Wearable and flexible electronic strain sensor

Flexible strain sensors or deformable strain sensors have attracted more and more attention for their excellent advantages and wide applications in personal electronics and industrial monitoring. The electronic skins and robotics are candidates for promoting the advancement of science and technology in recent years. Because of their biomedical applications, flexible strain sensors will significantly expand the application scope of stretchable electronics, particularly large-area electronics. The arbitrary surfaces covered with flexible electronics can be stretched in all directions, which are necessary for the next generation electronics. Howerer, conventional electronic strain sensors based on metal and semiconductor materials have poor portability, flexibility, and wearability, which is not applicable for stretchable sensors. With the rapid development of materials and electronic technology in the past decades, flexible strain sensors have become more and more popular in electronic skin, robotics, and other fields. Compared with conventional electronic sensors, flexible strain sensors have many unique advantages, such as good biocompatibility, wearable, real-time monitoring and non-invasive. In this review, the progress of wearable flexible strain sensors in materials, sensing mechanism, integrated output, and potential application are presented. After a brief introduction for the development and application of wearable flexible strain sensors, a simple comparison of old and new sensors is given. Then, studies on the different parts of the sensor and the materials are reviewed, including substrate, the dielectric layer, active layer and electrodes. Subsequently, the mechanism inclouding piezoresistivity, capacitivity, piezoelectricity, optical waveguide and other mechanisms of flexible strain sensors are presented. In addition, this review summarizes the integration and output of the flexible strain sensors. And the faced challenges of the flexible strain sensors and possible optimization solutions are proposed. Finally, challenges, important directions and perspectives related to flexible stretchable strain sensors are discussed. Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.