Incorporation of active ions into calcium phosphate coatings, their release behavior and mechanism

The dissolution and release of active ions from ion-doped apatites is currently gaining interest due to indications of a beneficial biologic response. The release of ions from apatite coatings is important because it influences the biological effect of these types of materials. In this study the ion release from three different ion-doped apatite coatings (iHA coatings), SrCaP, SiHA and FHA, has been studied. The coatings were prepared by a mineralization method based on immersion in modified PBS solutions containing additions of Sr, Si or F. The kinetics of ion release from the iHA coatings were studied in two different media with and without calcium and phosphate ions (phosphate buffer saline solution (PBS) and Tris-HCl). The amount of cumulative release of Sr, Si and F ions from the iHA coatings was SrCaP>SiHA>FHA in Tris-HCl, which could be also be related to the solubility for these iHA coatings. According to analysis using the Korsmeyer-Peppas model, the release of ions from the coatings was in most cases controlled by a combination of Fickian diffusion and dissolution of the coatings. The morphologies of the iHA coatings were not markedly changed after immersion in Tris-HCl. In the phosphate buffer solution, there was a concurrent redeposition of new apatite crystals on the surface of all of the iHA coatings, which means there were both a dissolution and a remineralization process acting, ultimately controlling the ion release rate.