X-ray diffraction and in situ pressurization of dentine apatite nanocrystals quantifies modulus stiffening upon carbonate removal.

Abstract Bone-like materials comprise carbonated-hydroxyapatite nanocrystals (c-Ap) embedding a fibrillar collagen matrix. The mineral particles stiffen the nanocomposite by tight attachment to the protein fibrils creating a high strength and toughness material. The nanometer dimensions of c-Ap crystals make it very challenging to measure their mechanical properties. Mineral in bony tissues such as dentine contains 2∼6 wt.% carbonate with possibly different elastic properties as compared with crystalline hydroxyapatite. Here we determine strain in biogenic apatite nanocrystals by directly measuring atomic deformation in pig dentine before and after removing carbonate. Transmission electron microscopy revealed the platy 3D morphology while atom probe tomography demonstrated carbon situated inside the crystals. High-energy X-ray diffraction in combination with in situ hydrostatic pressurization quantified reversible c-Ap deformations. Crystal strains differed between annealed and ashed (decarbonated) samples, following 1 or 10 hours heating at 250°C or 550°C respectively. Measured bulk moduli-(K) and a-/c-lattice deformation ratios-(η) were used to generate synthetic K and η identifying the most likely elastic constants C33 and C13 for c-Ap. These are then used to calculate the nanoparticle elastic moduli. For ashed samples, we find an average E11=107GPa and E33 =128GPa corresponding to ∼5% and ∼17% stiffening of the a-/c-axes of the nanocrystals as compared with the biogenic nanocrystals in annealed samples. Ashed samples exhibit ∼10% lower Poisson's ratios as compared with the 0.25∼0.36 range of carbonated apatite. Carbonate in c-Ap may therefore serve for tuning local deformability within bony tissues.