Formation of Strontium Hydroxyapatite by Homogeneous Precipitation Method

Abstract

Strontium hydroxyapatite (Sr-HAp) was prepared by homogeneous precipitation method using the mixed solutions of SrCl₂, NaH₂PO₄ and urea (Sr/P mole ratio = 1.67).Sr₅ (OH) (PO₄)₃ (Sr-HAp), α-SrHPO₄, β-SrHPO₄, Sr₃ (PO₄)₂, and SrCO₃ were obtained at 97 (± 1) °C depending on the concentrations of Sr²⁺ and urea and heating time. α-SrHPO₄ was obtained as the initial product in any synthesis condition. SrCO₃ once formed remained in the products even after aging in the solution. It is prefered, therefore, that the synthesis of Sr-HAp is carried out with low concentration of urea to prevent the formation of SrCO₃.
The following two processes were revealed for the formation of Sr-HAp: (i) Sr-HAp is formed via Sri (PO₄) ₂ from α-SrHPO₄ and (ii) Sr-HAp is formed via β-SrHPO₄ from α-SrHPO₄. These process es were governed by the pH change of the solution or the concentrations of Sr²⁺ and urea. Sr-HAp could be obtained as single phase by prolonged heating with low concentration of urea.
It was unavoidable to incorporate impurities such as Cl^- or CO₃^2- into crystal of Sr-HAp by homogeneous precipitation method utilizing the hydrolysis of urea. The amounts of these impurities, however, could be minimized by the selection of the starting materials and/or the control of the concentrations of Sr²⁺ and urea.