The human body is a very complex machine. It has plumbing, wiring, and motors, and, just like a car, needs fuel and produces waste. At the moment that is where the similarities end, but in the very near future machines may become a lot more similar to humans.
Standard electric motors have their limitations. They are noisy, costly, difficult to miniaturize, heavy, and prone to breakage. Consider a muscle, however. It moves relatively silently, is efficient, reaches its limitation of miniaturization basically at the cellular level, and can operate millions of times without breakage (for example, in the human heart). Muscles are the ideal medium for any number of applications, or at least the U.S. government thought so. In the early 1990s, SRI International was approached and tasked with producing an artificial version of biological muscle. In 1992 they began work on electroactive polymers.
Put simply, electroactive polymers are materials that expand or contract when an electric field is applied to them, just like a muscle. Furthermore, they rapidly return to their initial configuration when a field of the opposite polarity is applied to them. They were first conceived as a way to improve robotic movements (indeed, SRI is developing a set of plastic and polymer wings) but their applications are limitless. Artificial muscles have been used to power prosthetic limbs, as well as valves and actuators in aircraft.
Since 2007 researchers in the United States have developed an artificial muscle that heals itself and generates electricity, and "micromuscles" grown from the heart muscle cells of rats, which are capable of gripping, walking, and swimming. Such developments may lead to walking robots in the future.
