A Soft, Skin Interfaced Microfluidic Band with Colorimetric Sensor Suites for Monitoring Dynamic Sweat Biochemistry During Exercise

Traditionally, athletes monitor their muscle fatigue levels by measuring blood lactate concentrations with exercise intensity. Determination of blood lactate threshold, exercise intensity at which blood lactate starts to increase, allows the athlete to quantitatively track their fitness levels for determining endurance capacity, evaluating training routines, and predicting athletic performance. Blood lactate threshold determination, however, requires frequent multiple blood draws by lancets on fingertips or ear lobes within a short period, which can lead to discomfort with potential infection risks. Emerging non-invasive techniques based on alternative biofluids may benefit athletes by reducing the associated pain and allowing them to perform quantitative fitness monitoring on a routine basis.
Here, we present a soft, skin-interfaced microfluidic band with in-house colorimetric sensor suites for dynamic biochemical analysis of sweat for quantifying personal fitness levels. Our advanced sensor suites successfully capture temporal characteristics of essential sweat biomarkers including sweat lactate, pH, and sweat collection time in each microfluidic reservoir. These sensor suites include a sweat-activated colorimetric timer, a nanoplasmonic lactate sensor, and a dye-conjugated sweat pH sensor.
A sweat-activated colorimetric timer measures sweat collection time in the microfluidic analysis reservoirs, addressing the need to keep track of the dynamic sweat collection process for wearable colorimetric sweat-sensing devices. The timer utilizes purple-colored maltodextrin-iodine complexes loaded within cellulose fibers as the colorimetric transducing agent. As sweat fills the timing reservoir, maltodextrin-iodine complexes gradually dissociate and free iodine, iodide, and triiodide species then diffuse into surrounding sweat and polydimethylsiloxane substrates.
A nanoplasmonic lactate sensor provides a means to quantify sweat lactate concentration with minimal interference. This sensor utilizes silver nanoplates embedded within agarose/polyethylene oxide composite hydrogel as a colorimetric reporter. The operating mechanism is based on a molecular cascade reaction where the reaction of lactate and lactate oxidase produces hydrogen peroxide, which undergoes a subsequent chemical reaction with uric acid to generate cyanuric acid that effectively etches silver nanoplates within agarose hydrogels.
A dye-conjugated sweat pH sensor measures sweat pH utilizing bromothymol blue dye conjugated on mesoporous SiO₂ nanoparticles embedded within an agarose hydrogel. The large surface area of mesoporous SiO₂ nanoparticles supports covalent bonding of bromothymol blue dyes and serves as the basis for reliable, non-bleeding PH assay.
A series of human participant studies suggest the potential utility of sweat as an alternative biofluid for monitoring personal fitness. We find that iontophoresis-generated sweat pH from working muscle gradually decreases with cycling intensity and strongly correlates with blood lactate concentrations. In contrast, sweat pH from non-working muscle does not correlate with blood lactate concentrations. These differences in sweat biochemistry collected from different regions provide valuable information on local muscle activities, allowing athletes to monitor muscle fatigue status for a specific muscle of interest. We further demonstrate the ability of sweat pH to differentiate between fit and unfit individuals in a manner similar to blood lactate testing. Fit individuals exhibit less variation in sweat pH with increasing cycling speed than unfit individuals, which agrees with peripheral conventional blood lactate measurements. These observations provide interesting opportunities to quantify personal fitness levels through monitoring dynamic sweat biochemistry.