Fibres that Carry Light and Sense Pressure Could Be Used for Medical Imaging

Researchers have developed acoustic fibres with flat surfaces such as those shown here, as well as fibres with circular cross sections. The flat fibres could prove useful in acoustic imaging devices. Image courtesy: Greg Hren Photograph/MIT.

MIT professor Yoel Fink has announced plans to develop fibres that can detect and produce sound. 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. The research appeared on Nature Materials‘ website on July 11.

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

The heart of the new acoustic fibres is a plastic commonly used in microphones. By altering the plastic’s fluorine content, the researchers were able to ensure that its molecules remain configured so that their fluorine atoms lined up on one side and hydrogen atoms on the other. 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 fibre 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 than a metal would.

Not only did this prevent the mixing of materials, but it also made for fibres with a regular thickness. After the fibre 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 fibre 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 fibres in the lab. The researchers measured the fibre’s acoustic properties rigorously in the lab. 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 fibre and detect sound waves emitted by the fibre.

In addition to wearable microphones and biological sensors, applications of the fibres 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 fibres 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 the experimental fibres in a single fibre. Strong vibrations, for instance, could vary the optical properties of a reflecting fibre, enabling fabrics to communicate optically.

More information on the research is available from MIT News

Tags: acoustic fibres, fibres, MIT, optical fibres, Sensors