Passive Sensing for the Internet of Medical Things: A Low-Cost Battery-Less Wearable Sweat Rate Sensor

Continuous monitoring of the sweat rate can support the early diagnosis of several chronic diseases such as Parkinson's or diabetes. However, current wearable wireless devices use batteries that have a limited operating time, also increasing their cost, and thus, reducing their accessibility to a large part of the population. In that sense, passive IoT technologies could be exploited to produce battery-less low-cost antenna-based sensors (ABS). In this paper, we propose a UHF RFID battery-less antenna-based sensor that transforms the amount of sweat accumulated inside a microfluidic channel into a variation of the tag response. In order to provide multi-level sensing, we rely on the IC self-tuning sensor code, which is passively backscattered to the reader unit, instead of channel information which may provide insufficient resolution depending on the geographical location. The sensor performance is validated experimentally, achieving an accuracy of 90% in the estimation of the sweat loss and 95% in the estimation of the average sweat rate.