Audio speakers are showing up in a variety of unusual forms these days, from the incredibly tiny to the eye-catchingly bizarre, but a research group at Harvard University may have trumped them all with a speaker that's as clear as glass. Scientists at the college's School of Engineering and Applied Sciences have built a flexible speaker out of ionic gel that is almost invisible to the naked eye and can produce high-quality sound ranging across the full audible spectrum. In doing so, they also provided a proof of concept for electronics that can transfer electric signals in a similar manner to the human nervous system.

To construct the see-through speaker, the group took a transparent rubber sheet and added a layer of conductive saltwater gel to each side. Once a voltage is applied to opposite edges of the gel layers, the entire area of the rubber flexes rapidly, producing sounds ranging from 20 Hz to 20 kHz. Because of the flexibility and ionic properties of the materials, the speaker can be stretched to several times its normal area and still function properly. The scientists believe a more refined version of their invention could be fitted over a computer or tablet screen to provide sound and haptic feedback without the need for external speakers.

A transparent speaker may be impressive on its own, but the real point of the project was to demonstrate how an electric charge passing through ions instead of electrons could be used in electronic devices. In most cases, ionic substances tend to produce slow circuit connections when a voltage is applied to them, and too high a voltage can trigger a chemical reaction that destroys the material entirely. With this new system, however, the rubber acts as an insulator, which allows scientists to better control the voltage and speed up the connection in the process.

According to the research team, if ionic conductors were perfected, they could potentially offer quite a few advantages over conductors typically used today. A common issue with most current flexible conductors is that their resistance increases the more they are stretched, which severely limits their performance in some electronic devices. Ionic conductors on the other hand don't have this problem and can be pulled to several times their regular area without affecting the circuit. An ionic conductor won't be able to match a regular electronic one in terms of resistivity any time soon, but if the goal is to build a circuit that can be warped, then they could be a feasible alternative. As a bonus, ionic conductors can be made from entirely transparent materials.

"We’d like to change people’s attitudes about where ionics can be used," says Christoph Keplinger, a co-author on the project and postdoctoral fellow at Harvard SEAS. "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."

Connecting with our own biology

Aside from being clear and stretchable without hindering the circuit, the gels in the transparent speaker carry ions in much the same way as some biological systems, such as nerves in the human body. This could possibly open the door for merging biological systems with man-made ionic ones, such as artificial muscles or skin.

In the long term, the researchers see their discovery as potentially leading to more advanced "soft machines" that can alter their form on command.

"The big vision is soft machines," says Keplinger, "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."

Some examples of possible projects they've described include reading glasses that can change their own focal length and robots that reshape themselves to suit different tasks. For the time being, the team plans to continue its research by performing similar experiments with other ionic substances to find which ones can function under a charge longer and which materials can be combined for greater conductivity.

The research group recently explained its ionic speaker project in a paper published in the Aug. 30, 2013 issue of Science.

Professor of Mechanics and Materials at SEAS, Zhigang Suo, put together the research group behind the transparent speaker along with George M. Whitesides, a professor in Harvard's Department of Chemistry and Chemical Biology.

Check out the video below to hear just how crisp the audio from the researchers' ionic speaker sounds.

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