Chiral graded structures in biological plywoods and in the beetle cuticle

Biological chiral fibrous composites, known as biological plywoods, found throughout nature including the exoskeletons of insects and plant cell walls have optimized structural and functional properties, such as the iridescent colors observed in beetle cuticles. In many cases the micron-range chirality of the fibrous ordering is usually spatially graded, multi-periodic or layered as opposed to uniform. The challenge to discover structure–property relations in biological plywoods relies on the accuracy of determining the usually space-dependent chiral pitch of the plywoods. Here we use a recently developed geometric model and computational visualization tool to determine the complex spatial gradients present in beetle cuticle which is a canonical example of graded biological plywoods, extensively studied using optical methods. The proposed computational structural characterization procedure offers a complementary tool to optical and other experimental measurements. The new procedure has wide application in biological material characterization and in biomimetic engineering of structural and functional materials.

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