Selective preparation and characterization of nano-hydroxyapatite/collagen coatings with three-dimensional network structure

Nano hydroxyapatite/collagen (HA/COL) coatings were prepared on the surface of carbon/carbon (C/C) composites by a two-step process, i.e. electrochemically assisted co-deposition of calcium phosphate/collagen (CaP/COL) coatings and subsequent soaking CaP/COL coated C/C composites in Ca(OH)₂ aqueous solution. Both CaP/COL and HA/COL coatings on the C/C composites exhibit three-dimensional network structure consisting of collagen fibers and uniform CaP covered on the surface of collagen fibers. CaP in the composite coatings was deposited into different forms depending on the concentration, Ca/P molar ratio and pH value of initial electrolyte. The concentration and pH value of initial electrolyte have little impact on the shape of CaP though they significantly influence the size of CaP. Higher Ca/P molar ratio of initial electrolyte leads to spherical amorphous calcium phosphate (ACP)/COL coatings. Plate-like octacalcium phosphate (OCP)/COL coatings were prone to form with low Ca/P molar ratios in the electrolyte. After Ca(OH)₂-treatment as the second processing step, OCP/COL coatings on C/C were converted to HA/COL coatings. Their structure and compositions are similar to those of human bone tissue. SBF immersion tests indicate that HA/COL coated C/C composites have better in vitro bioactivity. In vitro cellular biocompatibility tests indicate OCP/COL and HA/COL coated C/C composites have almost the same biocompatibility.