Biomimetic calcium phosphate coating on Ti wires versus flat substrates: structure and mechanism of formation.

Biomimetic calcium phosphate (Ca-P) coatings improve the osteoconductivity of orthopedic implants and show promise as slow delivery systems for growth factors. This paper compares the structure and composition of biomimetic coatings on flat titanium coupons and on Ti wires/thin pins that are often used as model implants in small animal in vivo models. Ca-P coatings were grown on alkali-treated Ti substrates using a two-step deposition procedure. The coatings on wires consisted of a surface layer of octacalcium phosphate (OCP) and a layer of Ca-deficient hydroxyapatite (CDHA) underneath. The coating thickness and the proportion of CDHA decreased with increasing wire diameter. The coatings on flat coupons were the thinnest, and were comprised almost entirely of OCP. A mechanism of successive formation of the CDHA and OCP phases based on the interplay between nucleation, growth and hydrolysis of OCP crystals as a function of changing local supersaturation is proposed.