Transparent gel Speaker plays Music, paves way for Biocompatible 'Soft Machines'

A translucent speaker has been developed by Material scientists at Harvard University, which is made out of a thin sheet of rubber squished between two membranes of saltwater gel.

Transparent loudspeaker | Harvard School of Engineering and Applied 

By running a high voltage ionic signal across the surfaces, the rubber can be made to rapidly contract and vibrate, producing sounds that span the audio spectrum between 20 hertz and 20 kilohertz. The technology was used to play Edvard Grieg's Morgenstimmung in a demonstration video embedded in this post.

This is an important proof of concept for ionic conductors, since speakers require both a high voltage and high-speed vibrations i.e.  two criteria that are important for a wide range of applications, but have been ruled out of previous tests of ionic materials. This is because high voltages tend to set of electrochemical reactions, which cause damage to the materials.

This demonstration could pave the way for developing ionic conductors to replace some electronic systems. A key benefit to using ionic conductors is the fact that they can be stretched many times to cover a large surface area without affecting their resistivity.

Moreover, ions -- being larger and heavier than electrons -- have generally moved very slowly through circuits. The Harvard team has overcome these challenges with the combination of the polyacrylamide gel swollen with salt water and the rubber.

"The big vision is soft machines. 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", said by Co-lead author Christoph Keplinger, a postdoctoral fellow at Harvard School of Engineering and Applied Sciences (SEAS).

It has been added by Jeong-Yun Sun, a postdoctoral fellow at SEAS that it might seem counter-intuitive that ionic conductors could be used in a system that needs very fast actuation, as speakers generally do. "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."

The system doesn't need a lot of power and could be used as a soft transparent layer that deforms in response to electrical stimuli and even create active noise cancellation on windows so that rooms could be completely silent inside.

Harvard plans to commercialise the technology, working with companies involved in tablet computing, smartphones, wearable electronics, adaptive optics and consumer audio devices.

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Write by: RC - Monday, September 2, 2013

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