Since they are flexible, high-aspect-ratio nanostructures store elastic energy. When released, this energy could be used to destroy bacteria by physically stretching and rupturing their cell membranes. This technique was inspired by the bactericidal nature of insect wings and destroys both Gram-positive and Gram-negative bacteria at extremely high rates. The nanostructures could make for a new type of highly efficient mechano-responsive antibacterial surface.

Antibiotic resistance is an increasing problem worldwide. In Europe, for example, around 25,000 people die every year from resistant bacterial infections. Without new antibacterial agents, routine medical procedures and operations could soon become impossible.

Antibacterial surfaces are used to prevent the formation of bacterial biofilms on a variety of surfaces, including medical tools and implants. Until now, these generally worked thanks to a chemical coating that slowly releases a biocidal agent and kills bacteria that come into contact with it. This approach does have some drawbacks, however, with the main one being that bacteria may develop resistance over time.

Cicada wings are natural bactericides

A team of researchers led by Elena Ivanova of RMIT University in Melbourne, Australia, recently discovered that cicada wings, which are covered with arrays of pillars roughly 200 nm high and 60 nm in diameter, are highly bactericidal. This property is purely physico-mechanical – the bacteria cell membranes stretch and rupture between the pillars, which is the point subjected to the highest mechanical stress.

The researchers found that the bactericidal efficiency of the nanostructured surfaces depends on the spacing and size of the nanofeatures. This suggests that the nanopatterns can be modified to increase the surfaces’ antibacterial properties depending on which type of bacteria need to be treated. Gram-negative bacterial cells like Pseudomonas aeruginosa, for example, are destroyed relatively quickly on cicada wings, while Gram-positive bacteria such as Staphylococcus aureus are more tenacious since they resist the stretching process better. The differences in the two types of bacteria might come from the mechanical properties of the bacterial cell membranes themselves.

Inspired by these natural nanostructures, Ivanova and colleagues decided to design similar surfaces using high-aspect-ratio vertically aligned carbon nanotubes (VACNTs) grown by chemical vapour deposition as the pillars. Carbon nanotubes are very attractive for biotechnology applications thanks to their high strength and stiffness and high electrical and thermal conductivity. VACNTs, which share many of the same properties as CNTs, are also highly flexible and resilient, and can be easily functionalized using a plasma treatment. They can be made in different lengths by adjusting growth times, which allows their bending stiffness, and thus the amount of elastic energy they store, to be controlled.

Shorter nanotubes are best

When the VACNTs come into contact with a bacterial cell, they bend and release their elastic energy. This bending stretches the bacterial cell membrane and destroys it, so killing the bacteria.

The researchers observed the bactericidal effect of the nanotubes using a variety of techniques, including focused ion beam-scanning electron microscopy (FIB-SEM) and scanning transmission electron microscopy (STEM).

“We found that shorter nanotubes around 1 micron in length are more efficient at inactivating bacteria thanks to their ability to store and release more elastic energy compared to longer tubes,” says Ivanova. “We achieved bactericidal rates of as high as 99.3% for P. aeruginosa and 84.9% for S. aureus by modifying the lengths of the VACNTs, which allowed us to determine the optimal length for efficiently killing different types of bacteria.”

A new platform to combat antibiotic resistance

“Modulating the CNT characteristics in this way will further improve our understanding of mechano-bactericidal mechanisms. It also shows that the antibacterial activity of high-aspect ratio nanofeatures can outperform both natural bactericidal surfaces and other synthetic nanostructured multifunctional surfaces reported previously.

“Indeed, we believe that such high-aspect ratio features are promising as a new platform to combat antibiotic resistance in pathogenic bacteria,” she tells Physics World.

The team, reporting its work in ACS Nano 10.1021/acsnano.8b01665, says that it is now busy optimizing the height of its high-aspect ratio nanopatterns to make them more efficient against a wider range of bacteria.