Hierarchically Plied Mechano-Electrochemical Energy Harvesting Using a Scalable Kinematic Sensing Textile Woven from a Graphene-Coated Commercial Cotton Yarn.

Wearable sensing systems are suitable for monitoring human motion. To realize a cost-effective and self-powered strain-sensing fiber, we developed a mechano-electrochemical harvesting yarn and textile using hierarchically arranged plied yarns composed of meter-long graphene-coated cotton yarns. Such a fiber relies on the principle of electrochemical capacity change to convert mechanical energy to electric energy. Further, this harvester can be used as a self-powered strain sensor because its output depends on mechanical stimuli. Additionally, the yarn can be woven into a kinematic sensing textile that measures the strength and direction of the applied force. The textile-type harvester can successfully detect various human movements such as pressing, bending, and stretching. The proposed sensing fiber will pave the way for the development of advanced wearable systems for ubiquitous healthcare in the future.

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.

A transparent, glue-free, skin-attachable graphene pressure sensor with micropillars for skin-elasticity measurement.

Strong peeling resistance and water-drainable properties on rough and wet skin surfaces are highly desirable for realizing wearable and skin-attachable electronic sensors. Here, we propose a transparent, sensitive, glue-free pressure sensor for skin electronics. To achieve a thin, light-weight, transparent, and stretchable sensor patch, we laminated a single-layer graphene film as a sensing element on a thin polymeric supporter of polydimethylsiloxane. By assembling the graphene layer with densely populated micropillars, the pressure sensor achieved 10 times the sensitivity of a similar sensor without micropillars in the low-pressure range (<6 kPa). We then employed hexagonal patterns inspired by the toe pads of a tree frog, giving the assembled patch sensor with strong peeling resistance under both dry and wet conditions on surfaces such as silicon (15.5 J cm-2 for dry and 11.6 J cm-2 for wet conditions) and pig skin (2.0 J cm-2 for dry and 1.4 J cm-2 for wet conditions) without contamination after detachment. Our layered sensor patch also demonstrated successful measurement of water-dependent skin elasticity with transparent, conformal, and residual-free attachment, suggesting a variety of cosmetic and medical applications.

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.

A Flexible Graphene-Polydimethylsiloxane Nanocomposite Force Sensor with Linear Response Across a Wide Pressure Detection Range.

As a flexible force sensor operating in the pressure range covering the entire general human pressure detection range, we developed a piezoresistive nanocomposite using graphene flakes as the conducting filler with polydimethylsiloxane (PDMS) as the polymer matrix. The homogeneous dispersion of graphene flakes allows their uniform distribution in the PDMS matrix with a low percolation threshold owing to their geometrically high aspect ratio, thus resulting in a linear piezoresistive response across a wide pressure detection range (100 Pa-1,020 kPa), when static forces are externally applied. Furthermore, the sensor shows sensitive piezoresistive responses to dynamically applied forces. Based on the characteristics demonstrated and described in this study, graphene- PDMS nanocomposites can be considered promising materials for flexible force sensors capable of describing human pressure perception ability.

A Highly Efficient Absorbent for Various Organic Solvents Using Hydrophobic Graphene-Sponge Composite.

We propose a hydrophobic graphene-sponge composite produced by embedding graphene flakes within polyurethane sponge as a selective absorbent for hydrophobic liquids from a contaminated water mixture. The self-aggregation nature of graphene flakes effectively allows higher graphene content to be added inside the polyurethane sponge with repeated dip-coating. High hydrophobicity due to the intrinsic nature graphene surface allows the selective absorption of liquid contaminants from a water mixture. Given the porous structure of the composite, absorption capability of 134-233 times its own weight towards various hydrophobic liquids is possible.

Optical and Electrical Characteristics of Graphene Double Layer Formed by a Double Transfer of Graphene Single Layers.

We demonstrate formation of double layer graphene by means of a double transfer using two single graphene layers grown by a chemical vapor deposition method. It is observed that shiftiness and broadness in the double-resonance of Raman scattering are much weaker than those of bilayer graphene formed naturally. Transport characteristics examined from transmission line measurements and field effect transistors show the similar behavior with those of single layer graphene. It indicates that interlayer separation, in electrical view, is large enough to avoid correlation between layers for the double layer structure. It is also observed from a transistor with the double layer graphene that molecules adsorpted on two inner graphene surfaces in the double layered structure are isolated and conserved from ambient environment.

