[FoRK] MIT researchers create fibers that can detect and produce sound

Stephen D. Williams sdw at lig.net
Tue Jul 13 11:49:05 PDT 2010

Now that is cool. The applications are endless. Extensions into movement 
and robotics would be cool too: flagella, etc.

> The heart of the new acoustic fibers is a plastic commonly used in 
> microphones. By playing with the plastic's fluorine content, the 
> researchers were able to ensure that its molecules remain lopsided — 
> with fluorine atoms lined up on one side and hydrogen atoms on the 
> other — even during heating and drawing. The asymmetry of the 
> molecules is what makes the plastic "piezoelectric," meaning that it 
> changes shape when an electric field is applied to it.
> In a conventional piezoelectric microphone, the electric field is 
> generated by metal electrodes. But in a fiber microphone, the drawing 
> process 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 — it yields a thicker fluid — than a 
> metal would.
> Not only did this prevent the mixing of materials, but, crucially, it 
> also made for fibers with a regular thickness. After the fiber has 
> been drawn, the researchers need to align all the piezoelectric 
> molecules in the same direction. That requires the application of a 
> powerful electric field — 20 times as powerful as the fields that 
> cause lightning during a thunderstorm. Anywhere the fiber is too 
> narrow, the field would generate a tiny lightning bolt, which could 
> destroy the material around it.
> Despite the delicate balance required by the manufacturing process, 
> the researchers were able to build functioning fibers in the lab. "You 
> can actually hear them, these fibers," says Chocat, a graduate student 
> in the materials science department. "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. 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." For 
> their /Nature Materials/ paper, however, the researchers measured the 
> fiber's acoustic properties more rigorously. Since water conducts 
> sound better than air, they placed it in a water tank opposite a 
> standard acoustic transducer, a device that could alternately emit 
> sound waves detected by the fiber and detect sound waves emitted by 
> the fiber.
> In addition to wearable microphones and biological sensors, 
> applications of the fibers could include loose nets that monitor the 
> flow of water in the ocean and large-area sonar imaging systems with 
> much higher resolutions: A fabric woven from acoustic fibers would 
> provide the equivalent of millions of tiny acoustic sensors.
> Zheng, a research scientist in Fink's lab, also points out that the 
> same mechanism that allows piezoelectric devices to translate 
> electricity into motion can work in reverse. "Imagine a thread that 
> can generate electricity when stretched," he says.
> Ultimately, however, the researchers hope to combine the properties of 
> their experimental fibers in a single fiber. Strong vibrations, for 
> instance, could vary the optical properties of a reflecting fiber, 
> enabling fabrics to communicate optically.
> SOURCE <http://www.eurekalert.org/pub_releases/2010-07/miot-mr071210.php>

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