Water-Mediated Collagen and Mineral Nanoparticle Interactions Guide Functional Deformation of Human Tooth Dentin

Dentin in teeth is a bone-like nanocomposite built of carbonated hydroxyapatite (cHAP) mineral particles, protein, and water. It does not remodel nor heal and is excellently adapted for decades of mechanical function, due to the interplay between its constituents. Using samples of human origin, we combine heat treatments with synchrotron X-ray diffraction, second-harmonic generation microscopy, Raman spectroscopy, and phase contrast-enhanced nanotomography to study the water-assisted functional coupling of the biocomposite components. Across roots we find a gradual reduction in the c-lattice parameter of the cHAP nanocrystals, from 6.894 Å externally down to 6.885 Å on the inside. Thus, the tissue formed at later stages of tooth development around the pulp contains crystals with smaller unit cells. In all regions, a compressive strain of ∼0.3% is observed upon drying by mild heating (125 °C). Dehydration also results in a substantial increase in the averaged microstrain fluctuations in the mineral nanoparticles. The mineral crystallite platelet lengths fall off from ∼36 nm externally to ∼26 nm closer to the pulp. Our results suggest that both morphology and tight mineral–collagen coupling allow mineral nanoparticles in dentin to sustain rather large stresses of 300 MPa, far exceeding mastication loads.