Feeling extra sweaty from a summer heat wave? Don't worry—not all your perspiration has to go to waste. In a paper publishing July 13 in the journal Joule, researchers have developed a new device that harvests energy from the sweat on—of all places—your fingertips. To date, the device is the most efficient on-body energy harvester ever invented, producing 300 millijoules (mJ) of energy per square centimeter without any mechanical energy input during a 10-hour sleep and an additional 30 mJ of energy with a single press of a finger. The authors say the device represents a significant step forward for self-sustainable wearable electronics.