Mineralisation of two calcium phosphate ceramics in biological model fluids

Two ceramic phosphates, commercial hydroxyapatite and synthesised biphasic hydroxyapatite–14 wt.% α-tricalcium phosphate, were incubated in Hank's balanced salt solution (HBSS) and Kokubo's simulated body fluid (SBF) at 37 °C and the respective surface modifications investigated. The static contact angles (SCA) of water and diiodomethane on the surfaces of both incubated and non-incubated materials were measured as a function of time, showing that surface modifications were different for each type of phosphate and depended on the incubating solution. Dynamic contact angle (DCA) hysteresis provided information on the characteristics of possible deposits. These studies were complemented with SEM/EDS, FTIR and XRD analysis of the surfaces. Compositional changes in the incubating solutions with time were monitored by ICP spectroscopy. The main conclusions are (1) the presence of α-tricalcium phosphate in the synthesised biphasic hydroxyapatite enhances the deposition of calcium phosphate, (2) the precipitation of calcium phosphate is favoured by the pH increase of the incubating solution; consequently, buffer Tris inhibits the deposition because it avoids the pH increase and forms soluble complexes with Ca2+ ions. Another conclusion is that wettability measurements, especially DCA, provide a most sensitive method to detect surface transformations leading to mineralisation.