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.

A Tissue Adhesion-Controllable and Biocompatible Small-Scale Hydrogel Adhesive Robot.

Recently, the realization of minimally invasive medical interventions on targeted tissues using wireless small-scale medical robots has received an increasing attention. For effective implementation, such robots should have a strong adhesion capability to biological tissues and at the same time easy controlled detachment should be possible, which has been challenging. To address such issue, a small-scale soft robot with octopus-inspired hydrogel adhesive (OHA) is proposed. Hydrogels of different Young's moduli are adapted to achieve a biocompatible adhesive with strong wet adhesion by preventing the collapse of the octopus-inspired patterns during preloading. Introduction of poly(N-isopropylacrylamide) hydrogel for dome-like protuberance structure inside the sucker wall of polyethylene glycol diacrylate hydrogel provides a strong tissue attachment in underwater and at the same time enables easy detachment by temperature changes due to its temperature-dependent volume change property. It is finally demonstrated that the small-scale soft OHA robot can efficiently implement biomedical functions owing to strong adhesion and controllable detachment on biological tissues while operating inside the body. Such robots with repeatable tissue attachment and detachment possibility pave the way for future wireless soft miniature robots with minimally invasive medical interventions.

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.

Fabrication of a flexible and transparent touch sensor using single-walled carbon nanotube thin-films.

Reported herein are the fabrication and demonstration of a flexible and transparent touch sensor using carbon nanotube thin films (CNTFs). The CNTF was fabricated by vacuum filtration and was transferred CNTF to polydimethylsiloxane (PDMS) by water-assisted stamping method. The sheet resistance of the CNTF decreased by approximately 74% after HNO3 treatment. The CNTF touch sensor was fabricated similarly to the conventional four-wire touch screen structures. PDMS was used for the upper plate to absorb the tensile and compressive strain and polyethylene terephthalate (PET) for the lower plate to provide device stability during bending action. The CNTF touch sensor showed high optical transmittance (over 80%) and high sensitivity with the measured touch activation pressure of 23 kPa. Cyclic pressure (38 kPa) was applied at 0.5 Hz and good repeatability was found for several hundred cycles. The results show that the CNTF flexible touch sensor can be applied to future flexible electronic interfaces such as, e-paper and flexible displays.

An Open Source-Based BCI Application for Virtual World Tour and Its Usability Evaluation.

Brain-computer interfaces can provide a new communication channel and control functions to people with restricted movements. Recent studies have indicated the effectiveness of brain-computer interface (BCI) applications. Various types of applications have been introduced so far in this field, but the number of those available to the public is still insufficient. Thus, there is a need to expand the usability and accessibility of BCI applications. In this study, we introduce a BCI application for users to experience a virtual world tour. This software was built on three open-source environments and is publicly available through the GitHub repository. For a usability test, 10 healthy subjects participated in an electroencephalography (EEG) experiment and evaluated the system through a questionnaire. As a result, all the participants successfully played the BCI application with 96.6% accuracy with 20 blinks from two sessions and gave opinions on its usability (e.g., controllability, completeness, comfort, and enjoyment) through the questionnaire. We believe that this open-source BCI world tour system can be used in both research and entertainment settings and hopefully contribute to open science in the BCI field.

3D Printing of Elastomeric Bioinspired Complex Adhesive Microstructures.

Bioinspired elastomeric structural adhesives can provide reversible and controllable adhesion on dry/wet and synthetic/biological surfaces for a broad range of commercial applications. Shape complexity and performance of the existing structural adhesives are limited by the used specific fabrication technique, such as molding. To overcome these limitations by proposing complex 3D microstructured adhesive designs, a 3D elastomeric microstructure fabrication approach is implemented using two-photon-polymerization-based 3D printing. A custom aliphatic urethane-acrylate-based elastomer is used as the 3D printing material. Two designs are demonstrated with two combined biological inspirations to show the advanced capabilities enabled by the proposed fabrication approach and custom elastomer. The first design focuses on springtail- and gecko-inspired hybrid microfiber adhesive, which has the multifunctionalities of side-surface liquid super-repellency, top-surface liquid super-repellency, and strong reversible adhesion features in a single fiber array. The second design primarily centers on octopus- and gecko-inspired hybrid adhesive, which exhibits the benefits of both octopus- and gecko-inspired microstructured adhesives for strong reversible adhesion on both wet and dry surfaces, such as skin. This fabrication approach could be used to produce many other 3D complex elastomeric structural adhesives for future real-world applications.

