Purely electrical detection of electrolyte concentration through microfluidic impedance spectroscopy

Impedance spectroscopy enables the electrical properties of samples to be probed and is commonly used to characterize solids. Extending this technique to analyze fluids within microfluidic channels could enable the rapid characterization of bodily fluids such as sweat. Here, we present a low-cost microfluidic platform with integrated aerosol-jet printed electrodes for the electrical characterization of fluids via impedance spectroscopy. A novel analysis method is presented to accurately determine the concentration of several aqueous ionic chloride solutions, namely NaCl, KCl, CaCl2, and MgCl2. Importantly, we identify a key parameter, the turning point frequency of the capacitance-frequency graph, which is found to have a highly linear correlation with the solution concentration for each species spanning at least three orders of magnitude. This linear dependence is highly reproducible across different cationic species, making it useful for accurate fluid characterization. Applying this technique to analyze bodily fluids in real time has implications for remote health monitoring.