The world's smallest brushes, with bristles more than a thousand times finer than a human hair, have been created by researchers in the US.

The brushes can be used for sweeping up nano-dust, painting microstructures and even cleaning up pollutants in water.

The bristles' secret is carbon nanotubes, tiny straw-like molecules just 30 billionths of a metre across.

They are incredibly tough and yet flexible enough that they will yield when pushed from the side.

The researchers behind the brushes were led from Rensselaer Polytechnic Institute in Troy, New York. Their work is reported in the journal Nature Materials.

Flexible tools

The group of lead scientist Pulickel Ajayan has previously shown how carbon nanotubes can be grown controllably, and the team has now used the trick to make nanobrushes shaped like toothbrushes, bottle brushes and cotton-buds.

The scientists grow bristles from hot, carbon-laden gas on to threads of carbon silicide finer than baby's hair.

Thin coats of gold steer the carbon away from the brush handle and on to the brush head.

Like normal brushes, the nano varieties have many uses.

In their Nature Materials paper, the researchers show how the brushes can sweep up piles of nano-dust - though so far they have omitted to supply the nano-pan to collect it in.

They have also shown the brushes can be used to paint microstructures - dipped into a solution of iron oxide (rust), the minute brush hairs will pick up the red oxide particles which can then be wiped on to a bare surface.

Fast work

One of the great strengths of nanotubes, says group member Dr Anyuan Cao, is their extraordinarily high surface area per gram of material - a result, he explains, of their remarkable fineness.

In another of their demonstrations, the researchers show that with the bristles coated in absorbent materials, the brushes will soak up toxic silver ions from contaminated water.

And the carbon brushes could end up with larger-scale uses, too.

Many electric motors use metal brushes to conduct electricity to their spinning metal components.

Carbon nanotubes conduct electricity, and still do so on these brushes, the Rensellaer team proves.

Because of their strength, resistance to abrasion, and pliability, the nanobrushes may prove superior to macroscopic metal brushes in high-power motors.

Body clean-up

Dr Cao speculates the invention will also have medical uses.

However, the scientists will have to make sure their brushes do not shed bristles first - as there are concerns about possible health effects of nanotubes when loose in the environment - and have already started testing how easily they can be pulled off.

But tiny nanotube-tipped medical brushes might be used either to coat protective substances on to damaged surfaces in our bodies - for example veins - or to clean up unwanted deposits.

With appropriate chemical coatings, they might be able to pick out biomolecules such as DNA, specific proteins, or even whole viruses.

It seems likely that brushes, along with stone axes, were among the first inventions of our ancestors, if only as prehistoric fly swats.

The oldest surviving example dates from 30,000 years ago - the property of some ancient cave artist.

It seems fitting that miniature versions of these should adorn the burgeoning field of nanotechnology.