Chemical compounds that coat cicada wings also contribute to their ability to repel water and kill microbes, says a new study published in the journal Advanced Materials: Interfaces.
Neotibicen pruinosus and Magicicada casinnii and their wings with the relevant cells labeled (top). Scale representations of the material removed throughout the chemical extractions (orange: 1 min; dark gray: 5 min; blue: 20 min) and the remaining pillar height (light gray) after the extraction (bottom), based on SEM images. Image credit: Román-Kustas et al, doi: 10.1002/admi.202000112.
University of Illinois at Urbana-Champaign’s Professor Marianne Alleyne and colleagues looked at the physical traits and chemical characteristics of the wings of two cicada species: Neotibicen pruinosus (annual cicada) and Magicicada casinnii (emerges from the soil once every 17 years).
Previous studies have shown that both species have a highly ordered pattern of nanopillars on their wings. These tiny pillars contribute to the wings’ hydrophobicity and likely play a role in killing microbes that try to attach to the wings.
“We knew a lot about the surface structure of cicada wings before this study, but we knew very little about the chemistry of those structures,” Professor Alleyne said.
To study nanopillar chemistry, the researchers developed a method to gradually extract the compounds on the surface without damaging the overall structure of the wings.
They placed each wing in solvent in an enclosed chamber and slowly microwaved each one.
“We extracted all these different compounds over different time periods, and then we analyzed what came off. And we also looked at the corresponding changes in the nanopillar structure,” said Dr. Jessica Román-Kustas, an analytical chemist in the Sandia National Laboratories and the U.S. Army Engineer Research and Development Center.
The team found that cicada wings are coated in a stew of hydrocarbons, fatty acids and oxygen-containing molecules like sterols, alcohols and esters.
The oxygen-containing molecules were most abundant deeper in the nanopillars, while hydrocarbons and fatty acids made up more of the outermost nanopillar layers.
“Finding these particular molecules on the surface is not a surprise. Hydrocarbons and fatty acids on insect cuticle are fairly common,” Professor Alleyne said.
The ratio of surface chemicals differed between the two cicada species, as did their nanopillar structures.
The authors found that altering the surface chemicals also changed the nanopillar structure.
In Neotibicen pruinosus, the nanopillars began to shift in relation to one another as the chemicals were extracted, and later shifted back to a more parallel configuration. This also changed the wings’ wettability and anti-microbial characteristics.
The wings of Magicicada casinnii had shorter nanopillars and a higher proportion of hydrophobic compounds on their surface. Their nanopillar configuration orientation did not change as a result of extracting their surface chemicals.
“While preliminary, the new findings offer insight into the interplay of structure and chemistry in determining function,” Professor Alleyne said.
“By dissecting these characteristics, we hope to one day design artificial structures with some of the same surface traits.”
“Finding materials that shed water and kill microbes, for example, would be useful in many applications, from agriculture to medicine.”
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Jessica Román-Kustas et al. Molecular and Topographical Organization: Influence on Cicada Wing Wettability and Bactericidal Properties. Advanced Materials Interfaces, published online April 1, 2020; doi: 10.1002/admi.202000112
