Design and Fabrication of a Minimally Invasive Dielectric Sensor for Biological Environments

In this paper, a minimally invasive dielectric sensor with a compact footprint of
$24 \times 18$
mm2 is presented. It is based on two complementary split ring resonators (CSRRs) fed by a microstrip with coplanar waveguide configuration and placed on a
$200 \mu \text{m}$
thick Kapton substrate. The device has been fabricated by means of a multi-material 3D printer NanoDimension’s Dragonfly IV and is characterized by exploiting two different sensing methodologies. As a first step, the device has been characterized in the air by measuring the volume of deionized water droplets placed on one of the CSRRs in the range between
$3.6 \mu \text{l}$
and
$9.6 \mu \text{l}$
. This study has been performed through the analysis of the resonant frequency shift caused by the variation of the dielectric constant. The results show a Q-factor of 702 and a sensitivity of 0.3%
$\mu \text{l}^{-1}$
. Then, an additional step has been carried out for the measurement of the temperature of the water in which the sensor is totally dipped. We analyzed the frequency shift due to the water temperature variation in the range between 20°C and 40°C which corresponds to a variation in relative permittivity. In this case, the sensor shows remarkable results in terms of Q-factor, equal to 501, and a sensitivity to dielectric variations in out-of-body and in-body temperature ranges equal to 0.5% and 1.5%, respectively.