A new type of invisibility cloak made from a common material can work with larger objects.
There’s been a lot of excitement, both in the scientific community and in the popular media, about the possibility of creating cloaking materials that make people or military vehicles appear to vanish. But that goal seemed nearly impossible until Baile Zhang came up with a simple, promising solution. While Zhang’s technique has serious limitations—for one thing, it works only in an exotic medium called laser oil—it does suggest a possible path to making practical invisibility cloaks.
Most previously developed invisibility cloaks were made with materials painstakingly fabricated in the lab to have micro- or nanoscale patterns that bend light waves. But labs couldn’t turn out more than tiny amounts of these materials. What’s more, most existing examples of cloaking materials work only with microwaves and other nonvisible forms of light.
Reading about these exotic materials, Zhang, a professor at the Nanyang Technological University in Singapore, remembered a high-school physics demonstration of how calcite, an inexpensive natural mineral, bends light in strange ways. That, in turn, led him to come up with a simpler way to make a large cloak: gluing two pieces of calcite together.
Zhang demonstrated that his calcite sandwich could hide the middle section of a Post-it note rolled into a tube and placed on a mirror submerged in a liquid. The calcite cloak on top of the tube guides light from the space behind the tube to a point directly over it, so that the eye is, in effect, seeing right “through” the rolled-up paper. It turns out calcite’s crystal structure already resembles the sorts of artificial nanoscale patterns that other labs have been struggling to fabricate with electron beams.
“This shows better than any other experiment that the basic concept of cloaking can work,” says Steven Cummer, an engineering professor at Duke University, who was on the team that made the first cloaking device. But Cummer cautions that Zhang has a lot of work ahead to make this simple cloak more practical.
Right now the calcite trick works only if the medium around it helps to bend the light, which means the medium has to have just the right refractive index. The bath of laser oil used for the initial demonstration did the trick, but water or air won’t work.
Zhang is hoping, however, that some new tricks he has in mind will allow the cloak to work in air. That’s a project worth keeping an eye on.
Windows that block heat—but let it through when you want them to.
Is there a way for a window to reflect heat in the summer and let it through in the winter?
A window that changed in response to the heat might behave in just that way. Sarbajit Banerjee, a materials chemist at the University at Buffalo in New York, is applying his work on a compound called vanadium oxide to coat glass with a material that makes this possible.
Banerjee had been studying vanadium oxide because he was interested in the physics of phase transition—for example, the way water freezes as the temperature drops. When the temperature reaches 153 °F, this compound’s crystalline structure changes from one that’s transparent to infrared light—that is, radiated heat—to one that reflects the light.
Using nanofabrication techniques to change the microscopic structure of the crystalline material, Banerjee found a way to lower the temperature at which that change occurs. When the material is formed as long, thin nanowires, it undergoes the transition at a mere 90 °F. A researcher at a window company suggested that this version had good characteristics for a switchable window coating.
Banerjee was able to bring that temperature down even further by mixing tungsten into the material. And perhaps most promising of all, he found that he could trigger the transition at a range of temperatures by sending an electric current through the material—holding out the promise of changing a room’s temperature with the flip of a switch, and without racking up an energy bill.
Banerjee is now in the process of licensing his heat-blocking window coating to a U.S. building-materials company; he predicts that it will cost just 50 cents per square foot. He also has a partnership with Tata Steel, a global manufacturer headquartered in Mumbai, India, and they are looking at how to use the material to deflect heat from the corrugated-steel roofs that commonly turn houses stifling in India and other parts of the developing world.
Using sponges to improve and store alternative fuels.
Clean energy tends to come with drawbacks. Hydrogen has such low density that it’s hard to compress a useful amount of it into a container small enough to be practical; natural gas is more costly to transport and transfer than liquid fuels; batteries hold relatively little energy for their size and weight. But MIT chemistry professor Mircea Dincă has come up with a promising way to solve all these problems: sponges.
