Hyperspectral interference tomography of nacre

Structural characterization of biologically formed materials is essential for understanding biological phenomena and their environment, and for generating new bio-inspired engineering concepts. For example, nacre-the inner lining of some mollusk shellsencodes local environmental conditions throughout its formation and has exceptional strength due to its nanoscale brick-and-mortar structure. This layered structure, comprising alternating transparent aragonite (CaCO3) tablets and thinner organic polymer layers, also results in stunning interference colors. Existing methods of structural characterization of nacre rely on some form of cross-sectional analysis, such as scanning or transmission electron microscopy or polarization-dependent imaging contrast (PIC) mapping. However, these techniques are destructive and too time- and resourceintensive to analyze large sample areas. Here, we present an alloptical, rapid, and nondestructive imaging technique-hyperspectral interference tomography (HIT)-to spatially map the structural parameters of nacre and other disordered layered materials. We combined hyperspectral imaging with optical-interference modeling to infer the mean tablet thickness and its disorder in nacre across entire mollusk shells from red and rainbow abalone (Haliotis rufescens and Haliotis iris) at various stages of development. We observed that in red abalone, unexpectedly, nacre tablet thickness decreases with age of the mollusk, despite roughly similar appearance of nacre at all ages and positions in the shell. Our rapid, inexpensive, and nondestructive method can be readily applied to in-field studies.

Automated summary

This study presents an all-optical, rapid, and nondestructive imaging technique for mapping the structural parameters of nacre and other disordered layered materials. Through this technique, researchers observed a decrease in tablet thickness in red abalone shells at different stages of development.