Are Spray-On Antennas the Future of Wearables?

The ultra-thin, flexible antennas can be applied to nearly any surface using an airbrush

The antennas are made from a special two-dimensional metallic material called MXene. (Drexel University)
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We may not think about them much, but antennas are everywhere. In our phones, in our cars, in the anti-theft tags on the clothes we buy, and as the Internet of Things becomes an ever-more-present reality, they’re showing up in new places, like microwaves and lamps. Engineers have therefore been looking for methods of making antennas smaller, lighter and easier to apply.

Now, researchers at Drexel University have developed a method for creating nearly invisible antennas on almost any surface by literally spraying them on like paint. The antennas are made from a special two-dimensional metallic material called MXene. MXene powder can be dissolved in water to create a paint that is then airbrushed on. In tests, even a layer as thin as just 62 nanometers – thousands of times thinner than a sheet of paper – could communicate effectively. Performance maxed out at just 8 microns, a point at which the spray-on antennas worked just as well as those currently used in mobile devices and wireless routers.

The antennas are so thin they can be sprayed on without adding weight or bulk, even to tiny devices like medical sensors. And they’re flexible too, meaning they can go on non-flat surfaces, like curtains. The researchers say the antennas could make for huge improvements in wireless devices and the Internet of Things, especially when it comes to wearables—you could even spray an antenna on your socks to keep track of them.

“This will allow really wireless communication with any item,” says Yury Gogotsi, a professor of materials science and engineering who led the research. “This could make a real difference because we’re going towards a world where everything will be connected."

Imagine being able to instantly apply an antenna to any item you own and make it a communication device. You could put an antenna on your dog's collar to keep him from getting lost. Put one on your refrigerator so it can communicate with your phones. Put them on your tennis balls to monitor the speed of your serves.

The research was published recently in the journal Science Advances.

MXene, a two-dimension titanium carbide material, was discovered by Drexel researchers in 2011 and patented in 2015. Ultra-strong and conductive, it’s shown potential to be used in energy storage devices, like battery electrodes that could charge phones in seconds; preventing electromagnetic interference between devices; sensing dangerous chemicals in the air, and more. In the study, the MXene antennas performed 50 times better than ones made of graphene, the current "hot" nanomaterial.

Unlike other nanomaterials, MXene doesn't require any binders or heating to adhere the nanoparticles together. All it needs is to be mixed with water and sprayed with an airbrush. The resulting antennas can even work on materials that are moving and flexing, such as textiles, though it will affect the reception, much in the same way moving the antenna on an old TV did.

Spraying antennas is “an interesting approach,” says Josep Jornet, a professor of electrical engineering at the University at Buffalo who works on communications networks and the Internet of Things.

Most research on thin flexible antennas has involved printing, Jornet says. But spraying has the potential to be faster.

But while the antenna performance as shown in the paper is “very good,” Jornet says, “an antenna by itself is nothing but a piece of metal.”

To make the antennas maximally useful, he explains, they would be paired with types of flexible electronics – think stretchable phones or roll-up tablets – that don’t exist yet. This is something many researchers are working on, but has yet to come to fruition.

The Drexel team tested the spray-on antennas on a rough material, cellulose paper, and a smooth one, polyethylene terephthalate sheets. They now plan to test it on other surfaces, including glass, yarn and skin—yarn antennas could make for connected textiles, while skin could have applications for veterinary or human medicine. They hope to partner with investors or commercial partners interested in developing products that could benefit from the antennas.

While the antennas have the potential to be used for wearables or health monitors sprayed directly on the skin, Gogotsi advises caution, as MXene has little record of being used on humans.

“We are always a little bit concerned with novel materials,” he says. “Is it biocompatible? Are there long term consequences? I would suggest we should wait before putting it directly on the skin.”

The team is also looking at how to optimize the material in terms of conductivity and strength, potentially making it even thinner and easier to spray in more precise shapes, as well as making it work at different frequencies.

“There is plenty of room for improvement,” Gogotsi says. “The first one is never the best one.”

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