Highly Sensitive and Stretchable Resistive Strain Sensors Based on Microstructured Metal Nanowire/Elastomer Composite Films.

High sensitivity and high stretchability are two conflicting characteristics that are difficult to achieve simultaneously in elastic strain sensors. A highly sensitive and stretchable strain sensor comprising a microstructured metal nanowire (mNW)/elastomer composite film is presented. The surface structure is easily prepared by combining an mNW coating and soft-lithographic replication processes in a simple and reproducible manner. The densely packed microprism-array architecture of the composite film leads to a large morphological change in the mNW percolation network by efficiently concentrating the strain in the valley regions upon stretching. Meanwhile, the percolation network comprising mNWs with a high aspect ratio is stable enough to prevent electrical failure, even under high strains. This enables the sensor to simultaneously satisfy high sensitivity (gauge factor ≈81 at >130% strain) and high stretchability (150%) while ensuring long-term reliability (10 000 cycles at 150% strain). The sensor can also detect strain induced by bending and pressure, thus demonstrating its potential as a versatile sensing tool. The sensor is successfully utilized to monitor a wide range of human motions in real time. Furthermore, the unique sensing mechanism is easily extended to detect more complex multiaxial strains by optimizing the surface morphology of the device.

Wearable Resistive Pressure Sensor Based on Highly Flexible Carbon Composite Conductors with Irregular Surface Morphology.

Wearable pressure sensors are crucial building blocks for potential applications in real-time health monitoring, artificial electronic skins, and human-to-machine interfaces. Here we present a highly sensitive, simple-architectured wearable resistive pressure sensor based on highly compliant yet robust carbon composite conductors made of a vertically aligned carbon nanotube (VACNT) forest embedded in a polydimethylsiloxane (PDMS) matrix with irregular surface morphology. A roughened surface of the VACNT/PDMS composite conductor is simply formed using a sandblasted silicon master in a low-cost and potentially scalable manner and plays an important role in improving the sensitivity of resistive pressure sensor. After assembling two of the roughened composite conductors, our sensor shows considerable pressure sensitivity of ∼0.3 kPa-1 up to 0.7 kPa as well as stable steady-state responses under various pressures, a wide detectable range of up to 5 kPa before saturation, a relatively fast response time of ∼162 ms, and good reproducibility over 5000 cycles of pressure loading/unloading. The fabricated pressure sensor can be used to detect a wide range of human motions ranging from subtle blood pulses to dynamic joint movements, and it can also be used to map spatial pressure distribution in a multipixel platform (in a 4 × 4 pixel array).

Simple Approach to High-Performance Stretchable Heaters Based on Kirigami Patterning of Conductive Paper for Wearable Thermotherapy Applications.

Recent efforts to develop stretchable resistive heaters open up the possibility for their use in wearable thermotherapy applications. Such heaters should have high electrothermal performance and stability to be used practically, and the fabrication must be simple, economic, reproducible, and scalable. Here we present a simple yet highly efficient way of producing high-performance stretchable heaters, which is based on a facile kirigami pattering (the art of cutting and folding paper) of a highly conductive paper for practical wearable thermotherapy. The resulting kirigami heater exhibits high heating performance at low voltage (>40 °C at 1.2 V) and fast thermal response (<60 s). The simple kirigami patterning approach enables the heater to be extremely stretchable (>400%) while stably retaining its excellent performance. Furthermore, the heater shows the uniform spatial distribution of heat over the whole heating area and is highly durable (1000 cycles at 300% strain). The heater attached to curvilinear body parts shows stable heating performance even under large motions while maintaining intimate conformal contact with the skin thanks to the high stretchability and sufficient restoring force. The usability of the heater as a wearable thermotherapy device is demonstrated by increased blood flow at the wrist during operation.

Metal Nanowire-Coated Metal Woven Mesh for High-Performance Stretchable Transparent Electrodes.

This work presents a new template-assisted fabrication method to obtain stretchable metal grids for high-performance stretchable transparent conducting electrodes (TCEs). Readily accessible metal woven mesh (MWM) is used as a template to make the fabrication process simple, cost-effective, reproducible, and potentially scalable by combining it with silver nanowire (AgNW) coating and elastomer filling processes. Stretchable TCEs are made with the AgNW-coated MWM and show remarkable optoelectronic performance with a sheet resistance of ∼3.2 Ω/sq and optical transmittance of >80%, large maximum stretchability of 40%, and electrical and mechanical robustness even under repeated stretching and bending deformations (1000 cycles). The device is demonstrated in a highly flexible touch screen panel that can operate well even in a bent state.

Simple method for high-performance stretchable composite conductors with entrapped air bubbles.

We integrate air bubbles into conductive elastic composite-based stretchable conductors to make them mechanically less stiff and electrically more robust against physical deformations. A surfactant facilitates both the formation and maintenance of air bubbles inside the elastic composites, leading to a simple fabrication of bubble-entrapped stretchable conductors. Based on the unique bubble-entrapped architecture, the elastic properties are greatly enhanced and the resistance change in response to tensile strains can clearly be controlled. The bubble-entrapped conductor achieves ~80 % elongation at ~3.4 times lower stress and ~44.8 % smaller change in the electrical resistance at 80 % tensile strain, compared to bare conductor without air bubbles.

Simple and cost-effective fabrication of highly flexible, transparent superhydrophobic films with hierarchical surface design.

Optical transparency and mechanical flexibility are both of great importance for significantly expanding the applicability of superhydrophobic surfaces. Such features make it possible for functional surfaces to be applied to various glass-based products with different curvatures. In this work, we report on the simple and potentially cost-effective fabrication of highly flexible and transparent superhydrophobic films based on hierarchical surface design. The hierarchical surface morphology was easily fabricated by the simple transfer of a porous alumina membrane to the top surface of UV-imprinted polymeric micropillar arrays and subsequent chemical treatments. Through optimization of the hierarchical surface design, the resultant superhydrophobic films showed superior surface wetting properties (with a static contact angle of >170° and contact angle hysteresis of <3.5°) in the Cassie-Baxter wetting regime, considerable dynamic water repellency (with perfect bouncing of a water droplet dropped from an impact height of 30 mm), and good optical transparency (>82% at 550 nm wavelength). The superhydrophobic films were also experimentally found to be robust without significant degradation in the superhydrophobicity, even under repetitive bending and pressing for up to 2000 cycles. Finally, the practical usability of the proposed superhydorphobic films was clearly demonstrated by examining the antiwetting performance in real time while pouring water on the film and submerging the film in water.