Purely Electrical Detection of Electrolyte Concentration Through Microfluidic Impedance Spectroscopy

The COVID-19 pandemic has accelerated the drive towards delivering remote health monitoring (RHM) on a large scale. The advantages of RHM include earlier and improved detection and diagnosis, superior disease management, reduced healthcare costs, and increased convenience for patients. RHM could facilitate a shift from reactive healthcare to predictive, preventative and personalised methods.
Sweat contains a wealth of information about the metabolic state, infections, diseases and physical exertion of an individual. In particular, the concentration of salts such as NaCl and KCl in sweat can be used to monitor hydration, amongst other physiological markers. Dehydration or overhydration and associated electrolyte imbalances can lead to severe physical and mental impairment.
Current state-of-the-art wearable sweat monitoring devices rely on techniques such as colorimetry [1] or electrochemical detection [2]. These both require intermediary steps such as photographing the sensor or chemical reactions, which introduce errors, required patient interaction and limit sensor lifetime. Therefore, in this work, we report direct and purely electrical measurements of fluid properties, to reduce complexity and cost, increase ease-of-use and maximise accuracy and repeatability.
Here, we present a low-cost microfluidic platform with integrated electrodes made from silver nanoparticulate ink, produced using additive manufacturing techniques, for the electrical characterisation of fluids via impedance spectroscopy [3]. We employ this method to detect the concentration of various electrolytes in solution using small sample volumes within microfluidic channels, with the aim of enabling the rapid and real-time characterisation of bodily fluids such as sweat.
A novel analysis method is presented, facilitating accurate determination of the concentration of a range of aqueous ionic chloride solutions. Importantly, we identify a key parameter, namely the turning point of the capacitance as a function of frequency, which is found to have a highly linear correlation (R² > 0.99) across concentrations spanning at least three orders of magnitude. The linear dependence is found to be reproducible across different ionic species, making it highly useful for accurate fluid characterisation. Ion-selective membranes are employed as a species-specific filter to aid the selectivity of the platform. Applying this experimental technique to analyse bodily fluids in real-time has the potential to bring remote health monitoring to the masses.

References:
[1] Yue, X et al, Simple, Skin-Attachable, and Multifunctional Colorimetric Sweat Sensor, ACS Sensors, 7, p.2198 (2022)
[2] Chaffari, R et al, Soft Wearable Systems for Colorimetric and Electrochemical Analysis of Biofluids, Advanced Functional Materials, 30, p.1907269 (2020)
[3] Wade, T et al, Purely electrical detection of electrolyte concentration through microfluidic impedance spectroscopy, Cell Reports Physical Science, 5, p.102133 (2024)