Development, Characterization, and Circuit Modeling of Inkjet-Printed Coupled Ring Resonators for Application in Biological Samples

In the present study, the inkjet-printing (IJP) technique is used for the development of a planar microwave sensor aimed at the dielectric characterization of biological samples. The proposed sensor consists of two capacitively coupled split-ring resonators (SRRs) fabricated using the microstrip technology. A silver-based conductive ink is deposited by IJP on a conventional 1.6-mm-thick FR4 substrate, thus creating the resonant structure. The experimental analysis is carried out by considering a water-ethanol mixture as a test solution in which the ethanol volume fraction is varied, thereby changing the effective permittivity of the mixture. In this contribution, the authors focused their analysis on the dielectric constant of the solution under test (i.e., the real part of the complex relative permittivity). The water-ethanol mixture is placed into a low-density polyethylene (LDPE) sample vial that is arranged on the surface of the planar sensor. No direct contact is needed between the microwave device and the solution under test. The effect of the ethanol volume fraction change is related to the behavior of the two resonances occurring in the forward transmission coefficient (S21) of the sensor in the frequency range from 2 to 3 GHz. The permittivity change of the sample under test affects the capacitive coupling between the two SRRs altering the separation between two resonant frequencies. This enables the sensor to differential measurements, thereby improving its robustness. The frequency separation is used to track variations in the ethanol concentration and, thus, the dielectric constant of the solution under test. In addition, a lumped-element equivalent-circuit model is presented, and the changes in the values of the lumped elements with ethanol concentration are estimated and discussed.