Dincă uses organic and metallic materials to form his sponges, which are so thoroughly riddled with microscopic chambers that in some cases, the surface area of just a gram would cover a football field if laid out flat. By mixing and matching these building blocks, he is able to control the size of the tiny chambers. Different configurations have different chemical and electrical properties.
Getting enough hydrogen on board a hydrogen-powered car requires either ultrahigh-compression tanks or cryogenic fuel tanks, but neither of these methods stores enough hydrogen to meet the U.S. Department of Energy’s target: a vehicle that can travel 300 miles without refueling. Dincă came up with a sponge capable of trapping twice as much hydrogen as ordinary tanks in a given volume. Adding a bit of heat or relieving some pressure coaxes the sponge to release the hydrogen when it’s needed.
Dincă’s sponges also make great sites for catalytic reactions, because the whole inner surface can be coated with a catalyst; the reaction can be controlled by altering the size of the sponge’s pores. He is developing variants of the sponges that could transform methane into a liquid fuel by efficiently catalyzing reactions that strip oxygen from air. He is also working on turning these sponges into materials for batteries and for organic photovoltaics.
Tuning nanocrystals to make tinier, more efficient switches for optical computing and solar panels.
Quantum dots are crystal particles, with a diameter of tens to thousands of atoms, that can absorb and emit different wavelengths of light or move electric charges around. Now Prashant Jain, a chemistry professor at the University of Illinois, has figured out a way to create tunable quantum dots that can be adjusted on the fly. His innovation could be key to designing optical computers and ultra-efficient solar panels.
Jain makes quantum dots out of copper sulfide, varying the ratio of copper atoms to sulfur atoms. At certain ratios, the amount and distribution of electrical charges inside the dots becomes sensitive to small changes in voltage—and it’s that charge distribution that mostly determines the dots’ properties, such as which wavelengths of light they’ll absorb and emit. “You can controllably push and pull charges into these semiconductor nanocrystals and thus turn on and off their ability to interact with light,” he explains.
That means the dots could function as submicroscopic optical switches—potentially, core components of an ultrafast optical computer that replaces electricity with beams of light. Jain’s tunable-quantum-dot switch is about one-sixth the size of today’s smallest transistors, and about a hundredth the size of current optical switches. Jain is also making quantum dots out of titanium oxide mixed with bismuth. These dots absorb solar light and convert it to electrochemical energy, which is used to generate hydrogen fuel from water.
Jain’s dots are still very much in the research stage, and he predicts it will take an enormous amount of additional research to achieve practical optical computers or the super-efficient hydrogen production needed for energy applications. “There’s a lot more fundamental work to be done,” he says.
Soft, flexible electronics bond to skin and even organs for better health monitoring.
Better integrating electronics with human tissue holds out the promise of monitoring the body more conveniently and accurately than is possible with sensors that are worn or taped on. Nanshu Lu is developing long-lasting “electronic tattoos” that can bond to skin and track and report on the wearer’s vital signs or translate small muscle movements into commands for controlling devices. Future versions may play critical roles inside the body in watching for signs of disease or damage. They could even treat problems automatically.
Lu, an assistant professor in the department of aerospace engineering and engineering mechanics at the University of Texas at Austin, has solved a big problem in building electronics for biological tissue: silicon semiconductor circuits are flat, rigid, and brittle, making them a terrible match for the soft, pliable tissue. (See “Making Stretchable Electronics”) What is needed is a soft device better able to make intimate contact with skin.
To create a more tissue-friendly chip, Lu enlisted a flexible polymer substrate on which she could deposit small islands of silicon. That technique had been tried by other researchers, but these devices had limited flexibility, since ordinary wires used to connect the silicon tear as the substrate stretches or twists with the tissue’s movement. Lu solved the wiring problem by eliminating the islands and replacing them with a serpentine mesh of nanoribbons; this webbing stands up to twisting and pulling without breaking.
