How do cicadas protect themselves from bacteria and what does this have to do with packaging material?
Cicada wings are equipped with cone-shaped nanostructures that act like tiny needles and have a bactericidal effect by stretching or even piercing the membrane of bacteria. These nanostructures are hydrophobic, which means that they can repel water and thus create a self-cleaning surface. At JOANNEUM RESEARCH, master's student Tina Spirk investigated these natural structures and recreated them artificially. Research group leader Barbara Stadlober explains the background: ‘We have transferred the nanostructure of the cicada wing surface onto a film over a large area using roll-to-roll (R2R) UV nanoprinting. This enables the efficient production of an artificial cicada wing film which, when applied to the surfaces of everyday objects, furniture or public transport, makes them antibacterial without chemicals and is also water-repellent and anti-reflective.
The researchers demonstrated the hydrophobicity by measuring a high contact angle of water and oil droplets on the artificial cicada wing film. Light reflection is reduced by 1-2 per cent due to the nanostructured film surface, which was demonstrated by means of optical transmission measurements. In addition, tests such as bacterial colony counts, fluorescence and scanning electron microscopy were carried out to observe the behaviour of bacteria on the nanostructured films. It was shown that the cicada wing structures lead to a significant death of gram-negative bacteria, such as E. coli. These overstretch their cell membrane in an attempt to cling to the flexible nanocones, which leads to their death.
How does the structure get onto the film?
The researchers are pursuing a sustainable approach and using a bio-based UV coating consisting mainly of soya beans and castor oil, as well as pyruvic acid as a bio-based light-sensitive starter for polymerisation. The replication of the complex hierarchical structure of a cicada wing involves several steps, such as fixing and planarising the wing as well as different non-stick coatings. Challenges during the upscaling process, such as incomplete wetting of the nanostructure and accumulation of coating residues, were iteratively improved. The interaction between the varnish, non-stick coating and curing parameters had to be optimised in order to facilitate the removal of the stamp and ensure good impression quality during the production of the large, final stamp using step & repeat UV nano-embossing.
The film roll produced continuously with this stamp in a roll-to-roll process has a slightly inhomogeneous nanostructure due to different heights (up to 140 nm) and angles of the replicated nanocones. Nevertheless, the R2R cicada wing film has promising hydrophobic, antireflective and antibacterial properties, similar to those of the original cicada wing.
Tina Spirk: "The application of our research could have a major impact in many areas, from medical technology to food packaging. Our aim is to bring the technology to market maturity and thus significantly improve hygiene and safety standards."
