Surface instability of calcium phosphate ceramics in tissue culture medium and the effect on adhesion and growth of anchorage-dependent animal cells.

The surface of biocompatible ceramics made of synthesized hydroxyapatite (HAP) and beta-tricalcium phosphate (TCP) was found to be extremely active in tissue culture medium. Using mixed ceramics of HAP and TCP which had been prepared with different Ca/P molar ratios adjusted in stepwise fashion to values of 1.50, 1.55, 1.60, 1.64 and 1.67, the characteristics of the surface were investigated. The time-dependent variation of zeta potential of the TCP-HAP ceramics immersed in distilled water and in culture medium with and without addition of fetal bovine serum showed that the surface was unstable with significant changes in the charge being observed. Dry TCP powder had a zeta potential of -19 mV, which shifted to -7 mV after soaking in water and to -26 mV in culture medium. In contrast, HAP had a zeta potential of -11 mV in a dry state, -9 mV in water and -29 mV in culture medium. Concentrations of calcium and phosphate dissolved in distilled water showed the solubility was higher for TCP than for HAP. In comparison, it was found that dissolved calcium and phosphate in the medium were removed from the solution by deposition on immersed TCP-HAP ceramics. These results suggested that the stability of the surface was closely related to both reactions of association and dissociation of calcium and phosphate in tissue culture medium. The zeta potential analysis also suggested that Ca-deficient HAP, which has a similar crystal structure to HAP with a Ca/P ratio less than 1.67, was generated by degradation and reforming of the surface layer. The most stable structure which was the most suitable for adhesion of L-929 cells was obtained by the mixture of 20% TCP and 80% HAP ceramics. In conclusion, the stability of the surface structure was considered to be the dominant factor for the enhancement of the adhesiveness of cells on TCP-HAP ceramics.