Nanoindentation
To test the mechanical properties, nanoindentation experiments were
performed on three additional radulae (for detailed protocol see Krings
et al., 2022c, 2022d, 2023; Gorb & Krings, 2021). Radulae were arranged
on glass object slides and surrounded by a small metallic ring.
Afterwarde, each ring was filled with epoxy resin, which covered the
radula completely. After polymerization, samples were polished with
sandpapers until tooth sections were on display (see Supplementary
Figure 1), and smoothened with aluminum oxide polishing powder
suspension on a polishing machine. Samples were cleaned in an ultrasonic
bath for five minutes. A nanoindenter SA2 (MTS Nano Instruments, Oak
Ridge, Tennessee, USA) equipped with a Berkovich indenter tip and a
dynamic contact module (DCM) head was employed. Hardness (H) and Young’s
modulus (E) were determined from force-distance curves by applying the
continuous stiffness mode. All tests were performed under normal room
conditions (relative humidity 28–30%, temperature 22–24 °C) and each
indent and corresponding curve were both manually controlled. After
this, samples were smoothened and polished until the next target
localities were on display.
Overall, the inner structure of each tooth was tested at five localities
to receive data on mechanical property gradients within each tooth. E
and H were determined at penetration depths of 500–1000 nm. For each
site indented, we received ~60 values obtained at
different indentation depths, which were averaged to receive one H and
one E mean value per indent. 413 localities were overall tested:118 on the inner lateral, 59 on the outer lateral A, outer lateral B,
outer lateral C, outer lateral D and outer lateral E respectively.