Sweat Lactic Acid Monitoring System for Assessment of Exercise Intensity
Authors: Enomoto Keigo , Jingushi Yota , Kudo Hiroyuki 
Published: February 2020

A microfluidic lactic acid (LA) monitoring system for in-situ monitoring of LA excretion in sweat was fabricated and tested. LA is a chemical substance that is widely distributed in human body and it is known as a marker of metabolism which reflects the exercise intensity. For this reason, blood LA　is often used in the conditioning of elite athletes. However, it is still difficult to be used in the public sports, or daily diet because blood sampling is requested for blood LA determination. On the other hand, eccrine sweat is known as a biomarker-rich body fluid that is one of the most promising candidate for non-invasive biomonitoring. Previously numbers of wearable devices that measures ionic solutes in sweat (Cl-, K+, Mg2+, NH4+, and Zn2+) or other contents such as glucose or lactic acid. Although this sort of devises has been intensively investigated, there are several struggle points to be cleared for practical use. One of the most significant challenge is to stabilize the baseline of the signal. Since a part of sweat fluid immediately evaporates from the skin surface, the concentrations of sweat contents are influenced by the humidity or the secretion volume (i.e. surface area).
Our approach to solve this problem is using an open flow-channel fixed on the skin surface for transporting whole sweat contents to the sensing devise. Our system consists of two elements: a printed biosensor for LA based on a redox reaction of osmium wired horseradish peroxidase (Os-HRP) with a flexible micro flow channel formed using PDMS, and a sampling devise with the open flow-channel. We used polyethylene terephthalate (PET) as a substrate material of the biosensor and printed carbon graphite electrodes on the substrate by conventional screen printing method. Owing to the Os-HRP redox reaction, the sensor is operated under the low potential of 0V vs. silver / silver chloride electrode, which is possible to reduce the effect of interferences such as uric acid.
The biosensor has four electrodes (working electrode on which lactate oxidase (LOD) is immobilized, working electrode without LOD, silver / silver chloride reference electrode and counter electrode). LA is measured by amperometric method as a change of hydrogen peroxide which is produced by enzymatic reaction of LOD. For accurate sensing, differences of output signals between LOD electrode and electrode without LOD was recognized as an output current. Characterization of the biosensor was first conducted using standard LA solution. As a result, the biosensor had sufficient sensitivity (LA : 0.13nA µM-1) for monitoring of skin’s surface lactic acid continuously. The linear detection range was 10 µM -1.0 mM which contained the sweat LA level diluted in the PBS career flow. Selectivity to LA was also investigated. The biosensor showed sufficient selectivity (> 40.5) towards 0.5 mM LA under the presence of uric acid, urea, ammonia, glucose, ethanol (0.5 mM), because of specific activity of LOD from Aerococcus viridans.
Continuous monitoring of LA at the skin’s surface was also carried out. First, the PDMS flow-cell with an open flow-channel (1 mm wide and 20 mm long) was attached on the surface of the upper arm of healthy male volunteer. Phosphate buffer saline (PBS) was supplied into the flow-channel as carrier flow and the flow rate of PBS was regulated at 10 µL min-1. Excreted sweat content was solved into the PBS and transported to the biosensor. Monitoring of LA was conducted during a cycling exercise (20 min) and interval (20 min). Intensity of the exercises were adjusted by monitoring heartrates. As a result, elevation of the output signal was confirmed during the exercises and the signal decreased to the background level during the intervals (Figure). The output current of LA increased to 24.2 nA during the workout (heartrate : 160). This indicates that our system was suitable for continuous monitoring of exercise intensity. It is also possible to employ bluetooth-interface to construct of a wireless system.
Design and characteristics of the monitoring system, and the latest results of in-situ LA monitoring at the skin’s surface during the ‘bike’ exercises will be presented at the meeting.

Figure 1