In most people's sci-fi nightmares about robots trying to wipe out humanity, the robots tend to be big. But wouldn't they be more deadly if they were tiny? 0.2-0.5 millimeters is bigger than bacteria or viruses, but it's the size range of many single-cell protozoans.

That possibility is bad enough, but we'd better hope no one figures out how to make these things self-replicating. Think that sounds far-fetched? Evolution figured it out with single-cell organisms 2 billion years ago, and they haven't faltered since.

World’s smallest programmable robots perform tasks: Microscale swimming bots developed by U-M and Penn take in sensory information, process it, and carry out tasks, opening new possibilities in manufacturing and medicine.