An RF/microwave microfluidic sensor based on a center-gapped microstrip line for miniaturized dielectric spectroscopy

This paper reports on the design, fabrication and testing of a microfluidic sensor as part of a miniaturized measurement platform currently under development for dielectric spectroscopy (DS) at RF/microwave frequencies. The sensor employs a microstrip line with a 50-μm gap at the center as a parallel-plate capacitive sensing element and a 3-μL PDMS microfluidic channel on top for delivering the solution-under-test (SUT). Requiring only an S₂₁ measurement and after a 5-point calibration for a dielectric constant range of 1 to 48, sensor dielectric readings in the frequency range of 14MHz to 4GHz agree very well with bulk-solution reference measurements conducted with an Agilent dielectric probe kit. The maximum errors over the full frequency range for ethanol, ethylene glycol and ethyl acetate are -6.9%, -8.4% and -7.4%, respectively. The errors reduce to 1.7%, -4.7% and -7.1%, respectively, for frequencies above 2GHz.