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Soft strain sensors have attracted significant attention in wearable human motion monitoring applications. However, there is still a huge challenge for decoupled measurement of multidirectional strains. In this study, we have developed a biaxial and stretchable strain sensor based on a carbon nanotube (CNT) film and a microdome array (MA)-patterned elastomeric substrate. The MA structures lead to generating localized and directional microcracks of CNT films within the intended regions under tensile strain. This mechanism allows a single sensing layer to act as a strain sensor capable of decoupling the biaxial strains into axial and transverse terms. The ratio of resistance change between two perpendicular axes is about 960% under an x-directional strain of 30%, demonstrating the biaxial decoupling capability. Also, the proposed strain sensor shows high stretchability and excellent long-term reliability under a cyclic loading test. Finally, wearable devices integrated with the strain sensor have been successfully utilized to monitor various human motions of the wrist, elbow, knee, and fingers by measuring joint bending and skin elongation.

A number of flexible and stretchable strain sensors based on piezoresistive and capacitive principles have been recently developed. However, piezoresistive sensors suffer from poor long-term stability and considerable hysteresis of signals. On the other hand, capacitive sensors exhibit limited sensitivity and strong electromagnetic interference from the neighboring environment. In order to resolve these problems, a novel stretchable strain sensor based on the modulation of optical transmittance of carbon nanotube (CNT)-embedded Ecoflex is introduced in this paper. Within the film of multiwalled CNTs embedded in the Ecoflex substrate, the microcracks are propagated under tensile strain, changing the optical transmittance of the film. The proposed sensor exhibits good stretchability (ε ≈ 400%), high linearity (

Wearable pressure sensors have been attracting great attention for a variety of practical applications, including electronic skin, smart textiles, and healthcare devices. However, it is still challenging to realize wearable pressure sensors with sufficient sensitivity and low hysteresis under small mechanical stimuli. Herein, we introduce simple, cost-effective, and sensitive capacitive pressure sensor based on porous Ecoflex-multiwalled carbon nanotube composite (PEMC) structures, which leads to enhancing the sensitivity (6.42 and 1.72 kPa

Flexible and wearable pressure sensors have attracted a tremendous amount of attention due to their wider applications in human interfaces and healthcare monitoring. However, achieving accurate pressure detection and stability against external stimuli (in particular, bending deformation) over a wide range of pressures from tactile to body weight levels is a great challenge. Here, we introduce an ultrawide-range, bending-insensitive, and flexible pressure sensor based on a carbon nanotube (CNT) network-coated thin porous elastomer sponge for use in human interface devices. The integration of the CNT networks into three-dimensional microporous elastomers provides high deformability and a large change in contact between the conductive CNT networks due to the presence of micropores, thereby improving the sensitivity compared with that obtained using CNT-embedded solid elastomers. As electrical pathways are continuously generated up to high compressive strain (∼80%), the pressure sensor shows an ultrawide pressure sensing range (10 Pa to 1.2 MPa) while maintaining favorable sensitivity (0.01-0.02 kPa

In this research, the 3D printed package for optical transmittance change base strain sensor was developed. Optical transmittance based strain sensor is consisted by light source, light detector and functional film which transmittance changed by the external strain. Functional film is carbon nanotube embedded elastomer and it changes its visible light transmittance by the strain. To adjust the optical strain sensor on the human motion detection, we positioned light source and detector in the sensor package. To achieve this problem, we adopted the 3D printed polylactic acid (PLA) package for fabricating ring type strain sensor for finger bending detection.

This paper reports a wide range-sensitive, bending-insensitive pressure sensor based on a carbon nanotube (CNT)-coated porous elastomer for a wearable healthcare application. (1) Sensing performance was evaluated over ~1MPa, which includes whole human tactile and body weight levels. The sensor exhibits a very wide sensing range from ultralow to high pressure (50Pa-1MPa) with maintenance of favorable sensitivity and reliable dynamic responses. (2) To our knowledge this is the first to investigate bending-insensitivity of the CNTs-coated porous PDMS, which facilitates accurate detection of normal pressures even on curved surfaces like human skins. (3) Finally, we demonstrated a foot insole for real-time monitoring of foot plantar pressure distribution.

