Scientists Make a Bacteria-Size Machine Work
NY Times, Nov. 25
By KENNETH CHANG
In the continuing march toward miniaturization, scientists have now not
just built microbe-size contraptions. They have also found a way to
make them move.
Writing in Friday's issue of the journal Science, scientists at Cornell
University report that they hooked up a tiny motor to a metal propeller and
spun the propeller around at up to eight revolutions a second.
"This is the first true nano machine," said Dr. Carlo D. Montemagno,
professor of biological engineering at Cornell and senior author of the
Science paper.
"Nano" is a Greek prefix meaning "one-billionth," and nanotechnology
refers to devices that are a few nanometers — a few billionths of a meter
— in size. A single silicon atom, by comparison, is about one- quarter of a
nanometer wide.
Since the motor draws its energy from the same organic molecules that
power living cells, Dr. Montemagno suggests that scientists may one day
be able to build robots much smaller than bacteria that will be able to repair
cellular damage, manufacture medicines and attack cancer cells.
"This opens the door to make machines that live inside the cell," Dr.
Montemagno said. "It allows us to merge engineered devices into living
systems."
A second paper in today's Science captures another type of minuscule
motion: a clump of tin, pushed by chemical forces, scurries around like an
amoeba on a surface of copper, leaving behind a thin trail of bronze alloy.
"The tin island looks like it's alive as it's grazing along the copper surface,"
said Dr. Norman C. Bartelt, a staff scientist at Sandia National
Laboratories in Albuquerque and one of the researchers. "It moves to
clean regions of the surface, eating the substrate and spitting out the
copper atoms it eats in the form of bronze. It's amazing an inanimate
system on such a small scale emulates something that's living."
In an accompanying commentary, Dr. Flemming Besenbacher of the
University of Arhus in Denmark and Dr. Jens K. Norskov of the Technical
University of Denmark say the motion of the tin island can be considered
as a new type of nanomotor. They calculate that the system is roughly as
efficient as an automobile engine at converting chemical energy to
mechanical horsepower.
The motivation for the Sandia research was not nanomachines. "We're
interested in the reliability of nuclear weapons," Dr. Bartelt said. The
scientists were investigating electrical junctions between solder — a mix of
tin and lead — and copper wires.
In the experiments, hundreds of thousands of tin atoms were dropped, one
by one, onto a copper surface. At room temperatures, copper atoms
continually jiggle around, bouncing the tin atoms along the surface until they
coalesced into larger clumps.
At the same time, the tin atoms slowly swap places with some of the
copper atoms, forming a two- dimensional layer of bronze, an alloy of
copper and tin.
Because tin atoms are larger than copper atoms, they cause a bulge when
they enter the surface. This hill causes the tin clump to slide off to a
pristine copper section. After a few minutes, all of the tin atoms are
absorbed into the copper.
How the motion might be harnessed for a useful device is not at all clear.
Dr. Bartelt calls the notion of using the roaming tin islands as motors
"far-fetched." However, he suggests that the tin clumps could be used as
battering rams to push other tiny objects around or be assembled into larger
structures.
Dr. Besenbacher said the perspective piece was not meant as a prediction,
but to inspire researchers to brainstorm about the newly discovered
phenomenon.
"I can think of at least some ideas of how to do it," he said. "Whether it
works or not, I don't know. It certainly should stimulate people to think
along these lines in the future."
The Cornell work melds two lines of nanotechnology research that have
been pursued for the past few years. Just as electrical engineers have
been cramming smaller and smaller transistors onto computer chips,
nanotechnology scientists have crafted tinier and tinier sculptures, including
levers, beams, suspended wires and a model of a guitar with strings 100
silicon atoms wide. But without a way to make them move, the structures
were sometimes little more than tiny art pieces.
Meanwhile, other researchers have been building tiny motors inspired by
machinery inside living cells. The so-called biomolecular motors run on
adenosine triphosphate, or ATP for short, the same energy-rich molecule
that powers chemical reactions within cells.
Dr. Montemagno's group grafted nickel propellers onto the central shafts
of 400 biomolecular motors. Of those, 395 remained motionless, when
immersed in a solution full of ATP. But 5 spun.
The propellers are relatively long — 750 nanometers, or about
one-30,000th of an inch — which allowed the researchers to videotape
them spinning. In one section of the video, a dust particle can be seen
being sucked into the spinning propeller before being kicked out again.
"Today a propeller, tomorrow you can start putting other things on it," said
Dr. Ralph C. Merkle, a principal fellow at the nanotechnology company
Zyvex in Dallas. "It's moving in a direction where the end point might
actually be useful."
Potential applications might include "smart dust," sunlight- powered sensors
to detect dangerous chemicals. If activated, a tiny motor might open a
valve to release a visible warning dye.
Dr. Montemagno also envisions robots that interact with the machinery
inside living cells, somewhat like a virus, to produce healing drugs.
"We're going to have the device self-assemble inside the human cell," he
said. "That's what we're working on now."
To battle cancer, cells might be genetically modified by the nanorobots to
produce tumor-killing chemicals. But such chemicals are usually deadly to
healthy cells, too, so other nanorobots might swim through the cells,
collecting the toxic chemicals and then dump them directly onto the cancer
cells. For long trips to Mars and other planets, astronauts might also carry
an array of drug-producing nanorobots that can be injected into the body as
needed.
"This is 15-year or 20-year thinking that I'm talking about," Dr.
Montemagno said. "Life is really an orchestration of a bunch of
nanomachines running around." |
