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).

A Hierarchically Tailored Wrinkled Three-Dimensional Foam for Enhanced Elastic Supercapacitor Electrodes.

Recently, three-dimensional (3D) porous foams have been studied, but further improvement in nanoscale surface area and stretchability is required for electronic and energy applications. Herein, a general strategy is reported to form a tailored wrinkling structure on strut surfaces inside a 3D polydimethylsiloxane (PDMS) polymeric foam. Controlled wrinkles are created on the struts of 3D foam through an oxygen plasma treatment to form a bilayer surface of PDMS on uniaxially prestretched 3D PDMS foam, followed by relaxation. After plasma treatment for 1 h and prestretching of 40%, the wrinkled 3D foam greatly improves specific surface area and stretchability by over 60% and 75%, respectively, compared with the pristine 3D PDMS foam. To prove its applicability with improved performances, supercapacitors are prepared by coating a conductive material on the wrinkled 3D foam. The resulting supercapacitors exhibit an increased storage capacity (8.3 times larger), maintaining storage capacity well under stretching up to 50%.

Self-Powered Pressure- and Vibration-Sensitive Tactile Sensors for Learning Technique-Based Neural Finger Skin.

Finger skin electronics are essential for realizing humanoid soft robots and/or medical applications that are very similar to human appendages. A selective sensitivity to pressure and vibration that are indispensable for tactile sensing is highly desirable for mimicking sensory mechanoreceptors in skin. Additionally, for a human-machine interaction, output signals of a skin sensor should be highly correlated to human neural spike signals. As a demonstration of fully mimicking the skin of a human finger, we propose a self-powered flexible neural tactile sensor (NTS) that mimics all the functions of human finger skin and that is selectively and sensitively activated by either pressure or vibration stimuli with laminated independent sensor elements. A sensor array of ultrahigh-density pressure (20 × 20 pixels on 4 cm2) of interlocked percolative graphene films is fabricated to detect pressure and its distribution by mimicking slow adaptive (SA) mechanoreceptors in human skin. A triboelectric nanogenerator (TENG) was laminated on the sensor array to detect high-frequency vibrations like fast adaptive (FA) mechanoreceptors, as well as produce electric power by itself. Importantly, each output signal for the SA- and FA-mimicking sensors was very similar to real neural spike signals produced by SA and FA mechanoreceptors in human skin, thus making it easy to convert the sensor signals into neural signals that can be perceived by humans. By introducing microline patterns on the top surface of the NTS to mimic structural and functional properties of a human fingerprint, the integrated NTS device was capable of classifying 12 fabrics possessing complex patterns with 99.1% classification accuracy.

Serpentine-pattern effects on the biaxial stretching of percolative graphene nanoflake films.

Stretchable strain sensors based on percolative arrangements of conducting nanoparticles are essential tools in stretchable electronics and have achieved outstanding performance. Introducing serpentine patterns for strain-sensing materials is a very effective method for enhancing stretchability with a quantified structural resistance through a simple, reliable, and facile approach. Here, we investigate serpentine-pattern effects in the electrical responses to biaxial stretching for percolative graphene-nanoparticle films. Graphene nanoplatelet films are applied to a stretchable substrate using a facile spray-coating technique, for a variety of serpentine pattern shapes, aspect ratios, pattern frequencies, and number of coatings. The electrical responses after applying biaxial stretching (x-axis and y-axis) are measured and analyzed for comparison. The serpentine patterns that would be suitable for stretchable electrodes, sensitive sensors, and highly stretchable sensors are then identified. This work demonstrates the advantage of using serpentine patterns for stretchable strain sensors and offers guidelines for selecting suitable pattern types for strain sensors in stretchable-electronics applications.

A Stretchable Graphene Thin-Film Sensor for Detecting All of Lateral and Vertical Strains.

In this work, we propose a stretchable graphene film sensor that can detect all of lateral and vertical strain with unique architecture in single sensor element since most approaches so far are only available for detecting either lateral or vertical strain, but not both. The sensor is fabricated with percolative networks of graphene nanoplatelet using spray-coating method for constructing strain sensing channel and electrode simultaneously. The sensor exhibits a high stretchability of 150% with a gauge factor of 8.56 (0-100%) and 19.8 (100-150%) in the two regimes, for lateral strain. The sensor also presents a high sensitivity ((ΔR/R0)/ΔP of -0.026 kPa-1) for vertically applied pressure in the range of 100-20,000 Pa, belonging to general human pressure perception range. Based on the sensing properties demonstrated, the proposed graphene sensor is a promising candidate for sensor that can detect both lateral and vertical strains in single sensor element.

Ecoflex-Passivated Graphene-Yarn Composite for a Highly Conductive and Stretchable Strain Sensor.

