Although various groups have previously developed prototype tree-mounted forest fire sensors, the devices have mostly tended to be battery- or solar-powered. …Looking for a more practical alternative, scientists at Michigan State University looked to the triboelectric effect. …the phenomenon in which an electrical charge accumulates in one material, after it’s separated from another material with which it was in contact… The Michigan State device is a multilayered cylindrical triboelectric nanogenerator. In its simplest form, it incorporates two cylindrical sleeves of different materials, one nested inside of the other. While one of these is anchored solidly in place, the other is attached to the branch of a tree. As that branch sways back and forth in the wind, it pulls the cylinder with it, sliding it in and out of contact with the anchored cylinder. This generates a triboelectric charge, which is stored in a carbon-nanotube-based micro supercapacitor.