Study of Qᵤ-Based Resonant Microwave Sensors and Design of 3-D-Printed Devices Dedicated to Glucose Monitoring

A low-cost, additively manufactured, biocompatible glucose sensor based on the changes in the unloaded quality factor (
$Q_{\mathbf {u}}$
) with a single split-ring resonator is presented. An exposition of the fundamentals for the use of the
$Q_{\mathbf {u}}$
as sensing parameter with microwave planar resonant sensors is shown. The convenience of this sensing parameter is analyzed from the theoretical point of view, and practical design and optimization guidelines are inferred with a special focus on the optimization of
$Q_{\mathbf {u}}$
sensitivity to glucose concentration. For practical demonstration and experimental assessment, a novel inverted microstrip configuration is considered, built upon a customized structure made with a certified biocompatible material thanks to 3-D printing techniques, which is aimed to provide for a stronger interaction between the electromagnetic fields and the sample. Two metallization solutions are investigated, yielding devices operating at 4.50 and 4.62 GHz, with operating
$Q_{\mathbf {u}}$
of 16.36 and 22.00, relative
$Q_{\mathbf {u}}$
sensitivities of 1.377 and 2.727,
$Q_{\mathbf {u}}$
sensitivities to glucose content within the physiological range of
$0.3\times 10^{-3}$
per mg/dL and
$0.6\times 10^{-3}$
per mg/dL, sensing areas of approximately
$11.7\times8.8$
mm
²
and total structure sizes of 950.0
$\times 35.0\times3.5$
mm
³
. The devices show good performance with water–glucose solutions covering a wide range of concentrations, involving physiological as well as industry-related ones.