This fabric can hear your heartbeat

Someday our clothing may eavesdrop on the soundtrack of our lives, capturing the noises around and inside us.

A new fiber acts as a microphone — picking up speech, rustling leaves and chirping birds — and turns those acoustic signals into electrical ones. Woven into a fabric, the material can even hear handclaps and faint sounds, such as its wearer’s heartbeat, researchers report March 16 in Nature. Such fabrics could provide a comfortable, nonintrusive — even fashionable — way to monitor body functions or aid with hearing.

Acoustic fabrics have existed for perhaps hundreds of years, but they’re used to dampen sound, says Wei Yan, a materials scientist at Nanyang Technological University in Singapore. Fabric as a microphone is “totally a different concept,” says Yan, who worked on the fabric while at MIT.

Yan and his colleagues were inspired by the human eardrum. Sound waves cause vibrations in the eardrum, which are converted to electrical signals by the cochlea. “It turns out that this eardrum is made of fibers,” says Yoel Fink, a materials scientist at MIT. In the eardrum’s inner layers, collagen fibers radiate from the center, while others form concentric rings. The crisscrossing fibers play a role in hearing and look similar to the fabrics people weave, Fink says.

Analogous to what’s happening in an eardrum, sound vibrates fabric at the nanoscale. In the new fabric, cotton fibers and others of a somewhat stiff material called Twaron efficiently convert incoming sound to vibrations. Woven together with these threads is a single fiber that contains a blend of piezoelectric materials, which produce a voltage when pressed or bent (SN: 8/22/17). The buckling and bending of the piezoelectric-containing fiber create electrical signals that can be sent through a tiny circuit board to a device that reads and records the voltage.

The fabric microphone is sensitive to a range of noise levels, from a quiet library to heavy traffic, the team reports, although it is continuing to investigate what signal processing is needed to detangle target sounds from ambient noise. Integrated into clothing, this sound-sensing fabric feels like regular fabric, Yan says. And it continued to work as a microphone after washing it 10 times.

Woven into fabric, a specialized fiber (pictured, center) creates electrical signals when bent or buckled, turning the entire material into a microphone.Fink Lab/MIT, Elizabeth Meiklejohn/RISD, Greg Hren

Piezoelectric materials have “huge potential” for applications from observing the function of bodies to monitoring the integrity of aircraft materials, says Vijay Thakur, a materials scientist at Scotland’s Rural College in Edinburgh who was not part of this work. They’ve even been proposed for energy generation, but, he says, many uses have been limited by the tiny voltages they produce (SN: 10/1/15). The way the fibers are made in this fabric — sandwiching a blend of piezoelectric materials between other components, including a flexible, stretchy outer material — concentrates the energy from the vibrations into the piezoelectric layer, enhancing the signal it produces.

As a proof of concept, the team incorporated the fabric into a shirt, which could hear its wearer’s heart like a stethoscope does. Used this way, the fabric microphone could listen for murmurs and may someday be able to provide information similar to an echocardiogram, an ultrasound of the heart, Thakur says. If it proves effective as a monitoring and diagnostic tool, placing such microphones into clothing may someday make it easier for doctors to track heart conditions in young children, who have trouble keeping still, he says.

The team also anticipates that fabric microphones could aid hearing and communication. Another shirt the team created had two piezoelectric fibers spaced apart on the shirt’s back. Based on when each fiber picked up the sound, this shirt can be used to detect the direction a clap came from. And when hooked up to a power source, the fabric microphones can project sound as a speaker.

“For the past 20 years, we’ve been trying to introduce a new way of thinking about fabrics,” Fink says. Beyond providing beauty and warmth, fabrics may help solve technological problems. And perhaps, Fink says, they can beautify technology too.