Influence of ionic strength and carbonate on the Ca-P coating formation from SBFx5 solution

Biomimetic calcium-phosphate (Ca-P) coatings were applied on Ti6Al4V by using simulated body fluids concentrated by a factor 5 (SBFx5). The production of SBFx5 solution was possible by decreasing the pH of the solution to approximately 6 using CO2 gas. The subsequent release of this mildly acidic gas led to a pH rise and thus, increasing supersaturation. After immersion for 5(1/2) h a Ca-P coating on Ti6Al4V plates and a precipitate simultaneously formed at pH = 6.8. Sodium chloride (NaCl) and hydrogencarbonate (HCO3) contents were studied in relation to CO2 release and coating formation by changing their individual concentration in SBFx5 solution. On one hand, NaCl-free or low NaCl-content SBFx5 solution led to the earlier aspecific precipitation in the solution than for SBFx5 solution. In contrast, Ca-P coating was formed later and was thinner than the coating obtained in regular SBFx5 solution. High ionic strength delayed precipitation and favored Ca-P heterogeneous nucleation on Ti6Al4V. On the other hand, HCO3- content increased the pH of the solution due to its buffering capacity and influenced the release rate of dissolved CO2. Thus, HCO3- content strongly affected the supersaturation and Ca-P structure. Furthermore, HCO3- favored the attachment of Ca-P mineral on Ti6Al4V by decreasing Ca-P crystal size resulting in a better physical attachment of Ca-P coating on Ti6Al4V substrate.