from the November 21, 2001
Wires grown in liquid
may help find toxins
- Most people instinctively steer clear of
electrical wires in water. But not Orlin Velev. The North Carolina
State University professor and his colleagues recently figured out a
way to create tiny wires in liquid, a discovery that may lead to
much smaller electronics and sophisticated biochemical sensors.
Dr. Velev and fellow researchers at the University of Delaware
sparked the growth of the tiny wires by applying an alternating
electrical current to minuscule gold flakes suspended in liquid. The
flakes, each measuring several billionths of a meter in diameter,
assembled themselves into microwires too small for the eye to see
but able to carry a current. The process is known as
The gold flakes can self-assemble into wires very rapidly: They
grow faster than 50 millionths of a meter per second to lengths
topping 5 hundredths of a meter. The wires are about one millionth
of a meter in diameter, a fraction of the diameter of a human hair.
"When you miniaturize things, self-assembly is important," Velev
says. There is no chemical reaction, no need for soldering, and the
electrical connections are made spontaneously, he explains.
Typically, only one wire grows in a solution, but more than one
could be grown if the voltage were increased.
"One of the bottlenecks in microchip fabrication is connecting
wires to all of the elements inside the chip," says Thomas Jones, a
professor of electrical and computer engineering at the University
of Rochester in New York, who is familiar with Velev's work.
He explains that there are physical limits to how small chips can
be made, because there is a limit to how close to one another wires
can be placed without causing interference.
"But if something can self-assemble, we could potentially develop
microchips that are more complex, using this better way of
connecting things together," Mr. Jones says. "This is exciting."
Jones explains that the way the microwires are grown or assembled
is analogous to the well-known phenomenon of iron filings forming
chains under the influence of a magnet.
Velev says the assembly process for the microwires is relatively
simple and easy to control. "We have a fairly good degree of
control, because the electric voltage can control the direction of
the wire's growth, and the distance between electrodes can control
its length." Another advantage: The wires can automatically repair
The research still is in its early phases, so the scientists are
looking at controlling the wires to make rudimentary electrical
circuits. They also are studying other materials, such as silver,
platinum, plastic, semiconductors, and possibly carbon, for
Velev says the gold microwires could be used in biochemical
sensors, and they already can detect cyanide and thiol.
A prototype sensor device could be built in a year or so, but
commercial application of the technology is likely five or more
Any device would include a wire or wires in a solution, and
encased in glass.
"There are a lot of efforts now to develop new sensors to detect
bad things," says Eric Kaler, dean of the college of engineering at
the University of Delaware and a co-discoverer of the microwires
with Velev. "This technology is not yet mature. We need to see work
on an engineering prototype device that could take advantage of it."
Initially, Dean Kaler sees the technology being used to detect
contamination of water, such as reservoirs. Eventually, it also
could be used with living cells as a sensor of biological or
chemical agents, which cause changes in the surface of the gold.
Kaler says he already is working with the Army on nerve or chemical
warfare agent detection.
"Most people think water and electricity don't go together, but
they do in the biology of the body," Kaler says. "We've developed a
way to assemble electrical connections in a water environment. This
offers a new way to potentially interface living systems with
electronic readouts, so we could use a cell as a sensor."
(c) Copyright 2001. The Christian Science Monitor