Nanoscopic biomechanical properties: indentation modulus,
hardness and damage behaviorThe region-dependent chemical composition and microstructural
organization of the spike cuticle are reflected into spatially varying
mechanical properties, as measured by depth sensing nanoindentation
(Figure 5 A). The outer FAP-dominated region is clearly the
stiffest and hardest location (indentation modulus of 64.9 ± 3.5 GPa
and hardness of 4.0 ± 1.0 GPa). Both mechanical parameters show a
considerable drop (almost a factor of 2 for the modulus and of 5 for
hardness) in the thin outer helicoidal region, which displays the
lowest values of the entire spike, consistent with the low mineral
content and compositional modifications. Moving to the striated
region, indentation modulus and hardness increase and the former also
shows slightly higher values with respect to the inner twisted plywood
(Table 1), which is conceivable considering the highly ordered
arrangement of the fibers. Furthermore, a small negative gradient of
about 4 GPa is observed in the indentation modulus in this location,
probably reflecting the corresponding decrease in Ca content. To
better characterize the spatial tuning in local mechanical behavior
from the heavily mineralized to the parallel fibers region, we
performed high resolution nanoindentation with a sharp cube corner
probe (Figures 5B-D and Figure S8 ). The high-resolution maps
confirm the unusual drop in modulus within the outer twisted plywood
region, with values slightly below 20 GPa and no differences between
longitudinal and transverse sections, consistent with the isotropic
nature of the twisted plywood organization. In particular, the
transition region (featuring an individual lamella with fairly high
mineral content) has a clear signature in the mechanical maps (Figures
5C and D) with a modulus of about 40-45 GPa, allowing a gradual switch
from the highly mineralized to the outer helicoidal region. One
additional feature of the highly mineralized region is a quite
heterogeneous mechanical behavior (Figure S9 ), likely
reflecting the spatial variations of phosphate intensity and
crystallinity measured by Raman spectroscopy. A similar mechanical
modulation featuring stiffer crystalline domains and less stiff
amorphous islands has been reported for the heavily mineralized impact
region of the spearer appendage [35]. Although a
thin outer helicoidal region enclosing highly co-aligned fibers has
been found in other biological penetrating tools such as the uropod
back spike of the mantis shrimp [42] and the
spider fang [45], a novel feature observed here
is that this region, sandwiched between the hard outer cover and the
stiff parallel-fiber region, presents substantially lower mechanical
properties (modulus and hardness) than the neighborhood.