Among non-invasive wearable technologies with a large upside potential, sweat sensing technology is arguably the most underdeveloped. Even so, excitement is building, as many sweat electrolytes, metabolites, amino acids, proteins, and other biomarkers are showing promise for monitoring both mental and physical status and/or disease. However, unfortunately many of the commercially available technology solutions that could be used for sweat require ~10's of microliters sample volumes, which are entirely too large for continuous sweat analysis. Even the latest patch or tattoo advances in sweat sensing technology require several microliters of biofluid, resulting in slow sampling intervals (10's of minutes or even hours). If one of the biggest arguments for sweat sensing is the potential for continuous monitoring, then there is a clear and pressing need to discover powerful yet simple methods to continuously bring the tiniest amounts of sweat (nanoliters) to sensors. This is by no means a simple challenge, as the skin surface is highly variable and presents numerous factors that can confound sweat sampling. If such technology could be developed the payoff could be huge, allowing: (1) correlations with blood with as fast as ~2 min. time resolution; (2) for sedentary users, more sampling events per day of natural sweat; (3) for chemically stimulated sweat, a >100X reduction in the amount of chemical stimulant delivered compared to commercial products; (4) importantly, reducing confounding contamination coming from the dead skin surface. Clearly, improved sweat sampling is a central-challenge worthy of attention by researchers. Furthermore, such a research topic is also ideal for cultivating and inspiring the next generation of scientists and engineers through integration of research with both high-school and undergraduate student research and teaching.
The specific objective of this NSF proposal is to create for the first time, the ability to achieve chronologically correlated sweat sensing with minute-level resolution. The proposal's central hypothesis is that through a clever interplay of microfluidics, the fluidic volume between eccrine sweat ducts and sensors can be reduced from ~1's of microliters down to 10's of nanoliters (10-100X), resulting in only minutes before new sweat reaches the sensors. The rationale for pursuing this hypothesis is simple, sweat sensing has enormous upside potential for applications in patient and wellness monitoring, athletics, narrow therapeutic-range pharmaceutical monitoring, etc., but cannot reach its full potential unless lower sweat rates can be used, faster sampling times enabled, and analyte contamination from the skin surface mitigated. The proposal also advances knowledge in sweat sensing science by: (1) creating temporal models which predict the time for partitioning of ionic, molecular, and protein analytes from blood, into sweat, and onto the sensors;(2) advancing the frontiers of minimum biofluid sampling and sensing from the microliter to the nanoliter regime;(3 experimentally validating both the created models and technology, and therefore igniting further pursuit of sweat sensing science and technology.
