET) actuators, have become an important branch of next-generation soft actuators in bionic robotics. However, most reported EAP actuators could realize only simple movements, being restricted by the small area of flexible electrodes and simple designs. We prepared large-area flexible electrodes of high anisotropy, made of oriented carbon nanotube (CNT) paper, and carried out artful graphic designs and processing on the electrodes to make functional ET bimorph actuators which can realize large bending deformations (over 220°, curvature > 1.5 cm^-1) and bionic movements driven by electricity. The anisotropy of CNT paper benefits electrode designs and multiform actuations for complex actuators. Based on the large-area CNT paper, more interesting and functional actuators can be designed and prepared which will have practical applications in the fields of artificial muscles, complicated actuations, and soft and bionic robotics.

Interface-Controlled Conductive Fibers for Wearable Strain Sensors and Stretchable Conducting Wires

As an important subfield of flexible electronics, conductive fibers have been an active area of research. The interfacial interaction between nanostructured conductive materials with elastic substrates plays a vital role in the electromechanical performance of conductive fibers. However, the underlying mechanism has seldom been investigated. Here, we propose a fabricating strategy for a silver nanowire (Ag NW)/polyurethane composite fiber with a sheath-core architecture. The interfacial bonding layer is regulated, and its influence on the performance of conductive fibers is investigated, based on which an interfacial interaction model is proposed. The model underlines the significance of the embedding depth of the Ag NW network. Both supersensitive (gauge factor up to 9557) and ultrastable (negligible conductance degradation below the strain of 150%) conductive fibers are obtained via interface regulating, exhibiting great potential in the applications of wearable sensors and stretchable conducting connections.

High Durability and Waterproofing rGO/SWCNT-Fabric-Based Multifunctional Sensors for Human-Motion Detection

Wearable strain-pressure sensors for detecting electrical signals generated by human activities are being widely investigated because of their diverse potential applications, from observing human motion to health monitoring. In this study, we fabricated reduced graphene oxide (rGO)/single-wall carbon nanotube (SWCNT) hybrid fabric-based strain-pressure sensors using a simple solution process. The structural and chemical properties of the rGO/SWCNT fabrics were characterized using scanning electron microscopy (SEM), Raman, and X-ray photoelectron spectroscopy (XPS). Complex networks containing rGO and SWCNTs were homogeneously formed on the cotton fabric. The sensing performance of the devices was evaluated by measuring the effects of bending strain and pressure. When the CNT content was increased, the change in relative resistance decreased, while durability was significantly improved. The rGO/SWCNT (0.04 wt %) fabric sensor showed particularly high mechanical stability and flexibility during 100 000 bending tests at the extremely small bending radius of 3.5 mm (11.6% bending strain). Moreover, the rGO/SWCNT fabric device exhibited excellent water resistant properties after 10 washing tests due to its hydrophobic nature. Finally, we demonstrated a fabric-sensor-based motion glove and confirmed its practical applicability.

Quantitative characterisation of conductive fibers by capacitive coupling

The application of a capacitively coupled contactless conductivity sensor for the quantitative characterization of conductive fibers.

Design of a wearable and shape-memory fibriform sensor for the detection of multimodal deformation

A wearable and shape-memory strain sensor with a coaxial configuration is designed, comprising a thermoplastic polyurethane fiber as the core support, well-aligned and interconnected carbon nanotubes (CNTs) as conductive filaments, and polypyrrole (PPy) coating as the cladding layer. In this design, the stress relaxation between CNTs is well confined by the outer PPy cladding layer, which endows the fibriform sensor with good reliability and repeatability. The microcracks generated when the coaxial fiber is under strain guarantee the superior sensitivity of this fibriform sensor with a gauge factor of 12 at 0.1% strain, a wide detectable range (from 0.1% to 50% tensile strain), and the ability to detect multimodal deformation (tension, bending, and torsion) and human motions (finger bending, breathing, and phonation). In addition, due to its shape-memory characteristic, the sensing performance of the fibriform sensor is well retained after its shape recovers from 50% deformation and the fabric woven from the shape-memory coaxial fibers can be worn on the elbow joints in a reversible manner (original-enlarged-recovered) and fitted tightly. Thus, this sensor shows promising applications in wearable electronics.

