Biogenic hydroxyapatite powders: Effects of source and processing methodologies on physicochemical properties and bioactive response

Highlights: • CaO formation was observed without the destabilization of HAp in other CP's at 1100 °C. • The MgO and CaO phases formation strongly depend on the temperature. • Carbonate-rich HAp's growth analysis from the in-vitro test was performed. • Calcite phase appears after 3 soaking days in BHAp samples obtained with B-method. • Mechanism of calcite phase formation after the in-vitro assessment is proposed. This study compares and analyzes the biogenic hydroxyapatite (BHAp) powders obtained from bovine and porcine bones using two methodologies to determine the influence of the obtention process on BHAp properties. Despite the differences between the processing methods, the results obtained suggest that the thermal-treatment has a meaningful influence on physicochemical properties. Since the thermal-treatments performed (1100 °C for 2 h and 800 °C for 24 h for A- and B-methods, respectively) resulted in a hydroxyapatite crystalline phase formation as the main phase with the presence of magnesium oxide (MgO) as a secondary phase in both methods and calcium oxide (CaO) in samples obtained by A-method. The B-method samples also present higher carbonate group content (CO₃²⁻) and A- and B-type substitutions compared to the A-method. We found that the carbonate groups' changes can be mainly attributed to the thermal-treatment temperature. Subsequently, the BHAp samples were immersed in Hank Solution for 3, 7, 14, 21, and 28 days to study the bioactive response. We found a bone-like apatite layer formation on the surface as a function of the soaking time in all sets of samples, as was expected. However, we also found a calcite layer grown on the surface of the samples prepared by the B-method, which is associated with the carbonate ions release from these samples to the solution. Finally, a formation mechanism of calcite in samples obtained through the B-method during the exposure to Hank's solution is also proposed."