A Stretchable, Wearable Lactate Sensor with a Textile-Based Biofuel Cell
Authors: Kato Yuto , Ogawa Yudai , Kai Hiroyuki , Nishizawa Matsuhiko 
Published: February 2020

Wearable sensors for monitoring health status, exercise, etc. have received much attention. Commercial wearable sensors such as watches, patches, etc., measure physical quantities such as heart rate, body temperature, etc. In addition to those measurements, lactate is a promising chemical indicator of intensity of exercise. Lactate is generated in the body during exercise, and it is widely used for monitoring the level of exercise. Traditionally, blood samples have been taken for measurement of lactate concentration of athletes, but it is difficult to apply to continuous monitoring. Recently, it has been shown that the lactate concentration in blood is correlated with that in other body fluids such as perspiration, saliva, and tear, which shows feasibility of non-invasive and continuous lactate sensing with them. Biofuel cells, which use enzyme catalysts to convert chemical energy of biological molecules into electricity, can be a promising tool as a self-powered lactate sensor. Biofuel cells have advantages such as lightness, safety of materials, mild and ambient working conditions, high specificity to lactate by choosing appropriate enzymes. In a pioneering work of the lactate sensor for perspiration with a biofuel cell, a tattoo-shaped lactate sensor on skin was developed.1 The generated electrical current was correlated with the concentration of lactate, which enabled quantification of lactate concentration in perspiration. In this study, we developed a self-powered lactate sensor in perspiration using a stretchable biofuel cell. Stretchability of the sensor helps it fit to the skin, which enables the efficient interface with the skin during exercise (Fig.1).
First, we fabricated a stretchable porous electrode for a biofuel cell from a stretchable textile and carbon nanotubes (CNTs), where fiber of the textile was coated with CNTs.2 CNTs give high conductivity and porous structures to the electrode, which are essential for high performance of the biofuel cell. Scanning electron microscopy (SEM) observation indicated that CNTs between fiber were peeled off while those on fiber remained intact in the pre-stretching (the first stretching). After the initial drop in conductivity on the pre-stretching, conductivity of the electrode remained stable. Next, the anode and cathode were prepared. The lactate oxidase (LOx) anode was prepared by immobilizing LOx on the stretchable CNT electrode fabricated above. LOx oxidizes lactate to pyruvate accompanied by the release of electrons, and the electrons are carried to the electrode via tetrathiafulvalene (TTF), the electron transfer mediator. To fabricate a cathode, bilirubin oxidase (BOD) that reduces O2 in the air to H2O was immobilized on the CNT electrode. After that, we further deposited CNTs to make the surface hydrophobic to form the three-phase boundary between solution, air, and the electrode. This three-phase boundary is an important factor in current generation with O2 in the air. To keep the three-phase boundary upon stretching, we used poly(tetrafluoroethylene) (PTFE) as a binder of CNTs.
We investigated the performance of the LOx anode and the BOD cathode. Cyclic voltammetry (CV) of the LOx anode with and without lactate indicated that the anode generated current from lactate, and the anode maintained the current generation over 3 hours. We evaluated the stretchability of the anode and cathode by applying 50% stretching over 30 cycles. The generated current was constant after 5 cycles of pre-stretching. This is due to firm immobilization of the CNTs to the textile, the enzymes to the CNTs, and stable binding between the CNTs by PTFE. The LOx anode and BOD cathode were combined to assemble a biofuel cell, and it was put on the skin after intense exercise with O2-permeable medical tape. A LED device was connected to the biofuel cell on the skin, and blinking of the LED indicated that the biofuel cell generated current from lactate in perspiration (Fig.2). Finally, quantification of lactate concentration in perspiration by the biofuel cell was carried out. The biofuel cell was applied to the skin during exercise, and the current was measured and converted to lactate concentration from a calibration curve. The result obtained by the biofuel cell was correlated well with that by a commercial lactate sensor. Our stretchable lactate sensor could possibly be used for continuous monitoring of lactate on the skin.
[1] Jia et al., Anal. Chem., 85, 6553-6560 (2013).
[2] Ogawa et al., Biosens. Bioelectron., 74, 947-952 (2015).

Figure 1