Microstructures, mechanical properties and in vitro corrosion behaviour of biodegradable Mg–Zr–Ca alloys

The microstructures, mechanical properties, corrosion behaviour and biocompatibility of the Mg–Zr–Ca alloys have been investigated for potential use in orthopaedic applications. The microstructures of the alloys were examined using X-ray diffraction analysis, optical microscopy and scanning electron microscopy. The mechanical properties of Mg–Zr–Ca alloys were determined from compressive tests. The corrosion behaviour has been investigated using an immersion test and electrochemical measurement. The biocompatibility was evaluated by cell growth factor using osteoblast-like SaOS2 cell. The experimental results indicate that the hot-rolled Mg–Zr–Ca alloys exhibit much finer microstructures than the as-cast Mg–Zr–Ca alloys which show coarse microstructures. The compressive strength of the hot-rolled alloys is much higher than that of the as-cast alloys and the human bone, which would offer appropriate mechanical properties for orthopaedic applications. The corrosion resistance of the alloys can be enhanced significantly by hot-rolling process. Hot-rolled Mg–0.5Zr–1Ca alloy (wt %) exhibits the lowest corrosion rate among all alloys studied in this paper. The hot-rolled Mg–0.5Zr–1Ca and Mg–1Zr–1Ca alloys exhibit better biocompatibility than other studied alloys and possess advanced mechanical properties, corrosion resistance and biocompatibility, suggesting that they have a great potential to be good candidates for orthopaedic applications.     

A study of histological responses from Ti-6Al-7Nb alloy dental implants with and without plasma-sprayed hydroxyapatite coating in dogs

Titanium alloys are hoped to be used much more for applications as implant materials in the medical and dental fields because of their basic properties, such as biocompatibility, corrosion resistance and specific strength compared with other metallic implant materials. Thus, the Ti-6Al-7Nb alloy that has recently been developed for biomedical use, that is, primarily developed for orthopaedic use, is to be studied in this paper, for application in dental implants. The biocompatibility test in vivo was carried out in dogs and the osseointegration was verified through histological analysis of the samples of the Ti-6Al-7Nb alloy with and without hydroxyapatite coating that were inserted in the alveoli. Within the controlled conditions the samples did not show any toxic effects on the cells. © 2001 Kluwer Academic Publishers     

Evaluation of cast Ti–Fe–O–N alloys for dental applications

Good mechanical properties, biocompatibility and corrosion resistance make titanium an excellent material for biomedical applications. However, when better mechanical properties than those offered by commercially pure titanium (CPTi) are needed, Ti–6Al–4V is sometimes a good alternative. Some new titanium alloys, developed as industrial structural materials, aim at an intermediate range of strength between that of CP Ti and Ti–6Al–4V. Two of these alloys are Super-TIX800™ (Ti–1% Fe–0.35% O–0.01% N) and Super-TIX800N™ (Ti–1% Fe–0.3% O–0.04% N) (both produced by Nippon Steel Corp., Japan). Besides being stronger than CP Ti, the cost of manufacturing these alloys is reportedly lower than for Ti–6Al–4V since they do not contain any expensive elements. In addition, they are not composed of elements such as aluminum or vanadium, which have caused biocompatibility concerns in medical and dental appliances. To evaluate these alloys as candidates for dental use, it is helpful to compare them to CP Ti (ASTM Grade 2) and Ti–6Al–4V (ASTM Grade 5), which have already been employed in dentistry. We evaluated the tensile properties, mold filling capacity, corrosion characteristics and grindability of these industrial alloys prepared by investment casting. Compared to the strengths of cast CPTi, the yield strength and tensile strength of these cast alloys were more than 20% and approximately 30% higher, respectively. On the other hand, both of these properties were 30% lower than for Ti–6Al–4V. Better grindability and wear resistance were additional benefits of these new alloys for dental applications.     

