While recent sweat analysis has overwhelmingly focused on measuring biomarker concentrations, one of the most physiologically informative parameters is actually sweat secretion rate. Sweat rate is important to track as it modulates the concentrations of secreted analytes, but even stand-alone it can indicate evolving or unfavorable health conditions including cardiac complications, nerve damage, and dehydration. Precise and continuous sweat sensors are therefore an important component of wearable sweat sensing technology. Various sensing schemes and form factors can be used for sweat rate measurement. One model involves capturing sweat in a spiraling microfluidic channel that contains two parallel impedimetric electrodes. As sweat flows in the channel and increasingly covers and connects the two electrodes, the impedance between them drops at a rate that can be related to sweat rate. Another model has narrow metal fingers, or gates, that alternately protrude from each electrode. As sweat flows past each finger, the electrodes register a discrete jump in impedance. The rate of these jumps can be converted into a discrete but highly selective measure of sweat rate. Along with different sensing schemes, different device form factors and methods of attachment onto the body can ensure that sweat is collected fully, without loss or leakage, to ensure accurate sweat rate measurement. Flexible electrode and microfluidic layers can be stacked and attached to the skin surface as patches using aggressive and water-resistance medical adhesives. Alternately, they can be affixed to a rigid gasket that straps onto the forearm like a wristwatch. This gasket is shaped with a concave underside that collects and forces sweat up into the entrance of the microchannel, and the rigid seal against the body surface ensures no sweat escapes. The different fluidic pattern considerations must overall ensure that sweat rate can be measured accurately for prolonged on-body wear over a broad range of secretion rates. Roll-to-roll (R2R) fabrication processes are key for producing sweat rate sensors at high throughputs and volumes. This includes R2R printing of metallic inks for the electrodes, combined with R2R laser cutting to pattern microfluidic layers and spacers. Rigid gaskets can also be produced at scale through injection molding, and all components then rapidly assembled with adhesive tapes. Overall, we present mass produced sweat rate sensors with different sensing schemes and form factors to accommodate diverse wearable sweat sensing applications.
March 2, 2022
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