pH Watch - Leveraging Pulse Oximeters in Existing Wearables for Reusable, Real-time Monitoring of pH in Sweat

Sweat is a readily accessible bodily fluid for detecting biomarkers such as pH, glucose etc., enabling continuous and non-invasive assessment of the well-being of individuals. Our proposed work aims at leveraging pulse oximeter chips in current-day fitness trackers for real-time continuous monitoring of pH in sweat. We achieve that by fabricating a highly responsive and long-term reusable pH sweat sensor on a flexible material to achieve skin conformity, targeting the sensor to work at the reflected infrared (880nm) and red (660nm) photoplethysmograph (PPG) signal intensities recorded by pulse oximeters. The sensor can be readily mounted atop any wearable with a pulse oximeter. We have successfully demonstrated a low-cost, low-power, highly-responsive and long-term reusable wrist-worn wearable prototype, pH Watch, for real-time continuous monitoring of pH value of sweat. We conducted on-body trials with 10 participants and pH Watch achieves an accuracy of $\approx$91%. We also showed that the integration of our sweat sensor does not hinder the pulse oximeter from measuring heart rate and SpO\textsubscript2, and users can continue with their daily activities with motion artifacts removed efficiently from PPG signals using the TROIKA framework, resulting in heart rate and SpO\textsubscript2 measurements with an accuracy of $\approx$95% and $\approx$96% respectively when validated against commercial finger pulse oximeter measurements. To the best of our knowledge, pH Watch is the first demonstration of a reusable sweat sensor that can be readily integrated into today's smart watches with pulse oximeters, paving the way for ubiquitous sensing of biomarkers.