The simple E. coli bacterium shown can compute 1,000 times faster than the most powerful computer chip, its memory density is 100 million times higher and it needs 100 millionth the power to operate. Image: Jenni Ohnstad/Vanderbilt Univ. |
It’s
common knowledge that the perfect is the enemy of the good, but in the
nanoscale world, perfection can act as the enemy of the best.
In
the workaday world, engineers and scientists go to great lengths to make the
devices we use as perfect as possible. When we flip on a light switch or turn
the key on the car, we expect the lights to come on and the engine to start
every time, with only rare exceptions. They have done so by using a top-down
design process combined with the application of large amounts of energy to
increase reliability by suppressing natural variability.
However,
this brute-force approach will not work in the nanoscale world that scientists
are beginning to probe in the search for new electrical and mechanical devices.
That is because objects at this scale behave in a fundamentally different
fashion than larger-scale objects, argue Peter Cummings, John R. Hall Professor
Chemical Engineering at Vanderbilt Univ., and Michael Simpson, professor of materials
science and engineering at Univ. of Tennessee, Knoxville,
in an article in ACS Nano.
‘Noise’ makes a difference
The defining difference between the behaviors of large-scale and nanoscale
objects is the role that “noise” plays. To scientists noise isn’t limited to
unpleasant sounds; it is any kind of random disturbance. At the level of atoms
and molecules, noise can take the form of random motion, which dominates to
such an extent that it is extremely difficult to make reliable devices.
Nature, however, has managed to figure out how to put these fluctuations to
work, allowing living organisms to operate reliably and far more efficiently
than comparable man-made devices. It has done so by exploiting the contrarian
behavior that random behavior allows.
“Contrarian investing is one strategy for winning in the stock market,”
Cummings said, “but it may also be a fundamental feature of all natural
processes and holds the key to many diverse phenomena, including the ability of
the human immunodeficiency virus to withstand modern medicines.”
The cell mimic concept includes a microscale enclosure with nanoscale pores in the walls placed in a fluid channel. As shown in (a) the scientists seal DNA, RNA, and the other molecular machinery that the DNA needs to produce proteins in the cell. The pores allow the small molecules that DNA needs for replication to flow into the cell, allowing the DNA to function for up to 24 hours. The lower three images in (b) are microphotographs of the cell mimic. Image: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory |
In their paper, Cummings and Simpson maintain that in any given population,
random fluctuations—the “noise”—cause a small minority to act in a fashion
contrary to the majority and can help the group respond to changing conditions.
In this fashion, less perfection can actually be good for the whole.
Mimicking cells
At Oak Ridge National Laboratory, where the two researchers work, they are
exploring this basic principle through a combination of creating virtual
simulations and constructing physical cell mimics, synthetic systems
constructed on the biological scale that exhibit some cell-like characteristics.
“Instead of trying to make perfect decisions based on imperfect information,
the cell plays the odds with an important twist: it hedges its bets. Sure, most
of the cells will place bets on the likely winner, but an important few will
put their money on the long shot,” Simpson said. “That
is the lesson of nature, where a humble bacterial cell outperforms our best
computer chips by a factor of 100 million, and it does this in part by being
less than perfect.”
Following the lead of nature means understanding the role of chance. For
example, in the AIDS virus, most infected cells are forced to produce new
viruses that infect other cells. But a few of the infected cells flip the virus
into a dormant state that escapes detection.
“Like ticking bombs, these dormant infections can become active sometime
later, and it is these contrarian events that are the main factor preventing
the eradication of AIDS,” Simpson said.
“Our technology has fought against this chance using a brute force approach
that consumes a lot of power,” Cummings said. As a result, one of the factors
limiting the building of more powerful computers is the grid-busting amount of
energy they require.
Yet residing atop the cabinets of these supercomputers, basking in the heat
generated in the fight to suppress the element of chance, the lowly bacteria
show us another way.
Cummings and Simpson conduct research at the Department of Energy’s Center
for Nanophase Materials Sciences at Oak Ridge National Laboratory. CNMS is one
of five national DOE Nanoscale Science Research Centers.