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Singing Threads

Acoustic fibers can detect and produce sound

Yoel Fink, an associate professor of materials science, thinks that the threads used in textiles and even in optical fibers are much too passive. For the past decade, his lab has been working to develop fibers with ever more sophisticated properties, in the hope that fabrics will be able to interact with their environment.

Grooving fabric Yoel Fink’s lab has made acoustic fibers with flat surfaces, like these, as well as fibers with circular cross-sections.

In August, Fink and his collaborators unveiled fibers that can detect and produce sound. Applications could include clothes that capture speech or monitor bodily functions, tiny filaments that measure blood flow in capillaries, and nets that measure the flow of water in the ocean.

Like optical fibers, Fink’s acoustic fibers are made from a “preform,” a large cylinder of material that is heated up and drawn out. But unlike optical fibers, whose preform consists of a single material, the acoustic fibers derive their functionality from the elaborate geometrical arrangement of several different materials, which must survive the heating and drawing process intact.

The basis of the new fibers is a plastic commonly used in microphones. Because its molecules are lopsided–with fluorine atoms lined up on one side and hydrogen atoms on the other–the plastic is “piezoelectric,” meaning that it changes shape when an electric field is applied to it and produces a current when it’s manually deformed. Ordinarily, heating the plastic undoes that asymmetry. But by playing with the plastic’s fluorine content, the researchers were able to preserve it even during heating and drawing.

In a conventional piezoelectric microphone, the piezoelectric plastic is sandwiched between metal electrodes. But in a fiber microphone, the drawing process would cause those electrodes to lose their shape. So the researchers instead used a conducting plastic that contains graphite, the material found in pencil lead. When heated, this conducting plastic melts into a thicker, more viscous fluid than a metal would, so unlike a metal, it won’t mix with the piezoelectric plastic.

After the fiber has been drawn, the researchers subject it to a powerful electric field, which aligns all the piezoelectric molecules in the same direction. If the fiber were too narrow at any point, the field would generate a tiny lightning bolt, which could destroy the material. The viscous conductor, however, also helps keep the fibers at a uniform thickness, preventing that problem.

Despite the delicate balance required by the manufacturing process, the researchers were able to build functioning fibers in the lab. When they connected them to a power supply and applied a sinusoidal current (an alternating current whose period is very regular), they could make them vibrate at audible frequencies, producing different notes.

Ultimately, the researchers hope to combine the properties of several experimental fibers in a single fiber. Strong vibrations, for instance, could vary the optical properties of a fiber designed to reflect light, enabling fabrics to communicate optically.

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