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Scientists invent transparent, stretchy speaker

Posted: 02 Sep 2013     Print Version  Bookmark and Share

Keywords:Harvard University  speaker  ions 

Harvard University scientists have developed a transparent, artificial "muscle" that can be also turned into a 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 glass. 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, 20Hz to 20kHz."

But this is not an electronic device, nor has it ever been seen before, the researchers claimed. 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," said 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. 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 love to mesh artificial organs and limbs with that system.

"The big vision is soft machines," said co-lead author Christoph Keplinger, who worked on the project as a postdoctoral fellow at 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 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 that are important for applications but that would have ruled out the use of ionic conductors in the past.

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.

"We'd like to change people's attitudes about where ionics can be used," said Keplinger, who now works in Whitesides' research group. "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."

The researchers are working closely with the Suo and Whitesides labs to commercialise the technology.

Their plan is to work with companies in a range of product categories, including tablet computing, smartphones, wearable electronics, consumer audio devices, and adaptive optics.

"With wearable computing devices becoming a reality, you could imagine eventually having a pair of glasses that toggles between wide-angle, telephoto, or reading modes based on voice commands or gestures," suggested Liss.





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