Non-contact monitoring of D-glucose concentration in saline solution using a passive transmission-line based RF resonator

This paper investigates the relevance of an inductive sensor for non-contact blood glucose continuous monitoring purposes. The considered sensor is based on a flexible passive radiofrequency single-turn transmission-line resonator (STLR), which allows wearable, low cost, non-invasive and wireless sensing implementations to be considered. The STLR sensor is coupled to an inductive reading probe and used as a wireless transmit-and-receive high sensitivity sensor. The relevance of the used STLR sensor is evaluated using a high-Q 60 MHz STLR fabricated on a flexible Kapton substrate, implemented to monitor reference liquid samples mimicking the dielectric properties of blood, and featuring various D-glucose concentration in the 0.24 g/L to 5 g/L range, consistent with human blood glucose concentration levels. Thanks to an equivalent electrical circuit modelling the coupling between the STLR and the liquid sample, three sensor output parameters are defined and assessed at 60 MHz to monitor the dielectric changes induced by the modifications of the samples with D-glucose concentration. The obtained results are compared to those provided by a conventional open ended coaxial probe immersed in the liquid and operated at the same frequency, and to data published with similar coaxial sensors. The obtained results show full consistency between the proposed STLR non-contact sensor and contact dielectric sensors, and open the way to a new generation of non-contact and wearable sensors dedicated to low cost and easy to use continuous blood glucose measurement applications.