TECHNOLOGY FOR CHANGE

Acoustic Cloth Senses, Emits Sonic Signals

R. Colin Johnson | Date: 07-26-10 | Comments

Researchers at MIT have developed a way to weave piezoelectric fibers, which can send and receive sound, into cloth. Possible applications include wearable microphones, hospital gowns that monitor vital signs and sonar imaging systems.

Researchers at the Massachusetts Institute of Technology (MIT) claim to have perfected "acoustic cloth"�a man-made fabric mass-produced from piezoelectric fibers that enables textiles to both sense and emit sound. Applications range from wearable microphones and iPod speakers to hospital gowns that act as medical monitors and underwater "sails" that perform sonar imaging.

For the first time, piezoelectric fibers can be woven into fabrics containing millions of tiny acoustic transducers that act together to both sense and emit sound. Using the same equipment that makes optical fibers for communications, these new fibers can be woven into fabrics that act as wearable microphones and loudspeakers. Possible uses include biological sensors that measure medical quantities such as blood flow in capillaries, environmental monitors measuring the flow of water passing through them, and large-area sonar imaging systems for monitoring underwater activities.

A close-up of flat piezoelectric fibers that act as acoustic transducers, enabling fabrics to produce sounds as well as perform acoustic imaging by sensing sound, such as measuring blood flow in capillaries or pressure in the brain (source: Research Laboratory of Electronics at MIT/Greg Hren Photograph).

For over a decade, MIT professor Yoel Fink has sought to create such a piezoelectric fiber. Piezoelectric transducers have long been used to produce sound from electrical signals, such as in the tweeter of many loudspeakers. Piezoelectric transducers have also been used for sensing acoustic sound waves and producing electrical signals from them, as in piezoelectric microphones and musical instrument pickups. However, these applications required that the piezoelectric transducer be meticulously constructed from multiple layers of metal and plastic, making them unsuitable for fabrics.

The solution was to craft an artificial fiber that was all plastic and that used the same mass-production method as optical communication fibers. Today, such optical fibers are mass-produced using a "preform"�a large cylinder of the multilayer optical material, which is heated to the melting point and then drawn out through a small aperture and cooled. Unfortunately, the metal electrodes ordinarily required by piezoelectric transducers cannot be drawn out in such multilayer fibers, since the metal electrodes would deform at high temperatures.

The solution was discovered by Fink while working with lab members Noemie Chocat, Zheng Wang and Shunji Egusa (a former post-doctoral researcher). Instead of using metal for the necessary electrodes, the researchers invented a method that adds graphite (conductive carbon) to a polymer, thereby making the plastic conductive. Because the conductive plastic holds its shape better than liquid metals, the technique enables a piezoelectric fiber to be pulled from a preform complete with its polymer electrodes. The resulting fiber has a ferroelectric polymer core of 30 microns. This core is electrically contacted by the internal plastic electrodes and then encapsulated in an insulating cladding that is several hundred microns thick.

The researchers demonstrated their new material by successfully fabricating fibers that acted as a piezoelectric transducer at frequencies ranging from kilohertz to megahertz.

Funding was provided by the Institute for Soldier Nanotechnologies at MIT, the National Science Foundation (NSF) and the U.S. Defense Department's Defense Advanced Research Projects Agency (DARPA).