I just heard a webinar organized by Physics Today by Marc Miskin from U Penn on micron sized robots. The talk was very good and interesting. The abstract was

Nature routinely makes smart, complex, yet microscopically tiny machines: cells. But how can humans do it? And can we do it in a way that is easy to design and understand? This talk is about how to build a microscopic robot, one too small to see with the naked eye. We will discuss a platform that merges silicon-based microelectronics with a new technology for motion control to build simple, legged robots smaller than a hair’s width. Every step in this process can be performed massively in parallel, allowing us to produce over one million robots per 4-inch wafer. Combined, we will show how these results point to a future of intelligent, highly functional robots that are ten times smaller than the period at the end of this sentence.

There is a paper in Nature: Electronically integrated, mass-manufactured microscopic robots.