A Battery-Less and Low-Cost RFID Sensor for Unassisted Multilevel Detection of Sweat Loss

Emerging applications for wearable sweat loss measuring devices (SLMD) require real-time and long-term operation. These applications impose strict constraints to SLMD in terms of power consumption, size, and flexibility. Several efforts have been made to avoid battery limitations in electronic sensors. However, this remains an open challenge. This contribution presents a battery-less wireless data transfer device enabling sweat loss monitoring through a wearable, flexible, and low-profile antenna-based sensor (ABS). The proposed sensor design includes a small piece of fabric embedded in the antenna substrate to absorb sweat secreted by the skin in contact with the sensor inlet interface. This novel feature dramatically improves the frequency selectivity and frequency range of the ABS in response to the contact with liquids, which increases the measurement accuracy in comparison to the current state of the art, enabling real-time multilevel sweat loss detection. The sensor performance is assessed using a commercial radio frequency identification (RFID) system through laboratory and real-life experiments. The results show that the proposed sensor can track sweat loss within the typical range of the human perspiration rate (300–2100 g/m2/h) of healthy individuals using a battery-less and low-cost sensor.