We present a flexible strain sensor based on a graphene-yarn composite obtained by spray coating of graphene nanoplates. To improve the stretchability, graphene nanoplates were spray-coated instead of dip-coated on pre-stretched yarn. The spray-coating method yielded not only 3.68 times higher conductivity but also 2.1 times higher stretchability compared to the dip-coating method. The sensor spray-coated 400 times showed a high stretchability of 310%. Here, the relative resistance change (ΔR/R0) was 2.27 when a tensile strain of 50% was applied to the strain sensor. In addition, the fabricated sensor was coated with a protective layer of Ecoflex to minimize environmental effects. The passivated graphene-yarn composite sensor had a higher resistance than the unpassivated sensor because the Ecoflex film penetrated the conductive graphene nanoplates; however, the response to strains of up to 200% did not degrade after passivation. Furthermore, we demonstrated that our sensor can be used in wearable applications for monitoring individual finger movements and the wrist pulse.

Structurally Aligned Multifunctional Neural Probe (SAMP) Using Forest-Drawn CNT Sheet onto Thermally Drawn Polymer Fiber for Long-Term In Vivo Operation.

Neural probe engineering is a dynamic field, driving innovation in neuroscience and addressing scientific and medical demands. Recent advancements involve integrating nanomaterials to improve performance, aiming for sustained in vivo functionality. However, challenges persist due to size, stiffness, complexity, and manufacturing intricacies. To address these issues, a neural interface utilizing freestanding CNT-sheets drawn from CNT-forests integrated onto thermally drawn functional polymer fibers is proposed. This approach yields a device with structural alignment, resulting in exceptional electrical, mechanical, and electrochemical properties while retaining biocompatibility for prolonged periods of implantation. This Structurally Aligned Multifunctional neural Probe (SAMP) employing forest-drawn CNT sheets demonstrates in vivo capabilities in neural recording, neurotransmitter detection, and brain/spinal cord circuit manipulation via optogenetics, maintaining functionality for over a year post-implantation. The straightforward fabrication method's versatility, coupled with the device's functional reliability, underscores the significance of this technique in the next-generation carbon-based implants. Moreover, the device's longevity and multifunctionality position it as a promising platform for long-term neuroscience research.

Enhanced Hydro-Actuation and Capacitance of Electrochemically Inner-Bundle-Activated Carbon Nanotube Yarns.

Recently, several attempts have been made to activate or functionalize macroscopic carbon nanotube (CNT) yarns to enhance their innate abilities. However, a more homogeneous and holistic activation approach that reflects the individual nanotubes constituting the yarns is crucial. Herein, a facile strategy is reported to maximize the intrinsic properties of CNTs assembled in yarns through an electrochemical inner-bundle activation (EIBA) process. The as-prepared neat CNT yarns are two-end tethered and subjected to an electrochemical voltage (vs Ag/AgCl) in aqueous electrolyte systems. Massive electrolyte infiltration during the EIBA causes swelling of the CNT interlayers owing to the tethering and subsequent yarn shrinkage after drying, suggesting activation of the entire yarn. The EIBA-treated CNT yarns functionalized with oxygen-containing groups exhibit enhanced wettability without significant loss of their physical properties. The EIBA effect of the CNTs is experimentally demonstrated by hydration-driven torsional actuation (∼986 revolutions/m) and a drastic capacitance improvement (approximately 25-fold).

PdO-Nanoparticle-Embedded Carbon Nanotube Yarns for Wearable Hydrogen Gas Sensing Platforms with Fast and Sensitive Responses.

Hydrogen (H2) gas has recently become a crucial energy source and an imperative energy vector, emerging as a powerful next-generation solution for fuel cells and biomedical, transportation, and household applications. With increasing interest in H2, safety concerns regarding personal injuries from its flammability and explosion at high concentrations (>4%) have inspired the development of wearable pre-emptive gas monitoring platforms that can operate on curved and jointed parts of the human body. In this study, a yarn-type hydrogen gas sensing platform (HGSP) was developed by biscrolling of palladium oxide nanoparticles (PdO NPs) and spinnable carbon nanotube (CNT) buckypapers. Because of the high loading of H2-active PdO NPs (up to 97.7 wt %), when exposed to a flammable H2 concentration (4 vol %), the biscrolled HGSP yarn exhibits a short response time of 2 s, with a high sensitivity of 1198% (defined as ΔG/G0 × 100%). Interestingly, during the reduction of PdO to Pd by H2 gas, the HGSP yarn experienced a decrease in diameter and corresponding volume contraction. These excellent sensing performances suggest that the fabricated HGSP yarn could be applied to a wearable gas monitoring platform for real-time detection of H2 gas leakage even over the bends of joints.

Twist-Stabilized, Coiled Carbon Nanotube Yarns with Enhanced Capacitance.

