By Kenneth Chang

12 November 2003

The recipe for a computer chip of the future may read something like this: Take some wires. Add DNA. Stir.

In an advance that might provide a practical method for making molecular-size circuits, the smallest possible,
scientists in Israel used strands of DNA, the computer code of life, to create tiny transistors that can literally
build themselves.

"What we've done is to bring biology to self-assemble an electronic device in a test tube," said Dr. Erez Braun, a
professor of physics at the Technion-Israel Institute of Technology in Haifa, Israel, and a senior author of a paper
describing the research today in the journal Science. Scientists have in the last few years accomplished feats of the incredibly small, constructing devices not much larger than individual molecules, but they also realize that their current painstaking techniques are too slow and inefficient.

"In order to construct a circuit," Dr. Braun said, "you need to invent ways to tell molecules where to go and how
to connect to each other." To that end, many scientists have turned to the biologically inspired notion of self-assembly, using molecules like DNA and proteins that can automatically link together in the correct configuration.

"It's all of the dynamics on that scale rather than just making small stuff," Dr. Horst Stormer, a professor of
physics at Columbia, said. Dr. Stormer, who was not involved in the new research, described the work as a "good first step" toward self-assembling electronic devices. The Technion-Israel scientists constructed transistors out of carbon nanotubes, cylindrical molecules that are about one ten-millionth of an inch in diameter and resemble rolled-up chicken wire.

Current computer chip technology, which fashions transistors out of silicon, will hit fundamental limits in about a decade. To continue the progression toward ever faster computers, many scientists are looking to molecular electronics like the nanotube transistors to step in. Other researchers have made similar transistors, which already perform better than their silicon counterparts. But making them in quantity is a major unsolved challenge.

In the earliest work, the nanotubes were randomly placed. By chance, some made the correct electrical connections. Since then, researchers have looked for a more practical way to wire together the billions of transistors that would be needed for a computer chip. Scientists at Duke University reported in August that they had coated DNA with silver to produce ultrathin wires. The Israeli group is the first to use DNA to build a working electronic device.

"It's a very interesting demonstration of a completely new concept of assembling devices," said Dr. Cees Dekker, a professor of physics at the Delft University of Technology in the Netherlands who research group made the first nanotube transistor in 1998. The new technique takes advantage of a biological process known as recombination, where a segment of DNA is swapped out for an almost identical piece. The cell uses recombination to repair damaged DNA and to swap genes. A special protein helps connect the replacement DNA to the desired location.

By attaching a nanotube to the protein, the nanotube moves to an exact location along the DNA strand. "The DNA serves as a scaffold, a template that will determine where the carbon nanotubes will sit," Dr. Braun said. "That's the beauty of using biology." The scientists then coated the DNA with gold, producing a simple electronic device consisting of the nanotube connected to gold wires at each end. Current through the nanotube could be switched on or off by applying an electric field - the definition of a transistor.

In earlier work, the same researchers showed that they could stretch DNA across a surface to provide a template to hook together the transistors into a circuit. The next step would be to build the circuit, Dr. Braun said.

Other groups are looking at alternative ways to build molecular circuits. Dr. Dekker's group now lays catalyst
that will grow nanotubes at desired places. He is also exploring using DNA, although using a different approach. DNA molecules attached at the end of nanotubes would act like "smart glue." Each strand would be able to attach to only one other strand. "It's programmable Velcro," Dr. Dekker said.