We report a flexible and wearable pressure sensor based on the giant piezocapacitive effect of a three-dimensional (3-D) microporous dielectric elastomer, which is capable of highly sensitive and stable pressure sensing over a large tactile pressure range. Due to the presence of micropores within the elastomeric dielectric layer, our piezocapacitive pressure sensor is highly deformable by even very small amounts of pressure, leading to a dramatic increase in its sensitivity. Moreover, the gradual closure of micropores under compression increases the effective dielectric constant, thereby further enhancing the sensitivity of the sensor. The 3-D microporous dielectric layer with serially stacked springs of elastomer bridges can cover a much wider pressure range than those of previously reported micro-/nanostructured sensing materials. We also investigate the applicability of our sensor to wearable pressure-sensing devices as an electronic pressure-sensing skin in robotic fingers as well as a bandage-type pressure-sensing device for pulse monitoring at the human wrist. Finally, we demonstrate a pressure sensor array pad for the recognition of spatially distributed pressure information on a plane. Our sensor, with its excellent pressure-sensing performance, marks the realization of a true tactile pressure sensor presenting highly sensitive responses to the entire tactile pressure range, from ultralow-force detection to high weights generated by human activity.

There is a great demand for the development of self-powered physical sensors for wearable applications in recent years. However, it is still challenging to achieve self-powered sensors with a high stability, accuracy, and linearity. Here, a novel wearable self-powered pressure sensor based on the integration of a piezo-transmittance microporous elastomer (PTME) and a thin-film organic solar cell (OSC) is proposed. In contrast to the sensors based on other mechanisms such as piezoelectricity or triboelectricity, the proposed self-powered pressure sensor is cable of measuring static pressure continuously and stably, and utilizes the ambient light as the power source regardless of its intensity. The PTME shows the light transmittance changes by a gradual closure of micropores with compression in response to the applied pressure. This unique optical characteristics of the PTME enables the OSC to generate varying electrical current in response to the pressure. The proposed self-powered pressure sensor shows a high-performance with a sensitivity of 0.101/kPa, a linearity of R2 = 0.995, and fast and reversible response to the pressure up to ~100 kPa. As practical applications of the proposed sensor, a detection of flexion/extension of a human finger for the manipulation of a prosthetic robot finger and a wind detection for the continuous monitoring of the wind speed and direction have been demonstrated.

High-performance and low-power flexible Schottky diode-based hydrogen sensor was developed. The sensor was fabricated by releasing Si nanomembrane (SiNM) and transferring onto a plastic substrate. After the transfer, palladium (Pd) and aluminum (Al) were selectively deposited as a sensing material and an electrode, respectively. The top-down fabrication process of flexible Pd/SiNM diode H2 sensor is facile compared to other existing bottom-up fabricated flexible gas sensors while showing excellent H2 sensitivity (ΔI/I0 > 700–0.5% H2 concentrations) and fast response time (τ10–90 = 22 s) at room temperature. In addition, selectivity, humidity, and mechanical tests verify that the sensor has excellent reliability and robustness under various environments. The operating power consumption of the sensor is only in the nanowatt range, which indicates its potential applications in low-power portable and wearable electronics.

This paper reports a flexible optical pressure sensor based on a porous structured elastomer and its application to a wearable human motion detecting device, which is operated in a self-power-generating way by integrating with an organic photovoltaic cell. Complete investigation of material characteristics and sensing performances as well as optical simulation for understanding the sensing mechanism have been produced. Finally, a self-power-generating wearable motion sensing device was demonstrated as a sensor for detection of flexion and extension motion of a human finger.

The development of wearable technologies has been boosted considerably. On one hand, due to Internet of Things market expansion, and on the other, owing to health awareness on users demanding the convergence of wearable technology with applications that track their activity during the day, providing feedback on how to improve their consuming experience employing human vital signs. The availability of sensors on most of the wrist smartwatches and fitness bands, make this convergence a precondition for consumer devices industry and stakeholders. This article presents an analytical exploitation of this valuable data, and uses the embedded heart rate sensor from an attainable smartwatch to meet the prior requirements, parting the users according to the level of physical activity in pursuance of make a suitable recommendation from a specific consumer device edible catalog, according to the number of calories recommended for a healthy state.

In this work, we report a novel optical type flexible and stretchable strain sensor based on a carbon nanotube(CNT)-elastomer nanocomposite thin film. At the initial state, the percolated CNTs embedded on the surface of the elastomer film block most of the light illuminated on the film. As an external tensile strain is applied to the CNT-elastomer film, cracks on the film are generated and propagated, leading to an increase of optical transmittance of the CNT-elastomer film, which can be detected by a photodetector. The sensor shows a great strain sensing performances with high sensitivity, quick dynamic response, small hysteresis, high stability, and independence on light intensities.

