LEE- Nano Bilim ve Nano Mühendislik-Doktora
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ÖgeDevelopment of antibacterial coatings on titanium based biomaterials(Lisansüstü Eğitim Enstitüsü, 2022) Aydoğan, Dilek Teker ; Çimenoğlu, Hüseyin ; 723039 ; Nanobilim ve NanomühendislikSince the average human life is getting longer, research on long-lasting and biocompatible biomaterials has become more common. A biomaterial is a material that can replace any damaged tissue or organ and therefore, it is in continuous interaction with body fluids. There are various biomaterials for different application areas. The main issues to be considered are the mechanical properties, design, and biocompatibility of the developed biomaterial. Metal and metal alloys are the most frequently preferred biomaterials and they have been used as hip, knee, and dental implants for many years. These biomaterials are also expected to show superiour biocompatibility, corrosion and wear resistance, along with non-toxicity. Especially the long-term stability of orthopedic and dental implants depends on the bonding properties at the implant-bone interface and being free of any post-operative infections due to the implant features. Titanium and titanium alloys stand out among other metallic implant materials (such as stainless steel and cobalt-chromium alloys) with their excellent mechanical properties, biocompatibility, low densitiy, high corrosion and wear resistance. The most important feature that separates titanium from the other metals is the natural oxide film layer on its surface. Even though, this stable, dense, and continuous layer provides corrosion resistance and biocompatibility to the material, but, its ability to bond to bone is quite weak. In addition, due to the toxic effect caused by alloying elements that can be released from some titanium alloys (such as Ti6Al4V), titanium alloys may fail in long-term implant applications. For this reason, numerous surface treatment methods are used to enhanced the surface features of titanium and its alloys. Micro-arc oxidation (MAO), also called plasma electrolytic oxidation, is a convenient technic that is used to produce ceramic coatings on titanium, aluminum, magnesium, and their alloys. With this method, it is possible to get thick, porous, firmly attached ceramic coatings on the surface of titanium and its alloys. In addition to these, antibacterial oxide coatings can also be obtained by adding appropriate antibacterial agents into the electrolyte used during the process. The ability to coat materials with complex shapes, using environmentally friendly chemicals, and being a cost-effective process are the prominent advantages of this method. In the scope of this study, the formation of bioactive and antibacterial oxide coatings on the surface of titanium and its alloys was carried out via MAO process. In the first chapter of the thesis, it was aimed to produce multi-layer bioactive and antibacterial coatings on the commercially pure titanium (grade 4 quality, Cp-Ti) surface, which is frequently preferred in biomedical applications, by applying the MAO process. For this purpose, samples were subjected to a base electrolyte (which is containing calcium acetate hydrate (Ca(CH3COO)2.H2O) and disodium hydrogen phosphate (Na2HPO4)) during the MAO process. To obtain antibacterial properties on the coating surface silver acetate (AgC2H3O2) was added into the base electrolyte (the amount of silver (Ag) on the coating was measured as 4.6 wt.%). After the MAO process, a multi-layered oxide coating consisting of TiO2 (dense rutile-anatase phases) on the inner layer, and biocompatible compounds such as hydroxyapatite (HA) and calcium titanate (CaTiO3) just above oxide layer was obtained. It was observed that MAO treated samples in the base electrolyte formed biomimetic apatite structure faster in the simulated body fluid (SBF), as well as showed higher antibacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria. It has been determined that the addition of AgC2H3O2 to the base electrolyte increased the antibacterial activity of the samples without sacrificing bioactivity. In the second chapter of the thesis, optimization of AgC2H3O2 amount which had been added into the base electrolyte was studied in order to avoid toxic effect of Ag without sacrificing the antibacterial effect. The MAO process has been conducted in the base electrolyte containing Na2HP04 and Ca(CH3COO)2.H2O and AgC2H3O2 was added at different concentrations. Samples treated with an electrolyte containing 0.005 mol/L AgC2H3O2 concentration showed poor antibacterial activity against S. aureus bacteria, while samples treated with the electrolyte containing 0.001 and 0.002 mol/L Ag concentrations was determined strong antibacterial activity. Based on these results, considering the possible toxic effect of Ag, 0.001 mol/L AgC2H3O2 concentration