https://www.vice.com/en_us/article/wx5m5z/winged-microchips-are-the-smallest-flying-machines-made-by-humans

Tiny winged microchips that ride the wind like plant seeds are the smallest flying devices ever made by humans, a new study reports. 

The bio-inspired robots are about the size of sand grains and could be deployed in swarms with far-ranging applications, such as monitoring air pollution, tracking the spread of disease, or collecting scientific data. The fliers could also be constructed from biodegradable materials that are reabsorbed into environments upon landing to prevent contamination.