This paper reports a novel flexible optical pressure sensor based on a porous elastomer film as a light transmission medium with ultra-high sensitivity and simplicity. The pore-closing behavior under external pressure and corresponding change of light transmittance of porous elastomer film were investigated. The porous elastomer film based optical pressure sensor showed highly sensitive and stable performance with the sensitivity of 2.177 kPa−1 over the pressure range of 0–10 kPa. Finally, the optical pressure sensor was integrated with thin film solar cell to modulate the generated electrical current towards self-powered sensing platform.

We have developed a self-powered gas sensor operating without external power under a light source, unlike a sensor that requires an external power source, such as a chemiresistive gas sensor or an electrochemical gas sensor. The sensor consists of a colorimetric film and an organic solar cell. The sensor uses a color change caused by the reaction of the N, N,N',N'-tetramethyl-p-phenylenediamine (TMPD) with NO 2 gas. The color change caused by the redox reaction of the colorimetric material does not require a separate light source and photodetector, unlike the conventional colorimetric sensor, because it directly changes the output of the solar cell by changing the transmittance of the film. Using this sensor, 1, 5, 10, 20ppm NO 2 gas was detected without external power source, and the performance of the gas sensor could be improved by using microstructure. This work is the first demonstration of self-powered gas sensor based on the combination of photovoltaic cell and microstructured colorimetric film.

The practical utilization of soft nanocomposites as a strain mapping sensor in tactile sensors and artificial skins requires robustness for various contact conditions as well as low-cost fabrication process for large three dimensional surfaces. In this work, we propose a multi-point and multi-directional strain mapping sensor based on multiwall carbon nanotube (MWCNT)-silicone elastomer nanocomposites and anisotropic electrical impedance tomography (aEIT). Based on the anisotropic resistivity of the sensor, aEIT technique can reconstruct anisotropic resistivity distributions using electrodes around the sensor boundary. This strain mapping sensor successfully estimated stretch displacements (error of 0.54 ± 0.53 mm), surface normal forces (error of 0.61 ± 0.62 N), and multi-point contact locations (error of 1.88 ± 0.95 mm in 30 mm × 30 mm area for a planar shaped sensor and error of 4.80 ± 3.05 mm in 40 mm × 110 mm area for a three dimensional contoured sensor). In addition, the direction of lateral stretch was also identified by reconstructing anisotropic distributions of electrical resistivity. Finally, a soft human-machine interface device was demonstrated as a practical application of the developed sensor.

In this work, we developed micro-crack based stretchable strain sensors consist of polydimethylsiloxane (PDMS) with micro-dome structure arrays and carbon nanotube (CNT) film coated on its surface. Due to a stress concentration caused by micro-dome structure under a certain strain, micro-cracks of CNT film are generated and propagated more easily as compared with the case of a CNT film on the plane surface. In addition, because the micro-cracks are only positioned at designed region which is between micro-domes, electrical resistances in elongated and perpendicular directions are clearly different thanks to the novel mechanism. As results of these phenomena, strain decoupling is achieved and a sensitivity of the sensor can be modulated by adjusting the dimension of micro-dome arrays. We also conducted the numerical simulation based on finite element method (FEM) to verify the designed sensing mechanism and predict the performance of the sensor. This kind of work is important for the motion detecting sensor for acquiring decoupled information from complex signal. Finally, we fabricated a smart glove that could detect a biaxial strain applied on the back of a hand skin to demonstrate that this sensor could be used as wearable device.

We propose a fabrication process for extremely robust and easily patternable silver nanowire (AgNW) electrodes on paper. Using an auxiliary donor layer and a simple laminating process, AgNWs can be easily transferred to copy paper as well as various other substrates using a dry process. Intercalating a polymeric binder between the AgNWs and the substrate through a simple printing technique enhances adhesion, not only guaranteeing high foldability of the electrodes, but also facilitating selective patterning of the AgNWs. Using the proposed process, extremely crease-tolerant electronics based on copy paper can be fabricated, such as a printed circuit board for a 7-segment display, portable heater, and capacitive touch sensor, demonstrating the applicability of the AgNWs-based electrodes to paper electronics.