Antennas That Bend, Stretch and Twist

Researchers have created a new type of antenna made of liquid metal that can bend, stretch and twist, then return to its original shape, an advance in technology that could lead to new uses where resiliency is especially important--in the military, for example, or for rugged outdoor activities.

"It'd be great for anywhere an antenna could get beaten up," said Michael Dickey, assistant professor of chemical and biomolecular engineering at North Carolina State University, and one of the scientists who developed the device.  "It could work really well for people who go camping or rock climbing and need an antenna they can unfurl at their destination."

Modern antennas, which are used in everything from cell phones to GPS devices, are made from copper or other metals, but there are limitations to how far, and how often, they can be bent before they break completely. The new antennas can be stretched but don't break. "They are malleable and more mechanically robust," Dickey said.

The scientists made the antennas by injecting an alloy made of the metals gallium and indium, which remains in liquid form at room temperature, into very small hollow channels the width of a human hair. They used elastic silicone channels to hold their alloy, and then fashioned wire-like antennas out of the material. The channels, which resemble straws that are open at both ends, can be manipulated into a variety of shapes.

Once the alloy has filled the channel, the surface of the alloy oxidizes, creating a “skin” that holds the alloy in place, while allowing it to retain its liquid properties.  "Because the alloy remains a liquid, it takes on the mechanical properties of the material encasing it,” Dickey said.

"What most people do to make things flexible is make them thin--aluminum foil, for example, or paper, " he continued.  "Paper is more flexible than a two-by-four. But if you can make something into a liquid the right way, it won't break. You can bend aluminum foil, but you can't stretch it, and that's the difference. You can stretch this."

Another discovery: "If you cut this device just through the metal--not all the way through--it comes back together," Dickey said. "You can partially damage it, and it will self-heal."

Since the frequency of an antenna is determined by its shape, "you can tune these antennas by stretching them," Dickey said. "As you stretch the antenna and change its properties, it offers an extra degree of freedom. You can think of it as an antenna that can sense its surroundings, such as changes in the environment."

For example, an antenna in a flexible silicone shell could be used to monitor civil construction, such as bridges. As the bridge expands and contracts, it would stretch the antenna, changing the frequency of the antenna, and providing civil engineers information wirelessly about the condition of the bridge.

Flexibility and durability also would enhance its use in military equipment, since the antenna could be folded or rolled up into a small package for deployment and then unfolded again without any impact on its function. Dickey thinks these new applications are the most likely uses for the new antennas, since the alloy is more expensive than the copper typically used in most consumer electronics that contain antennas.

"It's more expensive than conventional materials," he said. "You would not take existing devices and replace them with this. This is more expensive, and it's one of its main limitations. But if you need the functionality it enables, then you can justify it."

Dickey's collaborators include North Carolina State University doctoral students, Ju-Hee So, Amit Qusba and Gerard Hayes; North Carolina State University undergraduate student Jacob Thelen; and University of Utah professor Gianluca Lazzi, who participated in the research while a professor at North Carolina State University.

The research, “Reversibly Deformable and Mechanically Tunable Fluidic Antennas,” appeared recently in Advanced Functional Materials, and was funded by the National Science Foundation.

—By Marlene Cimons, NSF

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