Interplay of organic matrix and amorphous calcium phosphate strengthens the isopod claw
Authors: Vittori Miloš , Srot Vesna , Bussmann Birgit , van Aken Peter A. , Štrus Jasna 
Published: December 2016

Animal skeletons are high‐performing composite materials that may help inspire materials and designs in a broad spectrum of industrial and biomedical applications. The study of various skeletal elements can provide important insights into evolutionary solutions to mechanical demands of animal locomotion, feeding and reproduction, as well as reveal the mechanisms controlling skeletal formation and biomineralization.

The extracellular matrix forming the crustacean exoskeleton comprises chitin‐protein fibers embedded in a calcified inorganic matrix that consists of calcite and amorphous calcium carbonate. In crustaceans, the cuticle can be subdivided into a thin external layer ‐ the epicuticle – and two internal layers, which are heavily calcified – the exocuticle and the endocuticle [1]. The crustacean cuticle generally consists of stacked sheets of parallel chitin‐protein fibers, which helicoidally shift their orientation in each sequential sheet, resulting in a structure referred to as the Bouligand pattern [2]. This organization of the fibers strengthens the cuticle in different directions.

In our study, we analyzed the structure and composition of the walking leg claw of the woodlouse
Porcellio scaber
. Woodlice are terrestrial crustaceans that support their bodies with 7 pairs of legs, each ending in a claw. The claws are thin skeletal elements predominantly subjected to unidirectional loads. To study the nano‐structure of the matrix, we imaged fractured claws with field‐emission scanning electron microscopy using a Jeol 7500F microscope. We then analyzed the elemental composition and the distribution of mineral components in the claws with energy‐dispersive X‐ray spectroscopy (EDX) and electron energy‐loss spectroscopy (EELS) combined with high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) at high spatial and high energy resolution using Zeiss SESAM and Jeol ARM200F microscopes at different accelerating voltages.

Our results demonstrate that the exocuticle of the claw is not calcified and is heavily brominated instead. The endocuticle, on the other hand, is mineralized predominantly with stable amorphous calcium phosphate, which is a highly unusual feature of an animal exoskeleton. Furthermore, we established that the claw endocuticle is highly structurally anisotropic, consisting of axially oriented chitin‐protein fibers and amorphous calcium phosphate particles, all oriented in the direction of loading.

The presence of amorphous calcium phosphate in the mineralized endocuticle and a non‐calcified, brominated external exocuticle may help increase fracture resistance of the claw cuticle. The brominated exocuticle, which is likely more elastic than the mineralized endocuticle, is distributed in areas subjected to maximum stress during axial loading of the claw. These structural and compositional features of the claw cuticle likely result in greater resistance of the claw to fracture and wear when exposed to axial loading.