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Shape-Shifting Antennas Flex Their Muscles

Digital natives probably don’t remember how home TV viewers had to manually adjust “rabbit ears”—those odd-shaped dipole antennas that sat atop a TV sprouting wires and sporting any number of dials to turn in the hope of improving the picture. But when a recently uncovered use for an alloy comprising gallium and indium becomes widespread as the go-to material for antennas, the newest antennas may be able to adjust themselves without a human hand. Although only in the second stage of research, the combination of these well-known materials already has demonstrated that when bent and twisted, antennas return to their original shape; when cut with a razor, they heal.

Combining gallium and indium, Dr. Michael Dickey, assistant professor of chemical and biomolecular engineering at NC State, has created a material that he is testing for an antenna that self-heals when cut. The antenna retains its conductivity through the cut because the wire is a liquid.

Doors to new uses open when well-known chemicals combine.

The story about how this way of creating antennas occurred is a basic “right-time, right-place” anecdote that Dr. Michael Dickey shares with some amazement still in his voice. While at Harvard University, Dickey discovered that combining gallium with indium created a metallic substance that could be injected into hollow channels the width of a human hair and open at each end. The substance remains in liquid form at room temperature, but when exposed to the air, it spontaneously forms a skin around it that keeps the alloy in place while retaining liquid properties.

Dickey, now an assistant professor of chemical and biomolecular engineering at North Carolina State University (NC State), Raleigh, North Carolina, and co-author of the research, suspected that this discovery could have some significance for antenna technology. But it was not until he came to NC State that he had the opportunity to meet up with an expert in antennas who could move the work forward.

Enter Dr. Gianluca Lazzi, a former professor at NC State. In fall 2008, Lazzi was part of a team that spoke to incoming faculty members during their orientation. Knowing that Lazzi was an expert in the area of antennas and electromagnetic devices, Dickey approached the senior professor immediately after the presentation. He described the properties of the gallium-indium alloy and asked Lazzi how a metallic substance of this kind could be used in electrical systems or antennas.

While he was skeptical about whether or not the substance would have beneficial properties, Lazzi agreed to a test the material as a possible antenna material. To create the initial dipole antenna, Dickey, Lazzi, an undergraduate student and a team of graduate students injected the alloy into elastic silicone channels. This created extremely thin antennas that were resilient and could be fashioned into different shapes because their mechanical properties were dictated by the elastomer and not the metal.

The results of the test were remarkable immediately. The researchers discovered that the alloy acted like other types of antennas. In fact, what surprised both Dickey and Lazzi was that the material “behaved beautifully and much better than expected” during the very first test, Lazzi relates.

“I said, ‘Michael, we’re on to something here, because it behaves very similarly to ideal antennas in terms of electrical properties. It does what anyone would want an antenna to do in terms of efficiency, radiation and low loss—the qualities that drive a dipole [antenna],’” he shares.

Not only did the material perform ideally as an antenna during the first trial, it also exhibited additional qualities that metal antennas do not possess. It could be stretched beyond anything Lazzi had seen before. “It did not break. … It was just wild—something that behaves like anything else we’ve seen before, but at the same time has properties that are totally different,” he states.

Dickey notes that unlike other liquids, the alloy does not evaporate because of its very high vapor rate. “Liquids like water want to form droplets, and most liquids oxidize when exposed to air. This stuff wants to do that as well, but it reacts with air and becomes solid, so you can mold it into shapes,” he explains. Although many people would not have a use for these properties, it is ideal for antennas, Dickey adds.

“This flexibility is particularly important when constructing antennas, because the frequency of an antenna is determined by its shape. So you can tune these antennas by stretching them. The longer the antenna, the more frequencies that can be obtained,” Dickey explains. And after stretching and bending these alloy antennas for a specific purpose, they independently return to their original shape.

In addition to the elasticity of this next-generation antenna, it exhibits a self-healing feature. When cut with a sharp object such as a razor blade, the material molds back together and becomes one piece again, he relates.

Once Lazzi was convinced that this alloy could be used to create antennas, the two scientists were ready to begin exploring the next research phase. For the initial test, a three-inch-long antenna was used. For the next step, the idea is to determine at what length the antenna can still remain effective. Currently, it appears to Dickey that the operational effectiveness of the alloy antenna is only limited by the length of the molds that are used to form the tiny antennas. “I don’t see any limits. There might be issues in stretching it for miles, but certainly it can be made several feet long, I believe,” he says.

One possible military use for the alloy antennas would be to wind a length of antenna into a ball for easy and light transport. Once a warfighter has reached the destination, the antenna could be rolled out to its full length.

Dickey admits that there is nothing particularly new about combining gallium and indium to create this substance. Rather, although it has been possible, no one has pursued work with this alloy, because they could not determine a use for it. However, now that at least one application has been found, that scenario is likely to change.

A three-year National Science Foundation grant of $300,000 is supporting the ongoing alloy antenna work. The researchers agree that creating a dipole antenna with this alloy is a useful but relatively simple test of its capabilities. However, if the mechanical properties remain the same as the professors and their teams create different types of antennas, the opportunities for usage may be as endless as the length the antennas can stretch. For example, the scientists already have been experimenting to determine if the alloy behaves as well when shaped into coiled antennas and have been testing gain efficiency. “Right now, we need to get a better understanding of the fluid itself,” Lazzi notes.

Once this understanding is achieved, alloy antennas could be used in flexible electronics. Dickey points out that perhaps it would be woven into fabric, so that clothes could in fact act as antennas. In addition, an antenna with this amount of flexibility could benefit the military because it could be deployed with military units in small, easy to carry packages, and then unfolded into a different form without losing its capabilities and functions.

Lazzi has additional ideas for its use. He believes it could be beneficial in a biological setting to help restore sight. The question Lazzi must answer is whether this flexible antenna can be affixed to the back of a blind person’s retina and still operate effectively.

The alloy antennas also could be used as sensors. Once embedded in structures such as buildings and bridges, the scattering from the antenna could be measured; changes would indicate that the material around them has expanded or contracted over time. As a result, civil engineers would be able to obtain information wirelessly about a structure’s condition, Lazzi notes.

Although Lazzi has moved to the University of Utah, Salt Lake City, since his initial meeting with Dickey, they and students located at both campuses continue collaborating on the project. Miles mean nothing to these antenna architects as they use videoconferencing to confer in between on-site meetings at the NC State campus. “Skype is great!” Lazzi exclaims.

In his new job, Lazzi is a professor and department chair of the university’s Department of Electrical and Computer Engineering. Among his current research interests are the interaction between biological media and electromagnetic fields, implantable micro antennas, neural stimulation and antennas for wireless transmissions. As a result, his work with Dickey continues to be of great interest to him.

When asked why someone has not come up with this application for the alloy before, Lazzi responds, “That’s a very good question. The material Michael is using was used before in other applications. Electrical engineers don’t always think of these things. Sometimes it takes brains from different specialties to get somewhere.”

WEB RESOURCES
North Carolina State University: www.ncsu.edu
University of Utah: www.utah.edu/portal/site/uuhome