Acoustic fabric can hear your heartbeat, may provide key health data

The fabric works like a microphone, converting audible sounds into electrical signals.

By Betsy Vereckey

Imagine a shirt that can sense your heartbeat even better than the wearable watch on your wrist, one that’s fashionable enough to make your heart race.

Researchers at the Massachusetts Institute of Technology have collaborated with the Rhode Island Institute of Design to do just that—create a fabric that can listen in to the subtleties of the human body and its surroundings, just like a microphone. It works by converting sounds—like the beating of your heart—into mechanical vibrations, then into electrical signals.

“The surface of our bodies is broadcasting a lot of information,” says Yoel Fink, a professor of materials science at MIT who worked on the research, which culminates five years of work, the results of which were published earlier this year in Nature.

“I would say it’s one of the most valuable surfaces we have,” one that can be used to monitor and better understand our health, he says.

Take the heart, for example: “The beat of your heart is not going to show up very well on your wrist,” Fink says. If you’re on a flight, and you start to develop a blood clot in your legs, “by the time my wrist knows about it, it’s kind of going to be pretty late in the game, right?”

The surface of our bodies is broadcasting a lot of information. I would say it’s one of the most valuable surfaces we have.

—Yoel Fink,  professor of materials science, MIT

Using the human eardrum as design inspiration

Fabrics are often used to muffle noise and soundproof spaces (think of a movie theater, for example), but they can also be used to detect sounds, as well. All fabrics vibrate in response to sound, but the vibrations are much too discrete to be sensed. Fink and his team were able to get around this by designing a flexible fiber that—when sewed into a piece of fabric—could bend with it and be converted into an electrical signal.

Fink teaches a class at MIT called Computing Fabrics, which challenges the aesthetics of technology and prepares the next generation of techies “for a future where computers are not in boxes with pieces of glass on them but actually look and feel like fabrics.”

Photo by Yoel Fink

When initially thinking about how they could design a fabric that could be used to sense audible noise, Fink and his team took inspiration from the human ear. When sound travels through the air as faint pressure waves, the eardrum uses a circular layer of fibers to transform these pressure waves into actual vibrations. Then, these vibrations travel through tiny bones in the inner ear, where they are converted into electrical signals that are picked up by the brain and processed.

“Low and behold—an ear, which you never really think about as a fibrous structure, utilizes fibers at the front, and then the back end does the two most important operations: take pressure and convert it to mechanical [energy] and take the output of that and convert it into electrical,” Fink says. “So that got us thinking: Could we make a fabric that could detect audible sound, these very slight pressure waves? And could we convert those waves into electrical energy? And that’s sort of what defined the fiber structure.”

Piezoelectric material that flows like seaweed in the ocean

Fink and his fellow researchers designed their fiber from malleable piezoelectric material. Piezo derives from the Greek work piezein, which means to press or squeeze, and this material can produce electrical energy when bent. Next, these signals are sent to a device that reads and records the voltage.

Fink likens the material to seaweed “that flows with the waves” because seaweed “doesn’t just bob up and down, but it really conforms with the waves,” he says. This is what allows the fabric to convert sound vibrations into electrical signals.

To design it, the team created a device called a preform, made from a layered block of materials the size of a thick marker, which included the piezoelectric material. It was then warmed up and stretched into thin, 40-meter-long fibers.

To test if the fiber could pick up sound, researchers attached it to a sheet of Mylar (a piece of polyester or plastic), played sound through a speaker, then used a laser to measure the vibration of the sheet. The material vibrated in response and generated an electrical current that was equivalent to the sound played.

To gauge whether the fabric could sense directional sound, researchers sewed the fiber into the fabric, then clapped while standing at various angles to the shirt. The fabric was able to detect the angle of the clapping sound as far as three meters away. Again, taking inspiration from the human body—just as having two ears spaced on either side of the head allows someone to hear with greater accuracy, the same theory could be applied in choosing to place fibers far from each other. Putting fibers on one arm of a shirt and on the opposite arm would provide much greater angular resolution.

“Directionality is a big issue in the ear,” Fink says. “It’s not just how loud things are. It’s also what direction they are coming from, and it has to do with how many detectors you have and how far they are spaced apart.”

The future of computing is fabrics

Acoustic garments made with this fiber have ample uses, especially because the fabric is machine-washable. It might seem strange to think of putting technology in water, but the fabric is able to withstand the washing machine because the technology itself isn’t attached to the fabric—it is the fabric.

One day, people might use acoustic garments like these to take a phone call or talk to someone. Given its ability to pick up on directional sound, it could be used by people who have hearing loss. Furthermore, because the fabric responds to sensitivity in a person’s skin, that means someone wearing it could monitor their heart rate or respiratory condition. It could even be used in maternity wear to keep tabs on a baby’s fetal heartbeat.

In addition to wearables, acoustic fabric could be incorporated into buildings with the goal of detecting cracks via sound before a building collapses. And given that it’s water-proof, it could be woven into a net to keep tabs on marine life or fish in the ocean.

Could computers become fabrics? Will we one day answer a phone call from the shirt on our back? Fink thinks so. “The future of computation as we see it is essentially fabrics.”

Lead photo by Greg Hren