Continuous Sweat Lactate Sensor Based on Screen-Printed MgO-Templated Carbon with Integrated Flow System
Authors: Loew Noya , Yoshihara Yuko , Mitsumoto Masaya , Hoshi Yoshinao , Watanabe Hikari , Shitanda Isao , Mikawa Tsutomu , Motosuke Masahiro , Itagaki Masayuki 
Published: December 2020

Lactate has long been known as the indicator for distinguishing aerobic and anaerobic metabolism, and thus is a useful marker for metabolic disorders and the state of exercise [1]. Especially, the continuous monitoring of sweat lactate is a useful non-invasive method for real-time tracking the progress of exercise, e.g. of athletes, or of physical therapy patients. In biosensors for continuous monitoring, the biological recognition element, e.g. lactate oxidase (LOx) has to be immobilized on the electrode. Our group previously reported of a glucose sensor in which glucose dehydrogenase (GDH) was immobilized covalently on screen-printed MgO-templated mesoporous carbon (MgOC) modified by graft polymerization, and showed improved stability [2]. Likewise, in this study, radicals were introduced on the surface of the MgOC by electron beam irradiation. Next, glycidyl methacrylate (GMA) was polymerized on the radicalized MgOC, which resulted in epoxy groups on the surface. Ink was prepared by dispersing this poly(GMA)-MgOC (GMgOC) and a polyvinylidene difluoride (PVdF) derivative in N-methyl-2-pyrrolidone (NMP). The ink then was used for the final layer of the working electrode (WE) of a screen-printed 3-electrode strip. Next, 1,2-naphthoquinone was deposited on the WE as mediator, which is insoluble in water and thus is unlikely to dissolve into sweat during the measurement. Finally, LOx derived from Enterococcus faecium was immobilized on the WE by binding covalently to the epoxy groups. This combination of LOx immobilized on screen-printed GMgOC and naphthoquinone promises the construction of stable lactate biosensors suitable for continuous measurements.
Furthermore, in this study, two other points were considered for the design and construction of the sensor device. Firstly, the sensor should be constantly supplied with a minimum amount of sweat, without the supply being interrupted. Secondly, the materials and texture of the parts of the sensor in direct contact with the skin should not be irritating. An integrated flow system can solve both points; it ensures a constant supply of sweat, and limits the material and texture in direct contact with the skin. For the construction of the flow system, first, a mold was formed on a silicon wafer using an epoxy-based negative photoresist. Next a degassed mixture of polydimethylsiloxane (PDMS) monomer and catalyst was poured over the mold and cured at 80°C for 1 hour.
In first trials, the sensor and flow system were pressed together tightly onto artificial sweat glands. Leakage was prevented by constructing a connector for the sensor out of conducting adhesive tape. Potassium phosphate buffer (100 mM, pH 7.0) containing various amounts of sodium lactate were pumped through the system at 80 µL/min using a syringe pump. Figure 1a shows the response of this system to varying concentrations of lactate. The spikes at the change of concentration are due to a change in the flow when switching the syringe. The delay between the switch of the syringe and the change in response current is due to the lag time until the lactate reached the sensor. The plot of the response current vs the lactate concentration (Fig. 1b) shows a concentration dependent response up to 20 mM lactate. These results indicate that this system is suitable for measuring sweat lactate continuously.

Acknowledgement: This work was partially supported by JST-ASTEP Grant Number JPMJTS1513, JSPS Grant Number 17H02162 and Private University Research Branding Project (2017-2019) from Ministry of Education, Culture, Sports, Science and Technology, and Tokyo University of Science Grant for President's Research Promotion．

References

[1] M.E. Payne, et al.; Sci. Rep., 9, 13720 (2019).
[2] I. Shitanda, et al.; Bull. Chem. Soc. Jpn., 93, 32-36 (2020).

Figure 1