How Atomic Layer Deposition Protects Astronauts in the ISS
Astronauts live and work in orbit alongside teeming populations of microorganisms, which might pose a serious threat to their health.
A project led by the ESA is developing coatings that kill microbes in spacecraft cabins to help combat this type of invisible stowaway.
German ESA astronaut Matthias Maurer in Europe's Columbus science module, part of the ISS. Image from esa.int
International Space Station crew members are not alone in space. Several bacteria and fungi species were found on the surfaces of the orbital outpost, including harmful pathogens like Staphylococcus aureus, which can cause skin and respiratory infections as well as food poisoning.
There is a possibility that these microbes could even make spacecraft sick, not just astronauts. Like plaque on your teeth, bacteria and fungi form 'biofilms' that can tarnish and eat away at metal, glass, plastic, and rubber.
On spacesuits, cable insulation, and even window seals, microbial colonies were observed growing on ISS's predecessor, the Mir space station.
UV-triggered self-cleaning surface
Microgravity suppresses astronauts' immune systems, so future long-duration space missions will require rigorous control of microbe populations, according to ESA material engineer Malgorzata Holynska. In order to accomplish this goal, ESA's Materials Physics and Chemistry Section is working with Istituto Italiano di Tecnologia, IIT, to develop antimicrobial surfaces for interior cabins.
As part of its research, the IIT team has begun working on titanium dioxide, which is used in self-cleaning glass down here on Earth and in hygienic surfaces. In the presence of ultraviolet light, titanium dioxide oxidizes water in the air, which produces free oxygen radicals that destroy bacterial membranes.
"Bacteria gets inactivated by the oxidative stress generated by these radicals,” explains Mirko Prato of IIT. Due to the fact that all microorganisms are affected without exception, there is no risk of increasing bacterial resistance, as is the case with some antibacterial materials.
Previous research into hospital antimicrobial coatings led to the selection of titanium oxide. Scientists are testing ways to 'dope' the compound, improving its recipe so that it can detect visible light more effectively.
ISS microbial samples
Self-cleaning surface triggered by UV light, image from esa.int
Malgorzata explained that antimicrobial coatings on Earth often use silver. "But we want to do without it here,” adds Malgorzata.
“The issue is that in the confined environment of a spacecraft, prolonged exposure to silver could have negative health effects for astronauts – we don’t want a heavy metal buildup in the onboard water, for instance, with soluble silver linked to skin and eye irritation, even changes in skin color at very high doses.” - Malgorzata Holynska
One of the advantages of titanium oxide as an alternative is its apparent long-term stability, says Fabio Di Fonzo at IIT: “But we will be performing artificial aging of coatings to see how they evolve. And part of the project results will be to see what are the photo-degradation products going back into the cabin atmosphere once the bacteria are oxidized - we don’t want end products that are more toxic than the microbes themselves.”
The IIT has successfully coated glass, silicon wafers, aluminum foil, and even paper tissue with titanium oxide.
Different techniques are used to lay down the coatings, including 'physical vapor deposition' and 'atomic layer deposition' - which involve slowly laying down thin films by exposing them to gaseous chemicals and are techniques more commonly used to fabricate semiconductors.
Mirko affirms that what they want to achieve for such surfaces is an antimicrobial layer as thin as possible. He added that they are targeting thicknesses of 50 to 100 nanometres, millionths of a millimeter.
It's incredible how Atomic Layer Deposition is such a versatile technique.
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Via the European Space Agency
Further reading on ALD antibacterial coatings:
Thesis by Viljami Pore: Atomic layer deposition and photocatalytic properties of titanium dioxide thin films
Even more references about antibacterial films and coatings made by ALD.