An ultrahigh frequency dielectric sensor for microdroplet detection using a split ring resonator-based microfluidic chip

Metamaterials have drawn interest in the sensor community due to their extreme dielectric-sensitive resonant behavior. Although these structures are studied in a wide range of frequencies, the ultrahigh frequencies are of special interest due to their compatibility with RF electronics. Unlike spectroscopic methods, where each material has its specific fingerprint, the response of these resonant structures depends on the electromagnetic properties, the volume of the material under test, and the resonator's design itself. Thus, implementing a metamaterial-based sensor for biological and chemical applications requires some mechanism to fix the sample's location and volume. Since most biological and chemical samples are liquids, microfluidics is the most promising candidate for this task. Here, we propose a dielectric sensing platform with a cost-effective fabrication method that allows fluid detection inside the microfluidic channel. The device proposed here is designed numerically, fabricated and measured, and finally validated via an analytical lumped model. It consists of a microstrip line coupled with a split ring resonator as the transducer and a microfluidic structure to control the sample and generate microdroplets. The fluid under test inside the microfluidic channel can be characterized based on the change in its dielectric constant or loss factor. The device shows a 600 kHz resonance shift in response to the dielectric change in sample volumes as low as 10 nl. We also demonstrate the platform's capability to generate and detect octanol–water microdroplets. The method reported here offers a fast prototyping method suitable for various microfluidic sensing applications.