Correlation of composition and structure of shark teeth

In contrast to mammalian teeth with the biomineral phase hydroxyapatite, the shark teeth contain harder mineral phase fluoroapatite with partial substitutions of phosphate by carbonate and of fluoride by hydroxide [1]. Their excellent mechanical properties are due to a special hierarchical structure of the constituting fluoroapatite crystals and organic matrix [2]. The two main structural elements of teeth, i.e. hard and mineral-rich enameloid on the outside and softer and less mineralized dentin on the inside, were structurally, chemically and mechanically characterized [3]. The teeth of two different shark species mako shark (Isurus oxyrinchus) and tiger shark (Galeocerdo cuvier) were investigated and their hierarchical structure by high-resolution scanning electron microscopy presented (Fig.1). X-ray diffraction showed that the inorganic matrix of both enameloid and dentin consisted of fluoroapatite, with a high crystalline phase in enameloid and nanocrystalline phase in dentin. FTIR-spectra of the shark teeth showed the characteristic bands of biological apatite. It was found by thermogravimetry that dentin had a higher content of water, organic matrix and carbonate than enameloid. To investigate the mechanical properties of the teeth in longitudinal and cross sections, nanoindentation and Vickers microhardness were carried out. Both methods gave comparable results: the enameloid of both shark teeth was approximately six times harder than the dentin with an isotropic hardness (longitudinal or cross section).