The role of residual stresses in biomineral morphogenesis revealed by 3D dark-field x-ray microscopy*

Residual stresses occur in numerous synthetic, geological and biogenic crystals having desirable or undesirable effects on materials performance. Recent studies have shown the significance of residual stresses in the mechanical functionality of a number of biomineralized tissues. However, the role of these forces in biomineral morphogenesis was never previously examined, mainly due to the lack of an appropriate multiscale characterization approach. Most of the state-of-the-art methods are either surface techniques yielding 2D information or allow limited 3D analysis of very small sample volumes.
In this work, we employed the recently developed technique—dark-field x-ray microscopy—to study the relationship between residual stresses and crystallographic properties of biogenic calcite in the prismatic ultrastructure in the bivalves Pinna nobilis and Pinctada nigra in 3D. This method, developed at the European Synchrotron Radiation Facility, utilizes magnifying refractive lenses to map lattice distortions with an angular resolution of 0.001° and allows to analyze millimeter sized samples with a spatial resolution of 50 nm. Whereas the prisms in P. nobilis have an almost perfect single crystalline character, the prisms in P. nigra gradually change their crystallographic orientation and split into sub-prismatic domains. Due to the high angular resolution of the method, we were able to obtain unprecedented detail on the mosaicity of prisms in the two organisms and to demonstrate a correlation between internal lattice strains and local crystallographic properties of biogenic calcite in 3D. By comparing the experimental data from the two species, we not only shed a new light on the relationship between structure and texture during biomineralized tissues formation, but also demonstrate the role of internal stresses in biomineral morphogenesis.