There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

Flexible and stretchable physical sensors that can measure and quantify electrical signals generated by human activities are attracting a great deal of attention as they have unique characteristics, such as ultrathinness, low modulus, light weight, high flexibility, and stretchability. These flexible and stretchable physical sensors conformally attached on the surface of organs or skin can provide a new opportunity for human-activity monitoring and personal healthcare. Consequently, in recent years there has been considerable research effort devoted to the development of flexible and stretchable physical sensors to fulfill the requirements of future technology, and much progress has been achieved. Here, the most recent developments of flexible and stretchable physical sensors are described, including temperature, pressure, and strain sensors, and flexible and stretchable sensor-integrated platforms. The latest successful examples of flexible and stretchable physical sensors for the detection of temperature, pressure, and strain, as well as their novel structures, technological innovations, and challenges, are reviewed first. In the next section, recent progress regarding sensor-integrated wearable platforms is overviewed in detail. Some of the latest achievements regarding self-powered sensor-integrated wearable platform technologies are also reviewed. Further research direction and challenges are also proposed to develop a fully sensor-integrated wearable platform for monitoring human activity and personal healthcare in the near future.

Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoringand Personal Healthcare.

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Carbon nanotubes have aroused great interest since their discovery in 1991. Because of the vast potential of these materials, researchers from diverse disciplines have come together to further develop our understanding of the fundamental properties governing their electronic structure and susceptibility towards chemical reaction. Carbon nanotubes show extreme sensitivity towards changes in their local chemical environment that stems from the susceptibility of their electronic structure to interacting molecules. This chemical sensitivity has made them ideal candidates for incorporation into the design of chemical sensors. Towards this end, carbon nanotubes have made impressive strides in sensitivity and chemical selectivity to a diverse array of chemical species. Despite the lengthy list of accomplishments, several key challenges must be addressed before carbon nanotubes are capable of competing with state-of-the-art solid-state sensor materials. The development of carbon nanotube based sensors is still in its infancy, but continued progress may lead to their integration into commercially viable sensors of unrivalled sensitivity and vanishingly small dimensions.

Carbon nanotube gas and vapor sensors.

Conventional metallic strain sensors are flexible, but they can sustain maximum strains of only ~5%, so there is a need for sensors that can bear high strains for multifunctional applications. In this study, we report stretchable and flexible high-strain sensors that consist of entangled and randomly distributed multiwall carbon nanotubes or graphite flakes on a natural rubber substrate. Carbon nanotubes/graphite flakes were sandwiched in natural rubber to produce these high-strain sensors. Using field emission scanning electron microscopy, the morphology of the films for both the carbon nanotube and graphite sensors were assessed under different strain conditions (0% and 400% strain). As the strain was increased, the films fractured, resulting in an increase in the electrical resistance of the sensor; this change was reversible. Strains of up to 246% (graphite sensor) and 620% (carbon nanotube sensor) were measured; these values are respectively ~50 and ~120 times greater than those of conventional metallic strain sensors.

Stretchable and Flexible High-Strain Sensors Made Using Carbon Nanotubes and Graphite Films on Natural Rubber

Polyaniline is one of the most promising conducting polymers for gas sensing applications due to its relatively high stability and n or p type doping capability. However, the conventionally doped polyaniline still exhibits relatively high resistivity, which causes difficulty in gas sensing measurement. In this work, the effect of carbon nanotube (CNT) dispersion on CO gas sensing characteristics of polyaniline gas sensor is studied. The carbon nanotube was synthesized by Chemical Vapor Deposition (CVD) using acetylene and argon gases at 600 degrees C. The Maleic acid doped Emeradine based polyaniline was synthesized by chemical polymerization of aniline. CNT was then added and dispersed in the solution by ultrasonication and deposited on to interdigitated AI electrode by solvent casting. The sensors were tested for CO sensing at room temperature with CO concentrations in the range of 100-1000 ppm. It was found that the gas sensing characteristics of polyaniline based gas sensor were considerably improved with the inclusion of CNT in polyaniline. The sensitivity was increased and response/recovery times were reduced by more than the factor of 2. The results, therefore, suggest that the inclusion of CNT in MA-doped polyaniline is a promising method for achieving a conductive polymer gas sensor with good sensitivity, fast response, low-concentration detection and room-operating-temperature capability.

The effect of carbon nanotube dispersion on CO gas sensing characteristics of polyaniline gas sensor.

The volatile-organic-compounds (VOCs) sensors based on In 2 O 3 sensing film were successfully produced by a simple and cost-effective sparking process in a single step. Two indium wires were sparked and scanned repeatedly above Al 2 O 3 substrates equipped with Au interdigitated electrodes for 10–200 cycles at a high sparking voltage of 4.5 kV under atmospheric conditions. The functional nanoparticles and sensing film were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The sensor response and response time of sensing films were systematically characterized toward ethanol (50–1000 ppm) and acetone (100–2000 ppm) with varying number of sparking cycles at 200–350 °C in dry air. The relationship between sensing characteristics and the number of sparking cycles was extensively discussed based on the concept of diffusivity and reactivity of gases inside the oxide films. It was found that the sensing film had well-developed porous structures when the number of sparking cycles was not larger than 100 and the film became denser due to particle agglomeration and grain growth at higher numbers of sparking cycles, leading to deteriorated gas-sensing response. The In 2 O 3 sensing film produced with 100 sparking cycles exhibited the highest sensor response and ultra-high detection speed with very short response time of less than a second at an operating temperature of 350 °C. In addition, the sensor response linearly increased and the response time decreased with increasing gas concentration. Moreover, sensors were highly sensitive to VOCs at low concentrations in the same range as the detection limit of human breath analyzer. Therefore, the sensor is a promising candidate as an ethanol and acetone detectors for drunken driving detection and diabetes diagnosis.

Ultra-rapid VOCs sensors based on sparked-In2O3 sensing films

Formation of CuO nanorods and their bundles by an electrochemical dissolution and deposition process

Multi-walled carbon nanotubes (MWNTs) were synthesized by infusing alcohol into a tube furnace. A nickel catalyst preparation was made by a reduction reaction of NiO powder by ethanol vapor at 450 °C for 30 min before the MWNT synthesis at 700 °C for 1–18 h. The as-grown MWNTs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Large quantities of MWNTs, having a diameter in the range of 20–50 nm can be produced from ethanol vapor without a carrier gas by this technique. A method to measure an electrical resistance of bulk MWNTs was carried out under stress between two conducting plates. The result shows an exponentially correlation between the resistivity and the D-band/G-band intensity ratio, suggesting that the measuring method provides a simple tool to monitor the degree of structural defects of MWNTs.

Pisith Singjai

Papers

Stretchable and Flexible High-Strain Sensors Made Using Carbon Nanotubes and Graphite Films on Natural Rubber

The effect of carbon nanotube dispersion on CO gas sensing characteristics of polyaniline gas sensor.

Ultra-rapid VOCs sensors based on sparked-In2O3 sensing films

Formation of CuO nanorods and their bundles by an electrochemical dissolution and deposition process

Electrical resistivity of bulk multi-walled carbon nanotubes synthesized by an infusion chemical vapor deposition method