Composition
The spike has a spatially tuned and graded chemical composition featuring 3 different minerals: fluorapatite (FAP), amorphous calcium phosphate (ACP) and amorphous calcium carbonate (ACC). The minerals are closely related to the underlying microstructural organization and fiber architecture. Elemental microanalysis by energy dispersive spectroscopy of X-rays (EDS) on representative locations highlights the presence of C, O, P and Ca as major chemical elements and of F, Na and Mg as minor components [36]. Those elements are unevenly distributed in the different regions as shown on a transverse cross-section (Figure 3 A) using 2D elemental mapping (Figures 3B-D) and line scans (Figures 3E, F and Figure S6 ). The highly mineralized region is rich in Ca and P together with F; a complementary Raman analysis, focusing on the outer part of the spike cuticle (Figure 4 A), shows a fairly sharp phosphate peak centered at 965 cm-1 (Figure 4B), indicating that FAP is most likely the predominant mineral phase of this region[7,46]. The sharpness of the peak further suggests a high degree of crystallinity [35]. Confocal Raman imaging reveals interesting spatial heterogeneities within the highly mineralized region. Firstly, the phosphate band presents higher intensity at the outer surface as well as in the transition region close to the individual lamella anchoring the highly mineralized region to the less mineralized thin outer helicoidal region (arrows in Figure 4C). This agrees with variations in P concentration detected by EDS (Figure 3E). The same zone co-localizes with an increased crystallinity of FAP, estimated by the full width at half maximum (FWHM) of the phosphate peak (Figure 4D). One additional feature of the highly mineralized region is a central band having relatively low phosphate intensity and crystallinity, along with some traces of ACC (arrows in Figure 4F). Comparison of the phosphate intensity at the 0° and 90° polarization angle confirms spatial differences in the orientation of the mineral phase, with less oriented mineral found in regions with higher FWHM (Figures 4E and Figure S7 ). A similar tuning of the mineral phases has been reported at the impact region of the spearing dactyl, with small amounts of sulfate co-localized with the more crystalline FAP regions [35]. The presence of fluorapatite, rather than the more common amorphous calcium carbonate, is a remarkable compositional modification at the surface of spearing and smashing appendages of mantis shrimp [35]. In decapods crustaceans, a similar modification involving FAP occurs in the external layer covering the molar area of the crayfish mandible[7]. This mineral is used not only to enhance the biomechanical performance of the cuticle, providing superior impact and wear resistance, but the presence of fluor decreases the solubility of apatite, thus improving its chemical stability. At the interface between the highly mineralized and the outer helicoidal regions there is an abrupt transition in the mineral from FAP to ACP as indicated by the sudden shift in phosphate position (Figure 4G). A central observation of the compositional analysis performed here is that the thin outer helicoidal region has the lowest mineral content as inferred by Ca and P concentrations both showing a minimum there (Figure 3E). Moving further towards the interior of the spike, ACP still predominates within the striated region, whereas in the inner helicoidal region it is gradually replaced by Mg-rich ACC [36].