Here's a bit from New Scientist:
I won't be first in line for this new medical treatment. I would need to be convinced that it isn't going to cause interesting problems like blocked capillaries or rogue microbots delivering drugs in the wrong locations.
... while microbots exist, and they can be made to swim, it's getting them to change direction that has been tricky so far - a bit of an issue if you're even planning on sticking them in a human body, for instance.
Now a system used to propel swimming microbots without the need for on-board fuel has brought this idea one step closer. Researchers at North Carolina State University have coaxed their bots to perform U-turns on command.
The microbot, a mere 1.3 millimetres long, is essentially a diode - an electrical element that only allows current to pass in one direction. The diode is exposed to an alternating electric field, which induces a voltage across it, creating an electric dipole. This dipole pushes on ions in the water, driving them backward and propelling the microbot forward.
Miniaturisation will be vital if the robots are ever to navigate human bodies., like in Fantastic Voyage. The group is also investigating "yeast-boats", which use metabolic reactions to motor through glucose or hydrogen peroxide solutions. A similar vehicle may one day carry a molecular doctor on board to make a diagnosis - or dock with diseased tissue to deliver a shipment of drugs.