Transparent artificial muscle plays Grieg to prove a point

In a materials science laboratory at Harvard University, a transparent disk connected to a laptop fills the room with music-it's the "Morning" prelude from Peer Gynt , played on an ionic speaker. No ordinary speaker, it consists of a thin sheet of rubber sandwiched between two layers of a saltwater gel, and it's as clear as a window. A high-voltage signal that runs across the surfaces and through the layers forces the rubber to rapidly contract and vibrate, producing sounds that span the entire audible spectrum, 20 hertz to 20 kilohertz. But this is not an electronic device, nor has it ever been seen before. Published in the August 30 issue of Science , it represents the first demonstration that electrical charges carried by ions, rather than electrons, can be put to meaningful use in fast-moving, high-voltage devices. "Ionic conductors could replace certain electronic systems; they even offer several advantages," says co-lead author Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS). For example, ionic conductors can be stretched to many times their normal area without an increase in resistivity-a problem common in stretchable electronic devices.