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
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