Continuously Producible Ultrasensitive Wearable Strain Sensor Assembled with Three-Dimensional Interpenetrating Ag Nanowires/Polyolefin Elastomer Nanofibrous Composite Yarn

Fiber-shaped strain sensors with great flexibility and knittability have been tremendously concerned due to the wide applications in health manager devices, especially in human motion detection and physiological signal monitoring. Herein, a novel fiber-shaped strain sensor has been designed and prepared by interpenetrating Ag nanowires (NWs) into polyolefin elastomer nanofibrous yarn. The easy-to-obtain structure and simple roll-to-roll process make the continuous large-scale production of nanofibrous composite yarn possible. The continuous and alternating stretching and releasing reversibly change the contact probability between AgNWs in this interpenetrating network, leading to the variations of electrical resistance of the sensor. The gauge factors of strain sensors are calculated to be as high as 13920 and the minimum detection limit is only 0.065%. In addition, the strain sensor shows excellent durability during 4500 cycles with the strain of 10%. The response times of stretching and releasing strains are 10 and 15 ms, respectively. Furthermore, the strain sensor has been successfully applied in human motion detections both in single yarn and knitted fabrics. The result shows the practicability in applications of monitoring limbs movements, eye motion changes, artificial vocal cords, human pulse, and complex motions, which shows great potential in wearable sensors and electronic skin.

Wet-Spun Biofiber for Torsional Artificial Muscles

The demands for new types of artificial muscles continue to grow and novel approaches are being enabled by the advent of new materials and novel fabrication strategies. Self-powered actuators have attracted significant attention due to their ability to be driven by elements in the ambient environment such as moisture. In this study, we demonstrate the use of twisted and coiled wet-spun hygroscopic chitosan fibers to achieve a novel torsional artificial muscle. The coiled fibers exhibited significant torsional actuation where the free end of the coiled fiber rotated up to 1155 degrees per mm of coil length when hydrated. This value is 96%, 362%, and 2210% higher than twisted graphene fiber, carbon nanotube torsional actuators, and coiled nylon muscles, respectively. A model based on a single helix was used to evaluate the torsional actuation behavior of these coiled chitosan fibers.

Secondary Sensitivity Control of Silver-Nanowire-Based Resistive-Type Strain Sensors by Geometric Modulation of the Elastomer Substrate

A secondary method for modulation of the sensitivity in silver nanowire (AgNW) resistive-type strain sensors without the need to change the material or coating process in the sensory layer is demonstrated. Instead of using a planar elastomer (polydimethylsiloxane is used in this study) substrate, diverse relief structures are introduced to induce nonuniform and complex strain within the elastic substrate and thereby different distributions of the crack density of the AgNWs upon stretching, which plays an important role in the modulation of the gauge factor (GF). Analysis of the sensory layer and mechanical studies reveal that a lower height ratio and greater number of trenches enhance the sensor sensitivity, for example, reaching a GF of 926 at 9.6% in this study. The demonstration of wrist-motion sensors using the technology illustrates the feasibility of using relief structures for various types of sensors and sensitivity ranges using an identical sensor layer.

Emerging Biofabrication Strategies for Engineering Complex Tissue Constructs

The demand for organ transplantation and repair, coupled with a shortage of available donors, poses an urgent clinical need for the development of innovative treatment strategies for long-term repair and regeneration of injured or diseased tissues and organs. Bioengineering organs, by growing patient-derived cells in biomaterial scaffolds in the presence of pertinent physicochemical signals, provides a promising solution to meet this demand. However, recapitulating the structural and cytoarchitectural complexities of native tissues in vitro remains a significant challenge to be addressed. Through tremendous efforts over the past decade, several innovative biofabrication strategies have been developed to overcome these challenges. This review highlights recent work on emerging three-dimensional bioprinting and textile techniques, compares the advantages and shortcomings of these approaches, outlines the use of common biomaterials and advanced hybrid scaffolds, and describes several design considerations including the structural, physical, biological, and economical parameters that are crucial for the fabrication of functional, complex, engineered tissues. Finally, the applications of these biofabrication strategies in neural, skin, connective, and muscle tissue engineering are explored.

