In vitro transformation of OCP into carbonated HA under physiological conditions

Octacalcium phosphate (OCP) is a precursor phase during the in vivo mineralization of biological tissues. OCP can easily convert into hydroxyapatite (HA), which is the main inorganic constituent of human bone and teeth. In this study titanium alloys (Ti6Al4V) were coated with a crystalline OCP layer at 37 °C by using a supersaturated calcification solution (SCS). The in vitro bioactivity and mineralization behaviour was investigated by soaking the OCP coatings into simulated body fluid (SBF) for various time periods between 10 and 168 h. Samples were characterized by SEM-EDX, FT-IR- and XRD-analyses. In the early mineralization stages of OCP hydrolysis and partial dissolution of the crystals results in the formation of carbonated hydroxyapatite (CHA). The plate-shaped morphology of the OCP crystals remains unchanged and the thickness of the crystals increases with soaking time. After 100 h in SBF a continuous CHA layer grew upon the OCP coating proving the in vitro bioactivity of thus processed coatings. Thus, OCP coatings might enhance the osteoconductivity of orthopedic titanium based implants.