Improvement of mechanical properties and corrosion resistance of biodegradable Mg–Nd–Zn–Zr alloys by double extrusion

Mg–Nd–Zn–Zr alloy is a novel and promising biodegradable magnesium alloy due to good biocompatibility, desired uniform corrosion mode and outstanding corrosion resistance in simulated body fluid (SBF). However, the corrosion resistance and mechanical properties should be improved to meet the requirement of the biodegradable implants, such as plates, screws and cardiovascular stents. In the present study, double extrusion process was adopted to refine microstructure and improve mechanical properties of Mg–2.25Nd–0.11Zn–0.43Zr and Mg–2.70Nd–0.20Zn–0.41Zr alloys. The corrosion resistance of the alloys after double extrusion was also studied. The results show that the microstructure of the alloys under double extrusion becomes much finer and more homogeneous than those under once extrusion. The yield strength, ultimate tensile strength and elongation of the alloys under double extrusion are over 270 MPa, 300 MPa and 32%, respectively, indicating that outstanding mechanical properties of Mg–Nd–Zn–Zr alloy can be obtained by double extrusion. The results of immersion experiment and electrochemical measurements in SBF show that the corrosion resistance of Alloy 1 and Alloy 2 under double extrusion was increased by 7% and 8% respectively compared with those under just once extrusion.     

Corrosion of bio implants

Chemical stability, mechanical behaviour and biocompatibility in body fluids and tissues are the basic requirements for successful application of implant materials in bone fractures and replacements. Corrosion is one of the major processes affecting the life and service of orthopaedic devices made of metals and alloys used as implants in the body. Among the metals and alloys known, stainless steels (SS), Co-Cr alloys and titanium and its alloys are the most widely used for the making of biodevices for extended life in human body. Incidences of failure of stainless steel implant devices reveal the occurrence of significant localised corroding viz., pitting and crevice corrosion. Titanium forms a stable TiO₂ film which can release titanium particles under wear into the body environment. To reduce corrosion and achieve better biocompatibility, bulk alloying of stainless steels with titanium and nitrogen, surface alloying by ion implantation of stainless steels and titanium and its alloys, and surface modification of stainless steel with bioceramic coatings are considered potential methods for improving the performance of orthopaedic devices. This review discusses these issues in depth and examines emerging directions.     

Nanostructured Composite Materials with Improved Deformation Behavior

The recent progress in the development of nanostructured composites is described for Zr‐base multicomponent alloys as a typical example for such materials. These advanced composite materials are attractive candidates for structural as well as functional applications. The combination of high strength with high elastic strain of fully nanocrystalline and glassy alloys renders them quite unique in comparison to conventional (micro‐)crystalline materials. However, one major drawback for their use in engineering applications is the often limited macroscopic plastic deformability, despite the fact that some of these alloys show perfectly elastic‐plastic deformation behavior. To improve the room temperature ductility of either fully nanocrystalline or amorphous alloys, the concept of developing a heterogeneous microstructure combining a glassy or nanostructured matrix with second‐phase particles with a different length‐scale, has recently been employed. This review describes the composition dependent metastable phase formation in the Zr‐(Ti/Nb)‐Cu‐Ni‐Al alloy system, which in turn alters the mechanical properties of the alloys. We emphasize the possibilities to manipulate such composite microstructures in favor of either strength or ductility, or a combination of both, and also discuss the acquired ability to synthesize such in‐situ high‐strength composite microstructures in bulk form through inexpensive processing routes.     

Nanotubular oxide surface and layer formed on the Ti-35Ta-xZr alloys for biomaterials

Titanium and its alloys are widely used as a dental implant material in clinical dentistry and as an orthopedic implant materials due to their good mechanical properties, corrosion resistance, and biocompatibility. In this study, nanotubular oxide surface and layer formed on the Ti-35Ta-xZr alloys for biomaterials have been investigated by using electrochemical methods. Ti-35Ta-xZr alloys were prepared by arc melting and heat treated for 24 hr at 1000 degrees C in argon atmosphere, and then water quenching. Ti oxide nanotubes were formed on the Ti-35Ta-xZr alloys by anodizing in H3PO4 containing 0.8 wt% NaF solution at 25 degrees C. Anodization was carried out using a scanning potentiostat. Microstructures of the alloys and nanotube surface were examined by FE-SEM, EDX, and XRD. Crystallization treatment of nanotube surface was carried out for 3 hr at 450 degrees C. Microstructures of the Ti-35Ta-xZr alloys were changed from beta phase to alpha' phase, and changed from an equiaxed to a needle-like structure with increasing Zr content. Nanotubular oxide surface and layers consisting of highly ordered nanotubes with a wide range of diameters (approximately 150-200 nm) and lengths (approximately 4-10 microm) can be formed on alloys in the Ti-35Ta-xZr alloys with Zr content. As the Zr content increased from 3% to 15%, length of step between the bamboo knob-like had increasing values of approximately 50 nm, 80 nm, and 140 nm, respectively. The nanotubes formed on the Ti-35Ta-xZr alloy surface were amorphous structure before heat treatment, but oxide surface had mainly an anatase structure by heat treatment.     

