Self-Powered Piezo-Transmittance Type Strain Sensor Based on an Auxetic Structure

Soft strain sensors hold great promise for a monitoring system in a soft system (e.g., human motion monitoring and healthcare monitoring) as well as an in a rigid system (e.g., structural healthcare monitoring). Representatively, the piezo-transmittance type strain sensors were developed to lessen the existed problems of piezo-resistive and piezo-capacitive associated with high hysteresis and environmental noise due to humidity, temperature, and electromagnetic fields. They promise advantages of high sensitivity, long-term stability, fast response, and negligible interference from environmental factors. However, they present low inter-sensor and intra-sensor uniformity as well as unpredictable responses. In addition, the current overall soft strain sensors have two general limitations. The necessity of a power source for reading the signal is the first one whereas the need for self-powered sensors increases for various applications. The second one is that the stiffness of the sensor material itself can hinder the movement of the target which should not be disturbed. These problems are in high demand to solve, but it persists in a challenge. In this paper, the self-powered piezo-transmittance type strain sensor using a gap control of mechanical metamaterial is developed to deal with these abovementioned problems. They can solve not only two general problems associated with power consumption and mechanical stiffness but also the limitation of piezo-transmittance type associated with uniformity and response predictability. An Au-deposited elastomer film with an auxetic pattern that changed its optical transmittance in response to changes in auxetic pattern gaps under externally applied strain. The light-blocking film fabrication method affects the uniformity of the sensor, so the proposed sensor was fabricated by the e-beam evaporated metal thin film which satisfies the remarkable uniformity and high reflectance. The gap opening mechanism of auxetic patterned film enables highly-low stiffness due to bending-dominated deformation as well as quantitative and rational design of sensor response using FEM simulation. Therefore, we achieved high inter-sensor uniformity (Coefficient of Variability <4%), high intra-sensor uniformity (Coefficient of Variability <6%), response predictability, and low mechanical stiffness during stretching. In addition, the sensor shows high sensitivity (gauge factor (GF) ≈ 10), low hysteresis (~0.508%), high linearity (R2 = 0.9), and long-term stability (>10,000 cycles). Finally, self-powered wireless practical applicability was verified by the consolidation of the developed sensor into a strain sensing system for structural health monitoring and human motion monitoring. As the structural health monitoring system application, the sensor shows the probability of monitoring the inflation/deflation of an aerostat and the cracks in a rigid specimen. As the human motion monitoring system application, the sensor uses to posture correction weight training, especially the aim of monitoring the dorsiflexion of joints at a high risk of injury and the changes of target muscles.