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).

Twistable and Stretchable Sandwich Structured Fiber for Wearable Sensors and Supercapacitors.

Twistable and stretchable fiber-based electrochemical devices having high performance are needed for future applications, including emerging wearable electronics. Weavable fiber redox supercapacitors and strain sensors are here introduced, which comprise a dielectric layer sandwiched between functionalized buckled carbon nanotube electrodes. On the macroscopic scale, the sandwiched core rubber of the fiber acts as a dielectric layer for capacitive strain sensing and as an elastomeric substrate that prevents electrical shorting and irreversible structural changes during severe mechanical deformations. On the microscopic scale, the buckled CNT electrodes effectively absorb tensile or shear stresses, providing an essentially constant electrical conductance. Consequently, the sandwich fibers provide the dual functions of (1) strain sensing, by generating approximately 115.7% and 26% capacitance changes during stretching (200%) and giant twist (1700 rad·m-1 or 270 turns·m-1), respectively, and (2) electrochemical energy storage, providing high linear and areal capacitances (2.38 mF·cm-1 and 11.88 mF·cm-2) and retention of more than 95% of initial energy storage capability under large mechanical deformations.

Free-standing nanocomposites with high conductivity and extensibility.

The prospect of electronic circuits that are stretchable and bendable promises tantalizing applications such as skin-like electronics, roll-up displays, conformable sensors and actuators, and lightweight solar cells. The preparation of highly conductive and highly extensible materials remains a challenge for mass production applications, such as free-standing films or printable composite inks. Here we present a nanocomposite material consisting of carbon nanotubes, ionic liquid, silver nanoparticles, and polystyrene-polyisoprene-polystyrene having a high electrical conductivity of 3700 S cm(-1) that can be stretched to 288% without permanent damage. The material is prepared as a concentrated dispersion suitable for simple processing into free-standing films. For the unstrained state, the measured thermal conductivity for the electronically conducting elastomeric nanoparticle film is relatively high and shows a non-metallic temperature dependence consistent with phonon transport, while the temperature dependence of electrical resistivity is metallic. We connect an electric fan to a DC power supply using the films to demonstrate their utility as an elastomeric electronic interconnect. The huge strain sensitivity and the very low temperature coefficient of resistivity suggest their applicability as strain sensors, including those that operate directly to control motors and other devices.

Omnidirectional Energy Harvesting Fleeces.

Underwater mechanical energy harvesters are of rising interest due to their potential for various applications, such as self-powered ocean energy harvesters, monitoring devices, and wave sensors. Pressure-responsive films and stretch-responsive fibers, which provide high electrical power in electrolytes and have simple structures that do not require packing systems, are promising as harvesters in the ocean environment. One drawback of underwater mechanical energy harvesters is that they are highly dependent on the direction of receiving external forces, which is unfavorable in environments where the direction of the supplied force is constantly changing. Here, we report spherical fleece, consisting of wool fibers and single-walled carbon nanotubes (SWCNTs), which exhibit repetitive electrical currents in all directions. No matter which direction the fleece is deformed, it changes the surface area available for ions to access SWCNTs electrochemically, causing a piezoionic phenomenon. The current per input mechanical stress of the fabricated SWCNT/wool energy harvester is up to 33.476 mA/MPa, which is the highest among underwater mechanical energy harvesters reported to date. In particular, it is suitable for low-frequency (<1 Hz) environments, making it ideal for utilizing natural forces such as wind and waves as harvesting sources. The operating mechanism in the nanoscale region of the proposed fleece harvester has been theoretically elucidated through all-atom molecular dynamics simulations.

Recent Advances in Carbon Nanotube-Based Energy Harvesting Technologies.

