The Brunauer-Emmett-Teller (BET) method was used to evaluate the specific surface area of the hydroxyapatite samples using a Gemini VII 2390t Micromeritics analyser. The analysis was based on the determination of 9 points of nitrogen adsorption isotherm in the pressure range of 0.05-0.25 p/p°, where p and p° are the equilibrium and the saturation pressures of adsorbates at the liquid nitrogen temperature, respectively. Prior to N2 adsorption, the samples were degassed at 105 °C in the N2 atmosphere for 1 h to dry and purify the samples.