Nanomedicine is a science fiction staple. Nanites in your blood keeping you healthy. Implants that detect illness and release the appropriate chemicals.
We're one step closer with something called Trail. It was developed at Cornell.
Nanoparticles are introduced into the patient's blood, where they hunt down stray cancer cells - to prevent cancer from spreading through the body. They do it by latching on to white blood cells, which can often spot the tumor cells...but not do anything about them.
They've already been used in some pilot trials...and certainly seem to be less unpleasant than chemotherapy.