The role of collagen in the dermal armor of the boxfish

This research aims to further the understanding of the structure and mechanical properties of the dermal armor of the boxfish (Lactoria cornuta). Structural differences between collagen regions underlying the hexagonal scutes were observed with confocal microscopy and microcomputed tomography (μ-CT). μ-CT revealed a tapering of the mineral plate from the center of the scute to the interface between scutes, suggesting the structure allows for more flexibility at the interface. High-resolution μ-CT revealed, for the first time, a 3D image of the dermal armor's complex collagen structure. Helical interfibrillar gaps in the collagen base were found that are similar to the Bouligand-type structure of the lobster, Homarus americanus, thereby suggesting that the collagen in the boxfish is also of a Bouligand-type structure. In situ scanning electron microscopy tests were performed in shear and tension between two connected scutes and suggest that the interfacial collagen is structurally designed to preferentially absorb energy during deformation to protect the internal collagen. Similarly, in situ small-angle x-ray scattering was performed in shear and tension and further corroborated the complex collagen structure. Lastly, these experimental results are coupled with finite element simulations that characterize the interfacial collagen and corroborate the non-linear deformation response seen during in situ testing. Overall, these findings further the understanding of the structure and mechanics of the dermal armor of the boxfish which may help provide a basis to synthesize bioinspired composites for impact-resistant materials, specifically with bioinspired Bouligand-type structures to create novel fiber-reinforced composites.