There has been enormous interest in technologies that generate electricity from ambient energy such as solar, thermal, and mechanical energy, due to their potential for providing sustainable solutions to the energy crisis. One driving force behind the search for new energy-harvesting technologies is the desire to power sensor networks and portable devices without batteries, such as self-powered wearable electronics, human health monitoring systems, and implantable wireless sensors. Various energy harvesting technologies have been demonstrated in recent years. Among them, electrochemical, hydroelectric, triboelectric, piezoelectric, and thermoelectric nanogenerators have been extensively studied because of their special physical properties, ease of application, and sometimes high obtainable efficiency. Multifunctional carbon nanotubes (CNTs) have attracted much interest in energy harvesting because of their exceptionally high gravimetric power outputs and recently obtained high energy conversion efficiencies. Further development of this field, however, still requires an in-depth understanding of harvesting mechanisms and boosting of the electrical outputs for wider applications. Here, various CNT-based energy harvesting technologies are comprehensively reviewed, focusing on working principles, typical examples, and future improvements. The last section discusses the existing challenges and future directions of CNT-based energy harvesters.

Plastic Shavings by Laser: Peeling Porous Graphene Springs for Multifunctional All-Carbon Applications.

Manufacturing strategies to create three-dimensional (3D) structures with multifunctional nanomaterials are of intense interest for fabricating building blocks in many electromechanical applications. A coil spring composed of graphene provides an important step toward the realization of all-carbon devices, as it is one of the essential elements for a wide range of systems. In this connection, here an unprecedented fabrication strategy to create a new type of 3D coil spring composed of laser-induced graphene springs (LIG-S) which is spontaneously produced via the pyrolytic jetting technique, is presented. Similar to wood or metal shavings observed in traditional machining processes, a pair of LIG-S with two opposite chiralities and controllable macroscopic dimensions is produced by a single scanning of a focused continuous-wave (CW) laser on a polyimide (PI) substrate. The resulting LIG-S, plastic shavings by laser, exhibits sufficient mechanical and electrical properties to enable many applications including strain-tolerant spring electrodes, antennas, supercapacitors, gas sensors, and luminescent filaments under extreme conditions. Without using any conventional fabrication techniques or other labor-intensive, time-consuming, and expensive processes, this novel approach enables a high-throughput mass production of macro-, micro-, and nanoscale featured LIG-S that can be manufactured within seconds to realize many open opportunities in all-carbon electromechanical systems.

Chemo-Mechanical Energy Harvesters with Enhanced Intrinsic Electrochemical Capacitance in Carbon Nanotube Yarns.

Predicting and preventing disasters in difficult-to-access environments, such as oceans, requires self-powered monitoring devices. Since the need to periodically charge and replace batteries is an economic and environmental concern, energy harvesting from external stimuli to supply electricity to batteries is increasingly being considered. Especially, in aqueous environments including electrolytes, coiled carbon nanotube (CNT) yarn harvesters have been reported as an emerging approach for converting mechanical energy into electrical energy driven by large and reversible capacitance changes under stretching and releasing. To realize enhanced harvesting performance, experimental and computational approaches to optimize structural homogeneity and electrochemical accessible area in CNT yarns to maximize intrinsic electrochemical capacitance (IEC) and stretch-induced changes are presented here. Enhanced IEC further enables to decrease matching impedance for more energy efficient circuits with harvesters. In an ocean-like environment with a frequency from 0.1 to 1 Hz, the proposed harvester demonstrates the highest volumetric power (1.6-10.45 mW cm-3 ) of all mechanical harvesters reported in the literature to the knowledge of the authors. Additionally, a high electrical peak power of 540 W kg-1 and energy conversion efficiency of 2.15% are obtained from torsional and tensile mechanical energy.

Single-Layer Graphene-Based Transparent and Flexible Multifunctional Electronics for Self-Charging Power and Touch-Sensing Systems.

