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MICROMACHINES could be made more cheaply and simply using a modified form of
electrolysis, say scientists in Germany. They have found that ultra-short pulses
of electric current let them sculpt metal structures only a few micrometres
across.

Electrolysis involves passing a current through a solution, removing ions
from one electrode and depositing dissolved ions at the other. This can be used
to do useful jobs such as refining chemicals or electroplating, in which layers
of metal are deposited onto an object.

By using pulses of current lasting just nanoseconds, Rolf Schuster and his
colleagues from the Fritz Haber Institute in Berlin were able to miniaturise the
electroplating process to make three-dimensional structures with an accuracy
down to a few hundred nanometres.

Their nanosecond pulses mean that chemical changes only have time to occur
extremely close to the surface of the electrodes. This allows the process to be
very tightly controlled. They used a platinum wire 10 micrometres in diameter as
one electrode—which they called the “tool”—and the metal surface on
which structures were to be sculpted as the other. The electrodes were immersed
in a mixture of copper sulphate solution and perchloric acid.

By making the platinum wire the anode, the researchers were able to deposit
tiny spots of copper onto a gold surface. When they reversed the current, making
a copper surface the anode, material was dissolved from the surface around the
platinum wire. By moving the wire in three dimensions, they could cut shapes
into the copper, just as if the wire were the tip of a miniature milling
machine. “You get the negative shape of the electrode imprinted into the
surface,” says Schuster.

Because the material is removed electrochemically, rather than mechanically,
the delicate structure is not subjected to stresses that might damage it. Also,
the tool doesn’t wear down. “We could imprint a thousand times into a surface,”
says Schuster. “It is not mechanical machining so the electrode doesn’t get
destroyed.” The platinum electrode could be replaced with more complex tools to
sculpt many structures in parallel, and so fabricate tiny sensors or pumps.

“People have done pulse electrochemical machining before,” says David Allen,
a microengineer from Cranfield University in Bedfordshire. But the German team
has taken it down an order of magnitude, he says. “It’s a nice way of doing
things.” He says the researchers will have to speed up their process before it
can be used in industry. “Time is money,” he says. “If you are talking about
millions or billions of structures, you have to be able to do it quickly.”