MIT researchers develop fibers that can detect and produce sound

Researchers from MIT announced a new milestone on the path to functional fibers – fibers that can detect and produce sound. Yoel Fink, an associate professor of MIT’s Department of Materials Sciences and Engineering, and his associates have been working to develop fibers with more sophisticated properties, in order to create fabrics that can interact with their environment.

Ordinary optical fibers are made from a “preform” (a large cylinder of a single material that is heated up, drawn out, and then cooled). The fibers developed in Fink’s lab, by contrast, derive their functionality from the elaborate geometrical arrangement of several different materials, which must survive the heating and drawing process intact.

Max Shtein, an assistant professor in the University of Michigan’s materials science department, points out that other labs have built piezoelectric fibers by first drawing out a strand of a single material and then adding other materials to it, much the way manufacturers currently wrap insulating plastic around copper wire. “Yoel has the advantage of being able to extrude kilometers of this stuff at one shot,” Shtein says. “It’s a very scalable technique.” But for applications that require relatively short strands of fiber, such as sensors inserted into capillaries, Shtein say, “scalability is not that relevant.”

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. In order to work around that problem, the researchers used a conducting plastic that contains graphite, the material found in pencil lead. When heated, the conducting plastic maintains a higher viscosity 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 needed to align all the piezoelectric molecules in the same direction. That required 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. If you make them vibrate at audible frequencies and put them close to your ear, you could actually hear different notes or sounds coming out of it. For their Nature Materials paper, 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.

Since the reversed operation of this invention could allow this device to generate electricity from movement. 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. Applications could include clothes that are sensitive microphones, for capturing speech or monitoring bodily functions, and tiny filaments that could measure blood flow in capillaries or pressure in the brain.