A typical sensor looks for the positive or negative charge in molecules, but that's not really an option with a chargeless substance like cortisol. The researchers tackled this with a membrane that binds only to cortisol and lets regular charged molecules pass through. The sensor then measures the cortisol-carrying molecules trapped by the membrane, rather than the cortisol itself. All you need to do is visibly sweat and apply the patch.

The technology isn't perfect in its current incarnation. It can work multiple times, but it struggles if bogged down in sweat. They also want to improve the overall reliability and try using it on your saliva, saving you from having to work out to gather data. Nonetheless, the potential is clear. This could help sports stars and fitness mavens quantify their abilities mere moments after finishing a sweaty workout, and it might provide clues to otherwise imperceptible illnesses.