Osmotic-Capillary Principle Inspired Epidermal Biofluid Sampling and Continuous Sensing

The current market for biosensor-based health monitoring is continuously rising. The demand for biosensors is proportionally 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 allow continuous tracking of one’s health status non-invasively and without the intervention of trained medical professionals. Despite the research progress with alternative (to blood) biofluids as essential candidates for disease biomarker detection, a majority of these remain difficult to be accessed continuously.^[1] As an alternative, sweat can be generated continuously on-skin via physical exertion and is an useful repository for disease biomarkers. However, conventional sweat-based devices function with active perspiration (sweat on skin prior testing), which makes them inoperative under exertion-free low sweat rate conditions. We demonstrate a new principle for the design of 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, hydrogel, and a paper microfluidic conduit with a site of evaporation at the end (evaporation pad). The hydrogel (pump) is equilibrated with a high concentrated solution to build up the desired osmotic strength to extract sweat from the skin under rest.² The sampled sweat analytes can be analyzed directly on the paper channel with both colorimetric^[3] and 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, glucose, vitamin C, and levodopa under both resting and non-resting conditions. I will discuss their correlation with blood concentrations and even focus on how osmotically sweat derived glucose can be a valuable candidate (in addition to ISF) towards next-generation continuous glucose monitor development. Furthermore, the concept can even be used in a biofuel cell for continuous bioenergy harvesting at rest. Our group is currently investigating how this sweat sampling concept can be further integrated with microneedle patches for prolonged interstitial fluid (ISF) sampling (US Patent App #18/217392). Overall, the successful execution of such novel sampling methods to point-of-care and wearable platforms holds great potential in laying the foundation for next generation epidermal devices for personalized metabolic monitoring. <br/> <br/>[1] T. Saha, R. Del Caño, E. De la Paz, S. S. Sandhu, J. Wang, <i>Small</i> <b>2023</b>, <i>19</i>, 2206064.<br/>[2] T. Shay, T. Saha, M. D. Dickey, O. D. Velev, <i>Biomicrofluidics</i> <b>2020</b>, <i>14</i>, 034112.<br/>[3] T. Saha, J. Fang, S. Mukherjee, M. D. Dickey, O. D. Velev, <i>ACS Appl. Mater. Interfaces</i> <b>2021</b>, <i>13</i>, 8071.<br/>[4] T. Saha, T. Songkakul, C. T. Knisely, M. A. Yokus, M. A. Daniele, M. D. Dickey, A. Bozkurt, O. D. Velev, <i>ACS Sensors</i> <b>2022</b>, <i>7</i>, 2037.