Applications in the field of portable and wearable electronics are becoming multifunctional, and the achievement of transparent electronics extensively expands the applications into devices such as wearable flexible displays or skin-attachable mobile computers. Moreover, the self-charging power system (SCPS) is the core technique for realizing portable and wearable electronics. Here, we propose a transparent and flexible multifunctional electronic system in which both an all-in-one SCPS and a touch sensor are combined. A single-layer graphene (SLG) film was adapted as an electrode for the supercapacitor, touch sensor, and a triboelectric nanogenerator (TENG), thus making an electronic system that is ultrathin, lightweight, transparent, and flexible. Capacitive-type transparent and flexible electronic devices can be simultaneously used as an electrochemical double-layer capacitance-based supercapacitor and as a sensitive, fast-responding touch sensor in a single-device architecture by inserting a separator of polyvinyl alcohol-lithium chloride-soaked polyacrylonitrile electrospun mat on polyethylene naphthalate between two symmetric SLG film electrodes. Furthermore, a transparent all-in-one SCPS was fabricated by laminating a TENG device with a supercapacitor, and high-performance electric power generation/storage capability is demonstrated.

Sheath-run artificial muscles.

Although guest-filled carbon nanotube yarns provide record performance as torsional and tensile artificial muscles, they are expensive, and only part of the muscle effectively contributes to actuation. We describe a muscle type that provides higher performance, in which the guest that drives actuation is a sheath on a twisted or coiled core that can be an inexpensive yarn. This change from guest-filled to sheath-run artificial muscles increases the maximum work capacity by factors of 1.70 to 2.15 for tensile muscles driven electrothermally or by vapor absorption. A sheath-run electrochemical muscle generates 1.98 watts per gram of average contractile power-40 times that for human muscle and 9.0 times that of the highest power alternative electrochemical muscle. Theory predicts the observed performance advantages of sheath-run muscles.

Magnetic torsional actuation of carbon nanotube yarn artificial muscle.

Magnetically driven torsional actuation of a multiwalled carbon nanotube (MWNT) yarn was realized by first biscrolling NdFeB magnetic particles into helical yarn corridors to make a magnetic MWNT yarn. The actuating device comprised a pristineMWNT yarn that was connected to the magnetic MWNT yarn, with a paddle attached between these yarns. The application of a magnetic field reversibly drove torsional actuation of up to 80° within ∼0.67 seconds. This magnetic actuator was remotely powered, and its actuation stroke was the same when the muscle array was at 20 °C and at -100 °C.

Harvesting electrical energy from torsional thermal actuation driven by natural convection.

The development of practical, cost-effective systems for the conversion of low-grade waste heat to electrical energy is an important area of renewable energy research. We here demonstrate a thermal energy harvester that is driven by the small temperature fluctuations provided by natural convection. This harvester uses coiled yarn artificial muscles, comprising well-aligned shape memory polyurethane (SMPU) microfibers, to convert thermal energy to torsional mechanical energy, which is then electromagnetically converted to electrical energy. Temperature fluctuations in a yarn muscle, having a maximum hot-to-cold temperature difference of about 13 °C, were used to spin a magnetic rotor to a peak torsional rotation speed of 3,000 rpm. The electromagnetic energy generator converted the torsional energy to electrical energy, thereby producing an oscillating output voltage of up to 0.81 V and peak power of 4 W/kg, based on SMPU mass.

Weavable asymmetric carbon nanotube yarn supercapacitor for electronic textiles.

Asymmetric supercapacitors are receiving much research interests due to their wide operating potential window and high energy density. In this study, we report the fabrication of asymmetrically configured yarn based supercapacitor by using liquid-state biscrolling technology. High loading amounts of reduced graphene oxide anode guest (90.1 wt%) and MnO2 cathode guest (70 wt%) materials were successfully embedded into carbon nanotube yarn host electrodes. The resulting asymmetric yarn supercapacitor coated by gel based organic electrolyte (PVDF-HFP-TEA·BF4) exhibited wider potential window (up to 3.5 V) and resulting high energy density (43 µW h cm-2). Moreover, the yarn electrodes were mechanically strong enough to be woven into commercial textiles. The textile supercapacitor exhibited stable electrochemical energy storage performances during dynamically applied deformations.

Harvesting electrical energy from carbon nanotube yarn twist.

