Abstract
Lactate is an essential biomarker for determining the health of the muscles and oxidative stress levels in the human body. However, most of the currently available sweat lactate monitoring devices require external power, cannot measure lactate under low sweat rates (such as in humans at rest), and do not provide adequate information about the relationship between sweat and blood lactate levels. Here, we discuss the on-skin operation of our recently developed wearable sweat sampling patch. The patch combines osmosis (using hydrogel discs) and capillary action (using paper microfluidic channel) for long-term sweat withdrawal and management. When subjects are at rest, the hydrogel disc can withdraw fluid from the skin via osmosis and deliver it to the paper. The lactate amount in the fluid is determined using a colorimetric assay. During active sweating (e.g., exercise), the paper can harvest sweat even in the absence of the hydrogel patch. The captured fluid contains lactate, which we quantify using a colorimetric assay. The measurements show the that the total number of moles of lactate in sweat is correlated to sweat rate. Lactate concentrations in sweat and blood correlate well only during high-intensity exercise. Hence, sweat appears to be a suitable biofluid for lactate quantification. Overall, this wearable patch holds the potential of providing a comprehensive analysis of sweat lactate trends in the human body.
Highlights
Our wearable patch functions based on the simultaneous action of three basic effects to deliver long-term sweat and biomarker sampling: osmosis, capillary wicking, and evaporation [14]
The hydrogel withdraws sweat and its associated biomarkers due to its higher osmotic strength (Figure 1). These withdrawn sweat and biomarkers are sampled on a paper microfluidic channel
Mglucose glucose hydrogel hydrogel patch patch and and blood blood lactate lactate Figure concentration. (a) Plot showing the estimated sweat lactate concentration in a 4 M glucose hydrogel patch under different concentration. (a) Plot showing the estimated sweat lactate concentration in a 4 M glucose hydrogel patch under different physiological conditions. (b) Plot presenting a comparative analysis of the lactate concentration readout from our patch physiological conditions. (b) Plot presenting a comparative analysis of the lactate concentration readout from our patch to the lactate concentration data from previously published lactate studies. (c) Plot showing the meas(dotted circular box) to the lactate concentration data from previously lactate studies
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Most of the current sweat lactate sensors function only under “active sweating”; i.e., they either require the subjects to undergo strenuous physical exertion, ingest sweat stimulating agents [36,57], or elevate body temperature [13] prior to testing As a result, these prototypes can quantify the lactate concentration in sweat only during exercise and not under any other physiological condition (such as rest or post-exercising conditions). Some prototypes have quantified sweat lactate release from rested subjects [34,36,58] These prototypes are not a wearable platform, have a low sampling duration, do not report their sweat lactate results post-exercise, and do not study the correlation between blood and sweat lactate levels. We seek to understand the correlation between sweat and blood lactate levels
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