On the mechanical properties of dual-scale microlattice of starfish ossicles: A computational study

While engineering cellular solids, either stochastic foams or architected lattices, are usually made of polycrystalline or amorphous materials, echinoderms (e.g., sea urchins, starfish, and brittle stars) build their microporous skeletal elements with single-crystalline calcite. This class of biomineralized cellular solid, known as stereom, also exhibits a vast diversity in pore morphology, ranging from random open-cell foams to fully periodic lattices. In particular, the skeletal elements (also known as ossicles) of some starfish species are composed of a diamond-triply periodic minimal surface (diamond-TPMS) microlattice. In addition, the crystallographic symmetries at both atomic (calcite) and lattice (diamond-TPMS) levels are precisely aligned. Here we investigate the mechanical performance of this unique dual-scale microlattice and discuss the synergistic effects of atomic- and lattice-scale crystallographic coalignment. Our computational and theoretical analysis suggests that the mechanical isotropy of ossicles is enhanced due to the property compensation between the atomic-level and latticelevel architectures. Moreover, the observed 50 vol% relative density in the diamond-TPMS microlattice of ossicles may be a result of achieving overall mechanical isotropy, minimal surface curvature, and maximized surface area. We believe the methodology introduced here will be useful for understanding the mechanical behavior of natural crystalline cellular solids such as echinoderms' stereom and designing dual-scale lattice materials.