"Researchers have created new plastic fibers that can detect and produce sound. When stretched, these strands could be used to make clothes that act as a microphone or generate electricity.

"You can actually hear them, these fibers," said Noémie Chocat, a graduate student in the materials science department at MIT and co-author of a paper describing the fibers.

"If you connected them to a power supply and applied a sinusoidal current" — an alternating current whose period is very regular — "then it would vibrate," Chocat said. "And if you make it vibrate at audible frequencies and put it close to your ear, you could actually hear different notes or sounds coming out of it."

Microphone check
The heart of the new acoustic fibers is a plastic commonly used in microphones. By playing with the amount of the element fluorine in the plastic, the researchers were able to ensure that the material's molecules remained "lopsided," with the fluorine atoms lining up on one side and hydrogen atoms on the other.

This asymmetry made the plastic " piezoelectric," meaning that it changes shape when an electric field is applied to it.

In a conventional piezoelectric microphone, this useful electric field is generated by metal electrodes. But in a fiber microphone, the drawing process — when the strand is pulled into being from a larger block of material — would cause metal electrodes to lose their shape.

So the researchers instead used a conducting plastic that contains graphite, the material found in pencil lead. When heated, the conducting plastic maintains a higher viscosity — meaning it yields a thicker fluid — than a metal would.
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Not only did this prevent the mixing of materials that might wreck the fibers' properties, but, crucially, it also made for fibers with a regular thickness.

After the fiber was drawn, the researchers needed to align all the piezoelectric molecules in the same direction. That required the application of an electric field 20 times as powerful as the fields that cause lightning during a thunderstorm.

Sound results
In addition to wearable microphones, the fibers could be used as biological sensors for monitoring bodily ......."