Hard copy

Researchers have discovered the secret behind the beetle’s strong but light armour, which, says
Stuart Nathan, could be the basis for applications in the aerospace and defence industries.

It’s a tough life, being a beetle. When you’re not eating or trying to make little beetles, you’re having to fend off attacks from all sorts of hostile wildlife who see you as a crunchy snack. And for that you need strong, lightweight armour. Humans, of course, have been trying to crack the secret of this armour for many years — and now a team from Kansas State University believes it has found the key.

Researcher Michael Kanost explained that the project, to find out how beetle armour changes from a soft, white substance after the insect has shed its shell into a tough, dark material, has two goals. The first is for pest control — preventing the shell from hardening would be a good way to control insect pests. The second involves using the findings to mimic the beetle armour, developing strong, lightweight materials for use in medical applications, aircraft and armour.

Beetle armour is made from a mixture of substances, including a soft network of proteins and chains of chitin, a hard material made from linked sugar molecules, which is also the main constituent of the shells of crustaceans. The armour is a natural composite, organised in a structure known as an interpenetrating network, where the strands of each polymer weave around and through the strands of the other. The strands are held together by cross-links formed from simple organic molecules called catechols.

The final substance is much stiffer than the protein alone — the Young’s modulus increases from around 8MPa to more than 1,600MPa — and is much more flexible than chitin. The secret seems to be in the cross-links, but how they form, turning the initial gel-like substance into a material that combines stiffness, flexibility and lightness, has until now been a mystery.

Kanost and colleagues knew that enzymes were responsible for the hardening process, which is known as tanning, because of the chemical reactions that had to occur to form the cross-links. They had identified two particular types of enzyme, tyrosinases and laccases, as the main possibilities. Using a pest that is widespread in Kansas, the red flour beetle, as their subject, they bred varieties that did not produce one of these two enzymes in their body chemistry. ‘When we knocked out tyrosinase, everything was normal,’ said co-researcher Karl Kramer. ‘When we knocked out laccase- 2, we prevented tanning taking place.’

Kanost has already made synthetic materials with the same structure as the beetle armour, and researched the properties. They all combine strength and flexibility, and many of them have physical characteristics similar to human cartilage. This, Kanost said, suggests they might find applications in the medical field, as part of replacement joint implants, for example.

But the discovery that the natural processes are catalysed by laccase-2 opens up new possibilities. The team is currently working on its own laccasemediated cross-linking experiments, and is hoping to find a formulation that will be harder and stronger than the previous versions.

Aircraft and military armour are the most likely applications for the materials, Kanost said — although he harbours ambitions to develop armour that would give an extra edge for the university’s football team.