Sr-substituted Hydroxyapatite Research Articles

Structural analysis of a series of strontium-substituted apatites

A series of Sr-substituted hydroxyapatites, (Sr x Ca 1− x ) 5(PO 4) 3OH, where x = 0.00, 0.25, 0.50, 0.75 and 1.00, were made by a standard wet chemical route and investigated using X-ray diffraction (XRD), Rietveld refinement and Raman spectroscopy. We report apatites manufactured by two synthesis routes under 90 °C, and only the fully Sr-substituted sample had a small amount of an impurity phase, which is believed to be strontium pyrophosphate. Lattice parameters ( a and c), unit cell volume and density were shown to increase linearly with strontium addition and were consistent with the addition of a slightly larger and heavier ion (Sr) in place of Ca. XRD Lorentzian peak widths increased to a maximum at x = 0.50, then decreased with increasing Sr content. This indicated an increase in crystallite size when moving away from the x = 0.50 composition ( d ≈ 9.4 nm). There was a slight preference for strontium to enter the Ca(II) site in the mixed apatites (6 to 12% depending on composition). The position of the Raman band attributed to v 1 PO 4 3 - at around 963 cm −1 in hydroxyapatite decreased linearly to 949 cm −1 at full Sr-substitution. The full width at half maximum of this peak also correlated well and increased linearly with increasing crystallite size calculated from XRD.

Sr-substituted hydroxyapatites for osteoporotic bone replacement

Porous apatites, which during resorption can release in situ Sr ions, were prepared to associate an anti-osteoporotic action with the peculiar features of the inorganic phase constituting the bone. Sr-substituted hydroxyapatite (SrHA) powder was directly synthesized using the classical neutralization route, but including Sr ions, and characterized. The higher solubility of SrHA granules of 400–600 μm size, potentially usable as a bone filler, was assessed compared with that of analogous stoichiometric HA granules. The Sr released in synthetic body fluid became constant after 1 week. The Ca release is improved for SrHA compared with stoichiometric HA, due to the higher solubility of the first material. Porous scaffolds with micro–macro interconnected porosity, which mimic the morphology of the spongy bone, were prepared by the impregnation of cellulose sponges with suspensions of the powder and a specific sintering process. A compressive strength of 4.52 ± 1.40 MPa was obtained for SrHA scaffolds characterized with 45 vol.% of porosity. Promising biomedical applications, such as resorbable bone filler or bone substitute releasing in situ Sr ions for a prolonged time, can be hypothesized for the SrHA materials when pathologies related with Sr deficiency are present.

Crystal Structure of Sr-Substituted Hydroxyapatite Synthesized by Hydrothermal Method

Single crystals of Sr-substituted hydroxyapatite, (Ca9.42Sr0.18 H0.8(PO4)6(OH)2, up to 0.2 × 0.2 × 5 mm3 maximum size were grown hydrothermally from calcium hydrogenphosphate (CaHPO4) and strontium hydrogenphosphate (SrHPO4) in a long tube-type autoclave using a temperature gradient technique. The single crystals, containing 1.92 mole% Sr to (Ca + Sr) and having a (Ca + Sr)/P atomic ratio of 1.60, had a hexagonal-prismatic shape with density of 3.207 g/cm3. The crystal had a hexagonal system, space group P63/m with a = 9.3920(7) Å, c = 6.890(1) Å, and Z = 1. A hexagonal-prismatic crystal, 165 × 165 × 231 μ3 in size, was used for crystal structure refinement. The atomic parameters in P63/m were refined to R = 5.9% and RwÅ = 6.3% with 2018 independent reflections. The preferential substitution site of Sr for Ca was determined as the Ca(1) site.

The Mechanical Properties and Solubility of Strontium-Substituted Hydroxyapatite

The mechanical properties and solubility of sintered Sr-substituted hydroxyapatites were examined in order to investigate the influences of Sr incorporation into hydroxyapatite in bones and teeth on them. Hydroxyapatite (HAp), Sr-substituted hydroxyapatite (Ca.SrAp), and strontium-hydroxyapatite (SrAp) were synthesized and sintered. The bending strength and Young's modulus were measured. Also, solubility in isotonic NaCl solution was examined. The Young's modulus of Sr-substituted hydroxyapatite (Sr wt% = 1.0) and hydroxyapatite surface treated with Sr ion were greater than that of hydroxyapatite. The bending strengths of Sr-substituted hydroxyapatites (Sr/Ca = 4/6 and less) were reduced slightly compared to that of hydroxyapatite. The bending strength of hydroxyapatite surface treated with Sr ion was increased compared to that of hydroxyapatite. The solubility in isotonic NaCl solution at 37 degrees C was increased with an increase in Sr content. The effects of Sr on the mechanical properties and solubility of bones and/or teeth are discussed.