SYNTHESIS OF BIOCOMPATIBLE COATINGS OF HIGH VALUE OF MICRO- AND NANOROUGHNESS BASED ON TITANOXIDE STRUCTURES ON THE SURFACE OF NANOSTRUCTURED TITANIUM
The coarse grain metallic materials are traditionally used in dental and orthopedic implants. However, they already reached their limit of the constructive strength, plasticity and other mechanical properties. In this regard, the great interest exists in recent years to nanostructured materials (NM). Especially interesting are the metals obtained by means of the intensive plastic deformation (IPD). Numerous works [1-3] demonstrate that the nanostructuring by means of IPD can provide the meaningful improvement of the mechanical properties of the pure titanium (that is the most suitable material for the medical implants.
However, the mechanical properties are not the only characteristic for the suitability of implant materials. Much more important characteristics are their bioinertness, biocompatibility, and bioactivity. Titanium bioinertness and biocompatibility are acceptable for the implantology due to the presence of natural oxide layer. At the same time, very important task is the improvement of titanium bioactivity. Bioactivity can be improved either by the surface modification and biocovering or by the variation of the surface relief [4, 5]. The developed bioactive relief is usually formed by means of mechanical treatment, electrochemical or chemical etching. To date a lot of research materials in this area are accumulated. However, there are only few works on the modification of highly promising nanostructured titanium. Nevertheless, physicochemical properties and the etching mode for nanostructured titanium are significantly different from ones for coarse grain titanium.
In this regard, first part of work is dedicated to the study of the dependence between titanium grain size, relief and composition of the surface while the treatment with different etchants in the range of etching times (from 5 min to 24 h). Initial coarse grain titanium Grade 4 with the average grain size of 25 nm and ultrafine titanium, obtained by IPD with the average grain size of 100 nm were disks of 12 and 6 mm in diameter and with 2-3 mm thickness, respectively. These disks were polished until roughness level of 3-5 nm. As the etching media, we used acidic (H2SO4/H2O2) and alkaline (NH4OH/H2O2) Piranha solutions.
It is shown by means of SEM and AFM that for the samples of etched ultrafine and coarse grain titanium, the etching medium nature as well as time of exposure influence significantly on the titanium surface morphology. In the case of acidic Piranha solution, the spongy structure is formed. If the etching time is increased to 24 h, this structure is destroyed and the surface roughness is increased significantly (RMS increases from 5-7 nm to 42.1 nm). Coral-like structures are formed while using alkaline Piranha solution with exposure times up to 2 h. Further treatment converts these structures to netlike ones (with net size of 50 to 200 nm depending on NH4OH\H2O2 ratio.
It is worth to note that the surface of the samples after treatment in NH4OH\H2O2 for more than 2 h is characterized by the presence of "holes" with 1-2 μm in diameter and depth up to 10-20 μm. Besides, the data of X-Ray Photoelectron Spectroscopy demonstrate the peaks assigned to Ti0 even after prolonged etching in ammonia solution. These peaks are not found for the samples treated in H2SO4\H2O2. This fact shows that the surface oxidation by H2O2 is actively proceed in acidic medium and is almost inactive in ammonia medium. The comparison of ultrafine and coarse grain titanium demonstrates that the nanostructured titanium is etched more actively and more uniformly. At the same time, qualitative difference is not observed.
The second part of work describes the formation of biocompatible layers on etched nanostructured titanium by means of ALD. This method provides differently shaped thin layers on the substrates keeping the surface relief unchanged.
Titanium isopropoxide and water have been used as initial reagents to get titanorganic and titanoxide structures. Tert-butyl phosphate and water provided phosphate groups addition. The samples were synthesized at 2500C and inert gas pressure of 20 mm Hg. 400 ALD cycles allowed to get 20-nm thick layers (data of spectral ellipsometry). The presence of titanorganic and phosphate groups has been confirmed by means of X-Ray Photoelectron Spectroscopy and X-Ray Fluorescent Analysis. The XPS data also demonstrate the absence of Ti0 characteristic peaks. This is the confirmation that the ultrafine titanium is covered by the film. According to SEM and AFM data, netlike and sponge-like structures are not preserved while layering, whereas the micron-sized “holes” remain unchanged after the film deposition. For all the samples the grains of 20-100 nm in diameter are indicative. It is worth to note that for ultrafine titanium etched in NH4OH\H2O2, the density of grain packing is much higher for 2 h etched sample compared to one etched for 15 min. For H2SO4\H2O2 – treated samples, the exposure prolongation leads to appearance of grains.
In conclusion, the influence of various etching media was studied on the manner of etching of ultrafine and coarse grain titanium samples. It was shown experimentally that the biocompatible material can be created that combines well mechanical properties (ultrafine titanium), developed relief both on nano and micro levels, as well as the presence of titanium-oxygen layer with phosphate groups. This new material is able to provide high biocompatibility and bioactivity.
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