Maybe this seems like science fiction, but I was wondering if some companies are developing robots that are so small, that they are about the size of a virus. You could imagine a swarm of robotic viruses, even reproducing using cellular material found in their hosts, just like real viruses.
I could see some applications, such as military. Or the robotic virus could be an helpful one, living in symbiosis with its host, and offering some advantages (e.g. killing bad viruses or cancer cells) or used for military purposes, in a way that is not as bad as biological warfare: it could be a virus that does not kill its host but make him incapacitated or infertile for instance, in a military context, to defeat an enemy. The benefit is that it would be hard to detect, I think.
There are two things I am aware of within this realm, and although not technically "robots" as we would think of them there are ideas of mechanized activity. My understanding is limited so I will leave you with the resources without trying to explain, but it is interesting.
Here is an excerpt from Self assembling nanoparticles:
"Self-organization phenomena at nanoscale bring out new experimental and theoretical results every day. This type of processes are important from both fundamental and practical perspectives because they (1) allow one to identify similarities between angstrom, nanometer, and micrometer scale building blocks of Nature and (2) can potentially simplify manufacturing process of nanodevices and nanomaterials with specialized optical/electronic properties."
Turn your military application around and you have a whole new field of medicine. We can already program bacteria - here's a good TED talk about it and the implications, e.g. for treating cancer: https://www.ted.com/speakers/tal_danino