A flexible battery made of silk films could power temporary medical sensors and implants in the body and then harmlessly melt away once its work is done (ACS Energy Lett. 2017, DOI: 10.1021/acsenergylett.7b00012). Caiyun Wang and Gordon G. Wallace of the University of Wollongong and colleagues made thin silk films for the battery by dissolving the protein fibroin, derived from silkworm cocoons, in water. They spread the solution in a mold and peeled off ultrathin sheets of silk after the water evaporated. To make a solid electrolyte for the battery, the researchers infused a piece of the silk film with the ionic liquid choline nitrate, a molten salt that conducts ions. Depositing a magnesium alloy on another piece of silk formed an anode, and depositing gold on still another piece formed a cathode. The team assembled the battery by sandwiching the electrolyte film between the two electrode films. The postage-stamp-sized, 170-µm-thick device generated a voltage of 0.87 V, which would be enough to power an implantable medical sensor. When placed in a saline buffer solution, the device nearly completely decomposed after 45 days.