In Korea a research team has built a really tiny robot. This robot, with six crab legs and being no thicker than a fingernail, is powered by rat cardiomyocytes, heart cells coated over its body. When the cells contract, the legs are pulled together, and the mini-cyborg inches forward, averaging about 100uM a second and close to 50 meters in a week. The robot is fabricated the same way we mold parts for things like cars or toys, just on a very small scale.
Check out the robot walker action here.
So what's the application, eh? Theoretically one could send these tiny robots into the smallest of blood vessels and gland ducts the way we send pipeline pigs into fuel lines for maintenance and information gathering missions. The tiny cyborgs could march in and collect valuable data and perhaps in later versions, effect repairs. That's a ways off, since the robot is going to need to be sturdy enough to force its way through flowing blood, but really, at this point its only a matter of engineering, not principle.