The newest warrior in the fight against antibiotic resistance may come with a copper shield. A team of nanotechnologists have developed a copper-based substance that kills more than 99.99% of bacteria on it within a couple of minutes.
Copper has long been known for its antimicrobial properties – it’s been used in water purification and wound sterilisation because it can kill bacteria. But it’s usually fairly slow-acting: an ordinary copper surface can kill around 97% of Staphylococcus aureus (golden staph) in roughly four hours.
This new copper surface, described in a paper in Biomaterials, can kill more than 99.99% of the same bacteria within two minutes, which means it could be highly effective against strains of bacteria that are resistant to antibiotics.
The material is made by alloying copper and manganese together, and then removing 99% of the manganese atoms in a process called dealloying.
Co-author Professor Ma Qian, a researcher in advanced manufacturing and materials at RMIT University, says that there are at least two reasons their material is so dangerous to microbes.
The first is because of its ‘micro-nano’ structure – that is, the shapes it takes on the scale of both micrometres and nanometres. (For context, golden staph bacterial cells are around one micrometre – 1000 nanometres – long, while copper ions are each roughly 0.1 nanometres in radius.)
“[At the microscale] It has comb-like peaks, and within the tooth of each comb, there are nanoscale small pores,” says Qian.
“So there are nano-scale cavities, and micro-scale cavities.”
These cavities-within-cavities mean that there is plenty of area for the copper to act.
“This copper surface has a massive surface area. For each gram of this copper surface, we have about 11 square metres,” says Qian.
The surface is ‘superhydrophilic’: it attracts water, and attracts it fast.
When a droplet of water falls onto a piece of paper, it usually sits on the surface as a whole drop for an amount of time before sinking into the paper.
When water is dropped onto this surface, it sinks in immediately – forming an angle of 0° between droplet and surface in a fraction of a second.
“The water would spread into a flat film in 0.18 seconds – which is super-fast,” says Qian.
Qian’s team has collected evidence to show that the water spreads into the surface so quickly that it damages bacterial cells by stretching them.
“This mechanical stretch would cause structural degradation of the bacterial cells.”
The second threat is the copper itself.
“Another feature is that the surface can release copper ions, which are very toxic to the bacteria,” says Qian.
These copper ions create a “bacteria-inhibition zone”, which isn’t seen around ordinary copper.
The chemical toxicity combines with the mechanical stretching to “make the bacteria more vulnerable”.
The researchers have shown the efficacy of the copper substance on 50 mm diameter discs in a lab. Eventually, they hope it can be used as an antimicrobial surface on doorhandles and other high-touch surfaces in hospitals, schools, and other public spaces. It could also be used in face mask and air filters.
Qian says the next step is to test the efficacy of this surface in a few public spaces – for instance, on handrails in train stations or cinemas.
“Ideally, we could test [the substance out] in Melbourne or in Australia.”
The team has applied for funding for these tests, and is planning to apply for more (the current research was funded by RMIT, CSIRO and the CASS Foundation).
They’re also planning to investigate the effect of this surface on SARS-CoV-2, to see if it can help to control the spread of COVID-19.