Can scientists use the binary of biology, DNA, to grow carbon nanotubes into more efficient circuits? IBM thinks so.

Scientists at IBM are conducting research into arranging carbon nanotubes -- tubes of carbon atoms that can conduct electricity -- into arrays with DNA molecules. Once the nanotube array has been constructed, the laboratory-generated DNA molecules could be removed, leaving an orderly grid of nanotubes. The nanotube grid, conceivably, could function as a data-storage device or perform calculations.

"These are DNA nanostructures that are self-assembled into discrete shapes. Our goal is to use these structures as bread boards on which to assemble carbon nanotubes, silicon nanowires, quantum dots," said Greg Wallraff, an IBM scientist and a lithography and materials expert working on the project. "What we are really making are tiny DNA circuit boards that will be used to assemble other components."

The work, which builds on the ground-breaking research on "DNA origami" conducted by the California Institute of Technology's Paul Rothemund, is only in the preliminary stages. Nonetheless, a growing number of researchers believe that designer DNA could become the vehicle for turning the long-touted dream of "self-assembly" into reality.

Chips made on these procedures could also be quite small. Potentially, DNA could address, or recognise, features as small as 2nm (nanometres). Cutting-edge chips today have features that average 45nm, or 45 billionths of a metre.

"There is nothing else out there that we can do that with," said Jennifer Cha, an IBM biochemist working on getting the biological and non-biological (inorganic) molecules to interact.

Today, products get manufactured in a top-down approach, with machinery and equipment manipulating raw materials. In self-assembly, the intrinsic chemical and physical properties of molecules, along with environmental factors, coax the raw materials into complex structures. It works with snowflakes, after all.

Getting the raw materials to behave in a precise, orderly manner, however, remains a challenge, which is where DNA comes in. DNA consists of specific chemical bases (guanine and cytosine, for example) that bind and react in somewhat predictable ways with each other.

"The sequence [of base pairs in DNA] is well known," said Cha. "Most people are acknowledging that DNA and these biological scaffolds are actually quite useful to at least pattern very small systems."

How it works
In creating chip arrays, DNA assembly might work as follows: scientists would first create scaffolds of designer DNA manipulated into specific shapes. Rothemund has made DNA structures in the shapes of circles, stars and happy faces.

A pattern would then be etched into a photo-resistant surface with electron beam lithography and the combination of several interacting thin films. A solution of the designer DNA would then be poured on the patterned surface and the DNA would space themselves out according to the patterns on the substrate and the chemical and physical forces between the molecules.

The nanotubes would then be poured in. Interactions between the nanotubes and the DNA would occur until they formed the desired pattern. Single-strand DNA, along with origami, could be used in concert.

Another key part in the system revolves around peptides that can bind to the DNA and a non-biologically inspired molecule, like a nanotube.

"Building a DNA scaffold is not trivial because you need the biological system to recognise something that doesn't exist at all in biology," said Cha. "We can also use these biomechanical scaffolds to position inorganic nanomaterials. Potentially, we could also use these biomechanical systems to synthesise inorganic materials."

Although it's early, progress is being made. Researchers have published papers on how DNA can coil around nanotubes and disperse them in water. Papers detailing how DNA can arrange nanotubes will come soon. Future experiments will need to be conducted into aligning nanotubes into arrays. Other researchers in this field include Nadrian Seeman at New York University and Thomas LaBean at Duke University.

IBM will also examine ways of employing DNA to sort nanotubes, said Cha. Not all nanotubes are equal. The arrangement and relative position of carbon atoms in a nanotube, called "chirality", can change the properties of that nanotube. Chirality refers to molecules that are mirror images -- for example your hands are chiral -- even thought they are identical one is "left" while the other is "right". Chiral molecules are notoriously difficult to sort from each other, but would have different interactions with DNA depending on their chirality.

In addition some nanotubes don't conduct, even though they were made with others that do conduct. Separating conducting from non-conducting nanotubes currently requires applying an electric field or soaking them in sorting solutions.

If DNA manufacturing can become a reality, worries about the pace of progress in the computing world slowing down because of the difficulties involved in following Moore's Law would probably fade, at least for a while. Chipmakers shrink the size of the features of their chips every two years. While this improves the performance, producing smaller circuits has strained the financial and technical resources of the industry. The limits of lithography -- used to "draw" circuits -- have prompted many, including Intel co-founder Gordon Moore, to predict that the pace of progress will slow down.

By using DNA, chipmakers could phase out multi-billion fabrication facilities stocked with lithography systems, which cost tens of millions of dollars, and the other "top-down" style equipment.

Potentially, DNA techniques could allow manufacturers to produce features that are smaller than patterns that could be achieved even with the most advanced lithography systems, said Wallraff. Electron beam lithography, which is extremely difficult to use in mass manufacturing, goes down to 10nm.

"Of course, the devil is in the detail," said Wallraff. "These are self-assembly procedures and error rates -- missing features -- could be the downfall."