Implementation of a microfluidic conductivity sensor — A potential sweat electrolyte sensing system for dehydration detection

As dehydration continues to plague performance athletes and soldiers, the need for improved dehydration detection is clear. We propose the use of a conductometric sensor as the foundation of a sweat-sensing patch to address this need. The conductometric sensor evaluates the conductivity of solutions with varying sodium concentrations. A lithographic process was used to fabricate a Polydimethylsiloxane (PDMS) microfluidic channel through which solution was flowed. The ionization of the solution that occurs when a voltage is applied results in an effective resistance across the channel. The measured resistance therefore, reflects the ionization of the solution and the corresponding sodium concentration. The potential application of the conductometric sensor in a sweat-sensing patch requires compatibility with a microcontroller and Bluetooth module. Thus, a circuit interface was created. A voltage divider was utilized to convert the output resistance of the sensor to a voltage that could be input into a microcontroller. An AC voltage signal with a frequency of 10 kHz was used as the source voltage of the voltage divider to minimize the faradaic impedance and the double layer effect of the ionized solution. Tests have revealed that the conductometric is capable of precisely measuring the conductivity of a sodium solution. The conductometric sensor will be applied to a sweat sensing patch through future work involving studying the link between sodium concentration in sweat and an individual's dehydration level, developing a sweat-collection method, and developing a method of consideration for the other ions contained in sweat.

I. Introduction

Often difficult to detect, dehydration continues to hamper athletes and soldiers in high-pressure environments. Current methods of dehydration include blood tests and urinalysis, both of which are inconvenient and fail to provide real-time dehydration feedback [1]. Thus, there is a strong need for real-time detection of hydration levels, which we propose can be addressed by the development of a sweat-sensing patch. This paper will introduce a conductivity sensor that has been developed as the foundation of the hydration technology utilized by the sweat-sensing patch. The application of the conductivity sensor to hydration detection relies upon the relationship between the electrolyte concentration of an individual's sweat and his or her hydration. In general, there are four major constituents of sweat; sodium, potassium, lactate, and urea [2], [3]. Compared with other constituents, sodium is the most concentrated electrolyte, ranging from 20 mM to 100 mM. This concentration increases significantly as the dehydration increases [4]. Although potassium ions are also present in the sweat, their concentrations are 1 to 2 orders of magnitude less than sodium ions [4], [5]. Therefore, sodium monitoring provides an efficient way for dehydration detection.