Transition from horizontal expansion to vertical growth in the oyster prismatic layer.

Biomimetic materials inspired by biominerals have substantial applications in various fields. The prismatic layer of bivalve molluscs has extraordinary flexibility compared to inorganic CaCO3. Previous studies showed that in the early stage, minerals expanded horizontally and formed prism domains as a Voronoi division, while the evolution of the mature prisms were thermodynamically driven, which was similar to grain growth. However, it was unclear how the two processes were correlated during shell formation. In this study, we used scanning electronic microscopy and laser confocal scanning microscopy to look into the microstructure of the columnar prismatic layer in the pearl oyster Pinctada fucata. The Dirichlet centers of the growing domains in mature prisms were calculated, and the corresponding Voronoi division was reconstructed. It was found that the domain pattern did not fit the Voronoi division, indicating the driving forces of the mature prisms evolution and the initiation stage were different. During the transition from horizontal expansion to vertical growth, the minerals broke through the inner periostracum and squeezed out the organic materials to the inter-prism space. Re-arrangement of the organic framework pattern was driven by elastic relaxation at the vertices, indicating the transition process was thermodynamically driven. Our study provided insights into shell growth in bivalves and pave the way to synthesize three-dimensional material biomimetically.