A self-assembling DNA motor developed by researchers from Bell Labs, Murray Hill, N.J., and the University of Oxford in the U.K. may provide the basis for building microprocessors when Moore's Law runs out of steam. Constructed from three strands of DNA molecules, the device functions as a motorized tweezer that opens and closes when acted on by other strands of DNA.
The DNA motor is analogous to a CMOS transistor in its switch-like operation, but with smaller dimensions than semiconductor fabrication can afford. As a result, scientists believe that someday they will be able to build complex structures such as electronic circuits that integrate many billions of devices. This will push computing power beyond what may be possible with semiconductors.
"This technology has the potential to replace existing manufacturing methods for integrated circuits, which may reach their practical limits within the next decade when Moore's Law eventually hits a brick wall," says physicist Bernard Yurke of Bell Labs. That brick wall is beginning to come into view. CMOS scaling is expected to carry semiconductor fabrication processes down to 0.035-µm design rules, which corresponds to a gate length of 0.020 µm (20 nm) for microprocessor de-signs. An ASIC manufactured in this pro-cess could have about 2 billion usable transistors/cm2.
Although last year's Semiconductor In-dustry Association (SIA) roadmap projected that this milestone would be reached around 2014, other sources say it may happen in 2010. At that point, there is the possibility that DNA-based computing could raise circuit density to the next level. That's because its basic circuit structures would have nanometer dimensions and rely on self-assembling chemical processes rather than the extremely precise photolithography needed for advanced silicon processes.
The self-assembling characteristics result from the way DNA molecules naturally form interlocking structures. As Yurke says, "We took advantage of how pieces of DNA—with its billions of possible variations—lock together in only one particular way, like pieces of a jigsaw puzzle." This quality makes it possible to build self-forming DNA motors just by bringing together the proper DNA components in a test tube.
To construct the motor, scientists synthesized three strands of DNA, labeled A, B, and C. These are individual strands, not the double-stranded helix or twisted pair associated with DNA when it is not replicating. The individual DNA strands are sequences of the bases adenine (A), cytosine (C), guanine (G), and thymine (T). These bases form stable double-helix bonds when complementary bases (A and T, or C and G) are brought together. So for a given DNA sequence of bases, there is a corresponding complementary DNA sequence that will bond to it.
The DNA sequences on strands A, B, and C are such that one portion (almost half) of strand A latches onto a part of strand B, while another section of strand A latches onto strand C. An intermediate section on A, though, doesn't bond with either B or C. This section then forms a hinge between what are essentially the two arms of the DNA motor, or tweezers (see the figure).
Normally, these structures float in a liquid medium with their arms open. But when strands of DNA fuel are introduced into the liquid, they attach to the dangling unpaired sections of strands B and C and pull them closed. To reopen the arms, additional DNA strands are introduced that mate with the fuel strands and remove them from the tweezer's arms. To get some idea of scale, the distance between the ends of the tweezer's arms is 6 nm, going from open to closed states, with a 50° angle between the arms.
"The entire population of 30 trillion DNA tweezers in a few drops of solution can be repeatedly closed and opened by successively adding fuel and removal strands," explains Andrew Turberfield, a physicist at the University of Oxford. Besides its main function, the fuel could possibly be used to configure the components of a complex structure or to conduct signals from one motor to another. These are critical requirements for transforming a building-block function such as the DNA motor into working circuits.
Researchers at Bell Labs are working on connecting DNA to electrically conducting molecules. Of course, there are many other issues that must be addressed to make DNA circuitry practical. Motor speed is one: switching time for the DNA motor was measured at 13 s.
A detailed description of the DNA motor research was published in the Aug. 10 issue of Nature. To order a copy of the article, go to www.nature.com. For more information, contact Steve Eisenberg, senior manager of media relations for Bell Labs, at (908) 582-7474.