| Implant-associated-infection remains a significant clinical problem.A novel class of Ti-xCu antibacterial alloys has been studied for years to overcome this challenge.The Cu-bearing Ti alloys show optimum mechanical properties,excellent antibacterial ability,and good biocompatibility and cytotoxicity.It is well known that the good biocompatibility of the Ti-alloys is due to the compact and tenacious oxide barrier film formed on the alloy surface.However,Ti-xCu alloys demonstrate a slight reduction in corrosion resistance,compared to the commercially pure titanium(CpTi).The decrease in the corrosion resistance of Ti-xCu alloy is caused principally by the removal of Cu-ions from the alloy surface which affects passive film properties of the Ti-xCu alloy.Previous work on the passive metals and alloys indicates the minor alloying elements also take a significant role in the growth and breakdown of the passive film besides the principal element in the alloy.These alloying elements could be present in the passive film structure as a separate oxide-layer(n-type or p-type in nature),and possible removed via cation-anion Schottky pair reaction according to Point Defect Model.On the other hand,the minor alloying element could be present in the passive film as segregated alloying elements and as metal interstitials(Mi+),and/or as substituent's(MMx')on the cation sublattice as a minority defect,and possibly removal of minor alloying element via preferentially oxidation and preferential dissolution.The Point Defect Model-?(PDM-?)and Quantitative analysis of the concentration depth profile would facilitate understanding the existing nature of the alloying element in the passive film and the localized breakdown(release of metal ions)of the passive film.It is vital to analyse the existing form of Cu in the passive film,its possible release mechanism and effect on the passivity of Ti-xCu alloy to understand the prime cause of the reduction of the corrosion resistance in Ti-xCu alloy.It is also known that the biological fluid not only contains inorganic ions(chloride or phosphate ions)but also holds organic molecules(protein).Therefore,the effect of adsorption of protein,and its effect on the antibacterial and electrochemical properties are also significant.Therefore,in this dissertation,a systematic research have been develop to better understanding of the alloy corrosion mechanism and is expected to be beneficial in the development of these novels,bio-metallic materials as implant products in clinic applications.In this dissertation,I do present the investigation on the;(?)existing form of Cu in the passive film and possible mechanism of Cu ion removal from Ti-7Cu alloy,(?)effect of Cu content on the passive film properties within the framework of the Point Defect Model(PDM-?),(?)study of the adsorption behavior of the BSA protein on the Ti-3Cu alloys,and effect of BSA protein addition on the antibacterial and electrochemical properties of Ti-3Cu alloy.To study the above-mentioned objectives various electrochemical analysis techniques have been employed in this study including,open circuit potential(OCP),Potantiostatic transient analysis,electrochemical impedance spectroscopy(EIS),Mott-Schottky analysis(MSA),cyclic voltammetry(CV),and potentiodynamic polarization(PD)test.The electrochemical results have been examined within the framework of the Point Defect Model-?(PDM-?).The antibacterial test was conducted according to the China Standard GB/T 21510.Cu ion release was examined via the inductively coupled plasma technique(ICP).Furthermore,the secondary ions mass spectroscopy(SIMS),and angle-resolved X-ray photoelectron spectroscopy(ARXPS)have also been used to analyze the surface chemical composition.Moreover,the Quantitative analysis of the depth profile of ARXPS has also been performed.Ti-3,5 and 7 wt%Cu alloys have been used in this dissertation.The major findings of this dissertation are presented below.(1)Study the existing form of copper(p-type oxide or segregation)in the passive film and its release mechanism from the passive film of Ti-7Cu alloy were studied.Mott-Schottky analysis and PDM-? criteria of the potential independency of the steady-state current density revealed that copper does not form any p-type(cuprous or cupric)oxide in the passive barrier layer,instead it revealed the n-type semiconducting nature of the barrier layer.Metal segregation factor(Zim 0.29<1)and system segregation factor(Zis 0.35<1)of the Quantitative analysis of the ARXPS depth profile revealed the direct consequence of the preferential oxidation and preferential dissolution of Cu.These observations were also confirmed in the cyclic voltammetry examination,which revealed the electro-oxidation of copper in Ti-7Cu alloy.The study observation also showed that the passive film formed on the Ti-7Cu alloy surface comprised an inner,oxygen-deficient TiO2-x barrier layer and a precipitated outer-hydroxide layer.The Cu segregated in the oxygen-deficient TiO2-x barrier layer as Cu interstitials Cui+,which make the barrier layer a highly Cu doped n-type tunnel diode.The transmission of Cu from the matrix to the metal/film interface occurred due to one of the dealloying or bi-metallic effect in the intermetallic Ti2Cu phase,as has been observed in CV analysis,the Ti firstly oxidized to TiO2-x,because Ti is less noble(more active)than is Cu,and Cu thereafter electro-oxidized.The transmission of Cu from the metal/film interface through the passive film to the film/solution interface occurred by migration phenomena as evident by potential independency of the electric field strength,which is due to band-to-band(Esaki)tunneling effect.(?)Effect of Cu content on the passive film properties within the framework of the Point Defect Model(PDM-?).The study demonstrated that the steady-state current density(Iss)remains steady as a function of applied potential,and the steady-state film thickness(Lss)depends linearly as a function of formation potential.These observations confirm that the passivity on Ti-xCu alloys is well-acknowledged by the Point Defect Model-II,which gives a realistic physiochemically explanation of the oxide layer formed on Ti-xCu alloys.The study within the frame work PDM-? also shows that the oxide layer produced on the surface of the alloy has the point defects within the oxide(barrier)layer of the passive film which are oxygen vacancies Vo**and/or metal interstitials Mi+.The study also demonstrates that as the amount of Cu wt%increases the donor density Nd values also increase.It is postulated that the higher donor concentration is due to copper interstitials Cuix+ in the defective TiO2-x lattice,which make the barrier layer a highly Cu doped n-type tunnel diode,resulting in the reduction the charge transfer resistance(Rct)and double-layer capacitance(Cdl)associated with the inner barrier and outer hydroxide layer,which demonstrates that the corrosion resistance decreases as the Cu content increased.The decrease in the steady-state current density after an initial increase upon the addition of 3%of Cu is attributed to the progressive substitution of copper on the cation sublattice of the film resulting in enhanced electrostatic interaction between the immobilized copper,CuTi3',and the mobile cation interstitials,Cui+,which carries the excess current over and that carried by oxygen vacancies in the barrier layer.This study also shows that over a hundred-year implant period,about 0.006 cm(0.06 mm)of Ti-3Cu alloy is expected to be lost due to general corrosion in the PBS solution under the given set of conditions,it can further be seen that as the Cu amount increased from 3 wt%to 5 wt%in the alloy,the steady-state corrosion rate decreased.(?)Study the adsorption behavior of the BSA protein on the Ti-3Cu alloys,and its effect on the antibacterial and electrochemical properties of Ti-3Cu alloy.It was observed that the surface charge density(QADS)was directly proportional to the amount of the adsorbed BSA protein,signifying that the protein adsorption was accompanied by the charge transfer,pointing to a chemisorptions phenomenon.BSA amino groups and other organic species were observed in the surface analysis examinations.It was shown that the formation of barrier complexes between the TiO2 oxide-layer and PBS solution resulted in decreasing the release of Cu-ions,which consequently reduced the antibacterial activity.On the other hand,these barrier complexes improved the corrosion resistance by increasing the charge transfer resistance and double-layer capacitance of the Ti-3Cu alloy. |
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