The general focus on improving air travel usually involves faster, safer, lighter, and more fuel-efficient aircraft. But another safety concern is disease transmission because the airtight flying metal cylinders that are passenger aircraft are also disease incubators. Studies have discovered infectious agents like MRSA surviving for up to a week on several of the surfaces that passengers come in contact with.
Historically, disease transmission of the worst varieties on passenger aircraft has been rare. However, air travel is expected to double over the next 20 years and with the looming threat of drug-resistant bacteria, precautions need to be taken now. With this in mind, researchers are looking to weaponize surfaces against pathogens.
The bactericidal property of the wings of many types of insects has been known for some time to kill bacteria, but no one knew why it was so. All that was known for sure was that the surface of the wings had a novel texture down at the nanoscale (left). The wings are covered in tiny spikes called nanopillars.
It was hypothesized that the spikes on the wing punctured the walls of any cells that came to rest against them. Like setting a plastic bag full of water on a bed of nails. But now, a group of researchers has published an article in the American Chemical Society’s publication, Applied Material & Interfaces, that details what they believe to be the actual mechanism of action behind this “death-on-contact” effect. They drew their conclusions by studying the interaction between the E. Coli bacteria and the wings of the dragonfly species, Orthetrum villosovittatum.
Bacteria, to a greater or lesser extent depending on the type, will exude EPS and filopodia. EPS, also known as extracellular polymeric substance, is essentially the glue holding an aggregation of cells together into a biofilm. Filopodia are thin little needles of material that cells use to hold onto surfaces, feel out their environment, and move around.
The researchers discovered that when a cell secures itself to the nanopillars with these films and feelers, it has metaphorically stepped on a landmine. As long as it doesn’t try to move, it’s fine… maybe. The adhesive force between the cell and the nanopillars is much stronger than the tensile strength of the cell itself. When the cell does try to move, it literally rips open and eviscerates itself.
Here is an imperfect thought experiment to help visualize this process:
Imagine a truck with two winches. One on the front and one attached to the rear. These winches are really strong. Now imagine extending the cables and not hooking them to anything but instead gluing the cable ends to some huge steel posts that are sticking out of the ground in front of and behind the truck. You are really going to have to suspend your disbelief now. Those glued joints are stronger than the tensile strength of the truck frame, and so are the posts and winches. Now, if just one winch is activated—let alone both of them—the truck will rip itself in half. Well, maybe you will just lose a bumper, but still, the setup is fairly representative of what a cell does to itself on the surface of a dragonfly wing.
Self-sterilizing surfaces aren’t a new concept. Copper and silver have long been known to be biocidal. A study last year demonstrated the efficacy of black silicon against gram positive and gram negative bacteria. Black silicon is silicon treated with specialized fabrication methods, and it has a pointy nanostructure very similar to that seen on a dragonfly wing. (It is called black silicon because its surface structure makes it reflect so little light that it appears black.) The problem is that coating everything with black silicon or copper or silver may not be feasible or cost-effective.
Because the nanopillars’ bactericidal effect is a consequence of shape rather than a property of the material itself, it promises to be cheaper to exploit for commercial purposes. Whether as a coating or fabricated onto the products themselves, the lethal geometry can be incorporated onto almost everything an aircraft passenger touches. The trays, armrests, window shades, absolutely everything in the washroom and as much of the upholstery as possible.
Germs are, of course, everywhere and it is unreasonable to expect a 100% kill rate. However, keeping the number of bacteria per unit of area below a threshold can mean the difference between infection or not. A disease carrier is just 24 hours of air travel away from anywhere else on earth, and currently, there is no bulwark against that threat. With the increasing prevalence of drug-resistant bacteria, it behooves us to limit possible avenues of transmission.