A Flexible Tuned Radio-Frequency Planar Resonant Loop for Noninvasive Hydration Sensing

In this work, a planar radio-frequency sensor based on a flexible polyimide substrate has been developed to monitor the water content changes noninvasively and efficiently in a human body or tissues. The sensor is based on the detection of electromagnetic resonance that is susceptible to dielectric property changes by water content variations. The planar loop resonator tuned with a metal pad features improved resonance, compact size, and flexibility to conform to a curved surface. Designs and experiments to continuously monitor human hydration processes in vivo have been demonstrated. The recorded data shows distinct trends when a person becomes hydrated from a dehydrated state. Discrete and continuous measurements are compared with the simulations conducted by using documented, generalized human skin permittivity properties. Discrepancies between the measurements and simulations are investigated and verified with directly measured skin permittivities. Measurements variations are also investigated. With the advantages of being compact, flexible, and planar, the sensor can be integrated into a wearable on the human forearm. Additionally, the sensor has been used to demonstrate its ability to detect water content changes in a phantom made of ground pork. The ex vivo results indicate the sensitivity and consequential variations in practical scenarios. The promising results show great potential for human body monitoring and also general applications in agriculture and the food processing industry from the demonstration of the sensing principle in the pork phantom.