Such devices are already possible because the scale needed is mm not nano. For example the devices to do angioplasties are pretty big because blood vessels are

Such devices are already possible because the scale needed is mm not nano. For example the devices to do angioplasties are pretty big because blood vessels are pretty big. There is also a lot of commercial and research work being done on "remote control" treatment delivery, esp. for cancer treatment. (By remote control I mean externally guided as opposed to joystick control.)