Australasian Science Magazine

Tiny bumps on cicada wings repel water and dirt.

By Gregory Watson & Jolanta Watson

The nanostructure of cicada wings is inspiring new materials that are self-cleaning, repel water and can kill bacteria.

On a bush walk more than 10 years ago we found a dead cicada, and started to ask questions about it. The trail we began to follow has now led to the discovery of materials that are self-cleaning, repel water, kill bacteria and can serve as a base for culturing cells. They may well provide the blueprint for many useful materials.

We began our investigation by exploring what the cicada surface looked like with an atomic force microscope. The wing consisted of tiny bumps of similar height spaced around 200 nm apart (Fig. 1). Why would this insect evolve such fine structuring of its wing?

We noticed that the wings didn’t seem to reflect light very well. We knew that moths had evolved similar nano-patterning on their eyes, so it seemed reasonable that the cicada structuring may serve a similar purpose. Not shining in the sunlight or moonlight would help the insect to avoid predators such as birds – a good stealth technology.

We used our instrumentation to physically scrape away small sections of the nanobumps to various heights, and shone a light to see what would happen. Sure enough, the more of the features that we removed, the more light bounced back. So, by using this technique we were able to measure the anti-reflective efficiency of the wing.

During our numerous walks through the bush we observed and collected many cicadas. During one of those walks it began to rain. Undeterred, we stayed and observed the insects to see how they coped with the rain, and again noticed that each insect did not fly off to hide under a leaf but remained on its tree. Furthermore, water did not stick to their wings.

As soon as we got back to the lab we placed a single droplet of water on a wing. It retained its perfect spherical shape (Fig. 2), and the droplet rolled off the surface at the slightest inclination or motion of the membrane. This phenomenon is termed “superhydrophobicity”.

When we observed the wings under various microscopes we also noticed they didn’t seem to get dirty. No pollen or dirt particles seemed to attach to the wing surface.

Some pollen grains are stickier than others. Anyone who sneezes severely during spring knows that pollen grains flying through the atmosphere stick to and irritate the airways.

Some dirt grains are also stickier than others. People on construction sites know to wear masks when working with fine dust in order to prevent lung diseases, including cancer.

But why would the cicada wings resist dirt contamination? If the wings become dirty, the cicadas may lose their anti-

reflective properties. Furthermore, water would now have far more favourable places to stick to them, such as silica and other hydrophilic particles. This could potentially restrict or immobilise the insect, making it vulnerable to predators. The insect also needs a thin aerodynamic membrane to remain airborne, and the extra mass of dirt and water may hinder their flight.

In order to understand how the insect keeps itself clean in an environment that is full of contaminating particles floating around in the air, we again used our atomic force microscope to measure the tiny forces responsible for particles attaching to a surface. When we did this using single pollen grains

(Fig. 3) and small beads of similar chemistry and size to contaminants such as silica from soils, we found that these forces are extremely small.

Of course, having low adhesion does not remove the particles unless some external force acts to remove them. The obvious candidates would be movement of the wings, wind and rain. Indeed, when we placed contaminants onto the cicada wings and exposed them to water droplets they were effectively cleaned. The droplets collected the particles and then rolled off the surface. However, since rain may be absent for extended periods of time, some insects with short life spans may never actually encounter these conditions.

We therefore suspected that nature would use other ways to clean these types of surfaces. Previous work and observations with small droplets on insect wings led us to hypothesise that when dew droplets merge on surfaces and roll off they could carry away contaminants and self-clean the surface. To test this we used high-speed cameras and witnessed dew droplets merging and spontaneously rolling off the cicada wing surface, in the process carrying dirt from the wings. This demonstrates a pathway that insects and other surfaces like plant leaves can use where contaminants can automatically be removed without the organism doing anything else other than their normal daily and nightly activities without rainfall.

On our walks we also noticed that the wings do not seem to decompose as fast as other parts of the cicada. To find out why we exposed the cicada wings to different bacteria, some of which died within a few minutes of landing on the wing surface.

The next step was to see whether it was the chemistry that was having this effect or the structure of the wing itself. We coated the wings with a very thin layer of gold, and sent these samples to our collaborators to see if the bacteria would die. We found that the bacteria still died with this new vastly different chemistry, so it must have been the wing structures that killed the bacteria (Fig. 4) by tearing apart their cell membrane. With the worldwide problem of bacterial resistance to drugs well and truly in focus, this finding now offers a physical alternative to the treatment of surfaces against bacterial attack.

While taking the opportunity to study the wing’s anti­bacterial properties, we were approached by a colleague who was interested in trying to grow living cells on the surface of insect wings. We sent some cicada wings (as well as other insect species) and found that human eye cells were quite happy to grow on the surface (Fig. 4, inset).

This type of surface, where bacteria are killed but other cells can still grow, may have uses in a wide range of medical applications. For example, imagine not having to worry about deadly infection when you have to have implant surgery. Your own body cells would quite happily grow and continue to heal while life-threatening bacteria died.

This story has only described the properties and functions of one type of insect – the cicada – demonstrating just some of the interesting features of the wing membrane. Our studies have also included other insects. For example, some dragonflies have revealed a magnificent diversity in the wings and body. Some of the amazing colours on their wings give clues to their structure at the micro- and nanoscale. On the transparent regions of the wing membrane, the structure typically comprises nanoscale pillars randomly orientated on the surface (Fig. 5), while the coloured regions can be quite different.

Interestingly we have found that these types of wing membranes – as well as termite wing membranes – can also be used to grow specific types of cells while not allowing adherence of others. We have now also begun to study other insect species such as butterflies, moths, wasps and even small vertebrates.

With our world inhabited by, and indeed dependant on, an amazing menagerie of insects, this provides us with an amazing range of natural surfaces to investigate. As Aristotle once said: “Nature does nothing needlessly”. Indeed, it seems Nature can go beyond this because it can, and does, multitask needfully.

Gregory Watson and Jolanta Watson carried out this research at James Cook University’s Scanning Probe Microscopy Laboratory.