Medical sensors that sample your sweat are great, as they can accurately gather some of the data you'd get from blood without having to poke through your skin. There's just one problem: this usually means going for a run or otherwise doing something taxing to produce that sweat. However, researchers have an elegant solution. They've devised a relatively small sensor (about the size of a Band-Aid) that stimulates the sweat glands on a small patch of skin so that you can get sweat data without exerting yourself. The device uses a tiny electrical current (0.2mA) to send carbachol, a chemical found in eyedrops, into the upper layer of your skin. From there, it's just a matter of measuring the electrolytes concentrated in your sweat.