(283e) Osmotic-Capillary Principle Enabled Epidermal Sweat Sampling and Sensing

The current market for biosensor-based health monitoring is on a booming trend. The demand for biosensors is ever rising majorly due to the following two reasons: (a) Biosensors as either wearables, or in-situ point-of-care (POC) testing platforms allow rapid, on-site monitoring of several disease biomarkers under decentralized settings, (b) Such platforms can even support continuous tracking of one’s health status without causing discomfort or intervention of trained medical professionals. Despite the progress of alternative (to blood) biofluids in proving to be essential repositories of diseases biomarkers, a majority of these remain difficult to release and sample from skin non-invasively.^[1] As an alternative, sweat can be easily generated on-skin via physical exertion and can prove to be a useful candidate for disease detection. However, conventional sweat-based devices function with active perspiration (sweat on skin prior testing), which makes them inoperative under low sweat rates, with enhanced chances of even facing biomarker dilution and contamination during sensing. We demonstrate a new principle for the design of flexible and wearable patches, which are capable of extracting sweat under both resting and actively perspiring conditions using osmotic pressure difference for pumping, and evaporation for liquid disposal.^[2] Our patch is composed of silicone, hosting polyacrylamide hydrogel patch, and paper microfluidic conduit with a site of evaporation at the end (evaporation pad). The hydrogel (pump) is equilibrated with glycerin, glucose, or NaCl solution to build up the desired osmotic strength to extract sweat from the skin.² In-vitro testing with gelatin-based model skin platform revealed that both glucose and glycerin-infused gels facilitate high analyte accumulation on the evaporation pad, with glucose as osmolyte having the highest driving pressure. The sampled sweat analytes can be analyzed directly on the paper channel with both colorimetric^[3] and enzymatic electrochemical sensing^[4] methods. The analyte amount also depends on the dimensions of the paper channel, hydrogel area and paper pore size. Human trials have shown the potential to extract sweat and analyze it for lactate under both resting and non-resting conditions within a period of two hours. We used lactate as a concept demonstrator, as sweat appears to be an established alternative informative medium for lactate quantification than blood. The ability to measure lactate enables monitoring metabolism and oxidative stress levels in athletes and military personnel. Our group is currently investigating how this sweat sampling concept can be further integrated with microneedle patches for long-term interstitial fluid (ISF) sampling and on the fingertip for biofuel cell applications.

[1] T. Saha, R. Del Caño, E. la De Paz, S. S. Sandhu, J. Wang, Small 2022, 2206064, 2206064.

[2] T. Shay, T. Saha, M. D. Dickey, O. D. Velev, Biomicrofluidics 2020, 14, 034112.

[3] T. Saha, J. Fang, S. Mukherjee, M. D. Dickey, O. D. Velev, ACS Appl. Mater. Interfaces 2021, 13, 8071.

[4] T. Saha, T. Songkakul, C. T. Knisely, M. A. Yokus, M. A. Daniele, M. D. Dickey, A. Bozkurt, O. D. Velev, ACS Sensors 2022, 7, 2037.

[5] J. Moon, R. Del Caño, C. Moonla, K. Sakdaphetsiri, T. Saha, L. Francine Mendes, L. Yin, A. Chang, S. Seker, J. Wang, ACS Sensors 2022, 7, 3973.