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.