Inkjet-Printed Capacitive Coupled Ring Resonators Aimed at the Characterization of Cell Cultures

The inkjet printing (IJP) is a highly attractive printing technology, consisting in the deposition of ink layers on flexible or rigid substrates. IJP technology is used in the present study to develop a planar microstrip sensor aimed at the dielectric characterization of biological samples, such as cell cultures. In this contribution we focused on the dielectric constant of the solution under test (i.e., the real part of the complex relative permittivity). The proposed sensor consists of two capacitive coupled ring resonators, which are achieved by printing a silver-based conductive ink on a 1.6-mm thick FR4-substrate. The measurement-based analysis is performed by studying the two resonances occurring in the forward transmission coefficient in the frequency range from 2 GHz to 3 GHz. By considering a water-ethanol mixture as a case study, the developed sensor is successfully validated for dielectric characterization of the material surrounding the prototype itself. No direct contact is required between the microwave transducer and the solution under test. In the present work, the water-ethanol mixture is put into a low-density polyethylene (LDPE) sample vial and placed over the sensor. It is found that the difference between the two resonant frequencies can be used as sensing parameter for monitoring variations in the ethanol concentration and then in the resulting dielectric constant of the solution under test.