Cleaner air-con thanks to antifungal aluminium

Korean engineers have shown that a specially-designed aluminium surface could help improve the air quality produced by air-conditioning units.

We’ve all become accustomed to using heating, ventilation and air-conditioning systems to manage the environment in our homes and offices. If improperly maintained, these systems can offer the perfect conditions for growth and circulation of some microbial contaminants that can aggravate respiratory conditions like asthma or bronchitis.

The issue is that aluminium – which most commercial evaporators are made from – isn’t inherently antimicrobial, so requires careful and frequent cleaning to minimise contamination. Other metals such as copper, silver, or titanium are anti-microbial but are also significantly higher in cost than aluminium, rendering them impractical for widespread use. But a recent paper in Materials Letters [DOI: 10.1016/j.matlet.2015.08.103] reports on a coated aluminium surface that obstructs the adhesion and spreading of microbes.

The idea is based on superhydrophobicity – whereby a rough surface provides a low-energy surface that water cannot stick to. Given the importance of water to the development of biological life forms, removing it from the surface should also stop the microbes from growing. And so the researchers designed a series of surfaces with varying wettability, using contact angle (CA) measurements to determine their wetting characteristics. Untreated aluminium is already weakly hydrophobic (water can partly stick to the surface, with a contact angle of 80°). By coating it with a polymer, the contact angle was increased to 110°, making it hydrophobic. The superhydrophobic surface (CA = 170°) was produced by treating the aluminium in oxalic acid and then coating the rough surface in another polymer. The result is a surface that is rough on both the nano and micro-scale.

Three common airborne fungi were then used to contaminate each of the surfaces – penicillium, cladosporium, and aspergillus. In the direct contamination experiment, equivalent to an evaporator in intermittent use, the superhydrophobic surface remained clean. In the indirect experiment, equivalent to recirculation of ‘old’ air, there was a small amount of contamination on the superhydrophobic surface, but hardly any of it spread. In both experiments, all other samples were heavily contaminated.

These results suggest that superhydrophobic aluminium is not only effective as an antifungal surface, but it may offer a low-cost way to keep our evaporators clean.

This article appeared in Materials Today in November 2015.

Original reference paper: Yeongae Kim, and Woonbong Hwang – “Wettability modified aluminium surface for a potential antifungal surface.” Materials Letters (2015) 161, 234–239. DOI: 10.1016/j.matlet.2015.08.103