Bursting the Bubble in Measuring Sweat Lactate Levels

Analyzing human sweat can reveal a lot about a person’s hydration level, electrolyte balance, and overall physiological state. For example, measuring the lactate levels in sweat can help estimate the amount of lactic acid in the blood and therefore predict muscle fatigue.

Wearable sensors can measure these levels and other biomarkers, and these devices often employ microfluidics technology that conveys small amounts of sweat to tiny chemical sensors. However, the microfluidic channels of such devices could trap air bubbles present in sweat, and if those bubbles cover the sensor’s electrodes, continuous measurements could be interrupted.

A research team led by Associate Professor Isao Shitanda from Tokyo University of Science (TUS) in Japan may have found a solution that could support athletic training management and health monitoring. Their study, published in ACS Sensors in June 2023, was co-authored by Dr. Masahiro Motosuke, Dr. Tatsunori Suzuki, Dr. Shinya Yanagita, and Dr. Takahiro Mukaimoto from TUS.

Their proposed wearable device consists of a conventional lactate oxidase sensor attached via double-sided tape to a microfluidic system made of a silicone polymer. Sweat enters the microfluidic channels via four inlets and flows toward a reservoir near which the electrodes are located. Old sweat exits the system through an outlet as new sweat enters, and a small wireless transmitter reports the measured lactate levels, the team explained in a news release.

The design employs a larger-than-usual sweat reservoir. “By increasing the length of the reservoir in the microfluidic channel, a space of approximately four microliters was created for trapping any air bubbles that infiltrate the device, thereby preventing them from contacting the electrodes of the sensor,” explained Shitanda in the release. 

In a series of laboratory experiments, the researchers verified that the bubble-trapping region worked as intended by injecting bubbles into the device while measuring lactate levels in artificial sweat. They found that the measurements were not affected by the sweat flow rate, and the response of the sensor remained stable for approximately two hours, they reported. The device was also tested on a male volunteer who exercised on a stationary bike for almost an hour. The results showed that the sensor captured a lactate concentration correlation ranging from 1 to 50 mM as well as a correlation between sweat and blood lactate levels.