Scientists at the US Department of Energy's national security facility have created a micro-machine etched from amorphous diamond, the hardest material in the world after crystalline diamond.

The methods used were the same as those used with current silicon chip manufacturing techniques.

The first diamond micro-machine is a comb drive whose tiny teeth move forward and back as an electrical current reverses constantly between positive and negative. The teeth are just two thousandths of a millimetre apart.

The researchers, at the Sandia National Laboratories in New Mexico, say diamond has a number of valuable properties.

Resistant to wear

Its resistance to wear makes diamond ideal for micro-machines that need to function for extended periods of time.

"One estimate in the literature claims that diamond should last 10,000 times longer than polysilicon," said group member Tom Friedmann.

Also, diamond is less susceptible to "stiction" than silicon. Stiction - a combination of stickiness and friction - can render micro-machines useless. This is because silicon is attracted to water, which acts as a kind of glue. Diamond does not have the same problem.

Another advantage is that diamond is biologically benign. A micro-machine such as a tiny drug-dispensing unit could be used in the body without triggering an allergic reaction.

Two types

There are two kinds of diamond: crystalline and amorphous. The Sandia researchers used amorphous diamond because crystalline diamond needs far higher temperatures to synthesise it, and also its surface roughness makes it unsuitable for micro-machines.

Amorphous diamond itself had been impractical because its tremendous internal stresses had made it impossible for the material to stand alone or to coat thickly any but the strongest surfaces.

However, a process developed by Friedmann and his group eliminated that problem.

Silicon MEMs (MicroElectroMechanical Systems) are already used in a variety of applications, ranging from air bags in cars to optical micro-mirrors intended for possible deployment on satellites.

It is hoped that diamond MEMs could eventually offer a harder wearing and more flexible alternative, and in the future could replace silicon micro-machines completely.