Mechanism of apatite formation on bioactive tantalum metal in simulated body fluid

Mechanism of the apatite formation on bioactive tantalum metal prepared by NaOH treatment was investigated by soaking the metal in a simulated body fluid and surveying the surface structual change using transmission electron microscope attached with energy dispersive X-ray spectrometry. The NaOH-treated tantalum metal was covered with nano-particles of crystalline sodium tantalate. Immediately after immersion in simulated body fluid, the sodium tantalate on the metal released sodium ions to form Ta-OH groups, which combined with calcium ions in the fluid to form an amorphous calcium tantalate. The calcium tantalate, after fairly a long immersing period, combined with the phosphate ions as well as the calcium ions in the fluid to form an amorphous calcium phosphate with a low Ca/P ratio. The amorphous calcium phosphate during immersion increases its Ca/P ratio subsequently to convert into crystalline apatite. These results indicate that the Ta-OHgroups on the metal induce the apatite nucleation not directly, but via formation of an amorphous calcium tantalate and calcium phosphate.