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Prototype Ionic Artificial Muscle Doubles as a Speaker
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Prototype Ionic Artificial Muscle Doubles as a Speaker

There are no electronics inside the muscle, and it is completely transparent under visible light. Yet, from a disc of the material, connected to a laptop via a single USB connection, comes the unmistakable sound of the “Morning” prelude from Peer Gynt , ranging in frequency from 20hz to 20khz as required.

The speaker system is a type of artificial muscle, and is able to produce sound as a side-effect of how it stretches. It is an ionic speaker, using the electrical charge carried by ions rather than electrons to power itself. Something that has not been attempted before due to a previous belief that ionic powered systems were extremely limited in application and power.

They're how neurons in organic systems transfer signals to one another; ions between dendrites in the central nervous system, and ionic charge moving down the axons of the peripheral nervous system. By creating devices that respond to the exact same type of control signal, the myriad applications become obvious:

Prosthetics and worn augmented reality devices which are directly in tune with the biological body's control signals. No need to use electrodes to translate the ionic signals into electrical data for a smart prosthetic to make use of, means such prosthetics could be radically reduced in size, and allowing a far higher fidelity interface and finer control signals because of this.

The prototype is not a very powerful muscle, but that's not really the point of the demonstration. It shows a new potential route for exploring robotic and augmented reality systems that are not even remotely electronic in origin.

“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.”

“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.”

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.”


Transparent artificial muscle plays Grieg to prove a point

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