(360b) Capillary-Osmotic Wearable Patch Assays for Sweat Analysis of K+, Lactate and Cortisol

The monitoring of human health and well-being with the use of wearable devices is the core of the next generation of biomedical devices. Sweat provides a facile source for the continuous and non-invasive measurements of biomarkers. Despite its advantages, sweat biomarker analysis is still challenging because most of the commercially available health-monitoring devices are either semi-invasive in nature (iontophoresis) or operate only during active sweating. Our group has introduced a simple and efficient platform for sweat sampling and handling based on osmotic-capillary principles and paper microfluidics. It can harvest sweat noninvasively without the necessity of active perspiration. Osmotic sweat withdrawal is achieved by interfacing the skin with a hydrogel disk containing concentrated solute. The extracted sweat is transported on a paper strip via capillary wicking. These principles were applied in the development of simple and inexpensive wearable skin patches for analysis of K⁺ and cortisol, based on lateral flow assays (LFAs). Our LFA platform is composed of silicone, a paper microfluidic conduit attached to a commercial K⁺ strip, and a polyacrylamide hydrogel with a higher osmotic strength than sweat. In-vitro testing on gelatin-based model skin allowed calibrating the platform with model sweat. Human trials revealed that it can function with very low sweat volumes (~2-3 µL) and can detect K⁺ levels under moderate intensity exercise and rest. We observed that sweat potassium levels are independent of the sweat rate and proportional to blood. Additionally, we are developing wearable skin LFAs for detection of cortisol (in sweat) as a key stress biomarker in individuals, soldiers in combat, athletes, and emergency personnel. The osmotic patch can also efficiently sample sweat lactate directly from the surface of skin. On-skin testing of the platform on both resting and exercising human subjects confirms that the patch can extract sweat and analyze the changing lactate levels. The results show that lactate in sweat increases with exercise and as a direct result of muscle activity. We were successful in interfacing the osmotic paper platform with electrochemical sweat lactate sensors, enabling long-term continuous electronic readout of the results from the wearable interface. Thus, the platform can find a broad range of applications in both self-contained patch assays and electronically monitored long-term health trackers.