Biomimetic apatite coatings—Carbonate substitution and preferred growth orientation

Abstract

Biomimetic apatite coatings were obtained by soaking chemically treated titanium in SBF with different HCO 3 − concentration. XRD, FTIR and Raman analyses were used to characterize phase composition and degree of carbonate substitution. The microstructure, elemental composition and preferred alignment of biomimetically precipitated crystallites were characterized by cross-sectional TEM analyses. According to XRD, the phase composition of precipitated coatings on chemically pre-treated titanium after exposure to SBF was identified as hydroxy carbonated apatite (HCA). A preferred c-axis orientation of the deposited crystals can be supposed due to the high relative peak intensities of the (0 0 2) diffraction line at 2 θ = 26° compared to the 100% intensity peak of the (2 1 1) plane at 2 θ = 32°. The crystallite size in direction of the c-axis of HCA decreased from 26 nm in SBF5 with a HCO 3 − concentration of 5 mmol/l to 19 nm in SBF27 with a HCO 3 − concentration of 27 mmol/l. Cross-sectional TEM analyses revealed that all distances correspond exactly to the hexagonal structure of hydroxyapatite. The HCO 3 − content in SBF also influences the composition of precipitated calcium phosphates. Biomimetic apatites were shown to have a general formula of Ca 10− x− y Mg y (HPO 4) x− z (CO 3) z (PO 4) 6− x (OH) 2− x− w (CO 3) w/2 . According to FTIR and Raman analyses, it can be supposed that as long as the HCO 3 − concentration in the testing solutions is below 20 mmol/l, only B-type HCA (0 < z < 1; w = 0 ) precipitates. At higher HCO 3 − concentration, it can be assumed that AB-type HCA ( z = 1; 0 < w < 1 ) is formed.