Wearable Textile Microwave Sensor for Wrist Pulse Wave and Heart Rate Monitoring

This paper presents a contact bracelet-type microwave sensor, implemented on fabric, able to monitor the wrist pulse wave (WPW) and, consequently, the heart rate of the individual under analysis. The device operates in transmission, and the working principle is the variation of the frequency response as consequence of pressure changes exerted on a diaphragm caused by blood pumping in the radial artery. The associated electronics to implement a stand-alone device (avoiding the use of a vector network analyzer) are also included. Specifically, by using a phase or an envelope detector, the system provides either the phase or the magnitude of the output signal at a specific frequency, namely, the WPW. The main relevant contribution of the paper is the implementation and validation of the sensor in a textile (and hence flexible and conformal) substrate, using also textile conductors. By this means, the device features a low cost, profile, and weight, making it ergonomic and user-friendly for real usage. Comparison of the recorded signal of the sensor on multiple individuals (at rest, under motion, and by wetting their wrist skin) with Photoplethysmography (PPG) further validates the approach.