High-performance hybrid carbon nanotube fibers for wearable energy storage

Wearable energy storage devices are of practical interest, but few have been commercially exploited. Production of electrodes with extended cycle life, as well as high energy and power densities, coupled with flexibility, remains a challenge. Herein, we have demonstrated the development of a high-performance hybrid carbon nanotube (CNT) fiber-based supercapacitor for the first time using conventional wet-spinning processes. Manganese dioxide (MnO₂) nanoflakes were deposited onto the as-prepared CNT fibers by electrodeposition to form highly flexible nanocomposites fibers. As-prepared fibers were characterized by electron microscopy, electrical, mechanical, and electrochemical measurements. It was found that the specific capacitance was over 152 F g^-1 (156 F cm^-3), which is about 500% higher than the multi-walled carbon nanotube/MnO₂ yarn-based supercapacitors. The measured energy density was 14.1 Wh kg^-1 at a power density of 202 W kg^-1. These values are 232% and 32% higher than the energy density and power density of MWNT/MnO₂ yarn-based supercapacitor, respectively. It was found that the cyclic retention ability was more stable, revealing a 16% increase after 10 000 cycles. Such substantial enhancements of key properties of the hybrid material can be associated with the synergy of CNT and MnO₂ nanoparticles in the fiber structure. The use of wet-spun hybrid CNT for fiber-based supercapacitors has been demonstrated.

Probe Sensor Using Nanostructured Multi-Walled Carbon Nanotube Yarn for Selective and Sensitive Detection of Dopamine

The demands for electrochemical sensor materials with high strength and durability in physiological conditions continue to grow and novel approaches are being enabled by the advent of new electromaterials and novel fabrication technologies. Herein, we demonstrate a probe-style electrochemical sensor using highly flexible and conductive multi-walled carbon nanotubes (MWNT) yarns. The MWNT yarn-based sensors can be fabricated onto micro Pt-wire with a controlled diameter varying from 100 to 300 µm, and then further modified with Nafion via a dip-coating approach. The fabricated micro-sized sensors were characterized by electron microscopy, Raman, FTIR, electrical, and electrochemical measurements. For the first time, the MWNT/Nafion yarn-based probe sensors have been assembled and assessed for high-performance dopamine sensing, showing a significant improvement in both sensitivity and selectivity in dopamine detection in presence of ascorbic acid and uric acid. It offers the potential to be further developed as implantable probe sensors.

Recent Advancements in Flexible and Stretchable Electrodes for Electromechanical Sensors: Strategies, Materials, and Features

Stretchable and flexible sensors attached onto the surface of the human body can perceive external stimuli, thus attracting extensive attention due to their lightweight, low modulus, low cost, high flexibility, and stretchability. Recently, a myriad of efforts have been devoted to improving the performance and functionality of wearable sensors. Herein, this review focuses on recent remarkable advancements in the development of flexible and stretchable sensors. Multifunction of these wearable sensors is realized by incorporating some desired features (e.g., self-healing, self-powering, linearity, and printing). Next, focusing on the characteristics of carbon nanomaterials, nanostructured metal, conductive polymer, or their hybrid composites, two major strategies (e.g., materials that stretch and structures that stretch) and diverse design approaches have been developed to achieve highly flexible and stretchable electrodes. Strain sensing performances of recently reported sensors indicate that the appropriate choice of geometric engineering as well as intrinsically stretchable materials is essential for high-performance strain sensing. Finally, some important directions and challenges of a fully sensor-integrated wearable platform are proposed to realize their potential applications for human motion monitoring and human-machine interfaces.

Conformable and ionic textiles using sheath-core carbon nanotube microyarns for highly sensitive and reliable pressure sensors

Conformable and reliable ionic textiles on the basis of sheath-core carbon nanotube microyarns are utilized for implementing highly sensitive and reliable piezocapacitive pressure sensors.