The effect of pH on the structural evolution of accelerated biomimetic apatite

The classic biomimetic apatite coating process can be accelerated by first immersing substrates into concentrated simulated body fluid, 5× SBF (SBF1), at 37°C, to form an initial coating of precursor apatite spheres, and subsequently transferring to a second 5× SBF (SBF2) solution which is devoid of crystal growth inhibitors to promote phase transformation of SBF1-derived precursor apatite spheres into final crystalline apatite plates. Since SBF1 governs the formation kinetics and composition of the initial precursor spheres, we hypothesized that the pH of the SBF1 solution will also influence the final structure of the SBF2-derived crystalline apatite. To test this hypothesis, polystyrene substrates were immersed into SBF1 with different pH (5.8 or 6.5), and then immersed into the identical SBF2 (pH=6.0). The resultant apatites exhibited similar 2θ XRD peaks; FTIR spectra in terms of hydroxyl, phosphate and carbonate groups; and Ca/P atomic ratio (1.42 for SBF15.8 apatite; 1.48 for SBF16.5 apatite). SEM, TEM and electron diffraction show that while SBF16.5 (pH 6.5) precursor spheres transform into larger, single crystals plates, SBF15.8 (pH 5.8) precursor spheres developed minute, polycrystalline plate-like structures over predominantly spherical precursor substrate.