Search

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).

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/cm

Nanomaterials-based flexible sensor array has been receiving tremendous interest for uses in the fields of wearable devices, healthcare monitoring, and biomedical application. However, flexible sensor technology, whose active sensing element is introduced by nanomaterials, presents common practical problems such as mismatch and deviation in integration for sensor array architecture. In particular, sensor elements in array that are attached to the skin and then experience different bending strains suffer from non-uniform pressure responses, thus being a significant obstacle to commercialization. To provide a solution to these problems, we here propose an electrical circuit method for obtaining uniform pressure responses in nanomaterials-based on-skin flexible pressure sensor array. The proposed method employed by cubic spline interpolation achieve a wide range of calibration for operation in sensor array. The uniform responses were confirmed in a 4 × 4 pressure sensor array with the piezo-resistive characteristics due to graphene nanoplatelet (GNP) used as active material. The calibrated responses are demonstrated on the flexible sensor array attached to a human arm having uneven surface curvatures on which each sensor in the array may experience different bending strain. This work has opened the way to give reliable responses in resistance for wearable electronic sensor array attached to the body despite different external stress, and is an approach that can be extended to various sensor technologies.

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%.

Humans detect tactile stimuli through a combination of pressure and vibration signals using different types of cutaneous receptor. The development of artificial tactile perception systems is of interest in the development of robotics and prosthetics, and artificial receptors, nerves and skin have been created. However, constructing systems with human-like capabilities remains challenging. Here, we report an artificial neural tactile skin system that mimics the human tactile recognition process using particle-based polymer composite sensors and a signal-converting system. The sensors respond to pressure and vibration selectively, similarly to slow adaptive and fast adaptive mechanoreceptors in human skin, and can generate sensory neuron-like output signal patterns. We show in an ex vivo test that undistorted transmission of the output signals through an afferent tactile mouse nerve fibre is possible, and in an in vivo test that the signals can stimulate a rat motor nerve to induce the contraction of a hindlimb muscle. We use our tactile sensing system to develop an artificial finger that can learn to classify fine and complex textures by integrating the sensor signals with a deep learning technique. The approach can also be used to predict unknown textures on the basis of the trained model. A tactile sensing system that can learn to identify different types of surface can be created using sensors that mimic the fast and slow responses of mechanoreceptors found in human skin.

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., Advanced Materials, 34 (13), ISSN:0935-9648, ISSN:1521-4095

We present a high-performance flexible triboelectric nanogenerator (TENG) based on an interlocked array of surface-functionalized indium tin oxide (ITO) nanohelix (NH) structures. The structural pr...

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.

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.

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.

Carbon allotropes are strong candidates for pressure sensing materials in flexible electronics due to their extraordinary mechanical and electrical properties. However, the complexity of the conventional transfer process for these allotropes, the success of which is strongly dependent on the surface conditions of the substrates, limits their feasibility for use as pressure sensors. Thus, we propose a method to create flexible pressure sensors using highly-oriented and free-standing hydrophobic carbon nanotube (CNT) sheets. When drawn from a sidewall of a carbon nanotube forest, these sheets only require a single transfer process without any chemical treatment, thereby facilitating a simple, cost-effective transfer method. The resulting sensors exhibit high sensitivity and fast response characteristics for both statically and dynamically applied pressures. In addition, the highly-oriented structure of these CNT sheets results in distinctive response characteristics for bi-axially applied bending strains. It was also confirmed that the sheets can be easily transferred onto any substrate, including those with rough surfaces, due to the naturally formed free-standing structure. In this work, we present the intact transfer of a CNT sheet onto a micro-patterned substrate representing a rough surface and demonstrate the accompanying typical piezoresistive responses of the resulting pressure sensor.

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., Advanced Materials, 33 (40), ISSN:0935-9648, ISSN:1521-4095

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 (

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.

Stretchable yarn/fiber energy storage devices with a high energy density are highly beneficial for use in wearable applications. Although some studies on stretchable fiber batteries have been conducted, the development of fiber batteries that combine outstanding stretchability with a high energy storage capacity has been restricted by the tradeoff between stretchability and the active material content. In the present study, a DNA-inspired supercoil battery is proposed and fabricated via the secondary twisting (i.e. supercoiling) of a fiber electrode consisting of an elastomeric core fiber and a conductive sheath containing nano-sized active materials. The structural changes induced by supercoiling facilitate the effective packing of the electrode into coil and supercoil loops that represent only 29% of the initial length. The resulting highly packed supercoil battery exhibits an outstanding stretchability (800%) and linear capacity (0.029 mAh/cm) compared to non-coil, coil, and previously reported stretchable yarn/fiber-based batteries.

We propose a transparent and stretchable all-graphene strain sensor that can detect various types of strain induced via stretching, bending, and torsion. The sensor is fabricated by introducing single-layer graphene as a force sensing material with a conductive film composed of graphene flake for the electrode. With the exclusive use of flexible materials, the completed strain sensor is fully flexible. Using a serpentine-shaped pattern for the single-layer graphene, the sensor is capable of stretching up to 20% with a high gauge factor (42.2). In addition, the sensor provides functional extension to bi-directional responses. This sensor can detect infinitesimal strain as low as 0.1% with a relative resistance change (ΔR/R0) of ∼0.005. Finally, sensitive detection of strains induced via bending and torsion are successfully demonstrated.

