The Invisibility Revolution
WHY YOU SHOULD CARE
Who doesn’t want an invisibility cloak like in Harry Potter? And with metamaterials, that’s just the beginning.
By Melissa Pandika
While science has gone Hollywood with Gravity and The Big Bang Theory, Hollywood has also gone science. When Harry Potter first donned his invisibility cloak, physicists around the world famously embarked on a quest to engineer the magical fabric. But invisibility cloaks are just the beginning. Scientists are designing many other materials that are allowing them to make the leap from reel to real.
Metamaterials are built from tiny metal or plastic structures arranged in precise patterns, which give them weird properties that aren’t seen in natural materials.
These so-called metamaterials are built from tiny metal or plastic structures arranged in precise patterns, which give them weird properties that aren’t seen in natural materials. Some metamaterials bend electromagnetic waves around objects, which can render them invisible to not only the naked eye but also to radar and sonar. Some metamaterials have unusual mechanical properties that allow them to “remember” their original shape, while others stretch when they’re compressed, and vice versa.
Invisibility cloaks have obvious military applications, but metamaterials also hold promise for drug delivery, energy-efficient power, improved cell phone signal strength and many other commercial uses. Metamaterial products could become mainstream within the decade, researchers say.
“The application of metamaterials will bring in a whole new era of next-generation materials research,” said Dan Luo, a professor of biological and environmental engineering at Cornell University.
Physicist David Smith and his colleagues at UC San Diego unveiled the first metamaterial in 2000 — an array of tiny copper wires and rings that deflected microwave beams. Since microwave scanners detect objects when microwaves bounce off them and hit the sensors, the material remained undetected by the scanner. The technology raised hopes for fabricating an invisibility cloak, since we detect objects similarly when light reflects off them and reaches our eyes.
The application of metamaterials will bring in a whole new era of next-generation materials research.
In November 2012, researchers from Duke University reported designing an invisibility cloak that can hide a 7.5-by-1-centimeter cylinder, while Nanyang Technological University scientists have shielded a cat and goldfish using cloaking devices made from conventional materials rather than metamaterials. But these cloaks work only in a single 2-D plane, so the object is hidden only to those looking from a certain direction.
So engineers at the University of Toronto designed a cloaking device made of a layer of tiny antennas that collectively radiate an electromagnetic field, cancelling out any waves that reflect off the hidden object over a wide range of wavelengths. Right now, the cloak only works for radio waves, but the antennas could theoretically be re-tuned to cancel out light, sonar and other waves. The cloak can mask objects of any size and can even deceive detection devices by transmitting signals to make the hidden object seem bigger, smaller or in a different location. Besides military applications, such as hiding vehicles or conducting surveillance operations, the technology could also be used to cloak buildings that interfere with cellphone and other signals.It may be years before we see a full-sized cloak that’s truly invisible to the naked eye, experts say. For one thing, the microscopic structures that make up the metamaterial need to be smaller than the type of wave they’re bending. It’s also hard to engineer materials that bend all wavelengths of light.
The cloak can mask objects of any size and can even make the hidden object seem bigger, smaller or in a different location.
If the wizardry of distorting electromagnetic waves doesn’t raise eyebrows, some metamaterials can transform one type of wave into another. Last November, Duke engineers created a material that can harvest energy from a microwave, satellite or Wi-Fi signal and convert it into enough electricity to recharge a small electronic device. They hope to build the material into a cellphone so you can recharge it, connecting to a Wi-Fi network on pesky days when you forget your charger or can’t find an outlet.
That metamaterial could hit the market in a few years, predicts Duke electrical and computer engineering professor Steven Cummer. For now it works only when there’s a Wi-Fi or other signal nearby. But “ultimately we would like it to be able to soak up power no matter where you are,” he said.
Some metamaterials have extraordinary mechanical properties. Take the material that Northwestern University researchers fabricated, which contracts when it’s pulled and vice versa. The design involves a row of four molecular particles. Since the force attracting the two inner particles is weak, stretching the material breaks the bond. Meanwhile, squeezing the fabric brings the two inner particles close enough to re-form their bond, causing the material to expand. Such materials could be used as protective coatings for military vehicles or shelters to push against explosive impacts.
In 2012, Luo and his colleagues created a mesh of synthetic DNA and other organic molecules that can flow like a liquid and then revert to its original shape. The hydrogel can be infused with drugs and poured into a wound to speed up the healing process, said Luo, who expects the hydrogel to emerge in the clinic within the decade.
The biggest hurdle to commercializing metamaterials? Finding quick, cheap ways to manufacture metamaterial elements on a nanometer scale. “It’s not like cement,” Luo said of his DNA hydrogel. “It’s not a commodity material, not yet.”
In the meantime, we’ll let our daydreams run amok with invisibility cloaks and wall-climbing suits. But science is gradually blurring the line between fantasy and reality. “I am cautiously optimistic that metamaterials … will prove truly revolutionary,” UC Berkeley physicist Xiang Zhang told Nature. “What we can do with metamaterials will be limited only by our imaginations.”
- Melissa Pandika Contact Melissa Pandika