The teeth of chitons resist cracking because of the highly ordered, submicroscopic architecture that features a partnership between hard mineral crystals and fibers.

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A remarkable characteristic of nature's hard materials is their ability to resist cracking under stresses and strains. Generally speaking, the secret is their highly ordered, submicroscopic architecture that features a partnership between hard mineral crystals and flexible protein molecules. The chiton tooth's wear and crack resistance is interesting because it's derived from an interactive foursome of carbohydrate, protein, metal ions, and mineral crystal. Magnetite mineral crystals impart the tooth with wear resistance while the carbohydrate, protein, and metal ions organize together to form long, thin fibers imbedded in the mineral crystal; the fibers impart the tooth with crack resistance. The interior of these fibers is an ordered carbohydrate (chitin) scaffold to which flexible proteins are tethered by amorphous chitin strings. The protein molecules, decorating the exterior of the chitin framework, are themselves adorned with metal ions (including sodium and magnesium), which are thought to foster a healthy connection between the fibers and the surrounding magnetite crystals.

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"Biological organisms possess an unparalleled ability to control the structure and properties of mineralized tissues. In many biominerals, an organic matrix interacts with the mineral as it forms, controls its morphology and polymorph, and is occluded during mineralization. The remarkable functional properties of the resulting composites—such as outstanding fracture toughness and wear resistance—can be attributed to buried organic–inorganic interfaces at multiple hierarchical levels. Consequently, our understanding of biological control over mineral growth and the resulting functional properties depends critically on characterizing buried organic–inorganic interfaces...In particular, we investigate the nano-crystalline magnetite cap of the chiton tooth, which is a classical model system for the study of matrix-mediated mineralization. Chitons (also known as sea cradles) are marine molluscs of the class Polyplacophora. Chiton teeth are arranged in rows along the radula (rasping tongue)...The outstanding fracture toughness and wear resistance of the tooth results from the organic–inorganic interfaces over multiple levels of hierarchy, which deflect and arrest cracks. Remarkably, the radula acts as a conveyor-belt on which an organic matrix scaffold composed of semi-crystalline a-chitin (poly-b-1,4-N-acetylglucosamine) and protein is first deposited. As the maturing tooth progresses along the radula (about one row per day), the scaffold is remodelled and filled in with mineral, which occludes the organic matrix in the process. All stages of tooth development are thus present in one animal." (Gordon et al. 2011:194)

"The hardness is notably about 3 times higher than that of enamel and nacre, which exhibit indentation...making this material exceptionally well suited for the continuous scraping activity of the radular teeth. Mechanical mapping of cross-sections through these two regions of the teeth reveals a distinct gradient in mechanical properties with the modulus of the leading edge of the tooth ca. 15% higher than that on the trailing edge. This design strategy results in an uneven wear pattern along the scrapping edge of the tooth and establishes a self-sharpening condition." (Weaver et al. 2010:45)

"This illustrates the important point that structural integrity can be attained only in the presence of the organic matrix that facilitates the anisotropic organization of the magnetite crystallites, binds them into a composite structure, and plays a critical role in crack blunting and deflection at interfaces...Small-scale sliding at the mineral/organic interfaces during external loading is a plausible cause to explain the larger compliance of radular tooth magnetite [compared to a geological magnetite mineral standard]." (Weaver et al. 2010:47)

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Journal article
Analysis of an ultra hard magnetic biomineral in chiton radular teethMaterials TodayFebruary 12, 2010
James C. Weaver, Qianqian Wang, Ali Miserez, Anthony Tantuccio, Ryan Stromberg, Krassimir N. Bozhilov, Peter Maxwell, Richard Nay, Shinobu T. Heier, Elaine DiMasi, David Kisailus

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Gordon LM; Joester D

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