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].