Coil-structured carbon nanotube (CNT) yarns have recently attracted considerable attention. However, structural instability due to heavy twist insertion, and inherent hydrophobicity restrict its wider application. We report a twist-stable and hydrophilic coiled CNT yarn produced by the facile electrochemical oxidation (ECO) method. The ECO-treated coiled CNT yarn is prepared by applying low potentiostatic voltages (3.0-4.5 V vs Ag/AgCl) between the coiled CNT yarn and a counter electrode immersed in an electrolyte for 10-30 s. Notably, a large volume expansion of the coiled CNT yarns prepared by electrochemical charge injection produces morphological changes, such as surface microbuckling and large reductions in the yarn bias angle and diameter, resulting in the twist-stability of the dried ECO-treated coiled CNT yarns with increased yarn density. The resulting yarns are well functionalized with oxygen-containing groups; they exhibit extrinsic hydrophilicity and significantly improved capacitance (approximately 17-fold). We quantitatively explain the origin of the capacitance improvement using theoretical simulations and experimental observations. Stretchable supercapacitors fabricated with the ECO-treated coiled CNT yarns show high capacitance (12.48 mF/cm and 172.93 mF/cm2, respectively) and great stretchability (80%). Moreover, the ECO-treated coiled CNT yarns are strong enough to be woven into a mask as wearable supercapacitors.

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.

Bioinspired Hairy Skin Electronics for Detecting the Direction and Incident Angle of Airflow.

The human skin has inspired multimodal detection using smart devices or systems in fields including biomedical engineering, robotics, and artificial intelligence. Hairs of a high aspect ratio (AR) connected to follicles, in particular, detect subtle structural displacements by airflow or ultralight touch above the skin. Here, hairy skin electronics assembled with an array of graphene sensors (16 pixels) and artificial microhairs for multimodal detection of tactile stimuli and details of airflows (e.g., intensity, direction, and incident angle) are presented. Composed of percolation networks of graphene nanoplatelet sheets, the sensor array can simultaneously detect pressure, temperature, and vibration, all of which correspond to the sensing range of human tactile perceptions with ultrahigh response time (<0.5 ms, 2 kHz) for restoration. The device covered with microhairs (50 µm diameter and 300 µm height, AR = 6, hexagonal layout, and ∼4400/cm2) exhibits mapping of electrical signals induced by noncontact airflow and identifying the direction, incident angle, and intensity of wind to the sensor. For potential applications, we implement the hairy electronics to a sailing robot and demonstrate changes in locomotion and speed by detecting the direction and intensity of airflow.

Water-Resistant and Skin-Adhesive Wearable Electronics Using Graphene Fabric Sensor with Octopus-Inspired Microsuckers.

Wearable and skin-attachable electronics with portable/wearable and stretchable smart sensors are essential for health-care monitoring devices or systems. The property of adhesion to the skin in both dry and wet environments is strongly required for efficient monitoring of various human activities. We report here a facile, low-cost, scalable fabrication method for skin-adhesive graphene-coated fabric (GCF) sensors that are sensitive and respond fast to applied pressure and strain. With octopus-like patterns formed on the side of the GCF that touches the skin, the GCF adheres strongly to the skin in both dry and wet environments. Using these characteristics, we demonstrate efficient monitoring of a full range of human activities, including human physiological signals such as wrist pulse and electrocardiography (ECG), as well as body motions and speech vibrations. In particular, both measurements of ECG and wrist-bending motions were demonstrated even in wet conditions. Our approach has opened up a new possibility for wearable and skin-adherent electronic fabric sensors working even in wet environments for health-care monitoring and medical applications in vitro and in vivo.

Highly twisted supercoils for superelastic multi-functional fibres.

Highly deformable and electrically conductive fibres with multiple functionalities may be useful for diverse applications. Here we report on a supercoil structure (i.e. coiling of a coil) of fibres fabricated by inserting a giant twist into spandex-core fibres wrapped in a carbon nanotube sheath. The resulting supercoiled fibres show a highly ordered and compact structure along the fibre direction, which can sustain up to 1,500% elastic deformation. The supercoiled fibre exhibits an increase in resistance of 4.2% for stretching of 1,000% when overcoated by a passivation layer. Moreover, by incorporating pseudocapacitive-active materials, we demonstrate the existence of superelastic supercapacitors with high linear and areal capacitance values of 21.7 mF cm-1 and 92.1 mF cm-2, respectively, that can be reversibly stretched by 1,000% without significant capacitance loss. The supercoiled fibre can also function as an electrothermal artificial muscle, contracting 4.2% (percentage of loaded fibre length) when 0.45 V mm-1 is applied.

Recognition, classification, and prediction of the tactile sense.

The emulation of the tactile sense is presented with the encoding of a complex surface texture through an electrical sensor device. To achieve a functional capability comparable to a human mechanoreceptor, a tactile sensor is designed by employing a naturally formed porous structure of a graphene film. The inherent tactile patterns are achievable by means of proper analysis of the electrical signals that the sensor provides during the event of touching the interacting objects. It is confirmed that the pattern-recognition method using machine learning is suitable for quantifying human tactile sensations. The classification accuracy of the tactile sensor system is better than that of human touch for the tested fabric samples, which have a delicate surface texture.