For most of medical history, if you wanted to know what was happening inside a person's body, you had to break the skin. Blood is the gold standard for almost every biomarker clinicians care about, and getting at it means needles, vials, laboratories and waiting. Even the most celebrated success in wearable biosensing, the continuous glucose monitor, still relies on a filament pushed under the skin to sample interstitial fluid. Sweat has always been the obvious alternative. I