Bioactive Ti Metal and its Alloys Prepared by Chemical Treatments: State‐of‐the‐Art and Future Trends

Titanium metal and its alloys are widely used as various implants in orthopaedic and dental fields, because of their good biocompatibility and high mechanical strength. However, they do not bond to living bone. Here, we review the literature showing that they will spontaneously bond tightly to living bone, if they are treated with an acid or alkaline solution and then subjected to heat treatment. This occurs due to the materials becoming negatively or positively charged on their surfaces in the bodily environment, which induces apatite deposition. Porous Ti metal subjected to such treatment exhibits not only osteoconductivity but also osteoinductivity. Examples and clinical applications are reviewed.     

Failure Analysis of Titanium-Based Dental Implant

Metallic materials designed for applications in orthopedic or dental surgical implants must show a group of properties, including biocompatibility, mechanical strength and resistance to degradation (by wear or corrosion) outstand. In order to assure that the properties are achieved, the implant materials must fulfill certain requirements, usually specified in standards. The standards also include chemical composition, microstructure and even macrographic aspects. The main aim of this work was to perform a failure analysis on a titanium-based dental implant and connect the possible causes of failure with the associated material requirements which were previously mentioned. Evaluation techniques included metallographic analysis by optical microscopy and fractographic analysis by scanning electron microscopy (SEM). The results of the examinations suggested that, in spite of their adequate microstructures, the implants fractured due to the overload generated by stress raisers which were found in the implants.     

Development and characterization of β-type Ti-Zr-Nb-Sn dental implant materials

To develop new materials of proper elastic modulus and biocompatibility for dental implants, Ti-2Zr-xNb-xSn (x = 0, 0.1, 0.2, 0.3) and Ti-2Zr-xNb-xMo (x = 0, 0.1, 0.2, 0.3) alloys were designed and fabricated. Effects of alloying elements on properties and the feasibility of application in dentistry are analyzed. It is indicated that Nb, Sn and Mo obviously influence the phase compositions of Ti-2Zr-based biological alloys. With the increase of alloying element content, all the alloys tend to form a single β-Ti phase. Ti-2Zr-xNb-xSn alloys exhibit better mechanical properties and corrosion resistance than the Ti-2Zr-xNb-xMo alloys. The Ti-2Zr-0.1Nb-0.1Sn alloy has proper elastic modulus (14.72 GPa) (which is very close to the natural bones), excellent corrosion resistance and comprehensive mechanical properties, and is considered as ideal candidate for implant materials.     

Surface modification of Ti-6Al-4 V alloy for biomineralization and specific biological response: part II,alkaline phosphatase grafting

Titanium and its alloys are the most widespread materials for the realization of orthopaedic and dental implants due to their good mechanical properties and biocompatibility. Surface functionalization of biomaterials aimed to improve and quicken implant integration and tissue regeneration is an active research field. The opportunity to confer biological activity (ability to directly stimulate cells with proper biological signals) to the Ti6Al4 V alloy, previously modified to be bioactive from the inorganic point of view (apatite precipitation), was explored in this research work. The alkaline phosphatase (ALP) enzyme was grafted to metal surface via tresyl chloride activation, maintaining its activity. A synergistic effect between biological functionalization and inorganic bioactivity was observed.     

