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The normal intersurface forces between nanosized probe tips functionalized with COO(-)-terminated alkanethiol self-assembling monolayers and dense, polycrystalline silicon-substituted synthetic hydroxyapatite (SiHA) and phase pure hydroxyapatite (HA) were measured via a nanomechanical technique called chemically specific high-resolution force spectroscopy. A significantly larger van der Waals interaction was observed for the SiHA compared to HA; Hamaker constants (A) were found to be A(SiHA) = 35 +/- 27 zJ and A(HA) = 13 +/- 12 zJ. Using the Derjaguin-Landau-Verwey-Overbeek approximation, which assumes linear additivity of the electrostatic double layer and van der Waals components, and the nonlinear Poisson-Boltzmann surface charge model for electrostatic double-layer forces, the surface charge per unit area, sigma (C/m(2)), was calculated as a function of position for specific nanosized areas within individual grains. SiHA was observed to be more negatively charged than HA with sigma(SiHA) = -0.024 +/- 0.013 C/m(2), two times greater than sigma(HA) = -0.011 +/- 0.006 C/m(2). Additionally, SiHA was found to have increased surface adhesion (0.7 +/- 0.3 nN) compared to HA (0.5 +/- 0.3 nN). The characterization of the nanoscale variations in surface forces of SiHA and HA will enable an improved understanding of the initial stages of bone-biomaterial bonding.

Abstract. Dense, polycrystalline, synthetic hydroxyapatite (HA) was incubated for 36 days in modified simulated body fluid (SBF) with increased HCO 3 -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), noncarbonate containing, and of intermediate hydrophilicity (advancing contact angle, θ a =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-to-valley 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). IntroductionSynthetic hydroxyapatite (HA, Ca 5 (PO 4 ) 3 OH), HA-based biomaterials, and HA coatings are used extensively for hard tissue applications due to their bioactivity [1]. Upon implantation in vivo [2] or incubation in vitro in simulated body fluid (SBF) [3], an apatite layer forms on the surface which is considered essential for the nucleation of biological apatite, the promotion of protein adsorption and cell adhesion, and ultimately, the creation of a strong bond with the surrounding tissue [4]. The objective of this study was to directly visualize and quantify the nanoscale topography of apatite precipitated in vitro from SBF onto dense, polycrystalline, phase pure HA using tapping mode atomic force microscopy (TM AFM) imaging, which enables spatial resolutions of <1 nm. New information is presented on the morphological heterogeneity of the apatite layer, as well as the nature of the transition boundarie s between topographically different regions. Such a methodology has great potential to contribute insights into the physiochemical mechanisms and temporal evolution of HA interfacial apatite layers and molecular origins of their bone bonding capability.

Dense, polycrystalline, synthetic hydroxyapatite (HA) was incubated for 36 days in modified simulated body fluid (SBF) with increased HCO 3 and reduced Clion 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), noncarbonate containing, and of intermediate hydrophilicity (advancing contact angle, θ a =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-to-valley 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).