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 (

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.

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 (

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.

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 (ε400%) 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.

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 (

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.

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., The development of electrically conductive fibres is attractive for wearable electronics, but performance should be maintained upon deformation and tensile strain. Here the authors fabricate flexible, stretchable, carbon nanotube-coated spandex fibres for supercapacitors and artificial muscles.

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.

Detecting variation in contact pressure is a separate sensing mode in the human somatosensory system that differs from the detection of pressure magnitude. If pressure magnitude and variation sensing can be achieved simultaneously, an advanced biomimetic tactile system that better emulates human senses may be developed. We report on a novel single-layer graphene based artificial mechanoreceptor that generates a resistance pulse as the contact stimulus passes a specific threshold pressure, mimicking the generation of action potentials in a biological fast-adapting mechanoreceptor. The electric field from a flexible membrane gate electrode placed above a graphene channel raises the Fermi level from the valence band as pressure deflects the membrane. The threshold pressure is reached when the Fermi level crosses the Dirac point in the graphene energy band, which generates a sharp peak in the measured resistance. We found that by changing the gate potential it was possible to modulate the threshold pressure and using a series of graphene channels, a train of pulses were generated during a transient pressurizing stimulus demonstrating biomimetic behaviour.

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.

In this paper, we propose a graphene sensor using two separated single-layered graphenes on a flexible substrate for use as a pressure sensor, such as for soft electronics. The working pressure corresponds to the range in which human perception recognizes surface morphologies. A specific design of the sensor structure drives the piezoresistive character due to the contact resistance between two graphene layers and the electromechanical properties of graphene itself. Accordingly, sensitivity in resistance change is given by two modes for low pressure (-0.24 kPa(-1)) and high pressure (0.039 kPa(-1)) with a crossover pressure (700 Pa). This sensor can detect infinitesimal pressure as low as 0.3 Pa with uniformly applied vertical force. With the attachment of the artificial fingerprint structure (AFPS) on the sensor, the detection ability for both the locally generated shear force and actual human touch confirms recognition of the surface morphology constructed by periodic structures.

We present a sensor of vertical forces based on single-layer graphene, with a working range that corresponds to the pressure of a gentle touch that can be perceived by humans. Polydimethylsiloxane (PDMS) is employed for the supporting material of the graphene layer and also works as a force absorber of the sensor. In spite of the low gauge factor that arises from the intrinsic electromechanical character of single-layer graphene, we achieve a resistance variation of about 17 ± 7% in response to an applied vertical pressure of 10 kPa by introducing a pressure-amplifying structure of patterned lines that induces additional structural deformation of the graphene layer for the applied vertical force. In addition, we demonstrate a method to enhance the sensitivity of the sensor by applying resistive single-layer graphene.

Tactile sensors capable of texture recognition are essential for artificial skin functions. In this work, we describe a tactile sensor with a single sensor architecture made of single layer graphene that can recognize surface texture based on the roughness of the interacting surface. Resistance changes due to the local deformation of a local area of the single layer graphene are reflected in the resistance of the entire sensor. By introducing microstructures inspired by human finger prints, surface texture was successfully defined through fast Fourier transform analysis, and spatial resolution was easily achievable. This work provides a simple method utilizing a single sensor for surface texture recognition at the level of human sensation without using a matrix architecture which requires high density integration technology with force and vibration sensor elements.

For sensors that emulate human tactile perception, we suggest a simple method for fabricating a highly sensitive force sensor using a conductive polyurethane sponge where graphene flakes are self-assembled into the porous structure of the sponge. The complete sensor device shows a sensitive and reliable detection response for a broad range of pressure and dynamic pressure that correspond to human tactile perception. Sensitivity of the sensor to detect vibration is also confirmed with vertical actuations due to slipping over micro-scale ridge structures attached on the sensors. Based on the sensor's ability to detect both pressure and vibration, the sensor can be utilized as a flexible tactile sensor.

The electromechanical properties of single-layer graphene have inspired specific application to force sensors, sinceit is capable of sensing within the range of human pressure perception. In this study, we present a pressure sensor for vertical force that is flexible and transparent by introducing a single graphene layer on a polyethylene naphthalate substrate. This substrate is commonly used as a force absorber in sensors. By employing it with a pressure amplifying structure, the performance of the sensor shows a reliable resistance change of 0.15% per 1 kPa of applied vertical pressure. Detection for the motion of the finger joint and touching are demonstrated with the sensor equipped on the human body.

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.

We report the use of poly(4-vinylphenol) (PVP) as a promising contact surface of transferred graphene, capable of sustaining the original performance found in as-grown graphene. Enhancement of surface tension obtained by O2 plasma treatment of the PVP surface also increases transferred graphene quality. With an easy coating method, PVP can be applied to any flexible substrate as the interlayer to increase transferred graphene quality. Owing to the mechanical flexibility and chemical inertness of PVP, the introduction of a PVP interlayer provides a general method for graphene soft electronics to be integrated into any flexible substrate.