A micro-scallop robot is only a fraction of a millimeter in size.
Simplicity is important when designing robots on the micro or nano scale (like, small enough to fit inside your body). There just isn’t room for complex motors or actuation systems. There’s barely room for any electronics whatsoever, not to mention batteries, which is why robots that can swim inside your bloodstream or zip around your eyeballs are often driven by magnetic fields.(Video)