CREATING OF TITANIUM DIOXIDE NANOCOATINGS WITH DIFFERENT MORPHOLOGY ON TITANIUM MATRIX FOR BONE IMPLANTS
Nowadays, products created from different alloys began increasingly using in orthopedics. However, the use of untreated metals or its alloys can be rejected by the body. For this reason it is very important that the materials used in medicine should have biocompatible and bioactive.
Examination of a possibility of the formation of a chemical bonds between an artificial implant surface, for example titanium, and a bone tissue is one of the major characteristics of modern biocompatible materials intended for orthopedics, reconstruction surgery, and dentistry. The important problem of the chemical material science is the creation of a such dental implant surface that would be able to create rational conditions for a contact osteogenesis (formation and evolution of a bone tissue), which is the optimal mechanism for the formation of an organotypic osteal material on the implant surface. It was shown earlier that the elemental composition of the implant surface and a surface relief of a titanium play an important role as in formation of osteoblasts (young osteal cells) and in increasing of their osteocompatibility . It is fair to say that qualitative and quantitative characteristics of osteointegration directly depend on surface topography of titanium implants, including nanostructured implants, and on their chemical composition . Therefore the important direction for the evolution of medical material science is the development of methods of surface modification, which will make it possible to increase osteocompatibility of metal implants and organism tissues. This can be implemented by two ways to create links between the implant and the living tissue:
1) mechanical cohesion as a result of the intergrowth of tissue in the implant structure;
2) chemical interaction with the tissue components of the elemental composition of the implant;
In the work it was carried out a second way to build links between the implant and the living tissue. The bioactive surface on titanium supports were created due to the simulation of the mesoporous structure of the oxide film and its composition, as well as the thickness of the coating in the nanorange. For directional control of biomedical properties due to the structure modeling of nanometal surface sol-gel technique was used. This method allowed to obtain a mesoporous film of titanium oxide on the metal surface in the nanorange.
Also, it was solved the problem of choosing nanocoating with an optimal surface structure in the context of accelerating of osteointegration period for implants based on titanium.
In the literature, it is presented the results of works on the creation of monolithic and transparent films on substrates , and in the conclusions researchers usually focus their attention on the fact that they found the synthesis conditions leading to the production of continuous defect-free films. Our task was to synthesize a film having a microcrack network penetrating the whole film volume using a dip coating technique in conditions of a high thermal gradient. This a coating texture can strengthen adhesion between cells and titanium surface.
The first stage of work was the testing of synthesis techniques of the starting titanium oxide sol. As a result, we chose the method described in  based on the hydrolysis of titanium tetraisopropoxide in the presence of diethanolamine in isopropyl alcohol. The next step was the testing of the mode of application of the film on the model titanium substrate by dip coating method. As a result films with various thickness were obtained. For comparison of measured film thicknesses we used a theoretical model of the Landau-Levich. When the film applied to the coating by different methods, it is often important to depicture what a film will be obtained, i.e. its structure and thickness. For dip-coating method, the film thickness depends on the characteristics of the liquid adsorbent according to Landau-Levich law. The thickness of the film depends on the rate of drawing the sample from liquid, viscosity, density and surface tension. The latter also depends on the concentration of the liquid, therefore the concentration also affects the thickness of the film produced. . In reality, there are deviations from Landau-Levich law, that depend on many parameters. However, considering the uncertainty, the experimental system can be described by a theoretical model of the Landau-Levich.
We have compared the theoretical and experimental results for the films with different content of titanium dioxide in the ash. As we have established a practical model differs from the theoretical hard enough. High uncertainty is taken when film thickness is measured by elipsometric method. Contribution in uncertainty is taken by surface irregularities - about 5%, and porosity - about 10%. Also, uncertainty occurs during measuring of viscosity and surface tension. With all of these uncertainties it can be said that the model of the Landau-Levich cannot describe our system yet. It was also noted that an increase in the drawing speed of the substrate from liquid increases divergence with Landau-Levich theoretical model. It can be explained that at high rates the viscous resistance induced by gravity cannot compensate the adhesion of layer .
For film deposition by dip-coating method drawing speed of 100 mm/min was chosen. According to electron microscopy of the cleaved film, the film had a thickness of 110 to 200 nm. Then, possibility of creating a film having a network of microcracks was explored. To solve this problem the film were obtained in the conditions of significant temperature changes and rapid drying of the precipitated gel. According to electron microscopy, a cracks passed through the entire thickness of the oxide layer at an angle close to 90о, their width was from 0.3 to 2 microns. At the same time all the cracks have been combined into a single network. It was also shown that the thickness of the film is an important factor affected on crack resistance of film, that is confirmed by other authors.
After the film deposition the samples were washed out from the organic components in the distillate water and then calcined. Scanning electron microscopy revealed that the uniformity of the coating as a result of this treatment is reduced, but not significantly.
Research of osteocompatibility of obtained samples with a TiO2 layer was carried out in comparison with a titanium sample with its own (real) titanium oxide.
Evaluation of cell state (the character of adhesion and spreading of the cells on the pattern surface) was performed using scanning electron microscopy. Despite the fact that the cell concentration during exposure on the sample surface was the same on the surface of the titanium sample with its own (real) titanium dioxide it was detected only single cells of osteoblasts. This may reflect the low adhesion properties of the surface of the sample to the test cell line. The titanium sample with deposited titanium dioxide coating was characterized by the formation of the cell monolayer of osteoblasts on a surface with good adhesion. This indicates a high degree osteocompatibility of our coating.
This work was supported by the Federal Target Program "Research and development in priority areas of Russian scientific and technological complex for 2014-2020" contract № 14.604.21.0084 (unique identification number RFMEFI 60414X0084).
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