Fig. 8: Comparison of toughness enhancements in the Ea. spicules and other SBMs with layered architectures.

a The initiation toughness enhancement metric R(0)^(arch)∕R(0)^(hom) of the Ea. spicules and other SBMs with layered architectures. We used the value R(0)^(hom) = 3 Jm⁻² of calcite as the control material for nacre and conch⁶⁸, and R(0)^(hom) = 10 Jm⁻² of hydroxyapatite⁶⁸ as the control material for bone and antler. For Ea., the solid circle represents the average and the cross represents the approximate value of the maximum enhancement which we observed for very short notches. b The average toughness enhancement metric 〈R〉^(arch)∕〈R〉^(hom) of the Ea. spicules and other SBMs with layered architectures. We used the value 〈R〉^(hom) = 22 Jm⁻² of limestone as the control material for nacre and conch⁶⁹, and 〈R〉^(hom) = 140 Jm⁻² of hydroxyapatite⁷⁰ as the control material for bone and antler. In both a and b, the values of R(0) and 〈R〉 for nacre, bone, antler, and conch (S. gigas) were obtained from the literature (see Supplementary Table 1). The multiple values for bone in a and for nacre and bone in b correspond to experiments performed by different research groups. The two green triangles connected by a line in a correspond to the range of values reported by³⁵. The references from which the data points were obtained are annotated in the insets. Note that in both a and b the toughness enhancement metrics are plotted using a logarithmic scale.