In school, you probably learned that materials like wood, glass, and water are made of atoms that stick together to make molecules. And that way those atoms stick together determines what the material can do. For example, super strong atomic bonds usually make a material solid.
But new manmade materials, called “metamaterials,” are tossing the laws of physics out the window. These materials have far-out capabilities, like bending sound and light around solid objects, and in some cases, making them invisible. Their behavior is not determined by individual atoms, but by larger artificial structures called meta-atoms.
Essentially, the whole is greater than the sum of its parts.
“These metamaterials are revolutionary in concept,” said John Pendry, professor of theoretical solid state physics at Imperial College London.
He notes that some applications are closer to reality than others. “If you’re a theorist like me you can have wild dreams,” he said. “If you’re an engineer, engineers tend to be quite rightly very cautious.”
Invisibility cloaks are one dreamy application. “Ten years ago we wouldn’t know how to make one,” said Steve Cummer, associate professor of electrical and computer engineering at Duke University. “Now we know what we have to do to make one.” Nanotechnology still has to advance significantly before that can happen, though. In the meantime, scientists like Cummer and Pendry are pondering these 10 future uses for metamaterials.
A painless, implantable sensor made of a silk-based metamaterial coated in gold could help keep diabetic patients healthy. The sensor, developed by a team of scientists from Tufts and Boston University, sends a wireless alert when glucose levels gets out of whack. Metamaterials could also improve medical imaging by cloaking internal organs and tissue -- making them invisible -- so that a doctor can see the problem point. Nicholas Fang, assistant professor of mechanical and science engineering at the University of Illinois, created an acoustic metamaterial that directs ultrasound, and could one day lead to higher resolution images.
Inspired by the desktop “toy” called Newton’s cradle (photo), California Institute of Technology scientists Chiara Daraio and Alessandro Spadonia came up with a metamaterial that focuses sound. Doctors could use the targeted sound to precisely annihilate tumors and kidney stones without damaging surrounding tissue.
The same technique that focuses sound to destroy cancer can also be used to destroy military targets. High-energy acoustic pulses or "sound bullets" could be potentially very destructive and extremely targeted, cutting through liquids and solids alike with powerful force. These sound bullets might even be able to get into areas that are difficult for metal bullets to reach, like underground caves where terrorists might be hiding.
Imperial College London nanophotonics professor Stefan Maier and his colleague John Pendry have been working on developing energy harvesting devices made with a special class of metamaterials, known as negative-index. These produce unusual light wave behavior such as negative refraction.
The team’s energy harvester would work similar to the way plants harvest light, except at a much smaller scale.
“What we want to do is to devise a system using negative materials that will harvest light on the scale of a wavelength,” Pendry said. Then the light could be concentrated and guided on a nanoscale to interact with electronic systems or even molecules.
Scientists are also working on creating metamaterials that manipulate heat flow. “One of the things people are trying to do with that is make materials that are better insulators than conventional materials,” said Duke University’s Steve Cummer. These metamaterials would be even better at insulating than a vacuum, which is what many thermoses use to keep liquids cold or hot.
In 2009, Stanford University electrical engineering professor Shanhui Fan and his team came up with a material (abstract) that blocks infrared radiation better than a vacuum. Their 100-micron-thick material created from photonic crystals only let in a fraction of the radiation that a vacuum would.
“The applications people have in mind right now are small-scale. We’re not talking about giant sheets you get at Home Depot and roll out in your attic,” Cummer said. Yet.
In the future, semiconducting devices made with metamaterials could move light -- instead of electrons -- through computer circuits. Engineers in the United States and abroad have been racing to develop photonic computer chips. Such chips could be many times faster than the chips in computers now. Recently researchers at the University of Bristol’s Center for Quantum Photonics in the U.K., built a two-photon silicon chip that they think could lead to a quantum computer.
“Getting the electron to talk to the photon, it’s a conversation between a mouse and an elephant,” Pendry said. “Using metamaterials, we are finding ways of mediating that conversation.”
Duke University’s Steve Cummer imagines that one day, metamaterials could be used in architecture to control acoustics without detracting from the appearance of a building.
“Acoustics are important in how you design a room with 50 people,” Cummer said. “You want a beam to be in a room for structural reasons, but it might be bad from an acoustic perspective.”
But if you were able to give the beam a different acoustic property, you could have your cake and eat it, too. Concert halls, and even large cubicle farms could be transformed into entirely new experiences.
Satellites are covered in metallized Mylar sheets, Steve Cummer notes. The gold foil is there to help control heat. “Sometimes they’re in the sun and that side bakes and gets hot, yet they’re in vacuum so it’s hard to get rid of excess heat,” he said.
Metamaterials could potentially allow for more heat control, Cummer said. When the satellite is in the sun, the metamaterial coating would be highly reflective, but then in the shadow switch to become strongly emitting. The challenge, however, will be constructing such a fine metallic structure on an extremely thin foldable sheet.
In early 2009, assistant professor of mechanical and science engineering Nicholas Fang led a group of scientists at the University of Illinois in constructing a metamaterial that can bend sound waves backward. Fang’s group crafted an array of tiny thin aluminum cavities filled with water to focus ultrasound.
While it is only several square inches in size, the metamaterial could bend sound waves around a structure. This gives it the potential to be turned into an application for ship hulls and submarines that hides them from sonar detection. Transforming Fang’s groundbreaking metamaterial into a real structure on a large scale, however, is still several years away.
A special fabric that makes a human truly invisible to others is still the stuff of Harry Potter, but some progress has been achieved. Scientists from Imperial College London and the Karlsruhe Institute of Technology in Germany successfully made an object disappear underneath a material they created from photonic crystals.
Minuscule lenses covering the material bent light to hide the object. The material was only 100 microns by 30 microns, and the object a fraction of that size, but still. Technology that works like magic has to start somewhere.