Calcium phosphates and glass composite coatings on zirconia for enhanced biocompatibility

Calcium phosphates (CaP) and phosphate-based glass (P-glass, xCaO–(0.55−x)Na2O–0.45P2O5 composition) composite coatings were obtained on a strong ZrO2 to improve biocompatibility, the mechanical strength and biological activity. Hydroxyapatite (HA) and P-glass mixed powder slurries were coated on the ZrO2 substrate, and subsequently heat-treated to obtain CaP- and P-glass composite coatings. The effects of glass composition (x=0.3, 0.4, 0.5 mol), mixing ratio of glass to HA (30%, 40%, 50% wt/wt), and heat treatment temperature (800°C, 900°C, 1000°C) on the coating properties were investigated. After heat treatment, additional calcium phosphates, i.e., dicalcium phosphate (DCP) and tricalcium phosphate (TCP), were crystallized, resulting in the formation of triphasic calcium phosphates (HA–TCP–DCP) surrounded by a glass phase. The relative amounts of the crystalline phases varied with coating variables. The higher heat treatment temperature and glass amount, and the lower CaO content in the glass composition rendered the composite coatings to retain the higher amounts of TCP and DCP while the initial HA decreased. These appearance of additional crystalline phases and reduction of HA amount were attributed to the combined effects, i.e., the melting-crystallization of P-glass and the reaction between glass liquid phase and HA powder during thermal treatment. As a result of the glass phase in the composite coatings, their microstructures became much denser when compared to the pure HA coating. In particular, a completely dense structure was obtained at coating conditions with large amount of glass addition (50 wt%) at the glass composition of lower CaO content (0.3 mol CaO), and the following heat treatment above 800°C for 2 h. As a result, the adhesion strengths of the composite coating layers were significantly improved when compared to the pure HA coating. The highest strength of the composite coating was 40 MPa, an improvement of 80% with respect to the pure HA coating. The composite coatings showed much higher dissolution rates than the pure HA coating due to the newly formed crystallines (TCP and DCP) and the remaining glass phase. The osteoblast-like cells grew and spread actively on the composite coating samples. The proliferation numbers and alkaline phosphate (ALP) activities of the cells on the composite coatings were improved by 30–40% when compared to Thermanox control and ZrO2 substrate, and were comparable to the pure HA coating. These findings suggested that the CaP and P-glass composites are potentially useful for hard tissue coating system, due to their morphological and mechanical integrity, enhanced bioactivity, and favorable responses to the osteoblast-like cells.