Internal structure of nanoporous TiO2/polyion thin films prepared by layer-by-layer deposition.

The internal structure of porous TiO2 films prepared by electrostatic layer-by-layer deposition was investigated. The films were prepared by alternate dipping of solid substrates into dispersions of TiO2 nanoparticles and polycations, polyanions, or pure buffer solution, respectively. The surface charge of the amphoteric TiO2 particles was controlled by the pH of the aqueous dispersions. The morphology of the film surface was investigated by means of scanning electron microscopy. It was found that the surface roughness strongly depends on the polymeric material used for the deposition process but is independent of the ionic strength of the solution or the molecular weight of the polyions. The samples with rough surfaces feature strong light scattering. The porosity and internal structure of the TiO2/polyelectrolyte films were investigated by adsorption/desorption of dye molecules. A crude estimate yields an internal surface that is up to 160 times the plane surface of the substrate for a film thickness of 1 microm. The composition of the films was investigated by X-ray photoelectron spectroscopy (XPS). Detection of the XPS signal after each deposition step of the first three dipping cycles shows a significant increase of the relative surface coverage of Ti after the TiO2 deposition step and of PSS after the PSS deposition step. For later dipping cycles, such an increase was also detectable but less prominent.

Testing the cytotoxicity of metal alloys used as magnetic prosthetic devices.

Technical magnetic materials are increasingly used for the development of magnetic retained dental prosthetic and orofacial epithetic devices. Since most of the magnets based on rare earth metals, such as samarium-cobalt based alloys have a high tendency for corrosion they were first coated by tin and then encapsulated by titanium. However, the high mechanical load particularly on dental devices may cause a rupture of the titanium capsule and the alloys contact directly biological fluids. Hence, it is important to know the cytotoxicity of these magnets to assess their potential effects on the surrounding tissue. In this study, the cytotoxicity of neodymium-iron-boron and samarium-cobalt (plain, tin and titanium coated) magnets was tested. First, magnets were incubated up to 7 days in culture medium to prepare extracts for cytotoxicity measurements. Changes in the surface morphology due to corrosion were visualized by scanning electron microscopy and analysis of the elemental composition. 3T3 mouse fibroblasts were cultured in the presence of extracts and their viability measured by neutral red and metabolic assays. To learn more about a possible toxic activity of the main components of magnets, salt solutions of different concentrations resembling those elements, which are main constituents of the magnets, were used. 3T3 fibroblasts were also cultured in direct contact with the materials and material induced effects on cell morphology and growth monitored by microscopy. As a result of this study it was found that samarium-cobalt magnets have a strong tendency for corrosion and exert a considerable cytotoxicity. Neodymium-iron-boron magnets have a lesser tendency for corrosion and are only moderate cytotoxic. Coating of samarium-cobalt magnets with tin or titanium makes the materials non-toxic. Application of salt solutions shows that cobalt has a tendency to be cytotoxic at higher concentrations, but enhances cell metabolism and proliferation at lower concentrations while the other magnet constituents had a lower or negligible cytotoxic potential.