A Micropillar-Assisted Versatile Strategy for Highly Sensitive and Efficient Triboelectric Energy Generation under In-Plane Stimuli.

For the application of portable and wearable devices, the development of energy harvesters sensitive to various types of local and subtle mechanical displacements is essential. One of the most abundant but difficult-to-harvest mechanical energies in everyday life is the in-plane kinetic energy that arises from a rubbing motion. Here, an efficient method is proposed to generate electrical energy from tiny horizontal forces by laminating microstructures on a conventional triboelectric nanogenerator (TENG). The microhairy structures serve to induce contact friction between the two dielectric materials, driven by reversible mechanical bending when a contact rubbing pressure or noncontact airflow is applied in the horizontal direction. Compared to TENG devices without microstructures, the introduction of microstructures greatly enhances the energy harvesting in the same situation. In addition, the TENG device with micropillars can generate electrical output under tiny mechanical variations (<0.2 Pa) induced by a local deformation below individual micropillars. A high energy-generation capability is demonstrated by rubbing textured samples on the micropillar-structured TENG devices to induce horizontal contact friction. The devices can also efficiently harvest electrical energy from noncontact fluidic airflow. By assembling the microhairy structures on a conventional TENG, more complex and realistic mechanical motion can be harvested.

Highly Air/Water-Permeable Hierarchical Mesh Architectures for Stretchable Underwater Electronic Skin Patches.

The development of an electronic skin patch that can be used in underwater environments can be considered essential for fabricating long-term wearable devices and biomedical applications. Herein, we report a stretchable conductive polymer composite (CPC) patch on which an octopus sucker-inspired structure is formed to conformally contact with biological skin that may be rough and wet. The patch is patterned with a hexagonal mesh structure for water and air permeability. The patch films are suited for a strain sensor or a stretchable electrode as their piezoresistive responses can be controlled by changing the concentration of conductive fillers to polymeric polyurethane. The CPC patch with a hexagonal mesh pattern (HMP) can be easily stretched for a strain sensor and is insensitive to tensile strain, making the patch suitable as a stretchable electrode. Furthermore, the octopus-like structures formed on the skeleton of the HMP allow the patch to maintain strong adhesion underwater by easily draining excess water trapped between the patch and skin. The sensor patch (<50 wt % carbon nanotubes (CNTs)) can sensitively detect the bending strain of a finger, and the electrode patch (50 wt % CNTs with addition of Ag flakes) can stably measure biosignals (e.g., electrocardiogram signals) under both dry and wet conditions owing to the octopus-like structure and HMP.

Carbon-Based, Ultraelastic, Hierarchically Coated Fiber Strain Sensors with Crack-Controllable Beads.

Fiber-based electronics or textronics are spotlighted as a promising strategy to develop stretchable and wearable devices for conformable machine-human interface and ubiquitous healthcare systems. We have prepared a highly sensitive fiber-type strain sensor (maximum gauge factor (GF) = 863) with a broad range of strain (ε < 400%) by introducing a single active layer onto the fiber. In contrast to other metal-based fiber-type electronics, our hierarchical fiber sensors are based on coating carbon-based nanomaterials with responsive microbeads onto elastic fibers. Utilizing the formation of uniform cracks around the microbeads, the device performance was maximized by adjusting the number of microbeads in the carbon-coating layer. We overcoated the carbon-based coating layer of the elastic fiber with a protective polymeric layer and verified no effects on the GF and the range of strain. Our fiber sensors were repeatedly tested more than 5000 times, exhibiting excellent cyclic responses to on/off switching behaviors. For practical applications, the hierarchical fiber sensors were sewed into electrical fabric bands, which are integrable to a wireless transmitter to monitor waveforms of pulsations, respirations, and various postures of level of bending a spinal cord.