Mechanical energy harvesters are needed for diverse applications, including self-powered wireless sensors, structural and human health monitoring systems, and the extraction of energy from ocean waves. We report carbon nanotube yarn harvesters that electrochemically convert tensile or torsional mechanical energy into electrical energy without requiring an external bias voltage. Stretching coiled yarns generated 250 watts per kilogram of peak electrical power when cycled up to 30 hertz, as well as up to 41.2 joules per kilogram of electrical energy per mechanical cycle, when normalized to harvester yarn weight. These energy harvesters were used in the ocean to harvest wave energy, combined with thermally driven artificial muscles to convert temperature fluctuations to electrical energy, sewn into textiles for use as self-powered respiration sensors, and used to power a light-emitting diode and to charge a storage capacitor.

Bio-inspired, Moisture-Powered Hybrid Carbon Nanotube Yarn Muscles.

Hygromorph artificial muscles are attractive as self-powered actuators driven by moisture from the ambient environment. Previously reported hygromorph muscles have been largely limited to bending or torsional motions or as tensile actuators with low work and energy densities. Herein, we developed a hybrid yarn artificial muscle with a unique coiled and wrinkled structure, which can be actuated by either changing relative humidity or contact with water. The muscle provides a large tensile stroke (up to 78%) and a high maximum gravimetric work capacity during contraction (2.17 kJ kg(-1)), which is over 50 times that of the same weight human muscle and 5.5 times higher than for the same weight spider silk, which is the previous record holder for a moisture driven muscle. We demonstrate an automatic ventilation system that is operated by the tensile actuation of the hybrid muscles caused by dew condensing on the hybrid yarn. This self-powered humidity-controlled ventilation system could be adapted to automatically control the desired relative humidity of an enclosed space.

Stretchable Triboelectric Fiber for Self-powered Kinematic Sensing Textile.

Stretchable fiber and yarn triboelectric nanogenerator are sought for such applications as wearable sensing system such as cloth communication devices, electronic textiles, and robotic sensory skin. Unfortunately, previously reported triboelectric fiber and yarn are difficult to have stretchable property. We introduce here a new type of stretchable and weavable triboelectric fibers with microdiameter dimensions. The stretchable triboelectric fibers can be reversibly stretched up to 50% in tensile direction while generating voltage output proportional to the applied tensile strain. The reversible distance change induced by the Poisson's ratio difference between the core fiber (silver-coated nylon/polyurethane) and the shell (wrinkled polyvinylidene fluoride-co-trifluoroethylene/carbon nanotube layer) during tensile deformation is the key working principle for electrical generation. Owing to exceptional structural stability, the stretchable triboelectric fibers show high performance retention after 10,000 times repeated stretching/releasing cycle. Furthermore, the stretchable triboelectric fibers are mechanically strong to be woven into a commercial textile for textile based sensors, which can detect magnitude as well as direction of the motion.

Torsional behaviors of polymer-infiltrated carbon nanotube yarn muscles studied with atomic force microscopy.

Torsional behaviors of polymer-infiltrated carbon nanotube (CNT) yarn muscles have been investigated in relation to molecular architecture by using atomic force microscopy (AFM). Two polymers with different stiffnesses, polystyrene (PS) and poly(styrene-b-isoprene-b-styrene) (SIS), were uniformly infiltrated into CNT yarns for electrothermal torsional actuation. The torsional behaviors of hybrid yarn muscles are completely explained by the volume change of each polymer, based on the height and full width at half maximum profiles from the AFM morphological images. The volume expansion of the PS yarn muscle (1.7 nm of vertical change and 22 nm of horizontal change) is much larger than that of the SIS yarn muscle (0.3 nm and 11 nm change in vertical and horizontal directions) at 80 °C, normalized by their values at 25 °C. We demonstrate that their maximum rotations are consequently 29.7 deg mm(-1) for the PS-infiltrated CNT yarn muscle (relatively larger rotation) and 14.4 deg mm(-1) for the SIS-infiltrated CNT yarn muscle (smaller rotation) at 0.75 V m(-1). These hybrid yarn muscles could be applied in resonant controllers or damping magnetoelectric sensors.

