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THE SCIENCE OF BEING UNSEEN
Have you ever seen a photograph of DNA? Most likely not. The biological blueprint molecule is so small that no optical lens can possibly provide enough magnification for our eyes to see it. In the future, however, metamaterials might change that. Generally speaking, metamaterials are composite structures that interact with waves in ways that are dependent on their structure. For example imagine a sheet of fiberglass circuit board imprinted with an array of tiny copper rings. The rings are at most a few millimeters in diameter. Such an array would bend electromagnetic waves that go through it with a certain wavelength. The wavelength is dependent on the size of the rings. Smaller rings affect radiation with smaller wavelengths (higher frequencies). When such a structure is arranged in concentric circles, it can be used to bend light around the center. As light passes through the layers of the metamaterial it is redirected around the center like a river around a boulder. An observer outside the metamaterial would not see the space in its center or any object placed there. For certain wavelengths of light an object would be invisible. At first this “cloaking” technology only worked for microwaves, but, since then, considerable effort has been devoted to developing metamaterials that work at smaller wavelengths. To have an invisibility effect in the visible light range would require much smaller structures, such as the copper rings, to the point where nanoscale fabrication techniques are required. Nanotechnology is neither cheap nor easy, but it is within our means. Barring the cliché mention of certain wizards, there is a definite possibility that someday we may be able to hide objects from plain sight, thanks to mind-boggling technology.
What does that have to do with DNA? Well, metamaterials can be designed to manipulate light in many ways. Conventional lenses have fundamental limits to their magnification capabilities. Metamaterials effectively break conventional rules and extend the limits of optical magnification to the point where molecules can be observed. A microscope equipped with a “superlens” would allow you to view DNA molecules and other similarly sized objects.
Metamaterials can also greatly improve the functionality of radio antennas, making them smaller and more efficient. With sufficient ability to manipulate light, metamaterial structures could one day replace the circuit boards we know so well. Replacing electric signals with light pulses, computers would work faster and produce less heat. There are claims that metamaterials could even function as artificial noses for computers to sniff out bombs.
As I said earlier, generally, metamaterials manipulate waves. Meaning, this field of technology is not limited to electromagnetic radiation; it can also apply to vibrations in matter. Think of the invisibility idea applied to sound; you could have a perfectly soundproof room. Sound waves can be redirected like light waves. How about water waves? Perhaps a very large metamaterial structure could “cloak” coastal areas from rough sea storms. Or ground waves? In the future, maybe buildings will be able to safely redirect seismic vibrations around them. It would seem the only limitations on metamaterials are our imaginations and our small-scale engineering abilities.
A FOUNDATION OF BUCKYTUBES
Carbon has a wide range of macroscopic forms—coal, ash, graphite, diamond. A relatively new form of carbon, nanotubes, is becoming a revolutionary material in many ways. Carbon nanotubes (CNTs), or buckytubes, are microscopic cylindrical cages of carbon atoms. Each nanotube is essentially a giant molecule with a diameter of about one nanometer and usual length of several millimeters. Multi-walled CNTs (which consist of nanotubes inside larger ones) have very little friction between layers. Individually a CNT has a greater strength to weight ratio than steel and can conduct heat and electricity better than copper.
So far the main use for CNTs has been to improve other materials. Adding them to mixture-based materials improves strength and durability, such as making crack-resistant concrete. On their own, however, carbon nanotubes can be much more impressive. Imagine a cable made of carbon nanotubes that extends many miles from the ground to a space station in geosynchronous orbit. Such a lengthy cable made of any other material would break from its own weight if not the tension of securing the space station. The incredible tensile strength of CNTs might make a cable of that magnitude possible. The cable would allow a “space elevator” to climb up into the heavens without need of rockets. Wouldn't it be incredible to ascend into outer space without the huge rocket engines but with a hi-tech elevator?
The electrical properties of CNTs make them attractive for improving electronic products. Recent research indicates that nanotubes could be used to speed up DNA sequencing. Chinese researchers have discovered that a sheet of CNTs can be used as a speaker. When a varying electrical current is applied, the CNTs heat up nearby air accordingly. The changing temperature causes rapid changes in pressure which we hear as sound. Normal speakers use vibration to produce changes in air pressure, and they can be rigid, fragile, and bulky. This new speaker, however, is as thin as paper, flexible as fabric, and turns transparent when stretched. Stretching, bending, and even cutting the CNT sheet into different shapes will not significantly affect sound quality.
Solar panels made with carbon nanotubes are also thin, flexible, and lightweight. These unique properties may one day lead to the replacement of today's thick, heavy, and fragile silicon solar panels. CNTs can not only improve how we obtain energy, but they can help us store it. Paper embedded with CNTs constitutes a “paper battery”. It looks and feels like black paper, but it can store electricity and function at greater temperature extremes than many conventional batteries. Medical researchers are interested in this technology because paper batteries can be powered by blood or urine. Imagine a pacemaker that is charged by blood passing through the heart.
While CNTs have astounding properties, there are some major hurdles in the way of success. The extraordinary capabilities are undermined by defects which can be difficult to prevent at the nano-scale. Over the last ten years the cost (per gram) has gone from $500+ to $50. This decreasing trend is promising, but for now it is still too expensive to produce CNTs in large quantities. About large quantities: it is still very difficult to make nanotubes longer than a few centimeters.
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