Most robots are powered by electrical motors that are big, bulky, heavy, and once they break, you'll have to replace them. Animals, on the other hand, use a biological motor—a muscle—that also requires electricity, but is far more efficient can repair itself.
Engineers at the University of Illinois at Urbana-Champaign have developed a tiny little "bio-bot" that uses muscle cells to walk.
The backbone of this robot is made of hydrogel, although it acts more like a combination of bone and tendon, providing both structure and flexibility. The muscle is anchored to hydrogel feet, and when external electrical impulses cause the muscle to contract, the feet get pulled together, and the bio-bot walks.
This is all very early stage research, and what's most exciting is how these little guys can be evolved into more sophisticated robots with all the things that you'd want in a robot. Long term, things might get even cooler.