Stretchable, weavable coiled carbon nanotube/MnO2/polymer fiber solid-state supercapacitors.

Fiber and yarn supercapacitors that are elastomerically deformable without performance loss are sought for such applications as power sources for wearable electronics, micro-devices, and implantable medical devices. Previously reported yarn and fiber supercapacitors are expensive to fabricate, difficult to upscale, or non-stretchable, which limits possible use. The elastomeric electrodes of the present solid-state supercapacitors are made by using giant inserted twist to coil a nylon sewing thread that is helically wrapped with a carbon nanotube sheet, and then electrochemically depositing pseudocapacitive MnO2 nanofibers. These solid-state supercapacitors decrease capacitance by less than 15% when reversibly stretched by 150% in the fiber direction, and largely retain capacitance while being cyclically stretched during charge and discharge. The maximum linear and areal capacitances (based on active materials) and areal energy storage and power densities (based on overall supercapacitor dimensions) are high (5.4 mF/cm, 40.9 mF/cm(2), 2.6 µWh/cm(2) and 66.9 µW/cm(2), respectively), despite the engineered superelasticity of the fiber supercapacitor. Retention of supercapacitor performance during large strain (50%) elastic deformation is demonstrated for supercapacitors incorporated into the wristband of a glove.

High-power biofuel cell textiles from woven biscrolled carbon nanotube yarns.

Biofuel cells that generate electricity from glucose in blood are promising for powering implantable biomedical devices. Immobilizing interconnected enzyme and redox mediator in a highly conducting, porous electrode maximizes their interaction with the electrolyte and minimizes diffusion distances for fuel and oxidant, thereby enhancing power density. Here we report that our separator-free carbon nanotube yarn biofuel cells provide an open-circuit voltage of 0.70 V, and a maximum areal power density of 2.18 mW cm(-2) that is three times higher than for previous carbon nanotube yarn biofuel cells. Biofuel cell operation in human serum provides high areal power output, as well as markedly increased lifetime (83% remained after 24 h), compared with previous unprotected biofuel cells. Our biscrolled yarn biofuel cells are woven into textiles having the mechanical robustness needed for implantation for glucose energy harvesting.

Artificial muscles from fishing line and sewing thread.

The high cost of powerful, large-stroke, high-stress artificial muscles has combined with performance limitations such as low cycle life, hysteresis, and low efficiency to restrict applications. We demonstrated that inexpensive high-strength polymer fibers used for fishing line and sewing thread can be easily transformed by twist insertion to provide fast, scalable, nonhysteretic, long-life tensile and torsional muscles. Extreme twisting produces coiled muscles that can contract by 49%, lift loads over 100 times heavier than can human muscle of the same length and weight, and generate 5.3 kilowatts of mechanical work per kilogram of muscle weight, similar to that produced by a jet engine. Woven textiles that change porosity in response to temperature and actuating window shutters that could help conserve energy were also demonstrated. Large-stroke tensile actuation was theoretically and experimentally shown to result from torsional actuation.

Ultrafast charge and discharge biscrolled yarn supercapacitors for textiles and microdevices.

Flexible, wearable, implantable and easily reconfigurable supercapacitors delivering high energy and power densities are needed for electronic devices. Here we demonstrate weavable, sewable, knottable and braidable yarns that function as high performance electrodes of redox supercapacitors. A novel technology, gradient biscrolling, provides fast-ion-transport yarn in which hundreds of layers of conducting-polymer-infiltrated carbon nanotube sheet are scrolled into ~20 µm diameter yarn. Plying the biscrolled yarn with a metal wire current collector increases power generation capabilities. The volumetric capacitance is high (up to ~179 F cm(-3)) and the discharge current of the plied yarn supercapacitor linearly increases with voltage scan rate up to ~80 V s(-1) and ~20 V s(-1) for liquid and solid electrolytes, respectively. The exceptionally high energy and power densities for the complete supercapacitor, and high cycle life that little depends on winding or sewing (92%, 99% after 10,000 cycles, respectively) are important for the applications in electronic textiles.