Hierarchy structure and fracture mechanisms of the wild wolf tusk's enamel

Tooth Diversity Underpins Future Biomimetic Replications

Although the evolution of tooth structure seems highly conserved, remarkable diversity exists among species due to different living environments and survival requirements. Along with the conservation, this diversity of evolution allows for the optimized structures and functions of teeth under various service conditions, providing valuable resources for the rational design of biomimetic materials. In this review, we survey the current knowledge about teeth from representative mammals and aquatic animals, including human teeth, herbivore and carnivore teeth, shark teeth, calcite teeth in sea urchins, magnetite teeth in chitons, and transparent teeth in dragonfish, to name a few. The highlight of tooth diversity in terms of compositions, structures, properties, and functions may stimulate further efforts in the synthesis of tooth-inspired materials with enhanced mechanical performance and broader property sets. The state-of-the-art syntheses of enamel mimetics and their properties are briefly covered. We envision that future development in this field will need to take the advantage of both conservation and diversity of teeth. Our own view on the opportunities and key challenges in this pathway is presented with a focus on the hierarchical and gradient structures, multifunctional design, and precise and scalable synthesis.

Mechanical properties, microstructure and chemical composition of naked mole rat incisors

The naked mole rat incisors (NMRI) exhibit excellent mechanical properties, which make it a good prototype for design and fabrication of bionic mechanical systems and materials. In this work, we characterised the chemical composition, microstructure and mechanical properties of NMRI, and further compared these properties with the laboratory rat incisors (LRI). We found out that (i) Enamel and dentin are composed of organic matter, inorganic matter and water. The ratio of Ca/P in NMRI enamel is higher than that of LRI enamel. (ii) The dentin has a porous structure. The enamel has a three-dimensional reticular structure, which is more complex, regular and denser than the lamellar structure of LRI enamel. (iii) Enamel has anisotropy. Its longitudinal nano-hardness is greater than that of transverse nano-hardness, and both of them are higher than that of LRI enamel. Their nano-hardness and elastic modulus increase with the increase in distance from enamel-dentin boundary. The nano-hardness of dentin is smaller than that of enamel. The chemical composition and microstructure are considered to be the reasons for the excellent properties of NMRI. The chemical composition and unique microstructure can provide inspiration and guide for the design of bionic machinery and materials.

Mammalian enamel: A universal tissue and diverse source of inspiration

The natural armors and weapons of the animal kingdom are serving as inspiration in the development of next-generation engineering materials. In this pursuit, seldom considered are the variations in properties across taxa that have evolved to meet their unique functional demands. Here, teeth from six different mammalian species were acquired and categorized according to their bite force quotient (BFQ), which accounts for the allometric scaling between bite force and body size. Selected chemical, microstructural, and mechanical properties of the enamel were quantified across the enamel thickness using spectroscopy and indentation techniques. Results showed that the chemical composition of enamel was significantly (P <  0.05) different between the Low and High BFQ groups, whereas the apatite crystallinity was not. The enamel of all animals exhibited a spatial gradient in mechanical properties that was consistent when evaluated using a normalized framework. Although the elastic modulus, hardness and indentation brittleness were significantly lower in the High BFQ group, the fracture resistance of enamel was significantly higher in this group, potentially reflective of bite force requirements related to diet and predation. Enamel rod decussation was present in all teeth, but there were differences in specific microstructural features. Overall, these results highlight that the diversity of tooth enamel across species should be considered in the pursuit of nature-inspired structural materials. STATEMENT OF SIGNIFICANCE: Natural weapons are serving as inspiration in the development of next-generation engineering materials. Tooth enamel is a viable candidate, but variations in the structure and properties of enamel across taxa have not been explored. Here, teeth from six different mammalian species were categorized according to their bite force quotient (BFQ), and the enamel was compared in terms of selected chemical, microstructural, and mechanical properties. We show that specific aspects of the chemical composition and properties of the Low and High BFQ groups are unique, which appears reflective of bite forces associated with diet and predation. Overall, the results highlight that the diversity of tooth enamel across species should be considered in the pursuit of nature-inspired structural materials.