How could you build an artificial tadpole? Or simulate the motion of a cilium? Those would be hard to do with mechanical means — even micromechanical because of their fluid motion. Researchers have been studying shape-programmable matter: materials that can change shape based on something like heat or magnetic field. However, most research in this area has relied on human intuition and trial and error to get the programmed shape correct. They also are frequently not very fast to change shape.
[Metin Sitti] and researchers at several institutions have found a way to make rapidly changing silicone rubber parts (PDF link) that can change shape due to a magnetic field. The method is reproducible and doesn’t seem out of reach for a hackerspace or well-equipped garage lab.