Nanoscale Morphology of Apatite Precipitated onto Synthetic Hydroxyapatite from Simulated Body Fluid

Abstract

Dense, polycrystalline, synthetic hydroxyapatite (HA) was incubated for 36 days in modified simulated body fluid (SBF) with increased HCO3 - and reduced Cl- ion concentrations (27 and 120 mM, respectively) closer to actual blood plasma than typical SBF. The resulting precipitated apatite layer was characterized by X-ray photoelectron spectroscopy (XPS) and contact angle measurements and found to be nonstoichiometric, calcium deficient (Ca/P~1.06), non-carbonate containing, and of intermediate hydrophilicity (advancing contact angle, qa=76.5±1.3°). The nanoscale surface topography of the SBF-incubated HA sample was imaged by tapping mode atomic force microscopy (TMAFM), observed to be ≤100 nm in thickness, and composed of three distinct morphologies. These topographically distinct regions were localized within individual grains and facets of the initial HA surface and included: hemispherical, globular structures (maximum lateral dimension, d=44.7±12.7 nm, peak-tovalley height, h=3.6±2.7 nm); elongated, needle-like structures (minimum lateral dimension, w=31.0±8.5 nm, d=104.4±31.1 nm, h=5.0±3.2 nm), and regions of larger, irregularly shaped structures that were relatively smooth (d=504.9±219.1 nm, h=104.0±51.7 nm).