Textured fluorapatite bonded to calcium sulphate strengthen stomatopod raptorial appendages

Shahrouz Amini,Luca Bertinetti,Admir Masic,Jefri Sanusi Teguh,Xi Zhu,Ali Miserez,Jason S Herrin,Haibin Su

doi:10.1038/ncomms4187

Abstract

Stomatopods are shallow-water crustaceans that employ powerful dactyl appendages to hunt their prey. Deployed at high velocities, these hammer-like clubs or spear-like devices are able to inflict substantial impact forces. Here we demonstrate that dactyl impact surfaces consist of a finely-tuned mineral gradient, with fluorapatite substituting amorphous apatite towards the outer surface. Raman spectroscopy measurements show that calcium sulphate, previously not reported in mechanically active biotools, is co-localized with fluorapatite. Ab initio computations suggest that fluorapatite/calcium sulphate interfaces provide binding stability and promote the disordered-to-ordered transition of fluorapatite. Nanomechanical measurements show that fluorapatite crystalline orientation correlates with an anisotropic stiffness response and indicate significant differences in the fracture tolerance between the two types of appendages. Our findings shed new light on the crystallochemical and microstructural strategies allowing these intriguing biotools to optimize impact forces, providing physicochemical information that could be translated towards the synthesis of impact-resistant functional materials and coatings.

Highlights

Stomatopods are shallow-water crustaceans that employ powerful dactyl appendages to hunt their prey
Recent investigations by Weaver et al.[2] have revealed the complex interplay between microstructure, mechanical properties and impact mechanics of the dactyl club from Odontodactylus scyllarus. These findings have showed that the club is divided into three distinct regions: (i) the impact surface made of highly crystalline apatite and that is near or in direct contact with the external environment, (ii) the periodic region made of a rotated plywood of chitin/amorphous calcium carbonate (ACC) whose main role is to protect the tool against internal damage, and (iii) the striated region made of mineralized chitin fibrils running in a nested fashion around the club, which presumably prevents expansion during impact
Moving from the bulk of the club to its surface, it was found that amorphous calcium carbonate is gradually replaced by amorphous calcium phosphate, which is itself progressively substituted by hyper-mineralized calcium phosphate, with nanoindentation data distinctly showing that this chemical gradation correlates with elastic modulus and a e

Summary

Introduction

Stomatopods are shallow-water crustaceans that employ powerful dactyl appendages to hunt their prey. Ultra-high mineralization[3,4] is an obvious choice to optimize the hardness and stiffness near free surfaces, and most species fine-tune the biomineralization process such that the mineral phase at the free surface exhibits a combination of mechanical properties that is optimized for impact or abrasion tolerance[5,6] Illustrative examples of this strategy include, among others, protective scales of fish[7], the chiton radular teeth[8] or the external protective armour of deep-sea gastropods[9]. Microstructural adjustments are commonly employed by species as a way to provide an extra level of control over the properties of their extracellular tissues and tools, with specific examples including porosity gradients in bones[10] or the preferred crystalline orientation in both vertebrate and invertebrate teeth[11] Stomatopods employ both strategies in their dactyl clubs. Indentation fracture studies indicate a higher damage tolerance for the smasher type of stomatopods, which is attributed to the residual organic chitin phase in the impact region that is absent in the spearer species

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Conclusion