title: Tailored order: the mesocrystalline nature of sea urchin teeth.
abstract: We investigated the pattern of crystal co-orientation at different length scales, together with variations in chemical composition and nanomechanical properties in the teeth of the modern sea urchin Paracentrotus lividus with electron backscatter diffraction (EBSD), electron probe microanalysis, energy-dispersive X-ray spectroscopy and nanoindentation testing. Modern sea urchin teeth are Mg-dominated calcite composite materials. They are distinctly harder than inorganically precipitated calcite. Some parts exceed even the hardness of dolomite. The teeth show a structuring of their mechanical properties that can be correlated to variations in major element chemical composition, such that their hardness is positively correlated to their magnesium contents. Mg/Ca ratio in Paracentrotus lividus varies between 10 and 26mol.%. Nanohardness of the tooth scatters between 3.5 and >8GPa compared to values of 3.0±0.2, 7.3±0.1 and 9.2±0.9GPa measured on the (104) planes of inorganic calcite, dolomite and magnesite, respectively. High-resolution EBSD shows that major structural units and subunits of the tooth of Paracentrotus lividus are tilted to each other by ∼3-5° and 1-2°, respectively. This indicates that the tooth is not a single crystal. With EBSD we can show that the tooth of the sea urchin Paracentrotus lividus is a hierarchically assembled biological mesocrystal with a mosaic texture. In comparison to the misorientation spread of 0.5° of calcite grown from solution, misorientation in the tooth varies between 2° and 4°. Thus, the self-sharpening feature of the tooth is enabled by a close interplay of its highly evolved micro- to nanostructure, structural unit size variations with a varying degree of crystal orientation, chemical structuring of the mineral component and a gradation of incorporated organic polymers.
authors.affiliation: Department für Geo- und Umweltwissenschaften, LMU, Munich, Germany.
authors.forename: Andreas J
authors.initials: AJ
authors.lastname: Goetz
authors.affiliation: Department für Geo- und Umweltwissenschaften, LMU, Munich, Germany. Electronic address: E.Griesshaber@lrz.uni-muenchen.de.
authors.forename: E
authors.initials: E
authors.lastname: Griesshaber
authors.affiliation: Natural History Museum, London, UK.
authors.forename: R
authors.initials: R
authors.lastname: Abel
authors.affiliation: Department für Geo- und Umweltwissenschaften, LMU, Munich, Germany.
authors.forename: Th
authors.initials: T
authors.lastname: Fehr
authors.affiliation: Zoologische Staatssammlung München, Munich, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Ruthensteiner
authors.affiliation: Department für Geo- und Umweltwissenschaften, LMU, Munich, Germany.
authors.forename: W W
authors.initials: WW
authors.lastname: Schmahl
mesh_terms: D000818:Animals; D003460:Crystallization; D018360:Crystallography, X-Ray; D004583:Electrons; D021621:Imaging, Three-Dimensional; D053758:Nanoparticles / Q000737:chemistry / Q000648:ultrastructure; D012617:Sea Urchins / Q000033:anatomy & histology* / Q000737:chemistry*; D014070:Tooth / Q000737:chemistry* / Q000648:ultrastructure
keywords: 3-D crystallographic orientational coherence; Biological mesocrystal; Hierarchical hybrid composite; High-resolution EBSD; Nanohardness and E-modulus of biomaterials