In this work, the properties of hydroxyapatite (HAP) nanoparticles (NPs) have been studied both theoretically and experimentally focusing on computational analysis. HAP is widely used to fabricate implants, for drug delivery, etc. The physical properties of the nanosized HAP particles play an important role in the interaction with cells in the human body and are of great interest. Computer simulation was employed to understand the properties of HAP clusters (Ca(5)(PO(4))(3)OH) including formation energies, dipole moments and polarization (surface charges) by molecular mechanics (MM + , OPLS) and mostly by quantum semi-empirical Hartree-Fock (PM3) methods. The size of the simulated cluster is found to affect its dipole moment, polarization, and, finally, the electron work function- ϕ. These parameters depend on the concentration of hydrogen atoms H (or protons) at the surface. Values of ϕ were experimentally estimated via photoelectron emission measurements. The magnitude of ϕ was demonstrated to have a positive correlation on sizes. The NPs demonstrated a capability to be gathered within conglomerates. This property is confirmed by the calculated data for various sizes. Their sizes have a positive correlation on ϕ by the native particles. The main results show that the distributions of dipole moments have very different space orientations (along the OX, OY and OZ axes, the OZ axis is oriented along the OH column) and change with the addition of hydrogen atoms, which saturate the broken hydrogen bonds. This electrical property of NP leads to different behaviors and motions with consequent aggregation: (1) for the case of NPs having dipole moment oriented preferably perpendicular to the OZ axis (with more hydrogen bonds saturated by added H)-the HAP NP aggregates with hexagonal orientation and forms a wider and more spherical shape (sphere-like or bundle-like); (2) for the case of NPs having dipole moment oriented along the OZ axis (as is the case in the absence of added protons or non-saturated hydrogen bonds)-the NPs firstly rotated and oriented along this axis to form the most elongated cylindrical shape (rod-like).

Photoluminescence, surface photovoltage spectroscopy and high-resolution characterization methods (Atomic Force Microscopy, Scanning Electron Microscopy, X-ray spectroscopy and DC conductivity) are applied to nanostructured Hydroxyapatite (HAp) bioceramics and allowed to study electron (hole) energy states spectra of the HAp and distinguish bulk and surface localized levels. The measured trap spectra show strong sensitivity to preliminary heat treatment of the ceramics. It is assumed that found deep electron (hole) charged states are responsible for high bioactivity of the HAp nanoceramics.

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Hydroxyapatite is known as a substrate for effective adhesion of various biological cells and bacteria as well implantable biomimetic material replacing defective bone tissues. It is found that low energy electron irradiation induces its strong surface potential variation and gives rise to pronounced wettability modification. The found electron-modulation method of the hydroxyapatite wettability enables both wettability switching and its microscopic patterning, which may be used for fabrication of spatially arrayed hydroxyapatite for biological cells immobilization, gene transfer, etc.

Correlation analysis demonstrated the role of inorganic parameters of the surfaces of calcium phosphate materials in the regulation of osteogenic differentiation of mesenchymal precursors. The progenitor stromal cells were isolated from syngeneic bone marrow immobilized in vitro on calcium phosphate surfaces with different structure, phasic, and elemental composition. After 45 days of subcutaneous ectopic osteogenesis in BALB/c mice, the tissues grown on these matrixes were characterized histologically. It was found that adhesion of bone marrow cells is the initial stage determining their future proliferation (conduction) over the artificial surface and the area of formed tissue plate. The success of histogenesis depends on surface roughness. The optimal roughness class was 4-5 (Russian State Standards), which enables differentiation of progenitor stromal cells under the specific microenvironmental conditions into the connective and adipose tissue cells. Differentiation of the progenitor cells into the stromal cells producing the hemopoiesis-inducing microenvironment also takes place in the foci of active hemopoiesis. Induction of osteogenic potential of the stromal precursors (osteoinduction) is determined by the ratio between calcium and phosphate atoms in surface coatings. In our experimental system, osteogenic differentiation of stromal mechanocytes was blocked only at Ca/P<0.5.