Effect of minor content of Gd on the mechanical and degradable properties of as-cast Mg-2Zn-xGd-0.5Zr alloys

RE-containing Mg alloys used as biodegradable medical implants exhibit good promising application due to their good mechanical properties and degradation resistance. In this work, effect of Gd on the microstructure, mechanical properties and biodegradation of as-cast Mg-2Zn-xGd-0.5Zr alloys was investigated. The results showed that there were mainly α-Mg, I-phase, W-phase and MgZn2 phase in Mg-Zn-Gd-Zr alloys. With increase of the Gd content, the strength of the alloys was enhanced due to the second phase strengthening and grain refinement. The degradation resistance of Mg-2Zn-0.5Zr alloy was increased by adding 0.5%–1% Gd due to the uniformly distributed second phases which acted as a barrier to prevent the pitting corrosion. However, increasing Gd content to 2% reduced the degradation resistance of the alloy due to the galvanic corrosion between the matrix and the second phases.The good degradation resistance and mechanical properties of as-cast Mg-2Zn-1Gd-0.5Zr alloy makes it outstanding for biomaterial application.     

Characterization and antibacterial performance of ZrCN/amorphous carbon coatings deposited on titanium implants

Titanium (Ti)-based materials have been used for dental/orthopedic implants due to their excellent biological compatibility, superior mechanical strength and high corrosion resistance. The osseointegration of Ti implants is related to their composition and surface treatment. Better biocompatibility and anti-bacterial performances of Ti implant are beneficial for the osseointegration and for avoiding the infection after implantation surgery. In this study, nanocomposite ZrCN/amorphous carbon (a-C) coatings with different carbon contents were deposited on a bio-grade pure Ti implant material. A cathodic-arc evaporation system with plasma enhanced duct equipment was used for the deposition of ZrCN/a-C coatings. Reactive gas (N₂) and C₂H₂ activated by the zirconium plasma in the evaporation process were used to deposit the ZrCN/a-C coatings. To verify the susceptibility of implant surface to bacterial adhesion, Actinobacillus actinomycetemcomitans (A. actinomycetemcomitans), one of the major pathogen frequently found in the dental implant-associated infections, was chosen for in vitro anti-bacterial analyses. In addition, the biocompatibility of human gingival fibroblast (HGF) cells on coatings was also evaluated by a cell proliferation assay. The results suggested that the ZrCN/a-C coatings with carbon content higher than 12.7 at.% can improve antibacterial performance with excellent HGF cell compatibility as well.     

Corrosion and in vitro biocompatibility properties of cryomilled-spark plasma sintered commercially pure titanium

Ti alloys, such as Ti6Al4V, are currently used in biomedical and dental implant applications. Ti alloys are used because they are stronger than commercially pure (CP) Ti due to the presence of alloying elements. However, toxicity of alloying elements during long-term use of implants is of concern. Another means of increasing the strength of materials is grain size refinement. In this study, ultrafine-grained (UFG, ~250 nm to 1 μm) CP Ti was produced by cryomilling followed by spark plasma sintering (SPS). Electrochemical impedance spectroscopy (EIS) and cell culture experiments were performed to compare the corrosion and biocompatibility properties of coarse grained (CG) Ti and UFG Ti. It was found that UFG Ti exhibited corrosion resistance comparable to CG Ti in Ringers solution. In addition, UFG Ti exhibited a reduced inflammatory response and enhanced cell adhesion compared to CG Ti. Investigation of surface roughness provided an explanation for enhanced cell adhesion.     

Effect of sintering parameters on microstructure,mechanical properties and electrochemical behavior of Nb–Zr alloy for biomedical applications

Despite the importance of Nb–Zr alloys as candidate materials for biomedical applications, little attention has been given to their processing and the development of new or improved structures. Here, we explore the viability of synthesizing a nano/sub-micron grain structured Nb–Zr alloy through the use of mechanical alloying (MA) and spark-plasma sintering (SPS). The sintered samples were characterized through measurements of densification, Vickers hardness (HV), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The effect of the SPS parameters on the microstructure and mechanical properties of the sintered alloys was also investigated. Moreover, electrochemical corrosion analyses were performed by a means of a conventional three-electrode cell to assess the corrosion resistance of the developed alloys in Simulated Body Fluids (SBF) medium. A nano/sub-micron grain structured Nb–Zr alloy with an average grain size of between 100 and 300 nm was produced using the MA-SPS techniques. A maximum hardness and relative density of 584 HV and 97.9% were achieved, respectively. Moreover, the nano/sub-micron grain structured Nb–Zr alloy exhibited higher corrosion resistance in SBF medium, which makes this alloy is a promising candidate for use in biomedical applications.     

