The bug-wing nanotechnology that beats bacteria

Insect wings have nanometre-sized structures that give them interesting properties – including being antibacterial. A team of Australian and Japanese researchers have figured out how to apply this property to plastic, paving the way for antibiotic-free antibacterial food containers.

The research leans on a decade of bio-inspired research on self-cleaning surfaces. Researchers have known for a while that plenty of living organisms have micro- and nano-structures that deflect bacteria.

“Lotus leaves are famous for their self-cleaning properties,” says Distinguished Professor Elena Ivanova, a researcher at RMIT University.

Other research at RMIT has focussed on water-repelling (hydrophobic) lotus leaves, which have also inspired self-cleaning plastic.

But Ivanova says that specifically for antibacterial purposes, the lotus leaf model “is not as promising”.

“We’re looking at other examples of bacteria-free surfaces in nature,” she adds.

Insect wings, also hydrophobic, have similar surfaces to lotus leaves. At the nanoscale, both are very rough: covered in bumps about ten thousand times smaller than a millimetre.

The researchers examined cicada and dragonfly wings. These wings have “nanopillars”: blunt spikes, between about a 100^th and a 10^th the size of bacterial cells. The researchers thought that these nanopillars, like lotus leaf surfaces, would be hydrophobic enough to clean bacteria off.

“What we thought would happen with bacterial cells is that there will be water droplets bouncing from the surface, [retaining] bacteria, and the surface will remain free of bacteria,” says Ivanova.

But when examining their surface with electron microscopy and a few other techniques, the team discovered something even more impressive was happening.

“To our big surprise, it was the other way around: bacteria actually were able to settle, attach on this surface, but when they attach on the surface, […] they are not viable,” says Ivanova.

“When we looked at the interface between bacterial cell and surface, you clearly see the bacterial membrane and the cell is broken, ruptured.”

This means that the surface itself was bactericidal: it “could in fact, rupture bacterial cells by simple mechanical strain which is imposed on the cell membrane”.

Nanopillars of around 60 nanometres in height seemed to be the most effective at destroying bacteria.

“When a bacterial cell is sitting on the surface, it’s stretched – and the strain which is imposed from this stretch is so strong that the membrane breaks. So basically, that’s quite an amazing way to rupture bacterial cells.”

Researchers at Tokyo Metropolitan University and Mitsubishi Chemical’s The KAITEKI Institute, both in Japan, then used this information to develop polymer films containing these bactericidal nanopillars that could be added to plastics.

“They developed a very precise nanofabrication technique,” says Ivanova. The RMIT researchers tested these films, finding they could kill up to 70% of the bacteria that lands on them.

The films are described in a paper in ACS Nanomaterials.

Ivanova says that, unlike traditional antibiotics, bacteria are unlikely to become resistant to these films.

“Depending on the nanopattern, the [membrane] could be ruptured in between five to 20 minutes. So basically, the cells don’t have a chance to multiply and adapt,” she says.

The researchers are now figuring out how to make the films even better at killing bacteria, as well as scale up their production. They’re also looking at how to apply them to soft plastics as well.

The coatings could end up extending the shelf life of food, as well as making it safer to store and transport.

“It’s not us who designed that – nature designed that particular pattern on the surfaces of the wings. We just found it!” says Ivanova.