Wearable and wireless monitoring of biomarkers such as lactate in sweat can provide a deeper understanding of a subject's metabolic stressors, cardiovascular health, and physiological response to exercise. However, the state-of-the-art wearable and wireless electrochemical systems rely on active sweat released either via high-exertion exercise, electrical stimulation (such as iontophoresis requiring electrical power), or chemical stimulation (such as by delivering pilocarpine or carbachol inside skin), to extract sweat under lowperspiring conditions such as at rest. Here, we present a continuous sweat lactate monitoring platform combining a hydrogel for osmotic sweat extraction, with a paper microfluidic channel for facilitating sweat transport and management, a screen-printed electrochemical lactate sensor, and a custom-built wireless wearable potentiostat system. Osmosis enables zeroelectrical power sweat extraction at rest, while continuous evaporation at the end of a paper channel allows long-term sensing from fresh sweat. The positioning of the lactate sensors provides near-instantaneous sensing at low sweat volume, and the custom-designed potentiostat supports continuous monitoring with ultra-low power consumption. For a proof of concept, the prototype system was evaluated for continuous measurement of sweat lactate across a range of physiological activities with changing lactate concentrations and sweat rates: for 2 h at the resting state, 1 h during mediumintensity exercise, and 30 min during high-intensity exercise. Overall, this wearable system holds the potential of providing comprehensive and long-term continuous analysis of sweat lactate trends in the human body during rest and under exercising conditions.

Biomarkers in sweat are a largely untapped source of health information. Most of the currently available sweat harvesting and testing devices are incapable of operating under low-sweat rates such as those experienced by humans at rest. Here we analyze the in vitro and in vivo sampling of sweat through osmosis via the use of a hydrogel interfaced with the skin, without need for active perspiration. The hydrogel also interfaces with paper-based microfluidics to transport the fluid via capillary forces toward a testing zone and then evaporation pad. We show that the hydrogel solute content and area of the evaporation pad regulate the long-term extraction of sweat and its associated biomarkers. The results indicate that the platform can sample biomarkers from a model skin system continuously for approximately 12 h. On-skin testing of the platform on both resting and exercising human subjects confirms that it can sample sweat lactate directly from the surface of skin. The results highlight that lactate in sweat increases with exercise and as a direct result of muscle activity. Implementation of such new principles for sweat fluid harvesting and management via wearable patch devices can contribute toward the advancement of next generation wearables.

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We construct and investigate paper-based microfluidic devices, which model long-term fluid harvesting, transport, sensing, and analysis in new wearables for sweat analysis. Such devices can continuously wick fluid mimicking sweat and dispose of it on evaporation pads. We characterize and analyze how the action of capillarity and evaporation can cooperatively be used to transport and process sweat mimics containing dissolved salts and model analytes. The results point out that non-invasive osmotic extraction combined with paper microfluidics and evaporative disposal can enable sweat collection and monitoring for durations longer than 10 days. We model the fluid flow in the new capillary–evaporative devices and identify the parameters enabling their long-term operation. We show that the transport rates are sufficiently large to handle natural sweat rates, while we envision that such handling can be interfaced with osmotic harvesting of sweat, a concept that we demonstrated recently. Finally, we illustrate that the salt film deposited at the evaporation pad would eventually lead to cessation of the process but at the same time will preserve a record of analytes that may be used for long-term biomarker monitoring in sweat. These principles can be implemented in future platforms for wearable skin-interfacing assays or electronic biomarker monitors.

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.