Electromechanical Muscle?

Mateorabi asks: "We just saw in a previous Slashdot article on how advances are being made in converting between electrical energy and torque motions. Does any sort of material have similar properties for contraction/expansion motions? I have in mind something like a plastic that would contract a good distance under an applied voltage: similar to the properties of human muscle tissue in behavior and strength. Wouldn't such a be a perfect for use in artficial limbs (+ lots of other stuff too)?"