The resulting device is a 30-micrometer-thick patch of supersoft, transparent silicone. Lu has built a prototype of the device that carries sensors to measure temperature, strain, and electrical signals. The patch could also be equipped with LEDs to enable visual signaling.
The circuits are printed onto silicone that’s supported by a stiffer layer of water-soluble polymer. When the patch is placed on dry skin and then wetted, the polymer layer dissolves; intermolecular attractions between the silicone and skin make the silicone adhere tightly. In tests, the silicone patches have adhered to skin for a week, hanging on even through showers and exercise. And the patches don’t irritate skin the way adhesives often do.
Lu and collaborators have already tested the devices in a few applications. For example, they have been attached to people’s necks to enable them to control Sokoban games simply by speaking commands; the patches measure the electrical activity of throat muscles during speech, with enough fidelity to distinguish between the spoken words “left,” “right,” “up,” and “down.”
Now Lu wants to see the patches used in a wide variety of health-related applications. She hopes to stick the devices on foreheads to directly monitor electrical activity in the brain, to place them on skin during plastic surgery so that strain gauges on the patch can alert surgeons if the procedure is overly stretching skin, to monitor heart rate and muscle activity during exercise, and to track the progress of healing in wounds and burns.
Lu is working on new versions of the devices. For example, she’s trying to create stronger physical and electrical connections by integrating arrays of microneedles on the bottom of the silicone patches. That, in turn, could enable the patches to stick to heart muscle so doctors could detect early signs of heart-attack risk, such as reduced blood flow.
Lu also hopes that a version of the patch with two-way communication capabilities might be able to sense heart arrhythmias and instantly respond by delivering small electric shocks to restore an even beat. And she envisions transdermal electronics that could detect the level of a protein in the body associated with a specific disorder and then release drugs to treat it.
A tiny roller coaster for light could help keep data centers cool.
Optical communications could be a boon for data centers, reducing electricity use and heat buildup by replacing electronic signals with light signals. But the technology has been cost-effective only over distances of a kilometer or more, and using it in data centers would mean sending signals mere meters or centimeters. Joyce Poon may have solved the problem by creating new optical modulators with microscopic loop-the-loops through which light can shuttle data between servers and even from chip to chip within a single server.
To make light-based data communications work over short distances, Poon, an assistant professor of electrical and computer engineering at the University of Toronto, knew she needed to come up with a much smaller version of an optical modulator, a device that converts an electronic signal into an optical one. She designed tiny rings that can be built onto computer chips. When laser light is sent into a ring, it races around the ring over and over before a bit of it emerges through a waveguide at the bottom. The trick was to control how much light came out. Other researchers working with micro-rings have tried to do that by adjusting the properties of the ring, in order to alter the length of the light’s path or the amount of light the ring absorbs. Poon realized she could leave the ring alone and simply control the gateway between the ring and the rest of the chip.
The resulting optical modulator can be both faster and more efficient. With a team from IBM, Poon is working to create a version that is competitive with today’s optical data rates.
The jump to optical data transmission in servers can’t come soon enough. Data centers consumed at least 200 billion kilowatt-hours’ worth of power in 2010, and the proliferation of smartphones and cloud storage is only going to push that higher, driving up costs and the risk of heat-related outages.
Mass-producible tiny machines snap into place like objects in a pop-up book.
Combining tools used to manufacture printed circuit boards with the spirit of origami, Pratheev Sreetharan has found a way to build tiny machines and complex objects that were previously impossible to fabricate without assembling them manually. Some of the results: a robotic bee created in a day, a tiny, precise icosahedron, and a small chain of interlocking carbon-fiber links. The small, intricate items demonstrate a fundamentally new fabrication approach that Sreetharan believes can be broadly applicable in making a range of new medical devices, robots, and components of analytical instruments.
