A Molecular-Scale Understanding of Misorientation Toughening in Corals and Seashells.

peer reviewedBiominerals are organic-mineral composites formed by living organisms. They are the hardest and toughest tissues in those organisms, are often polycrystalline, and their mesostructure (which includes nano- and microscale crystallite size, shape, arrangement, and orientation) can vary dramatically. Marine biominerals may be aragonite, vaterite, or calcite, all calcium carbonate (CaCO3 ) polymorphs, differing in crystal structure. Unexpectedly, diverse CaCO3 biominerals such as coral skeletons and nacre share a similar characteristic: Adjacent crystals are slightly misoriented. This observation is documented quantitatively at the micro- and nanoscales, using polarization-dependent imaging contrast mapping (PIC mapping), and the slight misorientations are consistently between 1° and 40°. Nanoindentation shows that both polycrystalline biominerals and abiotic synthetic spherulites are tougher than single-crystalline geologic aragonite. Molecular dynamics (MD) simulations of bicrystals at the molecular scale reveal that aragonite, vaterite, and calcite exhibit toughness maxima when the bicrystals are misoriented by 10°, 20°, and 30°, respectively, demonstrating that slight misorientation alone can increase fracture toughness. Slight-misorientation-toughening can be harnessed for synthesis of bioinspired materials that only require one material, are not limited to specific top-down architecture, and are easily achieved by self-assembly of organic molecules (e.g., aspirin, chocolate), polymers, metals, and ceramics well beyond biominerals

See-Through Teeth, Clearly

The teeth of the deep-sea dragonfish are sharp, hard, stiff, and transparent. Here we compare them to other teeth and their structure, which may determine both light scattering and mechanical behavior of teeth in diverse animals

Loss of biological control of enamel mineralization in amelogenin-phosphorylation-deficient mice

Amelogenin, the most abundant enamel matrix protein, plays several critical roles in enamel formation. Importantly, we previously found that the singular phosphorylation site at Ser16 in amelogenin plays an essential role in amelogenesis. Studies of genetically knock-in (KI) modified mice in which Ser16 in amelogenin is substituted with Ala that prevents amelogenin phosphorylation, and in vitro mineralization experiments, have shown that phosphorylated amelogenin transiently stabilizes amorphous calcium phosphate (ACP), the initial mineral phase in forming enamel. Furthermore, KI mice exhibit dramatic differences in the enamel structure compared with wild type (WT) mice, including thinner enamel lacking enamel rods and ectopic surface calcifications. Here, we now demonstrate that amelogenin phosphorylation also affects the organization and composition of mature enamel mineral. We compared WT, KI, and heterozygous (HET) enamel and found that in the WT elongated crystals are co-oriented within each rod, however, their c-axes are not aligned with the rods' axes. In contrast, in rod-less KI enamel, crystalline c-axes are less co-oriented, with misorientation progressively increasing toward the enamel surface, which contains spherulites, with a morphology consistent with abiotic formation. Furthermore, we found significant differences in enamel hardness and carbonate content between the genotypes. ACP was also observed in the interrod of WT and HET enamel, and throughout aprismatic KI enamel. In conclusion, amelogenin phosphorylation plays crucial roles in controlling structural, crystallographic, mechanical, and compositional characteristics of dental enamel. Thus, loss of amelogenin phosphorylation leads to a reduction in the biological control over the enamel mineralization process

Deep learning virtual indenter maps nanoscale hardness rapidly and non-destructively, revealing mechanism and enhancing bioinspired design

Over evolution, organisms develop complex material structures fit to their environments. Based on these time-tested designs, human-engineered bioinspired structures offer exciting possible materials configurations. However, navigating diverse structure spaces for attaining desired properties remains non-trivial. We focus on the hardest biological tissue in humans, tooth enamel, to examine the structure-property relationship. While typical hardness measurements are time consuming and destructive, we propose that artificial intelligence models can predict properties directly and enable high-throughput, non-destructive characterization. We train a deep image regression neural network as a surrogate model and visualize with gradient ascent and saliency maps to identify structural features contributing most to hardness. This model demonstrates improved spatial resolution and sensitivity compared with experimental hardness maps. Using this rapid hardness testing model, a generative adversarial model, and a genetic algorithm that operates in latent space, allows for guided materials design, yielding proposed designs for bioinspired structures with precisely controlled hardness

Crystal misorientation correlates with hardness in tooth enamels.

The multi-scale hierarchical structure of tooth enamel enables it to withstand a lifetime of damage without catastrophic failure. While many previous studies have investigated structure-function relationships in enamel, the effects of crystal misorientation on mechanical performance have not been assessed. To address this issue, in the present study, we review previously published polarization-dependent imaging contrast (PIC) maps of mouse and human enamel, and parrotfish enameloid, in which crystal orientations were measured and displayed in every 60-nm-pixel. By combining those previous results with the PIC maps of sheep enamel presented here we discovered that, in all enamel(oid)s, adjacent crystals are slightly misoriented, with misorientation angles in the 0°-30° range, and mean 2°-8°. Within this limited range, misorientation is positively correlated with literature hardness values, demonstrating an important structure-property relation, not previously identified. At greater misorientation angles 8°30°, this correlation is expected to reverse direction, but data from different non-enamel systems, with more diverse crystal misorientations, are required to determine if and where this occurs. STATEMENT OF SIGNIFICANCE: We identify a structure-function relationship in tooth enamels from different species: crystal misorientation correlates with hardness, contributing to the remarkable mechanical properties of enamel in diverse animals

The hidden structure of human enamel.

Enamel is the hardest and most resilient tissue in the human body. Enamel includes morphologically aligned, parallel, ∼50 nm wide, microns-long nanocrystals, bundled either into 5-μm-wide rods or their space-filling interrod. The orientation of enamel crystals, however, is poorly understood. Here we show that the crystalline c-axes are homogenously oriented in interrod crystals across most of the enamel layer thickness. Within each rod crystals are not co-oriented with one another or with the long axis of the rod, as previously assumed: the c-axes of adjacent nanocrystals are most frequently mis-oriented by 1°-30°, and this orientation within each rod gradually changes, with an overall angle spread that is never zero, but varies between 30°-90° within one rod. Molecular dynamics simulations demonstrate that the observed mis-orientations of adjacent crystals induce crack deflection. This toughening mechanism contributes to the unique resilience of enamel, which lasts a lifetime under extreme physical and chemical challenges

Corrigendum ‘Black Drum Fish Teeth: Built for Crushing Mollusk Shells’ [Acta Biomaterialia 137 (2022) 147-161]

The authors regret to report that several errors were discovered in the manuscript. - Supplementary note: The calculated bite force quotients (BFQs) in Tables S1-S4 were incorrect. The corrected supplementary materials are now updated online.- Manuscript Introduction paragraph 4 sentence 4: The corresponding explanation of the Figure 1E should be corrected as “Based on the comparative results, the black drum has one of the highest BFQ values among all fish species (∼134–891, Table S1, and Fig. 1E) [13,16,18,19].”- Figure 1E: The dashed reference lines (BFQ = 90/110/120) were incorrect, and should be corrected by deletion. The corrected full Figure 1 is now provided below along with the original figure caption.Although the corrections do not affect any findings and conclusions reported in this work, the authors sincerely apologize for any inconvenience and confusion caused. DOI of original article: https://doi.org/10.1016/j.actbio.2021.10.02