Oct 25, 2013
Plasma reduction enables nanoscale platinum printing
Platinum is the material of choice for the electrodes that electrically communicate with living cells in implanted bionic devices. A major roadblock to device development is the difficulty in achieving nanoscale (sub 100 nm) patterning of platinum on soft biomaterials. Researchers at the University of Wollongong in Australia have now developed a versatile printing strategy that can create nanoscale patterns of platinum on a range of conductive, insulating, hard or soft materials.
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).
About the author
Research was performed at the University of Wollongong-based IPRI, the lead node of the Australian Research Council Centre of Excellence for Electromaterials Science (ACES). Cathal O’Connell is a PhD student researching the fabrication of nanoelectrodes using AFM. Ryan Sullivan and Sina Jamali are also PhD students and they performed SEM and SECM characterisation. Associate professor Simon Moulton designed experiments and interpreted data. Associate professor Michael Higgins designed experiments and directed the project. Professor Gordon Wallace is Executive Director of IPRI/ACES and guided the research. ACES is on Facebook and Twitter. Professor Gordon Wallace is on Twitter.