Today you may check your clothes in the mirror. Tomorrow you could be checking the mirrors on your clothes. MIT researchers have created high-performance mirrors in the shape of hairlike flexible fibers that can be woven into cloth or incorporated into paper. The technology could one day reside in clothing with an embedded reflective code or in protective gear for emergency personnel.

The mirror fibers are based on a nonmetallic "dielectric mirror" developed by Yoel Fink, an MIT assistant professor of material science and engineering. Like a conventional mirror, Fink’s invention can reflect light from all angles and polarizations. Unlike its traditional counterparts, however, the fiber mirror can be tuned to reflect certain wavelength ranges. This means an array of mirror fibers, or even a single fiber, can reflect light at different wavelengths to create a kind of optical code (a code that could be discretely detected at security checkpoints, for example).

To develop a fiber version of the dielectric mirror, Fink and his coresearchers constructed a cylinder?measuring about 30 centimeters long by 25 millimeters in diameter?to pack 21 layers of dielectric materials around a polymer core. They then fed the package into a furnace and fiber-optic "draw tower" and pulled the bundle into threadlike fibers. Fink says the process requires exacting control. "Typical glass fiber has only a few, fairly large internal features," he notes. "The mirror threads contain over 21 features, and thickness control needs to be kept to under one micron." By using materials that can be thermally processed at the same temperature, Fink and his team were able to minimize defects while drawing mirror fibers hundreds of meters long.

Mirror fibers have potential applications that go beyond wearable identification. Scientists could also design fibers to reflect thermal radiation over various temperature ranges, useful in heat-protective clothing worn by firefighters, soldiers, astronauts and others who work in searing environments. "It’s a low cost process, so we think it will lead to a variety of practical applications," says Fink.