Flow injection system for real-time monitoring of lactic acid at the outer surface of the skin

A flow injection system using electrochemical biosensor for in-time monitoring of lactic acid at the skin's surface was fabricated and tested. The system consists of a microfluidic `skin-patch' sampling device and an electrochemical biosensor using a carbon graphite electrode. In order to enhance the stability of monitoring, lactic acid excreted at the skin's surface was directly solved into the career flow (phosphate buffered saline) in the `skin-patch' sampling device. Therefore our system measures the amount of lactic acid secretion instead of concentrations of lactic acid in the sweat. The electrodes of the biosensor was modified using osmium wired horseradish peroxidase (Os-HRP) redox polymer. The detection limit of the biosensor for lactic acid was 1.0 μM and the sensitivity was 3.125 μA / mM at 1.0 mM. Real-time monitoring of excreted lactic acid during the stationary bike workout was carried out. The flow injection system was attached on the upper arm of the volunteer. Owing to the `skin-patch' flow-channel, transudation of lactic acid was successfully monitored continuously in real-time with sufficient stability. As a result, the lactic acid level increased during the exercise and decreased to the initial level after the exercise. From this result, the system is expected to be used in sports science field such as scientific sports training.