KUnfortunately, they are no longer just meant to keep their wearer warm, cool or protect them from wind and rain. Some textiles are now equipped with plenty of electronics. There are jackets and shirts that have built-in heart rate monitors, movement monitors, humidity and temperature sensors, and veritable navigation aids. Shirts can be equipped with flexible displays and a small keyboard – luminous and conductive fibers make it possible for some. In the meantime, woven-in thin batteries also supply the power required for operation.
Fans of these textiles, known as smart, can now look forward to another technical achievement that scientists from the Massachusetts Institute of Technology (MIT) in Boston are presenting in the journal “Nature”. Yoel Fink and his colleagues have developed a substance that can process acoustic signals in a similar way to a microphone. The wearers of this acoustic sensor could even “hear” their own heartbeat.
The MIT researchers used the ear as a model for their invention. In the hearing organ, acoustic signals are first converted into mechanical vibrations via the eardrum. Electrical stimuli are then generated in the sensory cells of the cochlea, which are then transmitted to the brain by nerve fibers.
A special fiber with piezoelectric properties takes over some functions of the inner ear in hearing textiles. It generates an electrical signal when it is only slightly bent or deformed, such as by incoming sound waves. The fiber unfolds its full effect when it is woven into a fabric. This then acts like a membrane: incoming sound waves are converted into mechanical vibrations. These vibrations are barely noticeable, but are sufficient to generate electrical signals in the piezoelectric fiber. The electrical signals can be made audible with a loudspeaker.
More than just technical gimmicks
The piezoelectric fiber is manufactured in one step. The starting point is a flat, centimeter-sized blank that contains all the central components: a piezoelectric polymer, fine particles of barium titanate, which is also piezoelectric, a copper wire and an elastomer for the later plastic cladding. The blank is heated, pressed through a narrow nozzle and thereby stretched. The original diameter shrinks to around two millimeters. Although everything around “tightens”, the components remain intact. The result is an elastic fiber about 40 meters long whose components are protected by a plastic sheath. During manufacture, the barium titanate is distributed evenly throughout the piezoelectric polymer, increasing the sensitivity of the fiber.
Tests have shown that the fiber responds to sounds across a wide acoustic range, from whispers in a library to the roar of traffic on a busy street. The noise causes the fiber to vibrate, which creates an electrical current. The current varies linearly with the sound level. “The acoustic properties of the fiber are similar to those of a microphone,” says Noel.
The researchers processed the fiber into cloths using normal yarn. Sewn on the back and front of a jacket, it was possible to determine the direction from which a handclaps came. The angular resolution was about one degree at a distance of three meters from the sound source. The heartbeat of a test subject could be detected with a fiber that the researchers attached to the chest area on the inside of the T-shirt with simple needle pricks.
According to Fink’s researchers, what at first glance appears to be a gadget for technology freaks could be used, for example, to monitor sleeping babies in the first few months. MIT’s achievement could also make it easier for people with hearing or speech impairments to communicate. The researchers found that the fiber can be used not only as a microphone, but also as a speaker. A t-shirt or shirt could be transformed into a pull-on hearing aid in this way.