Why there aren't yet nanobot doctors

For years, the cutting edge of medicine has promised nanobots. Tiny little machines that could run around your body delivering drugs, checking up on arteries, and generally keeping people healthy. But so far, those machines haven't quite come to dominate the way some people thought they might. The human body is vastly more complicated than any robot we've ever made. So creating a miniscule robot to go inside of it, to work with that vast infrastructure, and to do our bidding, is a huge challenge.