Textured fluorapatite bonded to calcium sulphate strengthen stomatopod raptorial appendages

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.

A Sinusoidally Architected Helicoidal Biocomposite

“…[20] Additionally, supporting nanomechanical studies revealed the anisotropic stiffness response and quasi-plastic nature of the impact region. [20,21] However, key details regarding the microand nano-structural features and the corresponding mechanical response have yet to be revealed.Here, we uncover a novel ultrastructural design within the impact region of the dactyl club, which exhibits a notable departure from the helicoidal structure found within most crustacean exoskeletons and affords superior mechanical advantages. We also reveal previously unreported structural and mechanical details from this region that not only provide new insights to the design of impact resistant and damage-tolerant composite materials, but also hint at the mechanisms of self-assembly and biomineralization in complex biological architectures.…”

“…This variation could be attributed to the substitution of fluorine, sulfur, and perhaps carbonate into the apatite crystal Submitted to 5 closer to the club surface, which, as reported previously, would explain the higher elastic modulus and hardness within the impact surface. [20] Analysis of a fractured dactyl club (along its sagittal plane) by scanning electron microscopy (SEM, Figure 2) reveals that the characteristic helicoidal arrangement of mineralized chitin fibers seen in the periodic region is highly compacted laterally within the impact region, forming a herringbone pattern ( Figure 2A). Fibers continue to rotate in the plane (x-y) of the micrograph ( Figure 2B) about an axis (z) normal to the club surface; however, the laminations appear to be compacted in the azimuthal directions, yielding sinusoidally bent fibers forming the herringbone pattern (see also Figure S2).…”

Structure and mechanical properties of selected protective systems in marine organisms

“…The impacting surface of the smashing mantis shrimp species, the dactyl club, changes in structure from an inner bulk area of chitin fibers to an outer, heavily calcified surface, with the highest degree of crystallinity at the impact surface (Fig. 29b) [204,205]. The bulk area primarily contains amorphous calcium carbonate that transitions to amorphous calcium phosphate [205,206] and then ultimately a hypermineralized fluorapatite with calcium phosphate at the surface [204].…”

“…29b) [204,205]. The bulk area primarily contains amorphous calcium carbonate that transitions to amorphous calcium phosphate [205,206] and then ultimately a hypermineralized fluorapatite with calcium phosphate at the surface [204]. These changes in crystallinity and mineral result in increasing stiffness and hardness toward the impact surface, with hardness six times greater in the impact region [204][205][206].…”

Large area sub-micron chemical imaging of magnesium in sea urchin teeth

“…Vibrational spectroscopies, namely Raman and infrared (IR) spectroscopy, have proved to be excellent characterization tools for extracting short range chemical information from crystalline and amorphous mineral phases in biological systems (Bentov et al, 2012;Long et al, 2012;Politi et al, 2004;Stock et al, 2012;Amini et al, 2014). IR spectroscopy, while very sensitive to low concentrations of analytes, is characterized by limited spatial resolution because of its longer incident wavelength, the required preparation of sample thin sections, and the difficulty of performing experiments under hydrated conditions due to the overwhelming intensity and width of water's intrinsic OH vibrational signatures.…”