“The big vision is soft machines”
Gel-based audio speaker demonstrates capabilities of ionic conductors, long thought limited in application.
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 (see video below).
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 theHarvard 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. 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 spurring the heart to beat. Bioengineers would dearly love to mesh artificial organs and limbs with that system.
“The big vision is soft machines,” says 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. “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.”
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 traditional constraints are well known: high voltages can set off electrochemical reactions in ionic materials, producing gases and burning up the materials. Ions are also much larger and heavier than electrons, so physically moving them through a circuit is typically slow. The system invented at Harvard overcomes both of these problems, opening up a vast number of potential applications including not just biomedical devices, but also fast-moving robotics and adaptive optics.
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