MOST of us dread computer viruses, but what about a computer that鈥檚 made of
viruses? Strange as it sounds, plant viruses can now be turned into the building
blocks of microprocessors. Their inventors say the tiny biocircuits could even
be used inside the body to make a new breed of sensors that detect blood glucose
levels, for instance.
But why use a virus? The big attraction is that at just 30 nanometres across,
they are far smaller than the 130-nanometre wide components in today鈥檚
microchips. It鈥檚 getting tougher and more expensive to shrink conventional
microchip technology, so the viral approach could bring the breakthrough in
miniaturisation that chip manufacturers have been searching for.
The viruses provide the perfect scaffold for tiny electronics systems because
they can be made to arrange themselves into crystal-like arrays. This raises the
tantalising possibility of self-organising circuits, which need little or no
intervention to help them build useful three-dimensional structures that can be
populated with circuit components. Until now, nanotechnologists have only
constructed flat nanocircuits, using components like carbon nanotubes as transistors
(快猫短视频, 17 November 2001, p 26),
but what they really want is to find a way for these molecular circuits to build
themselves.
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To make their living, 3D microcircuits, chemist MG Finn and virologist Jack
Johnson, both of the Scripps Research Institute in La Jolla, California, chose
to work with the cowpea mosaic virus, a common pathogen which stunts the growth
of the black-eyed pea plant.
This virus is encased in a protective protein coat that has 20 faces and 12
corners, or vertices. The researchers inserted DNA segments into the virus鈥檚
genome that caused the pathogen to produce cysteine amino acids on the vertices
of its viral shell. The resulting cysteine complex at each vertex sports
sulphur-containing thiol groups, which bind readily to gold. So when the team
added ultrafine gold particles to the cysteine-loaded viruses, they ended up
with viruses studded with a pattern of gold electrodes
(see Diagram).
The researchers are now working with scientists at the US Naval Research
Laboratory in Washington DC to bridge the electrodes with wires and organic
molecules that can act as electrical switches, effectively doing the job of a
transistor. In this way the researchers hope to build logic gates and combine
them to perform complex operations. 鈥淏uilding the molecular electronics becomes
an exercise in connecting the dots,鈥� says Finn.
If they succeed, the microchips of the future could come from farms instead
of high-tech factories. You can cultivate the virus cheaply by growing a few
hectares of its host plant, the black-eyed pea, and then isolating the virus
from the leaves.
鈥淚t will be very exciting to see such circuits patterned on something nature
has provided us with, like a virus,鈥� says Uzi Landman, who researches nanoscale
computer circuit materials at the Georgia Institute of Technology in
Atlanta.
But it鈥檚 not only viruses that are being harnessed for computer research.
Last year, a team led by Michael Simpson at the Oak Ridge National Lab in
Tennessee, succeeded in getting modified Pseudomonas bacteria to behave
like logic gates.
(快猫短视频, 26 May 2001, p 24)
The Scripps team wants to go beyond viral electronics, however, and use the
viruses as tiny chemical reaction vessels. To do this, they will denature the
virus鈥檚 protein surface, allowing them to attach molecules to the inside
instead鈥攅ffectively hollowing it out, perhaps to allow drugs to be ferried
into the body.