Calcium phosphate formation on TiO2 nanomaterials – finding a suitable biomimetic route of nanocomposites preparation

Among different materials for bone engineering, calcium phosphates (CaPs) attract special attention because the main inorganic component of bones is poorly crystalline, non-stoichiometric, calcium deficient, Na-, Mg- and carbonate hydroxyapatite [1]. Despite their excellent bioactivity and osteoconductivity, CaPs" applications are limited to nonstress bearing applications due to their poor mechanical properties [2]. In recent years, the addition of inorganic nanomaterials (NMs) in CaPs based ceramics, cements and coatings is proposed as a solution to this problem [2]. TiO2 nanomaterials (TiNMs) attract attention due to their favorable properties for biomedical applications [3], and the possibility to prepare TiNMs of different morphologies, structure and composition. In order to rationalize the design of CaP/TiNM composites, the influence of TiNMs" surface structure and charge density on CaPs formation should be investigated. Until now, most of the studies were performed with TiO2 (TiNPs) nanoparticles and arrays of nanotubes (TiNTs) in simulated body fluid [6, 7]. In order to contribute to the understanding of CaPs formation on TiNMs of different dimensionality, in this research the formation of CaPs in the presence of TiNPs and TiNTs was investigated in two sets of experiments [8]. The ability of TiNMs to induce CaPs formation was tested in corrected simulated body fluid, while the influence of TiNMs on the kinetics of formation and properties of formed CaPs was investigated in the spontaneously precipitating system. The influence of TiNMs on CaPs precipitation has been investigated by potentiometric measurements, powder X-ray diffraction (PXRD), electron paramagnetic resonance spectroscopy (EPR), scanning (SEM) and transmission (TEM) electron microscopy, dynamic (DLS) and electrophoretic light scattering (ELS). Both TiNMs showed poor ability to promote CaPs formation in c-SBF which can be attributed to their low absolute zeta-potential value. However, during spontaneous precipitation, they exhibited unusual dual influence on precipitation kinetics, i.e. inhibition of initially formed amorphous calcium phosphate (ACP) transformation at lower and promotion at higher concentrations. After 1 hour reaction time, in presence of both TiNMs, calcium deficient hydroxyapatite was formed (CaDHA). Morphology of the CaDHA depended on the type and concentration of TiNMs. However, PXRD and EPR characterization confirmed that there is no significant difference between CaDHA formed in the presence of different TiNMs. Obtained results confirm the importance of surface structure and charge in CaPs – TiNMs interactions and point to a versatile method for biomimetic synthesis of novel biocomposites.