I wrote this previous post complaining that I had not seen biomedical nanobots in spite of predictions from prominent futurists.
To do this, Martel used bacteria that naturally contain magnetic particles. In nature, these particles help the bacteria navigate toward deeper water, away from oxygen. “Those nanoparticles form a chain a bit like a magnetic compass needle,” says Martel. But by changing the surrounding magnetic field using an extended set-up coupled to an MRI machine, Martel and his colleagues were able to make the bacteria propel themselves in any direction they wanted. -from here
How you control a (magnetizised) bacteria with MRI, is beyond me, but I guess this can count as a proof of principle nevertheless.
The medical-nanobot scenario just became much more likely.