Design principles of the beetle’s exoskeleton

Much research time goes into investigating synthetic nanomaterials that mimic the hierarchical structures of natural materials. One interesting development in this area is a new study by scientists at Northwestern University in the US who have been examining the exoskeleton of the beetle to better understand the design principles and mechanics that provide the structure with its unique properties. Tools that enable a fundamental awareness of such natural materials could lead to improvements in the automotive and aerospace industries, which rely on novel, lightweight and multi-functional materials.

The team, whose work featured in the journal Advanced Functional Materials [Yang et al. Adv. Funct. Mater. (2017) DOI: 10.1002/adfm.201603993], took as their subject the Cotinis mutabilis, a beetle native to the western US. Its exoskeleton, in common with all insects and crustaceans, is made up of twisted structures – known as Bouligand structures – that resemble plywood, being comprised of multiple layers, each composed of aligned fibers. The fibers are actually bundles of chitin polymer chains wrapped with proteins with a higher density along its length than its transverse.

They had previously researched the mechanical properties of natural materials such as seashells, before realizing that some of these materials have evolved to achieve useful combinations of properties based on hierarchical features. While many techniques have been used to characterize the geometry and chemistry of constituents, it has been more difficult to identify mechanical properties such as constituent elasticity and hardness at the nanoscale.

The team therefore combined atomic force microscopy with a theoretical approach to achieve this in their research into the fibers of the beetle’s exoskeleton. As team leader Horacio Espinosa points out, “It is very challenging to characterize the properties of such fibers given that they are directionally dependent and have a small diameter of just 20 nanometers. We had to develop a novel characterization method by taking advantage of the spatial distribution of fibers in the Bouligand structure”. Their approach involved cutting along a plane, producing a surface composed of tightly packed cross-sections of fibers with different orientations so they could analyze the mechanics of the fibers.

They had previously shown that constituent geometry and mechanical properties play a synergistic role in the emergent properties of materials. Having the ability to characterize properties across scales is key for inferring design rules that emerge from studies into natural materials. As Espinosa stresses, “such design rules are a framework of understanding based on our knowledge of materials and mechanics, but ultimately it is what we need to be able to achieve the design of novel materials across scales”.

"We...develop[ed] a novel characterization method by taking advantage of the spatial distribution of fibers in the Bouligand structure”Horacio Espinosa