To print a metal circuit, the trick is in the ink. You need to first deposit the metal as a processable liquid—the ink—and then convert the pattern to its functional solid form. Many inks use solutions of metal salts, which are then heated to reduce the salt to pure metal. The problem with platinum salts is that this reduction step requires much higher temperatures than other metals, such as gold or silver. These high temperatures may degrade or destroy biomaterial substrates.

Now, scientists at the University of Wollongong, Australia, have discovered that a simple, two-component ink can be reduced using treatment by a mild nitrogen plasma. This type of plasma treatment is often used in standard substrate cleaning protocols, but it has not previously been used as a reduction method. Though the precise mechanism is not yet understood, the researchers suggest it may involve localised heating and the reductive environment provided by nitrogen ions.

The team next illustrated that this strategy can achieve feature sizes in the nanodomain. Using a technique known as dip-pen nanolithography, they deposited miniscule volumes of liquid via the tip of an atomic force microscope (AFM) cantilver. The droplets are on the order of an attolitre—one million times smaller than the smallest ink-jet droplet. By treating the printed droplets with nitrogen plasma, they could reduce the ink to form platinum metal features with a diameter of down to 60 nm.

The platinum blonde experiment

The group found that their technique was versatile over a range of substrates, including PDMS and Parylene C, two materials used in bionic implants. In another demonstration, the group succeeded in printing the ink on a strand of human hair. The plasma reduction strategy was mild enough to reduce the ink to platinum metal without damaging the hair.

The next step is to fully characterise the electrical properties of the platinum features, and to explore their use as cell-stimulation platforms in bionic devices.

More information can be found in the journal Nanotechnology (in press).

Further reading

Creating large-area nanoscale array patterns by nanosphere lithography (Jun 2008)