Gel-based audio speaker demonstrates capabilities of ionic conductors, long thought limited in application (w/ Video)

August 29, 2013
Transparent loudspeaker, capable of producing sound across the entire audible range. As shown in the image, the speaker is placed in front of a laptop screen. An electrical signal from the audio output of the laptop is fed through an amplifier to the loudspeaker. The loudspeaker transforms the electrical signal into sound. Music videos are played from YouTube, where the sound is generated by the transparent loudspeaker in front of the screen. The screen is clearly visible through this fully transparent device. Credit: Christoph Keplinger and Jeong-Yun Sun, Whitesides and Suo Research Groups, Harvard University

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 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 .

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 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 . 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 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 and limbs with that system.

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Demonstration of a soft membrane loudspeaker with near- perfect transparency and frequency response, from 20 Hz to 20 kHz, across the entire audible range. The speaker is placed in front of a laptop screen. An electrical signal from the audio output of the laptop is fed through an amplifier to the loudspeaker. The loudspeaker transforms the electrical signal into sound. Music videos are played from YouTube, where the sound is generated by the transparent loudspeaker in front of the screen. Thickness changes in the soft membrane are induced by electrical signals. These thickness changes can – in addition to generating sound – provide localized haptic feedback in front of a screen, where the nearly transparent device will not impede the viewing experience. Credit: Christoph Keplinger and Jeong-Yun Sun, Whitesides and Suo Research Groups, Harvard University

"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.

To make the speaker, a membrane of transparent, insulating rubber is sandwiched between two layers of transparent, conductive gel. The electrical connection to the power source is established outside of the active region of the device, where it does not need to be transparent. Credit: Christoph Keplinger, Jeong-Yun Sun

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.

"It must seem counterintuitive to many people, that ionic conductors could be used in a system that requires very fast actuation, like our speaker," says Sun. "Yet by exploiting the rubber layer as an insulator, we're able to control the voltage at the interfaces where the gel connects to the electrodes, so we don't have to worry about unwanted chemical reactions. The input signal is an alternating current (AC), and we use the rubber sheet as a capacitor, which blocks the flow of charge carriers through the circuit. As a result, we don't have to continuously move the ions in one direction, which would be slow; we simply redistribute them, which we can do thousands of times per second."

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Side view of the transparent loudspeaker performing an audio test, where the frequency is ramped up from 20 Hz up to 20 kHz (across the entire audible range). At very low frequencies (initial part of the video) changes in area (with the frequency of the applied audio signal) of the active region of the device are visible. As the frequency rises, out-of-plane vibrations become visible. The wave structure of these vibrations changes as the frequency increases. Credit: Christoph Keplinger and Jeong-Yun Sun, Whitesides and Suo Research Groups, Harvard University

Sun works in a research group led by Zhigang Suo, the Allen E. and Marilyn M. Puckett Professor of Mechanics and Materials at Harvard SEAS. An expert in the mechanical behaviors of materials, Suo is also a Kavli Scholar at the Kavli Institute for Bionano Science & Technology, which is based at SEAS.

Suo teamed up with George M. Whitesides, a prominent chemist who specializes in soft machines, among many other topics. Whitesides is the Woodford L. and Ann A. Flowers University Professor in the Department of Chemistry and Chemical Biology, co-director of the Kavli Institute at Harvard, and a Core Faculty Member at the Wyss Institute for Biologically Inspired Engineering at Harvard.

"We'd like to change people's attitudes about where ionics can be used," says 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."

Jeong-Yun Sun (left) and Christoph Keplinger (right), Harvard University, demonstrate their transparent ionic speaker. It uses a signal conducted by ions, rather than electrons, to vibrate a rubber membrane. Credit: Eliza Grinnell

Sam Liss, Director of Business Development in Harvard's Office of Technology Development, is working closely with the Suo and Whitesides labs to commercialize 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," suggests Liss.

For now, there is much more engineering and chemistry work to be done. 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.

Explore further: Tough hydrogel stretches to 21 times its length, recoils, and heals itself

More information: "Stretchable, Transparent, Ionic Conductors," by C. Keplinger et al. Science, 2013.

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not rated yet Aug 29, 2013
Ions have a long history in musical reproduction. William Duddel made singing arcs around 1900. US Patent #3,105,124, "Inertialess Transducer" granted to William R. Torn September 24, 1963- became the Dukane Ionovac plasma tweeter (limited to between 3.5- 400KHz- yes, 400,000 Hz!) followed in later years by the Hill Type I "Plasmatronics" helium ion tweeters.


Hopefully, this new device is less fiddly and dangerous than previous attempts!
2.3 / 5 (3) Aug 29, 2013
Non-linear distortion is extreme and the low end doesn't couple to the air. Interesting as a phenomenon but not as an acoustic transducer.
rocky j squirrel
5 / 5 (1) Aug 29, 2013
Not mentioned is that the speaker is showing the tympani drum head vibration modes and many more in addition. Call in the Percussion Specialist from the Music Department!
not rated yet Aug 30, 2013
Mention of adaptive optics makes me wonder how small these transducers can be. Could an array of these, working in concert, change the way something flows across that surface?
1 / 5 (2) Sep 01, 2013
The description alone- an array could perform the work of an artificial muscle, and retain the same dimensional alteration (contraction in length, increase in cross section), in addition to being potentially biocompatible.
1 / 5 (3) Sep 01, 2013
clearly the first step in the development of an ocular implant that will replace computer screens and make google glass a dated and outmoded concept the moment glass hits the market.

About the same time, Jeong-Yun Sun's tech company will announce they have incorporated LCD tech into their ionic conductor, enclosed it all in a modified soft contact lens about the size of a quarter to create the first fully functional ocular implant computer display.

It will look like the cyborg eyes of science fiction with the clear area about the size of the pupil surrounded by circuitry with a gold coating only a few atoms thick.

Extending the concept behind the ionic conductor, tears will be used in a similar way to create the current needed to run the ocular display contact lens.

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