An Ultra-Sensitive Flexible Resistive Sensor with Double Strain Layer and Crack Inspired by the Physical Structure of Human Epidermis: Design, Fabrication, and Cuffless Blood Pressure Monitoring Application

The cuffless blood pressure (BP) monitoring device has attracted much attention because of its comfortable and real-time monitoring. Inspired by the physical structure of the human epidermis, an ultra-sensitive flexible resistive sensor with a double strain layer and crack structure is prepared by screen printing in this study. The strain material of the sensor is mainly prepared by blending Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with waterborne polyurethane and reduced graphene oxide. The sensor exhibits a high gauge factor (GF, approximate to 1682), fast response (approximate to 48 ms), and long-term stability (> 2000 cycles) at low strain (approximate to 0-5%). The sensor is placed on the human radial artery and can accurately measure the pulse wave. The features of the pulse wave are automatically extracted using a convolutional neural network. Then the features are corrected using a long short-term memory neural network, and the current BP value is predicted using a fully connected network layer. The BP result meets the A-level standards of the Association for the Advancement of Medical Instrumentation and the British Hypertension Society. This study provides an efficient device and method for continuous and non-invasive BP monitoring.