Technically inclined people in the baby boom generation no doubt remember the crystal (more properly known as piezoelectric) phonograph cartridge or microphone. Often temperamental, these devices put out a voltage when the crystal was stressed by mechanical motion such as a phonograph stylus or a microphone diaphragm. But the piezoelectric effect also works in reverse: Apply a voltage to a piezoelectric substance, and it contorts, producing motion.
This effect has worked well for Parker Hannifin’s Pneumatic Division North America; its engineers learned how to use it to open and close pneumatic valves. The effect takes little power and can move an actuator up to one-eighth of an inch. That may not sound like much, but 1.5mA of current at 6.8VDC can exert a force up to seven pounds.