Conductive nanotube threads are being stitched into regular apparel to add functions such as heart rate monitoring and continuous electrocardiogram (EKG) readings.

Developed by the Brown School of Engineering lab of Matteo Pasquali at Rice University, the threads are said to be just as conductive as metal wires, but washable, comfortable and far less likely to break when a body is in motion, according to the researchers.

A shirt enhanced with them has proved better at gathering data than a standard chest-strap monitor taking live measurements during experiments. When matched with commercial medical electrode monitors, the carbon nanotube shirt gave slightly better EKGs.

“The shirt has to be snug against the chest,” said Rice graduate student Lauren Taylor, lead author of the study. “In future studies, we will focus on using denser patches of carbon nanotube threads so there’s more surface area to contact the skin.”

The researchers noted that nanotube fibres are soft and flexible, and clothing that incorporates them is machine washable. The fibres can be machine-sewn into fabric just like standard thread and a zigzag stitching pattern allows the fabric to stretch without breaking them.

The fibres provide not only steady electrical contact with the wearer’s skin but also serve as electrodes to connect electronics like Bluetooth transmitters to relay data to a smartphone or connect to a Holter monitor that can be stowed in a user’s pocket.

Carbon Hub

Pasquali’s lab first introduced carbon nanotube fibres in 2013. Since then, the fibres – each containing tens of billions of nanotubes – have been studied for use as bridges to repair damaged hearts, as electrical interfaces with the brain, for use in cochlear implants, as flexible antennas and for automotive and aerospace applications. Their development is also part of the Rice-based Carbon Hub, a multiuniversity research initiative led by Rice and launched in 2019.

The original nanotube filaments, at about 22 microns wide, were too thin for a sewing machine to handle. A rope making machine was used to create a sewable thread – essentially three bundles of seven filaments each, woven into a size roughly equivalent to regular thread.

“We worked with somebody who sells little machines designed to make ropes for model ships,” said Taylor, who at first tried to weave the thread by hand, with limited success. “He was able to make us a medium-scale device that does the same.”

The zigzag pattern can be adjusted to account for how much a shirt or other fabric is likely to stretch. The team is working with Dr Mehdi Razavi and his colleagues at the Texas Heart Institute to figure out how to maximise contact with the skin.

Fibre woven into fabric can also be used to embed antennas or LEDs, according to the researchers, and minor modifications to their geometry and associated electronics could eventually allow clothing to monitor vital signs, force exertion or respiratory rate.

Taylor noted other potential uses could include human-machine interfaces for vehicles or soft robotics, or as antennas and ballistic protection in military uniforms.

 “We demonstrated with a collaborator a few years ago that carbon nanotube fibres are better at dissipating energy on a per-weight basis than Kevlar, and that was without some of the gains that we’ve subsequently made in tensile strength,” she said.

“We are seeing that, after two decades of development in labs worldwide, this material works in more and more applications,” Pasquali said. “Because of the combination of conductivity, good contact with the skin, biocompatibility and softness, carbon nanotube threads are a natural component for wearables.”

The wearable market, he added, although relatively small, could be an entry point for a new generation of sustainable materials that can be derived from hydrocarbons via direct splitting – a process that also produces clean hydrogen. The development of such materials is a focus of the Carbon Hub.

“We’re in the same situation as solar cells were a few decades ago,” Pasquali said. “We need application leaders who can provide a pull for scaling up production and increasing efficiency.”

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