At the moment this can’t be done unfortunately. But, it might be possible in the future to kill microbes this way. There’d be 2 problems I can see with this though:
1) We’ve still got a long way to go until we can make machines that small
2) There’d have to be a way of shutting the robots off after the infection is gone and stopping the immune system panicking at all the metal in the bloodstream. I guess it’s also possible that the robots might rip the blood vessels as they bounce along it…
So actually I’m going to say no, I don’t think it can be done lol! It would be incredible if it was possible though – would certainly help people out when they’re infected by the microbe I work on!
As for attacking cancer cells, cancer is caused by problems in the DNA. So it’d be very difficult for robots to tell which cells are the cancer cells. They’d look exactly the same as healthy cells.
Sorry to disappoint 🙁 It’s a nice idea though and I hope I’m proved wrong at some point!