Orientation Of Hydroxyapatite Crystals Research Articles

Bending Fibers of Hydroxyapatite for Ordered Parallel Architecture in Bovine Tooth Enamel

The nanometer-scale bending structure in the enamel–dentin interfacial region of bovine teeth was clarified using detailed electron micrographs. We found that enamel prisms of width ∼5–10 μm in the enamel are regarded as a bundle of nanometer-scale hydroxyapatite (HA) fibers of width ∼20–30 nm that are connected to the enamel–dentin interface. The radial arrangements of HA fibers elongated in the c-axis were formed on the dentin surface at the boundary. The orientation adjustment of the HA fibers was observed through stepwise bending with a gradual change of the c direction in the ordered parallel array of the enamel prisms. Our observation suggests that the crystallographic ordering in the enamel layer originates from the growth-induced orientation of HA crystals in the interfacial region.

Orientation of Hydroxyapatite Rods Formed by Hydrothermal Treatment of Calcium Carbonate

Morphological control on hydroxyapatite (HAp) crystals is one of attractive researches to produce novel bioactive materials for repairing bone defects. Hydrothermal processing has been applied to fabricate well-grown crystals of HAp, through a reaction from appropriate calcium and phosphate sources. We recently found that oriented structure was observed when single crystal of calcium carbonate was used as a starting material for the hydrothermal processing. However, the detailed process on the orientation of HAp crystals has been not clarified yet. In the present study, we investigated the effects of the crystalline planes of calcite on HAp formation through the hydrothermal condition. Calcite single crystals with either of {100} or (111) plane was used as a starting material to examine the difference of the reactivity in a phosphate solution. After the hydrothermal treatment at 160 °C for 24 h, the surfaces and cross-sections of starting materials were characterized. Rod-shaped HAp crystals were oriented on the surface of the starting material with {100} plane, whereas small HAp crystals formed on the starting material with (111) plane followed by growth to same direction that were observed on the sample with the {100} plane. The difference in the morphology of the formed HAp was caused by the reactivity of each crystal plane.

Understanding the influence of structural hierarchy and its coupling with chemical environment on the strength of idealized tropocollagen–hydroxyapatite biomaterials

Hard biomaterials such as bone, dentin, and nacre have primarily an organic phase (e.g. tropocollagen (TC)) and a mineral phase (e.g. hydroxyapatite (HAP) or aragonite) arranged in a staggered arrangement at the nanoscopic length scale. Interfacial interactions between the organic phase and the mineral phase as well as the structural effects arising due to the staggered arrangement significantly affect the strength of such biomaterials. The effect of such factors is intricately intertwined with the chemical environment of such materials. In the present investigation, an idealized TC–HAP composite system under tensile loading is analyzed using explicit three-dimensional (3-D) molecular dynamics (MD) simulations to develop an understanding of these factors. The material system is analyzed in three different environments: (1) in the absence of water molecules (non-hydrated), (2) in the presence of water molecules (hydrated), and (3) in the presence of water molecules with calcium ions (ionized water). The analyses focus on understanding the correlations among factors such as the structural arrangement, the peak stress during deformation, Young's modulus, the peak interfacial strength, and the length scale of the localization of peak stress during deformation. Analyses show that maximizing the contact area between the TC and HAP phases results in higher interfacial strength as well as higher fracture strength. Due to the staggered arrangement, the orientation of HAP crystals has insignificant effect on the biomaterial strength. Analyses based on strength scaling as a function of structural hierarchy level reveal that while peak strength follows a multiscaling relation, the fracture strength does not. The peak strain for failure was found to be independent of the changes in levels of structural hierarchy. Overall, the analyses, being limited in size due to the computational time constraint, point out important correlations between the mechanical strength and chemically influenced structural hierarchy of biomaterials.