Scientists at the Raytheon-UMass Lowell Research Institute ( RURI ) have developed printable inks for producing tunable, electromagnetic filters on a variety of surfaces. The result can be printed directly on military equipment, including tanks, armored vehicles, and aircraft, serving as anything from a radio antenna to a radar array to even an "invisibility cloak".

The technology involves a ferroelectric nano-ink—a material whose properties can be tuned by an applied voltage—printed directly onto sheets or the surface of objects. The ink is sprayed down via aerosol jet into gridlike patterns, and researchers are working on a robotic arm that can help guide the ink dispenser.

The defense-related applications of this new process are huge. Radars arrays on fighter planes and AWACS-type aircraft could be printed directly onto the aircraft itself, doing away with nose or mast-mounted radar systems. Dr. Chris McCarroll, RURI co-director, told Popular Mechanics, "Near term, there is potential to print antennas on the outer surface with feeds to internal electronics below the surface. We can put the very flat phased array system on the skin of an aircraft, it will have to be thin enough to not impact aerodynamics and other concerns."

Meanwhile on the ground, armored vehicles could do away with tall, conspicuous whip antennas, and ride into battle with radar arrays printed directly onto their turrets or other surfaces, allowing them to detect enemy drones, missiles, and aircraft with ease. Infantry radios could have their antennas printed directly onto the device, making them less conspicuous.

The inks could even be used on warships, printing radar arrays and antennas directly onto the superstructure of warships, eliminating radar dishes and antennas that make the ship less stealthy. Salt water is a problem, but not an unsolvable one. "We are developing coatings to our electronics that protect from the environmental factors without complex packaging," said Dr. McCarroll. "These coating are also potentially formed as inks and could be printed in the near future as another layer of the printing process."

But the most interesting outgrowth of this technology is the potential to create an anti-radar "invisibility cloak" for military vehicles, a form of active stealth that can hide the wearer from different radars at different angles.

Radar works by sending out pulses of electromagnetic radiation tuned to a certain frequency, and analyzing the pulses that return after they have bounced off a ship, aircraft, or armored vehicle. Current so-called "passive" stealth technology relies upon permanent, faceted surfaces built into an object redirecting radar waves away from the sender. The exterior of the F-35 Joint Strike Fighter, for example, is designed to prevent radar waves from returning to enemy radar receivers, thus rendering the aircraft effectively invisible to radar.

There are two problems with this method. For one, passive stealth is not completely efficient at all angles—that is, a stealth aircraft become may become more detectable depending on whether the radar is viewing the aircraft head-on, from behind, or some other angle. Second, U.S. stealth aircraft—including the F-35—are permanently optimized towards high frequency radars. As a result, both Russia and China are working on low frequency radars that would render this sort of passive protection useless.

With Raytheon's new technology, a fighter, ship, or tank coated with the new functional inks could achieve a form of active stealth by redirecting radar waves away from the sender and changing as the situation does. It would also have the ability to tune the stealth to whatever frequency the enemy radar is operating at, giving the aircraft a wider range of protection against different radar systems. Unlike passive stealth, active stealth could also offer equal protection at all angles.

The benefits of tunable inks are considerable. Printing 2D and 3D surfaces and objects will be cheaper than other forms of manufacturing. Active stealth alone could keep U.S. military aircraft protected from enemy radar threats not envisioned just twenty years ago, when U.S. designers felt that low frequency radars were incapable of targeting aircraft.

RURI has filed a provisional patent on the ink technology and is exploring commercial use as well. Stealthy cars anyone? Maybe not, but how about radars that watch your blind spot, seamlessly and invisibly built into the car's body styling? The multitude of uses for tunable inks means that the technology will eventually reach into nearly all of our lives.

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