November 25, 2000

 

 
 
 

Scientists Make a Bacteria-Size Machine Work

By KENNETH CHANG

 

 
 
 

n 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."
  Source: New York Times