In-Situ Self-Aligned NaCl-Solution Fluidic-Integrated Microwave Sensors for Industrial and Biomedical Applications

This work presents, for the first time, an in-situ self-aligned fluidic-integrated microwave sensor for characterizing NaCl contents in NaCl-aqueous solution based on a 16-GHz bandpass combline cavity resonator. The discrimination of the NaCl concentration is achievable by determining amplitude differences and resonant frequency translations between the incident and reflected microwave signals at the input terminal of the cavity resonator based on the capacitive loading effects of the comb structure inside the cavity introduced by the NaCl solution under test. Twelve NaCl-aqueous liquid mixture samples with different NaCl concentrations ranging from 0% to 20% (0 - 200 mg/mL), which are generally exploited in most industrial and biomedical applications, were prepared and encapsulated inside a Teflon tube performing as a fluidic channel. The Teflon tube is subsequently inserted into the cavity resonator through two small holes, fabricated through the sidewalls of the cavity, which can be used to automatically align the fluidic subsystem inside the combline resonator considerably easing the sensor setup. Based on at least five repeated measurements, the NaCl sensor can discriminate the NaCl content of as low as 1% with the measurement accuracy of higher than 96% and the maximum standard deviation of only 0.0578. There are several significant advantages achieved by the novel NaCl sensors, e.g. high accuracy, traceability and repeatability; ease of sensor setup and integration to actual industrial and biomedical systems enabling in-situ and real-time measurements; noninvasive and noncontaminative liquid solution characterization as well as superior sensor reusability due to a complete physical separation between the fluidic and microwave subsystems.