A gel-based audio speaker in a materials science laboratory at Harvard University represents the first demonstration of an electrical charges carried by ions, rather than electrons. Two major advantages of ionic conductors over electronics are the ability to be very stretchy and completely transparent.
The speaker consists of a thin sheet of rubber sandwiched between two layers of a saltwater gel. A high-voltage signal runs across the surface and through the layers forces the rubber to rapidly contract and vibrate, producing sounds that span the entire audible spectrum, 20 Hz to 20 kHz.
The audio speaker represents a robust proof of concept for ionic conductors because producing sounds across the entire audible spectrum requires both high voltage (to squeeze hard on the rubber layer) and high-speed actuation (to vibrate quickly)—two criteria which are important for applications but which would have ruled out the use of ionic conductors in the past.