Silica- and zirconia-hybridized amorphous calcium phosphate: effect on transformation to hydroxyapatite

The goal of this study was to determine the effect that silica and zirconia have on the stability of bioactive amorphous calcium phosphate (ACP) mineral, i.e., in retarding its transformation to hydroxyapatite (HAP). The glass-forming agents, tetraethoxysilane and zirconyl chloride, were introduced individually during the low-temperature preparation of ACP. These hybrid ACPs (Si-ACP and Zr-ACP, respectively) as well as the control, unhybridized ACP (u-ACP), were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, specific surface area measurements, and chemical analysis (Ca/PO(4) ratio of the solids) before being dispersed in one of the following four test solutions: N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered (pH = 7.40) saline solutions with 0 microg/g fluoride (test solution A1), 1 microg/g fluoride (test solution A2), and 10 microg/g fluoride (test solution A3), or a lactic acid-containing solution (pH = 5.10, adjusted with NaOH; test solution B). Aliquots were taken at predetermined time intervals for solution Ca and PO(4) analysis. Solids isolated after 30 and 90 min exposure to solution B as well as the final dissolution/transformation products from all four solution experiments were analyzed by Fourier transform infrared spectroscopy and X-ray diffraction. Regardless of the type of experimental solution used, slower conversion to HAP was observed with the hybrid ACPs compared with u-ACP. The retarding effect of the Si or Zr species in the hybridized ACPs is probably due to these ions specifically blocking, by adsorption, potential sites for HAP nucleation and growth. The stability of ACP toward HAP conversion increased in the following order: u-ACP < Si-ACP < Zr-ACP. Hybrid ACP fillers, especially Zr-ACP, could be utilized in applications in which it is desired to enhance performance of composites, sealants, and/or adhesives in preventing demineralization or actively promoting remineralization.