Acoustic Fibers  (Source: Research Laboratory of Electronics at MIT)
Fibers can interact with environment

Researchers at the Massachusetts Institute of Technology (MIT) have been working on giving fibers more elaborate properties over the past decade and have finally succeeded in doing so with their most recent discovery: Fibers that can interact with their environment by producing and protecting sound.

These fibers were discovered by associate professor of Materials Science and principal investigator at MIT's Research Lab of Electronics Yoel Fink, along with his collaborators. The study was published in the August issue of Nature Materials, and featured on their website on July 11. Other authors of the study include Shunji Egusa, Noémie Chocat and Zheng Wang.

The newly developed fibers, known as acoustic fibers, acquire their functionality from the geometrical pattern of many different materials and have to endure the heating and drawing process while remaining intact. Regular optical fibers are made from a "preform," where it is placed in a large cylinder of a single material and heated up, drawn out, and cooled.

Plastic that is most commonly found in microphones lies at the center of the acoustic fibers. Researchers make this plastic "piezoelectric" (meaning that applying an electric field causes it to change shape) by making sure that the molecules remained lopsided within the fluorine content (even during heating and drawing) where fluorine atoms lined up on one side and hydrogen atoms lined up on the other side. The electric field in a traditional piezoelectric microphone is generated by metal electrodes, but in the case of this fiber microphone, metal electrodes would lose their shape during the drawing process. To stop this from happening, Fink and his researchers used a conducting plastic with graphite, and when heated, the conducting plastic upholds a higher viscosity than metal, which prevented the mixture of materials.

After drawing the fiber, researchers aligned the piezoelectric molecules in the same direction by using a powerful electric field that is 20 times as strong as fields that produce lightning in a thunderstorm. If a fiber is too thin in any particular area, the electric field generates a small lightning bolt that could destroy its surrounding material. This is a very fragile balance that researchers had to control, but were finally able to create functioning fibers

Fink and his fellow contributors measured the fibers' acoustic properties closely by putting them in a tank of water (since water conducts sound better than air) "opposite a standard acoustic transducer," which can emit sound waves both detected by the fiber and emitted by the fiber. 

"You can actually hear them, these fibers," said Chocat. "If you connected them to a power supply and applied a sinusoidal current (an alternating current whose period is quite 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."

In addition to sound sensitive fibers, MIT has developed camera-like fibers that are sensitive to light. They are a mesh of fibers that can detect two frequencies of light and will produce signals that "when amplified and processed by a computer," reproduce images near the mesh.

These fibers can be used for wearable microphones, biological sensors, large-area sonar imaging systems with high resolutions and loose nets that monitor the flow of water in the ocean. 

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