If Sreetharan is successful, he could open up the manufacturing no-man’s-land between the micrometer-scale features of silicon chips and the centimeter-plus scale of everyday items. It’s a size range that’s of critical importance in biology and medicine. But today there’s simply no practical way to mass-produce three-dimensional objects and complex machines on this in-between scale.
Sreetharan’s prize creation is the robot bee, fabricated through a series of steps inspired by pop-up books. As a graduate student in the lab of Harvard microrobotics pioneer Robert Wood (a member of the 2008 TR35), Sreetharan was familiar with the task of laboriously gluing the miniature robots together under a microscope, and his fabrication approach was born of his determination to find a better way.
He began by adapting standard lamination and micromachining techniques from circuit board manufacturing to carve the needed parts into a flat substrate. But the real trick came in adding features that allowed the parts to pop up and lock into place in one step, creating the bee.
Sreetharan, who spent a recent summer in the Indian region of Tamil Nadu teaching Sri Lankan refugees about renewable energy and designing a solar-powered computer charger, recently got his PhD from Harvard and founded a startup called Vibrant Research in Cambridge, Massachusetts, to adapt his fabrication methods to advanced manufacturing.
He is still deciding which specific products the company will focus on, but he says he is able to routinely make objects that have never before existed. And he hopes the novel production methods will create new opportunities in manufacturing. That would be a pretty good way to build on the buzz from his robot bee.
Artificial tissue that can monitor and improve health down to the level of individual cells.
“Cyborg tissue could allow us to put multifunctional prosthetics in humans,” says Bozhi Tian. That goal is still a long way off, but Tian has taken a key step by creating artificially grown tissue that’s intelligent. So far, he’s developed a synthetic blood vessel that can detect the pH of solutions flowing through it. And with different nanoelectric sensors embedded in that and other tissue replacements, Tian thinks, the technology could one day wirelessly monitor proteins linked to cancer and other diseases.
Tian’s cyborg tissue project grew out of another impressive feat: an innovative method for detecting electrical changes in living cells. Instead of sticking fine-tipped glass pipettes into the cells, a conventional technique that ends up killing them within a few hours at most, Tian created a semiconductor device made of a kinked nanowire less than 50 nanometers wide at the tip.
He then coated the tip of his probe with molecules similar to those found in cell membranes, enabling the device to enter the cell with minimal damage. The implanted nanowires can potentially send information for days, and cells can tolerate multiple wires, making it possible to map complex changes across the cell.
By coating the wire with antibodies, which can be designed to latch onto a specific molecule, researchers could enable the tool to detect the presence of specific proteins seen when a particular disease state is getting better or worse. That could be useful for monitoring how cells respond to different compounds being considered for use as drugs.
Tian, an assistant professor at the University of Chicago, is currently working on equipping cells with electronic components that don’t merely monitor activity but actively affect it. Get ready for the cyborg cell.
Slowing light to help chips cope with optical data.
Light beams are so fast that using them to replace electrons would make for vastly more powerful and energy-efficient chips, even paving the way for quantum computing. At times, though, light is too fast. That’s why Zheng Wang decided to slow it down. “The speed is very good for optical communications but very bad for processing signals on-chip,” he says.
To slow light, Wang, an assistant professor of electrical and computer engineering at the University of Texas at Austin, created nanometer-size ridges on a chip. The ridges are so slender and flexible that they can be deformed by electric fields. When light is delivered by optic wire to the ridges at the edge of the chip, they convert the light waves to high-frequency sound waves, which travel at about a hundred-thousandth the speed of light. The same trick works in reverse after the sound waves have traversed the chip, with the ridges converting the sound back into light to continue its higher-speed journey via optic wire.
Other researchers had accomplished similar feats with light—but only by enlisting a high-powered pulse laser that generates acoustic pulses, a much less efficient and larger-scale process that can’t be handled on a chip.
Sound waves are much easier to read and route within the tiny confines of a chip. And they offer the huge advantage of not generating the heat that electronics do. That makes Wang’s approach promising for applications in information processing, as well as in nanoscale microscopy.