Lithium can alleviate the symptoms of bipolar disorder and depression—if taken in just the right amount. Too little won't work, while too much can bring on dangerous side effects. To precisely monitor the amount of this medication in the body, patients must undergo invasive blood tests. But today, scientists report the invention of a tiny sensor that detects lithium levels from sweat on the surface of a fingertip in as little as 30 seconds, without a trip to the clinic.