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ZAP a bath of water and gold with electricity and you can make tiny wires
grow spontaneously. If they break, the strands can even fix
themselves—which could make them perfect for connections between implants
and living tissue.

This weird wire effect was discovered accidentally by Jacob Williams while he
was a student doing summer work in Orlin Velev’s lab at the University of
Delaware. Williams, now at Carnegie Mellon University, was experimenting with
chunks of latex and gold nanoparticles in a box of water sandwiched between two
metal electrodes. When he applied a voltage, he expected the latex to act as a
scaffold, holding the gold together in a bridge. But to his surprise, the gold
started forming a wire between the electrodes all on its own, sprouting small
branches as it traversed the gap. When the power was turned off, the strand
stayed in place.

This isn’t the first time researchers have made gold link up into a chain.
But those previous experiments involved bigger, millimetre-sized particles which
stuck together because of the electrostatic attraction between the opposite
charges that develop in an electric field. When the current was turned off, the
chain flew apart. Velev reckons smaller particles of gold just nanometres wide
won’t attract each other strongly enough to accumulate this way.

What’s happening instead, he says, is that the gold particles are drawn to
places inside the box where the electric field changes the most. Even the
tiniest bump on the surface of an electrode reduces the distance to the opposite
electrode and increases the strength of the electric field at that point. As
gold particles are drawn into the stronger electric field gradient, they get so
close that attractive van der Waals forces stick them together. So they stay
stuck together.

“It’s not some mysterious process once you know what’s going on, but you
wouldn’t expect it,” says Velev. The wires are self-repairing: a tiny break
in the chain warps the electric field, herding in more metal to fill the gap.
The same trick works with silver dust and should work with other particles.

The team also found they could make wires leapfrog between islands of
conducting carbon inside the box, forming a network of circuitry. They hope that
one day they could connect implants to living cells in this way. “Eventually
you’ll want to do this in medical systems,” says Velev.

For now, Velev thinks their most likely potential use is as chemical sensors.
When gold adsorbs a contaminant like cyanide, its resistance changes. Since the
wires are very porous and have a huge surface area because of their branching
fronds, he says they should be able to detect very small amounts of chemicals.