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Plasma Processing Yields Antibacterial Implant Coating

Published: June 26, 2011

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Using plasma processing, an Australian physicist has produced titanium dioxide (TiO2) doped with nitrogen atoms that is sensitive to visible light. The resulting process and doped TiO2 coating can promote osseointegration of implants while providing antibacterial properties to combat nosocomial or hospital-acquired infections.

When exposed to ultraviolet light, a TiO2 crystal generates electron-hole pairs that can migrate to the surface and act as a source of free electrons and electron absorbers. Resulting oxidation and reduction reactions can, subsequently, produce free radicals that attack the cell walls of bacteria.

“One of the limitations of TiO2 is that its large band gap requires ultraviolet light to generate the electron hole pairs,” explains Christian Sarra-Bournet, a physicist working at Australian National University‘s Space Plasma, Power, and Propulsion laboratory. “So, what we wanted to do was devise a method of generating a doped version that would work in visible light and also be economical to produce.” Sarra-Bournet notes that an antibacterial material that was sensitive to visible light rather than ultraviolet light was much more practical for use in hospital and clinical environments.

To achieve this visible-light-sensitive version of TiO2, Sarra-Bournet employed a process called helicon assisted reactive sputtering, or HAReS, which entails the development of a plasma of oxygen and argon ions. This plasma is then used to sputter material from a titanium metal target. “The sputtered titanium and oxygen vapor reacts in its plasma phase to create TiO2 while the argon provides a dense and effective plasma,” according to a university article. “And because of its inert noble gas properties, it doesn’t get in the way of the chemistry. By adding nitrogen to the plasma, [Sarra-Bournet] was able to create TiO2 doped with nitrogen atoms that was sensitive to visible light.”

While conventional TiO2 has been used as a coating for hip and knee implants, for example, it is flawed in that it can only be used on metal parts, according to Sarra-Bournet. Polymers, fabrics, and other heat-sensitive parts can not withstand the high-temperature annealing step that provides the TiO2 with its crystal structure, he says. His method, in contrast, is performed at room temperature and thus can be employed with a broader range of materials.

The new method also allows for the incorporation of nitrogen or other elements by varying the plasma conditions. “We’ve seen that the visible absorption, bioactivity, and photocatalytic properties of the coatings are really dependent not only on the amount of dopant but also if the dopant replaces oxygen in the TiO2 structure or lies in one of the interstitial sites of the structure,” says Sarra-Bournet. “Plasma processing allows us to fine tune the material properties.”

Related reading

Visible-light Photocatalyst – Nitrogen-doped Titanium Dioxide – download as a PDF

 

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