A mass-customizable dermal patch with discrete colorimetric indicators for personalized sweat rate quantification

In this paper, we present a disposable, colorimetric, user-friendly and mass-customizable dermal patch for chronological collection and discrete real-time in situ measurement of sweat secretion over a small area of skin. The patch consists of a laminated filter paper patterned into radially arranged channels/fingers with water-activated dyes at their tips. As channels are filled during perspiration, their tips change color once fully saturated, providing easily identifiable levels of water loss which in turn can be mapped to personal dehydration levels. The patch can be manufactured at low cost in a variety of sizes to allow hydration monitoring for individuals participating in activities under different conditions (intensity, temperature, humidity, etc.). Furthermore, we describe an analytical model that enables mass customization of such a flexible wearable system accommodating a broad range of sweat rates and volumes to generate patch designs that are personalized to an individual's sweat rate, desired time of usage, and the temporal resolution of the required feedback. As a proof-of-concept demonstration, we characterized laser-fabricated patches that cover (7 cm × 5 cm) area of skin having various wicking materials, thicknesses (180-540 µm), and pore sizes (3-11 µm). Tests were conducted at various flow rates simulating different sweating intensities in the range of 1.5-15 mg/cm²/min. Experimental results for the case of a half-marathon runner targeting 90 min of usage and sweating at a rate of 1.5 mg/cm²/min indicated measurement accuracy of 98.3% when the patch is completely filled.