Silicon Bugs - They’re Half Bacterium, Half Microchip
Source: New Scientist Magazine
August 12, 2000
A LIVING semiconductor that could sniff out poison gas
in a bio-terrorist attack has been accidentally discovered by
researchers in the US and Northern Ireland. The discovery followed the
scientists’ failure to eliminate some particularly persistant bacteria
from computer chip production lines.
The researchers had tried everything to destroy the
microbes, from ultraviolet light to powerful oxidants. But the
bacteria survived every attempt.
"The micro-organisms were protecting themselves against
our heroic measures to kill them," says biophysicist Robert Baier,
director of the Center for Biosurfaces at the State University of New
York in Buffalo.
When microchips are cleaned with ultra-pure water, the
water can dissolve some semiconducting materials, such as germanium
oxide, which can then crystallise around the bacteria. The bacteria
survive extremely well inside their crystal homes, impervious to the
best human efforts to eradicate them. But the problem has a silver
lining. The microbes have created a "living cell" out of
semiconducting material.
"This is where the imagination runs wild," says
physicist John O’Hanlon, project leader and director of the Center for
Microcontamination Control at the University of Arizona in Tucson.
O’Hanlon and Baier believe the semiconductor-encrusted bacteria can be
used for building bio-transistors.
In a normal three-terminal transistor, the current flow
between the source and the drain is controlled by the voltage across
the gate terminal. In the bio-transistor, the gate would be replaced
by the bacteria-semiconductor crystal.
The idea is to tap into biological processes, such as
respiration and photosynthesis, which result in electron transfers,
says Baier. The bacteria could be induced to produce electrons when
exposed to light or organic vapours--which would turn on the
bio-transistor. Such an exquisitely sensitive device could detect
poison gas during bio-terrorist attacks, says Baier.
Michael Larkin, a microbiologist at the Queen’s
University of Belfast, has been identifying the bacteria. He declines
to name them, as the results of the research have yet to be published.
However, he says that the bacteria are extremophiles, "capable of
growing in areas with very low nutrients, fixing nitrogen and
surviving in clean water".
Meanwhile, Baier’s group is making the crystals in the
lab by trickling bacteria-infested ultra-pure water over semiconductor
wafers. Bio-crystals form on the surface of the wafers and can be
scraped off. "It looks like diamond dust. It’s absolutely beautiful,"
says Baier.
The next step is to get the crystals to behave like a
transistor, says Baier. Sandip Tiwari, director of the Cornell
Nanofabrication Facility at Cornell University in Ithaca, New York,
says that while the device should work there’s still a lot of
development needed.
by Anil Ananthaswamy