https://techcrunch.com/2018/07/23/researchers-design-self-powered-robots-the-size-of-human-cells/

The latest robots out of MIT are small enough to float “indefinitely” in the air. Researchers accomplished the feat by attaching 2D electronics to colloids — tiny particles measuring around one-billionth to one-millionth of a meter. All told, the devices are roughly the size of a human egg cell.

What’s more, the addition of photodiode semiconductors means the tiny individual systems are able to to be self-powered, without the need for a battery. The system converts light into a small electrical charge that’s enough to keep the device’s on-board environmental sensors running, while storing on-board information.