into the base electrolyte is sufficient for MAO process. It was determined that used optimum AgC2H3O2 concentration into the base electrolyte caused 1.14% Ag on the oxide coating surface after MAO process. Cell culture experiments were performed using SAOS-2 (a human primary osteogenic sarcoma cell line) to understand the effect of the amount of silver measured from the coating surface on the cell. As a result, it was observed that the amount of Ag determined in the oxide coating did not prevent cell growth however retarted it and also showed high antibacterial efficiency against S. aureus bacteria. In the third chapter of the thesis, after determining the optimum amount of AgC2H3O2 concentration in the base electrolyte containing Ca(CH3COO)2.H2O and Na2HPO4 in the previous section, the biological properties of the oxide coating formed in the MAO process using Ti6Al4V alloy were investigated by biofilm formation and cell culture experiments. After adding 0.001 mol/L AgC2H3O2 to the base electrolyte during the MAO process, it was determined that there was 0.76 wt.% Ag on the oxide coating surface. It was observed that the alloying elements in the Ti6Al4V alloy effect the structure of the oxide coating formed by the MAO process conditions. The low amount of Ag measured from the coating surface was explained by the precipitation of Ag particles mostly around the pores of the thick TiO2 layer, and the formation of a thick HA layer on it. The presence of Ag particles between TiO2 and HA layer effected Ag release behavior in simulated body fluid (SBF). Compared with Ag-free coatings, the presence of 0.76 wt. % Ag in oxide coatings exhibited antibacterial activity to some extent against Streptococcus mutans (S. mutans) bacteria and did not adversely effect the proliferation of SAOS-2 cells. However, in order to obtain enhanced antibacterial efficiency, higher amount of silver must be incorporated into the MAO coating. In the fourth chapter of the thesis, the structural features, Ag release behavior and bioactivity of HT treated oxide layer with different amounts of Ag nanoparticles formed via MAO process on Ti6Al7Nb alloy have been investigated. While MAO process was applied in the base electrolyte (containing (Ca(CH3COO)2.H2O) and (Na2HPO4)) with and without the addition of AgC2H3O2 to obtain oxide layer, HT treatment was performed in an alkaline solution (pH = 11) at 230 ºC to improve bioactivity. After the MAO process, HA structure with a low degree of crystallinity and TiO2 layer containing rutile and anatase structures was formed on the surface. Nano-sized Ag particles were detected on the coatings formed over Ag incorporated oxide coatings. Moreover, higher AgC2H3O2 concentration in the base electrolyte caused a higher number of Ag nano-particles in the MAO coating. Afterwards, application of the HT treatment fabricated an 1-2 m thick exterior surface layer that is composed of nano-rod TiO2 and hexagonal HA crystal morphologies on oxide surface and increased degree of HA crystallinity. When samples treated with MAO and MAO+HT are compared, it was observed that HT treatment not only accelerated biomimetic apatite accumulation on Ti6Al7Nb alloy but it also eliminated the negative effect of Ag, which delayed the apatite formation on the MAO coatings. In addition, unlike the oxide coatings formed with MAO, HT treatment considerably reduced the amount of Ag released from the oxide coating into the SBF solution. As a result, thick, microporous and multi-layered oxide coatings containing bioactive components have been successfully produced on the surface of titanium and its alloys, which are frequently prefered in implant applications. Generally, various additives such as ions or particles can be introduced into the electrolyte to fabricate antibacterial oxide coatings with biocampatible properties via MAO process. Within the scope of the thesis, studies have shown that oxide coatings which were fabricated using different amount of Ag agent in the base electrolyte exhibited antibacterial efficiency on bacterial cultures and bioactive components support the bioactivity. Especially in obtaining antibacterial coatings, the importance of Ag agent amount in the base electrolyte has been demonstrated by antibacterial tests. In addition to the MAO process, when MAO process is combined with HT treatment, it is possible to fabricate highly bioactive surfaces without obtaining multi-layered coatings on the substrates. Moreover, Ag agent has been introduced into the oxide coating to give antibacterial properties to the surface. The Ag agents and their amounts is still one of the biggest concerns for health. In the future studies, the living body applications (in vivo) will guide the evaluation and development of short and long term effects of TiO2 based bioactive and antibacterial coatings fabricated with MAO process and HT treatment.