New Nano-Brushes Keep the Tiny Tidy

John Roach
for National Geographic News
June 17, 2005
Even at the nano-scale—where machines and materials can be the size of atoms and molecules—there are messes to sweep, walls to paint, tubes to unclog, and electronics to power. And now there's a way to make the tiniest of brushes to do these chores.

Made with bristles more than a thousand times smaller than a human hair, they are the tiniest brushes in the world. Yet they are durable and flexible enough to perform any brushing chore.

Conventional brush bristles, made of animal hairs, synthetic polymer fibers, and metal wires, are flimsy and prone to breaking down at the nano-scale, researchers say.

To work at the nano-scale, researchers realized that a different kind of material was needed.

"It dawned on us that [carbon] nanotubes would make excellent bristle material," Pulickel Ajayan, a professor of materials science and engineering at the Rensselaer Polytechnic Institute (RPI) in Troy, New York, said in an e-mail to National Geographic News.

Ajayan has worked with carbon nanotubes—or cylinders of carbon molecules—for more than a decade. Their small size, strength, elasticity, and ability to conduct electricity make them ideal bristle material at the nano-scale, he says.

Together with colleagues at RPI and the University of Hawaii at Manoa, Ajayan has made brushes with nanotube bristles in a variety of shapes and sizes. The researchers used them to perform tasks such as sweeping up nanoparticles in a narrow trench, coating the inside of a tube, and serving as electrical contacts in a nano-motor.

The researchers explain how they made the new brushes in the July issue of the academic journal Nature Materials.

Eric Grulke, a professor of chemical and materials engineering at the University of Kentucky in Lexington, said in an e-mail that the research was "quite innovative with respect to potential applications."

Nano-Brush Market

According to Ajayan, as materials and machines get smaller and smaller, the potential market for nano-sized brushes will blossom. Even the smallest amount of dirt can render machines at the nano-scale useless.

Interest in cleaning up nanoparticles has intensified lately due to growing health concerns over their release in the environment.

With the brushes, researchers will be able to sweep trenches in miniature computer chips or clean mirrors used in nano-motors. When coated in absorbent material, the brushes could also soak up particles in contaminated waters.

"Certainly I believe that the brushes would have commercial uses, particularly when micro-technologies become [the] norm in the future," Ajayan said.

Making Brushes

In previous research, Ajayan grew carbon nanotubes using a technique known as chemical vapor deposition. He and his colleagues applied the technique to make the brushes.

The researchers grew the bristles onto a thin brush handle by sticking the handle into a furnace of carbon-rich vapors.

To control the shape of the brushes, the researchers wrapped the fibers in gold, except where they wanted the bristles to attach to the brush. When the brush handle was placed in the furnace, the fibers grew only on the unmasked areas.

In this case, the nanotubes were grown onto handles of silicon carbide fibers, though other materials can be used, Ajayan said.

Using this process, "we could fabricate these miniature brushes in pretty much any morphology you want," Ajayan said. Some of the brushes look like mini-toothbrushes, others like mini-paintbrushes, others like mini-fans.

Each nanotube bristle is tiny, about 30 nanometers in diameter. The handles, however, could be made relatively thick—about the diameter of a human hair—allowing the brushes to be manipulated either manually or with robotic motors.

Ajayan said the bristles might break off used in a particularly abrasive task, such as stirring a highly viscous substance. But in the demonstrations conducted by the researchers, the brushes held up under a variety of sweeping, painting, cleaning, and electric current-carrying tasks.

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