By leveraging cutting-edge additive manufacturing techniques and shape memory alloys, researchers at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, have created an antenna that can change its shape based on its temperature. This technology — described in a recent online publication in ACS Applied Engineering Materials and which will be featured on the cover of an upcoming print issue — has transformative potential in a wide range of military, scientific and commercial applications.
The shape of an antenna’s front end dictates many of its operating parameters. Once it’s manufactured, those characteristics are locked in. A shape-changing antenna would enable communications across a wider array of radio-frequency (RF) bands, opening up new realms of operational agility. Among the possibilities, a single shape-shifting antenna could do the work of multiple fixed-shape antennas, adapt dynamically to spectrum availability and change beamwidth to switch between short- and long-range communications.
Inspired by science fiction technology, the novel antenna is the result of creative cross-disciplinary collaboration across APL.
Electrical engineer Jennifer Hollenbeck said she got the idea from “The Expanse” series, where alien technology is organic and shape-changing. “I have spent my career working with antennas and wrestling with the constraints imposed by their fixed shape,” she said. “I knew APL had the expertise to create something different.”
In 2019, Hollenbeck reached out to Steven Storck, now chief scientist for additive manufacturing in the Lab’s Research and Exploratory Development Department, who at the time led an Independent Research and Development project to create a promising methodology to additively manufacture shape memory alloys. These unique materials deform at lower temperatures but return to a “remembered” shape when heated and are used in a wide variety of applications, ranging from medical uses such as orthodontic wires, vascular stents and bone implants to actuators for control surfaces in spacecraft.
Mechanical engineer and materials scientist Andy Lennon had used nitinol — a shape memory alloy of nickel and titanium — to create coils that would extend down through a person’s esophagus to assist with heart imaging. As Lennon and others worked on applications for nitinol, a desire arose to 3D-print complex shapes with it. But that presented a problem: Nitinol and other shape memory alloys conventionally require extensive mechanical processing — known as cold work — to achieve the shape memory effect, and as a result they are typically only available as wire or in thin sheets.
“Doing an extreme amount of cold work would defeat the whole point,” Lennon said. “If you take that complex shape and pass it through a die to stretch it out, you’re back to a wire.”
The APL team initially conducted research to tackle the fundamental challenges associated with scalable additive manufacturing of nitinol components, later applying these techniques to create shape-changing structures that could be deployed in space applications. After extensive experimentation toward the antenna application, the team altered the ratio of nickel and titanium, but the first attempt to create a shape-shifting horn antenna using 3D-printed nitinol fell short. While the antenna did technically expand and contract and change its frequency, it was also rigid and difficult to expand.
“It turned out to be a really complicated design, and it didn’t work as well as I would have liked,” Hollenbeck said.
Undeterred, Hollenbeck and the team submitted a proposal for a Propulsion Grant, one of APL’s internal funding opportunities designed to support the development of revolutionary solutions to critical challenges.
This time, Hollenbeck had a new antenna design in mind. Lennon’s team had been able to 3D-print nitinol with what’s known as two-way shape memory, in which the alloy can be heated and cooled to alternate between two remembered shapes. With critical design and prototyping support from Kyle Sibert, an electrical engineer in APL’s Force Projection Sector, Hollenbeck’s team developed an antenna that was shaped like a flat spiral disk when cool but became a cone spiral when heated.
Heating the spiral proved to be a challenge. The team had to determine how to heat the metal of the antenna enough for it to change shape, but without interfering with the RF properties or burning out the structure. To solve the problem, the team, led by RF and microwave design engineer Michael Sherburne, had to invent a new form of power line.
“For peak heating, the power line has to handle a lot of current,” Sherburne said. “We had to go back to fundamentals to make this work.”
The final piece of the puzzle was working out how to 3D-print the antenna in a consistent, repeatable fashion. Lennon’s modified nitinol, with its higher concentration of nickel, made it challenging to print at scale.
“We have a lot of experience optimizing processing parameters and designs for alloys, but this was a step beyond,” explained additive manufacturing engineer Samuel Gonzalez. “There aren’t many people out there, if anyone, printing this material, so there’s no recipe for how to process it.”
“We made shrapnel in the printer a few times because the antenna is trying to change shape as you’re printing it, due to the heat,” added colleague Mary Daffron. “It wants to peel apart.”
Typically, the team can process an alloy in less than four days, but Daffron and Gonzalez said this particular material took two to four weeks of build time.
Now that they have optimized the processing parameters, they’re already looking for ways to build on their initial success.
“We want to optimize the parameters to work on multiple different machines, to make this more widely applicable, and we know we’ll need to optimize for different variations of the material that might actuate at different temperatures,” Daffron said.
The hard work put in by teams across APL has yielded a radically innovative technology that could have wide-ranging applications, supporting special operators in the field, mobile network telecommunications and even space missions to distant celestial bodies.
APL is pursuing a full patent on behalf of the team for the shape-adaptive antenna technology. The Lab has also provisionally decided to pursue patents for the novel power line for heating the spiral, a method for controlling the antenna, and a method and process for using shape memory alloys to create a phased array antenna.
“The shape-shifting antenna capability that has been demonstrated by this APL team will be a game-changing enabler for many applications and missions requiring RF adaptability in a low-size and -weight configuration,” said APL Chief Engineer Conrad Grant. “This is yet another powerful example of the innovation that occurs at the Laboratory through motivated, highly capable, multidisciplinary teams.”
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