Sweat monitoring beneath garments using passive, wireless resonant sensors interfaced with laser-ablated microfluidics

• Main Authors: Adam R. Carr, Yash H. Patel, Charles R. Neff, Sadaf Charkhabi, Nathaniel E. Kallmyer, Hector F. Angus, Nigel F. Reuel
• Format: Article
• Language: English
• Published: Nature Publishing Group 2020-04-01
• Series: npj Digital Medicine
• Online Access: https://doi.org/10.1038/s41746-020-0270-2

Abstract Sweat loss can help determine hydration status of individuals working in harsh conditions, which is especially relevant to those who wear thick personal protective equipment (PPE) such as firefighters. A wireless, passive, conformable sweat sensor sticker is described here that can be worn under and interrogated through thick clothing to simultaneously measure sweat loss volume and 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. A four-person study is then used to determine level of sensor variance caused by local tissue dielectric heterogeneity and sensor-reader orientation.