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Ceramics In Vitro Mineralisation Protocols: a Supersaturation Problem

Two ceramics: a 25CaO 2.5P2O5 72.5SiO2 mol% sol-gel glass (S72.5P2.5) and a commercial hydroxyapatite (HAP), both bioactive in Kokubo ́s Simulated Body Fluid (SBF), were studied according to two recently proposed in vitro protocols. The first one (SBF-dynamic) avoids the variations of the ionic concentration in the assay solution with a continuous renewal of SBF. The second protocol, uses a carbonated simulated inorganic plasma (CSIP), prepared by substituting the tris(hydroxymethyl aminomethane)/HCl buffer (TRIS) of SBF for a CO2/HCO 3 buffer, whereby a physiological concentration of HCO 3 in solution is reached (24 to 27 mM) whereas the pH is maintained between 7.3 and 7.4. After the in vitro immersions, S72.5P2.5 exhibited a hydroxycarbonate apatite (HCA) surface layer in both cases, but some differences were found depending on the in vitro protocol. In CSIP, a higher HCA crystallization rate was initially observed (1 day vs 3 days in SBF-dynamic). Nevertheless, after 7 days in SBF-dynamic, the new layer was thicker (6 μm vs 1 μm) and presented a Ca/P molar ratio lower than in CSIP (1.7 vs 2.1). In the case of HAP ceramic, a HCA layer was observed only in CSIP. In SBF-dynamic the local supersaturation degree of the solution and/or the system dynamics were not appropriated for HCA layer formation.