There are many challenges to developing robots that could operate within your body, not the least of which is finding a power source -- you can't exactly strap a big battery on them. That might not be an issue thanks to Georgia Tech researchers. They've developed minuscule "bristle-bots" that move by tapping vibration from a variety of sources, whether it's ultrasound or a nearby speaker. The trick was to mate a tiny piezoelectric actuator to a 3D-printed polymer body whose bristle-like legs are angled to move in specific directions in a resonant response to vibrations.
The bots are only 2mm (about 0.08in) long and weigh just 5mg (less than 0.0002oz), but they can move relatively briskly at about 8mm (just over 0.31in) per second. They're flexible, too. The actuators are made out of lead zirconate titanate that can turn voltage into vibration for movement, or the reverse if they need to power sensors.
Georgia Tech's existing design wouldn't be much useful in its current form. It can only move in one direction, and there isn't a payload. Movement could be solved by combining robots together to respond to different frequencies, though, and the creators hope to develop bots that can jump or swim. If the technology continues to advance, though, you could see unintrusive bots that could fix health problems from within, or achieve simpler feats like tracking environmental conditions and moving small objects.