wileyonlinelibrary.comSkin-mountable and wearable sensors can be attached onto the clothing or even directly mounted on the human skin for the real-time monitoring of human activities. [ 10 ] Besides their high effi ciency, they must fulfi ll several minimum requirements including high stretchability, fl exibility, durability, low power consumption, biocompatibility, and lightweight. These demands become even more severe for epidermal electronic devices where mechanical compliance like human skin and high stretchability ( ε > 100% where ε is the strain) are required. [ 11,12 ] Recently, several types of skin-mountable and wearable sensors have been proposed by using nanomaterials coupled with fl exible and stretchable polymers. Indeed, nanomaterials are utilized as functional sensing elements owing to their outstanding electrical, mechanical, optical, and chemical properties while polymers are employed as fl exible support materials thanks to their fl exibility, stretchability, humanfriendliness, and durability. [ 13 ] Examples of those innovative sensors include strain sensors, [ 10,[12][13][14] pressure sensors, [ 5,[15][16][17] electronic skins (e-skins), [ 3,[16][17][18][19][20] and temperature sensors. [ 2,21,22 ] Particularly, various skin-mountable and wearable strain sensors have been developed because of their broad applications in personalized heath-monitoring, human motion detection, human-machine interfaces, and soft robotics. [ 10,12,[23][24][25][26][27][28][29][30][31] This paper aims to survey fabrication processes, working mechanisms, strain sensing performances, and applications of stretchable strain sensors. The article is organized as follows: fi rst, common operation mechanisms of stretchable strain sensors are described. Here, we summarize novel functional nanomaterials and techniques for the fabrication of stretchable strain sensors in details. Second, mechanisms involved in the strain-responsive behavior of resistive-type and capacitive-type sensors are explained. We show that the infl uence of traditional mechanisms like geometrical changes and piezoresistivity of materials on the strain sensing performance of fl exible strain sensors are very small whereas mechanisms such as disconnection between sensing elements, crack propagation in thin fi lms, and tunneling effect can potentially be employed for highly stretchable and sensitive strain sensing. Third, we emphasize the performance parameters of stretchable strain sensors in terms of stretchability, sensitivity, linearity, hysteresis behavior, response time, overshooting, and durability. We demonstrate

The demand for flexible and wearable electronic devices is increasing due to their facile interaction with human body. Flexible, stretchable and wearable sensors can be easily mounted on clothing or directly attached onto the body. Especially, highly stretchable and sensitive strain sensors are needed for the human motion detection. Here, we report highly flexible, stretchable and sensitive strain sensors based on the nanocomposite of silver nanowire (AgNW) network and PDMS elastomer in the form of the sandwich structure (i.e., AgNW thin film embedded between two layers of PDMS). The AgNW network-elastomer nanocomposite based strain sensors show strong piezoresistivity with tunable gauge factors in the ranges of 2 to 14 and a high stretchability up to 70%. We demonstrate the applicability of our high performance strain sensors by fabricating a glove integrated with five strain sensors for the motion detection of fingers and control of an avatar in the virtual environment.

Super-stretchable, skin-mountable, and ultra-soft strain sensors are presented by using carbon nanotube percolation network-silicone rubber nanocomposite thin films. The applicability of the strain sensors as epidermal electronic systems, in which mechanical compliance like human skin and high stretchability (ϵ > 100%) are required, has been explored. The sensitivity of the strain sensors can be tuned by the number density of the carbon nanotube percolation network. The strain sensors show excellent hysteresis performance at different strain levels and rates with high linearity and small drift. We found that the carbon nanotube-silicone rubber based strain sensors possess super-stretchability and high reliability for strains as large as 500%. The nanocomposite thin films exhibit high robustness and excellent resistance-strain dependency for over ~1380% mechanical strain. Finally, we performed skin motion detection by mounting the strain sensors on different parts of the body. The maximum induced strain by the bending of the finger, wrist, and elbow was measured to be ~ 42%, 45% and 63%, respectively.

Wearable strain sensors for human motion detection are being highlighted in various fields such as medical, entertainment and sports industry. In this paper, we propose a new type of stretchable strain sensor that can detect both tensile and compressive strains and can be fabricated by a very simple process. A silver nanoparticle (Ag NP) thin film patterned on the polydimethylsiloxane (PDMS) stamp by a single-step direct transfer process is used as the strain sensing material. The working principle is the change in the electrical resistance caused by the opening/closure of micro-cracks under mechanical deformation. The fabricated stretchable strain sensor shows highly sensitive and durable sensing performances in various tensile/compressive strains, long-term cyclic loading and relaxation tests. We demonstrate the applications of our stretchable strain sensors such as flexible pressure sensors and wearable human motion detection devices with high sensitivity, response speed and mechanical robustness.

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.

In this review article, we focus on the various types of materials used in biomedical implantable devices, including the polymeric materials used as substrates and for the packaging of such devices. Polymeric materials are used because of the ease of fabrication, flexibility, and their biocompatible nature as well as their wide range of mechanical, electrical, chemical, and thermal behaviors when combined with different materials as composites. Biocompatible and biostable polymers are extensively used to package implanted devices, with the main criteria that include gas permeability and water permeability of the packaging polymer to protect the electronic circuit of the device from moisture and ions inside the human body. Polymeric materials must also have considerable tensile strength and should be able to contain the device over the envisioned lifetime of the implant. For substrates, structural properties and, at times, electrical properties would be of greater concern. Section 1 gives an introduction of some medical devices and implants along with the material requirements and properties needed. Different synthetic polymeric materials such as polyvinylidene fluoride, polyethylene, polypropylene, polydimethylsiloxane, parylene, polyamide, polytetrafluoroethylene, poly(methyl methacrylate), polyimide, and polyurethane have been examined, and liquid crystalline polymers and nanocomposites have been evaluated as biomaterials that are suitable for biomedical packaging (section 2). A summary and glimpse of the future trend in this area has also been given (section 3). Materials and information used in this manuscript are adapted from papers published between 2010 and 2015 representing the most updated information available on each material.

Recent advances in the design and implementation of wearable resistive, capacitive, and optical strain sensors are summarized herein. Wearable and stretchable strain sensors have received extensive research interest due to their applications in personalized healthcare, human motion detection, human–machine interfaces, soft robotics, and beyond. The disconnection of overlapped nanomaterials, reversible opening/closing of microcracks in sensing films, and alteration of the tunneling resistance have been successfully adopted to develop high‐performance resistive‐type sensors. On the other hand, the sensing behavior of capacitive‐type and optical strain sensors is largely governed by their geometrical changes under stretching/releasing cycles. The sensor design parameters, including stretchability, sensitivity, linearity, hysteresis, and dynamic durability, are comprehensively discussed. Finally, the promising applications of wearable strain sensors are highlighted in detail. Although considerable progress has been made so far, wearable strain sensors are still in their prototype stage, and several challenges in the manufacturing of integrated and multifunctional strain sensors should be yet tackled.

One-step direct nanoimprinting of metal nanoparticles was investigated to fabricate nano-/microscale metallic structures such as nanodot and nanowire arrays. This was done at low temperatures and pressures, utilizing the low melting temperature and viscosity of metal nanoparticle solutions. Through precise control of the fluidic properties of the nanoparticle solution and the mold design, high-quality nanoscale features with no or negligible residual layer were nanoimprinted. Nanoscale electronic devices were also demonstrated, including nanowire resistors and nanochannel organic field effect transistors with an air-stable semiconducting polymer.