(150f) Conformable Resonant Sensors for Contact-Free Sweat Analysis through Clothing and Protective Gear

Maintaining proper hydration is important to overall health and especially in work situations which require periods of high exertion. Monitoring sweat loss can help determine hydration status of individuals working in harsh conditions (high temperature, high humidity), which is especially relevant to those who wear thick personal protective equipment (PPE). This study presents a wireless, passive, skin-conformable sweat sensor sticker that can be worn under clothing and interrogated through thick PPE to simultaneously measure sweat loss volume and sweat conductivity. The sticker consists of a laser-ablated, microfluidic channel and a resonant sensor transducer. The resonant sensor is wirelessly read with a handheld vector network analyzer coupled to two, co-planar, interrogation antennas that measure the transmission loss. A sweat proxy is used to fill the channels and it is determined that the sensor can orthogonally determine the sweat conductivity and volume filled in the channel via peak transmission loss magnitude and frequency, respectively. Signal offset can be calibrated for by linear relationship between separation distance between reader and sensor by measuring the PPE thickness. A model is used to calibrate the sensor response to sweat conductivity and sweat loss with an R² of 0.964 and 0.934 respectively. A four-person study was performed to determine level of sensor variance caused by local tissue dielectric heterogeneity and sensor-reader orientation which has led to different resonator designs to diminish the effect of tissue dielectric and reader/sensor orientational sensitivity. This work provides a wearable sensor platform for human health monitoring which is passive (no on-board power), wireless, and can be interrogated through PPE; progress on extending this to skin temperature monitoring will be shared. It has potential applications in areas requiring thick PPE including in chemical industry, emergency response personnel, and military as well as competitive athletics (e.g., American football).