Time Transient of Calcium and Phosphate Ion Adsorption by Rutile Crystal Facets in Hanks' Solution Characterized by...

For the elucidation of the mechanism of calcium phosphate formation on commercially pure titanium (CP Ti) in the human body, rutile TiO₂ single crystal plates with (001), (110), and (111) facets, namely, TiO₂(001), TiO₂(110), and TiO₂(111), and polycrystalline plates (TiO₂(poly)) were immersed in a simulated body fluid, Hanks' solution (Hanks), for 10⁰-10⁵ s, and the adsorption of calcium and phosphate ions was precisely characterized employing X-ray photoelectron spectroscopy (XPS). Previously published CP Ti data were used for comparison. Prior to immersion in Hanks, oxygen content was more than twice as high as that of titanium due to the existence of hydroxyl groups and water on the oxides. After immersion in Hanks, the composition and chemical state of the TiO₂ substrates remained unchanged. Among the electrolytes contained in Hanks, only calcium and phosphate ions were adsorbed by and incorporated onto TiO₂ surfaces. Adsorption of calcium ions onto rutile did not exhibit any systematic increase of calcium with immersion time except TiO₂(poly). Adsorption of phosphate ions was initially constant, followed by an increase with the logarithm of immersion time. The adsorption rate of phosphate ions decreased in the following order: TiO₂(001), TiO₂(poly), TiO₂(111), CP Ti, and TiO₂(110). The coordination number and band gap of each crystal facet of rutile is important for the adsorption and incorporation of phosphate ions. Regular calcium phosphate formation on CP Ti is possibly enabled by the surface oxide film, which consists chiefly of amorphous TiO₂. However, calcium phosphate formation kinetics on CP Ti differed from those on the TiO₂ crystalline phase. These findings may further the understanding of CP Ti hard tissue compatibility.