Materials science engineers at Harvard University, Cambridge, MA, have created a transparent audio speaker consisting of a thin sheet of rubber sandwiched between two layers of a saltwater gel that can carry a high-voltage signal across the surfaces and through the layers forcing the rubber to rapidly contract and vibrate, and producing sounds that span the entire audible spectrum. However, this is not an electronic device; 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.
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. Secondly, they can be transparent, making them well suited for optical applications. Thirdly, the gels used as electrolytes are biocompatible, so it would be relatively easy to incorporate ionic devices—such as artificial muscles or skin—into biological systems.
After all, signals carried by charged ions are the electricity of the human body, allowing neurons to share knowledge and allowing the heart to beat. The researchers say that engineered ionic systems can achieve a lot of functions of the human body: they can sense, conduct a signal, and actuate movement. 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.
The system invented at Harvard opens up a vast number of potential applications including biomedical devices, as well as fast-moving robotics and adaptive optics.
The Harvard team chose to make its audio speaker out of very simple materials—the electrolyte is a polyacrylamide gel swollen with salt water—but they emphasize that an entire class of ionically conductive materials is available for experimentation. Future work will focus on identifying the best combinations of materials for compatibility, long life, and adhesion between the layers.
