sNails: sweat-sensing nails for unobtrusive, wearable microfluidic sweat monitoring from the dorsal distal phalanges.

We present a fingernail-mounted microfluidic sweat sensor, the sNail, that leverages the anatomical and mechanical advantages of the nail plate and adjacent dorsal finger skin-regions that are uniquely stiff and stable yet underutilized for wearable biosensing. In contrast to fully soft, skin-mounted sensors, our device exploits the rigidity of the nail to host microfluidics that remain stable under typical daily finger motion. The platform incorporates a hydrogel-filled sweat collection well and stretchable thermoplastic polyurethane (TPU) microfluidics, fabricated via scalable laserjet printing and laser cutting. We demonstrate frequent sweat rate monitoring across multiple fingers and subjects during activities ranging from walking to typing, and capture dynamic changes in sweat rate associated with both physical exertion and psychological stress. This nail-mounted approach enables unobtrusive, informative sweat biosensing, paving the way for broader adoption in everyday settings.