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Shape-Shifting Titanium Provides Hospitals with a Germ-Proof Sliding Doors Moment

A University of Canterbury research team is another step closer to developing germ-proof surface coatings after an unexpected development in the lab. Once it is commercially available, an antimicrobial coating applied to high-traffic surfaces, such as door handles, will help minimise infections that spread within hospitals.

Research leader Prof Susan Krumdieck had been working with titanium oxide (TiO2) for more than a decade when the element suddenly changed form. “TiO2 is famously bright white or transparent, but one day the coating came out all black,” she says. “We set it aside because we really didn’t know what had happened. But then some undergraduate project students tested it for the self-cleaning performance, and it was so photo­catalytically active without any UV radiation that we knew we had discovered something new.”

TiO2 is used in sunscreens because it has the ability to absorb radiation. This action creates energy, which is expressed as oxygen ions, which are deadly to bacteria. TiO2 is therefore ideal for use on surfaces such as door handles in environments where sterility is a priority, such as hospitals.

Krumdieck pioneered the innovative coating technology during her PhD at the University of Colorado, and continued her research at UC. However, there were still two challenges to overcome – how to fix a TiO2 coating onto something like a door handle, and how to activate it without UV radiation. The new black TiO2 held the key to both.

Research collaborator Tim Kimmett at Callaghan Innovation helped to solve the puzzle. “We spent a fun science day playing with the Scanning Electron Microscope and X-ray diffractometer and really marvelling at how different this material was. We knew we had a new material due to the strange nanostructures we were seeing, and of course the striking black colour,” Krumdieck says.

A few months later she was awarded a visiting researcher fellowship at Université Grenoble Alpes in France, and took a few of the black coating samples with her. Researchers at the SiMAP Institute there were intrigued that the material could be the same as white TiO2 according to analysis, but instead of the typical smooth pyramid crystals of TiO2, the French team, led by Prof Raphaël Boichot, found that the crystals were nano­structured in ways previously only possible by the hydrothermal growth of individual nanoparticles.

“Professor Boichot suggested that the material could have visible light antimicrobial activity. When I got back to UC, I was lucky to run into Prof Jack Heinemann, who is an expert in microbiology, and he worked with his students to set up a testing system,” Krumdieck says. “Sure enough, the bacteria did not stand a chance – even after a short time in visible light.”

With no need for radiation to energise the new form of TiO2 and an altered nanostructure that enables the compound to be fixed in coatings, the conditions are right for the team to move ahead to commercial applications. They have successfully deposited the black coating onto a door handle, and are now working with several companies to complete the engineering development needed to design and upscale it for advanced manufacture.

The research was published in Scientific Reports (https://goo.gl/hzQyqV).