The finished fiber has a core that can carry light, a piezoelectric layer, and electrodes that can carry electricity to and from the piezoelectric layer. The MIT researchers can send pulses of electrical current down the fiber, causing the piezoelectric layer to squeeze the fiber. The resulting vibrations can be used to create acoustic waves, and the fibers can also detect vibrations and changes in pressure, because these, in turn, generate an electrical signal. This work is described this week in the journal Nature Materials.

Fink believes there are many possible applications for the new fibers. They could be woven into carpets that can count the number of people walking across them, or integrated into structural composites and used to sense cracks before they become serious. But one of the most promising applications, Fink believes, is in biomedicine. The fibers are less than a micrometer wide--narrow enough to be snaked into blood vessels or inserted into organs to monitor heart rate, blood flow, or biomarkers in the blood. Their ability to carry infrared light and to perform acoustic sensing offers a combination of properties similar to an ultrasound imager, a heart-rate monitor, and chemical spectrometer.

"Having the piezoelectric and the optical fiber completely integrated makes the sensor much smaller," says Juan Hinestroza, professor of fiber science and leader of the Textiles Nanotechnology Laboratory at Cornell University. "This is important--especially in a blood vessel or in a composite material where you have very little room."

The piezoelectric layer of the MIT fiber can be used to modulate the optical signals bouncing off the insides of the fiber. Fink's group has also made fibers containing a reflective layer that act as a sort of optical switch. The reflective layer interacts with specific wavelengths of light, determined by the thickness of the layers. Running an electrical pulse through the fiber squeezes the mirror, changing the color of light with which it will interact. If woven into a fabric the fiber could produce different visible patterns of color. "If you wanted to read information off a piece of clothing, or a plane or car, you could integrate these fibers," says Fink.