Design And Development Of A Novel Antibacterial Cu-bearing TiZr-based Alloy For Biomedical Applications

Metallic biomaterials are known to possess comprehensive excellent mechanical properties;including high strength-to-weight ratio,adequate plasticity and toughness,moderate elasticity,high hardness,and desirable fatigue resistance.However,these biomaterials behave biologically inert when required to play the supporting roles in human bodies.In addition,they are unable to prevent and solve the implant-related infections(IRIs).Deficient or poor early bone healing at the interface of bone/implant and bacterial-related infections often occur in the post-implantation period,and once the infection occurs,it brings heavy economic burden and great pains to the patients.To date,no treatment can guarantee rapid and complete elimination of the bacteria infections/biofilm colonization,or prevent these implants from secondary infections,and therefore the antimicrobial properties become essential to improve long-term clinical stability of the employed biomaterials.Surface modifications,such as incorporating antibiotics or antibacterial agents into the surfaces of the implants,is a common method to provide antibacterial properties.However,the lack of binding force,limited drug loading ability and difficulty in controlling the release rate of antibacterial agents have always hindered the clinical applications of these coatings.Therefore,it is of vital importance to develop integral antibacterial materials by immobilizing antibacterial agents into materials for long-term effective anti-bacterial activity.Copper(Cu)has been proven to have many useful bio-functions in human bodies,such as promoting osteogenesis and angiogenesis,reducing the occurrence of in-stent restenosis and thrombosis,as well as possessing impressive antimicrobial properties.For these reasons,this dissertation systematically details the design and development of a series of Cu-bearing TiZr alloys for use as biomedical implants.By immobilizing appropriate amounts of Cu and informed heat treatments,novel antibacterial Ti-15Zr-xCu(3 ? x? 7,wt.%)alloys with competitive mechanical properties,biological responses and corrosion resistance were achieved.Bond order(?)and metal d-orbital(?)method was used in designing the alloys.Data obtained from this hypothesis were used to predict the properties of the different alloys.Hot forging of the billet was done in temperature range of 950?-970?,followed by hot rolling at 840?-870? and then different heat-treatments were applied within,and at slightly above,the beta transus temperatures(BTT).Sectioning for examination was done by cutting the samples using electric discharge machining(EDM)workstation.Chemical composition analysis was performed using inductive couple plasma mass spectrometry(ICP-MS),and the other gaseous elements that might be present were also determined with an Oxygen/Nitrogen/Hydrogen analyzer.Differential scanning calorimetry(DSC)was employed to characterize the phase transformations in the alloys between 0?-1100? heating and cooling range.Phase identification was carried out by using X-ray diffraction,and the microstructural morphologies of the alloys were examined using optical microscopy(OM),field emission scanning electron microscopy(SEM)equipped with energy dispersive spectroscopy(EDS),and transmission electron microscopy(TEM).Room temperature mechanical testing was performed with a universal testing machine and a microhardness indenter.Buoyancy method of density determination was used to determine the strength-weight ratio of the alloys,and ultrasonic resonance frequency technique for the Young's modulus determination.The antibacterial and antibiofilm tests were carried out by deploying the Gram-positive Staphylococcus aureus(S.aureus)and Gram-negative Escherichia coli(E.coli)bacteria.The amount of Cu ions released was measured using ICP-OES axial spectrometer.Cell proliferation to assess the toxicity of the designed alloys was evaluated with cell counting kit-8(CCK-8)and their morphologies on the samples' surfaces were examined by observing the cytoskeleton through F-actin filament staining.The corrosion behaviours of the alloys were investigated by using a potentiostat/galvanostat Gamry Instruments,employing the classical three-electrode system.Samples' wettabilities were determined by using a contact angle goniometer.All data were expressed as mean and standard deviations,and the statistical evaluations were carried out by variance(ONE WAY ANOVA)statistical package for social science(SPSS).In the first aspect,the values of ? and ? obtained predictively suggested that the alloys would possess structures and properties composed of an entirely ?-type phase and moving towards the ?+? phase as the amount of Cu increased.Annealing heat treatment on Ti-15Zr-7Cu(TZC-7A)alloy,after holding for 2 h at slightly above its beta transus temperature(BTT),ensured its tensile strength(UTS),yield strength(YS)and hardness(HRV)were improved by 31.2%,20%and 12.3%respectively compared to the control without Cu,Ti-15Zr(T-15ZA).Although the 3 wt.%Cu alloy displayed the highest elongation(26%),the TZC-7A alloy also possessed a good ductility.Presence of evenly dispersed Ti2Cu and Zr2Cu Cu-rich intermetallic phases formed as interwoven and alternating lamellae within the ?+? matrix as a result of Cu addition,as revealed by X-ray diffraction(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).These greatly contributed to their strengthening and bactericidal properties.Over 98%antibacterial rate against E.coli and S.aureus have been imparted,coupled with excellent biofilm inhibition.Potentiodynamic polarization curves showed that the TZC-7A alloy possessed higher corrosion resistance than commercially pure titanium,cp-Ti;contact angle test revealed enhanced hydrophilicity;while confocal laser scanning microscopy(CLSM)and cell counting kit(CCK-8)assays also displayed drastically lowered bacterial adhesion rate with comparatively no cytotoxicity.Cell attachment on all alloys were similar but the best spread was obtained on TZC-7A after 24 h.In the second aspect of the investigation,by using different aging conditions,the strengths and ductility of Ti-15Zr-xCu(3? x?7)(TZC)alloys were further optimized.The different aging conditions resulted in significant precipitations of the Cu-rich Ti2Cu intermetallic compounds,mild formations of Zr2Cu and Zr7Cu10 secondary phases and pockets of segregates of Cu and Zr in the various Cu-bearing alloys,when investigated by XRD and SEM-EDS.Despite the different clusters of Cu phases,the amount of Cu ions released was well below the lethal dose,LD50,as obtained by ICP-OES.The resulting microstructures and the effects were investigated on the alloys' mechanical properties,corrosion resistance,and antibacterial abilities.The best combination of properties was obtained after solution treatment and aging(STA)at 660 0C for 4 h with significantly improved tensile strength-UTS(945.88±24.8)MPa,yield strength-YS(902.97±19.6)MPa,and over 8%elongation compared to the solution-treated(ST)alloy with zero ductility.Electrochemical techniques confirmed better corrosion resistance of the Cu-containing STA Ti-15Zr-7Cu(TZC-7),Ti-15Zr-5Cu(TZC-5)and Ti-15Zr-7Cu(TZC-7)alloys respectively treated at 450? for 10 h,540?for 6 h and 660? for 4 h compared with their 0 wt.%Cu(TZ)counterparts.The antibacterial abilities,boosted by the Cu-rich precipitates,were measured by count plate method and very high bactericidal activity of the aged samples was observed against Staphylococcus aureus compared to the control without Cu.The results obtained in this study showed that STA considerably increased and optimized the strengths and antibacterial property as well as the general corrosion resistance of the TZC alloys.This study thus further establishes a probability for application of antibacterial TZC alloys with attractive and improved strength,corrosion resistance and biological responses in the field of biomedicine after different heat-treatment conditions.