Gel-based Audio Speaker Carries Electrical Charges Using Ions -- Not Electrons [VIDEO]

Harvard University researchers have developed a gel-based audio speaker system in which electrical charges are carried by ions, rather than electrons.

Consisting of a thin sheet of rubber sandwiched between two layers of saltwater gel, the transparent speaker is brought to life by a high-voltage signal that runs across the surfaces and through the layers, forcing the rubber to contract and vibrate. The result is sounds that span the entire audible spectrum.

The achievement marks the first clear demonstration that electrical charges carried by ions, rather than electrons, can be put to practical use in fast-moving, high-voltage devices, the scientists explain.

Ionic conductors hold several advantages, including the ability to be stretched far beyond their normal size without experiencing an increase in resistivity -- a problem that plagues stretchable electronic devices. Furthermore, they can be transparent and are more easily incorporated into biological systems, such as artificial muscle or skin, due to the biocompatible nature of their gels.

"The big vision is soft machines," co-lead author Christoph Keplinger, who worked on the project as a postdoctoral fellow at Harvard SEAS and in the Department of Chemistry and Chemical Biology, said in a statement. "Engineered ionic systems can achieve a lot of functions that our body has: they can sense, they can conduct a signal, and they can actuate movement. We're really approaching the type of soft machine that biology has to offer."

One of the largest hurdles that has long faced ionic conductivity is the tendency of high voltages to set off electrochemical reactions in ionic materials, causing them to burn up. A second is that ions are much larger and heavier than electrons, making it difficult to move them through a circuit at high speeds.

The new study and the resulting speaker overcomes both these issues and, in doing so, opens up the door for a wide range of potential applications, including robotics and adaptive optics, in addition to biomedical devices.

"We'd like to change people's attitudes about where ionics can be used," Keplinger, who now works in Whitesides' research group, said. "Our system doesn't need a lot of power, and you can integrate it anywhere you would need a soft, transparent layer that deforms in response to electrical stimuli-for example, on the screen of a TV, laptop, or smartphone to generate sound or provide localized haptic feedback -- and people are even thinking about smart windows. You could potentially place this speaker on a window and achieve active noise cancellation, with complete silence inside."