A review on nickel-free nitrogen containing austenitic stainless steels for biomedical applications

The field of biomaterials has become a vital area, as these materials can enhance the quality and longevity of human life. Metallic materials are often used as biomaterials to replace structural components of the human body. Stainless steels, cobalt–chromium alloys, commercially pure titanium and its alloys are typical metallic biomaterials that are being used for implant devices. Stainless steels have been widely used as biomaterials because of their very low cost as compared to other metallic materials, good mechanical and corrosion resistant properties and adequate biocompatibility. However, the adverse effects of nickel ions being released into the human body have promoted the development of “nickel-free nitrogen containing austenitic stainless steels” for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel and emphatically the advantages of nitrogen in stainless steel, as well as the development of nickel-free nitrogen containing stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength, better corrosion and wear resistance and superior biocompatibility in comparison to the currently used austenitic stainless steel (e.g. 316L), the newly developed nickel-free high nitrogen austenitic stainless steel is a reliable substitute for the conventionally used medical stainless steels.     

The effects of calcium and yttrium additions on the microstructure,mechanical properties and biocompatibility of biodegradable magnesium alloys

In this study, the effects of calcium (Ca) and yttrium (Y) on the microstructure, mechanical properties, corrosion behaviour and biocompatibility of magnesium (Mg) alloys, i.e. Mg–xCa (x = 0.5, 1.0, 2.0, 5.0, 10.0, 15.0 and 20.0%, wt%, hereafter) and Mg–1Ca–1Y, were investigated. Optical microscopy, X-ray diffractometry (XRD), compressive and Vickers hardness testing were used for the characterisation and evaluation of the microstructure and mechanical properties. The in vitro cytotoxicity of the alloys was assessed using osteoblast-like SaOS2 cells. The corrosion behaviour of these alloys was evaluated by soaking the alloys in simulated body fluid (SBF) and modified minimum essential medium (MMEM) at 37 °C in a humidified atmosphere with 5% CO₂. Results indicated that the increase of the Ca content enhances the compressive strength, elastic modulus and hardness of the Mg–Ca alloys, but deteriorates the ductility, corrosion resistance and biocompatibility of the Mg–Ca alloys. The Y addition leads to an increase in the ductility; but a decrease in the compressive strength, hardness, corrosion resistance and biocompatibility of the Mg–1Ca–1Y alloy when compared to the Mg–1Ca alloy. Solutions of SBF and MMEM with the immersion of Mg–xCa and Mg–1Ca–1Y alloys show strong alkalisation. Our research results indicate that Mg–xCa alloys with Ca additions less than 1.0 wt% exhibited good biocompatibility, low corrosion rate as well as appropriate elastic modulus and strength; whilst the Y is not a proper element for Mg alloys for biomedical application due to its negative effects to the corrosion resistance and biocompatibility.     

The estimation of corrosion behaviour of ZrTi binary alloys for dental applications using electrochemical techniques

Titanium and zirconium are in the same group in the periodic table of elements and are known to have similar physical and chemical properties. Both Ti and Zr usually have their surfaces covered by a thin oxide film spontaneously formed in air. However, the cytotoxicity of ZrO₂ is lower than that of TiO₂ rutile. Treatments with fluoride are known as the main methods to prevent plaque formation and dental caries. The corrosion behaviour of ZrTi alloys with Ti contents of 5, 25 and 45 wt.% and cp-Ti was investigated for dental applications. All samples were tested by linear potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) performed in artificial saliva with different pH levels (5.6 and 3.4) and different fluoride (1000 ppm F⁻) and albumin protein (0.6%) contents. In addition, scanning electron microscopy (SEM) was employed to observe the surface morphology of the test materials after linear potentiodynamic polarisation. The corrosion current densities for the ZrTi alloys increased with the titanium content. The Zr5Ti and Zr25Ti alloys were susceptible to localised corrosion. The role that Ti plays as an alloying element is that of increasing the resistance of ZrTi alloy to localised corrosion. The presence of 0.6% albumin protein in fluoridated acidified artificial saliva with 1000 ppm F⁻ could protect the cp-Ti and ZrTi alloys from attack by fluoride ions.     

Study on the biodegradability and biocompatibility of WE magnesium alloys

Magnesium alloys have been widely applied in biomedical devices because of their high strength, toughness, processing performance and the trace release of Mg^2 +. In this study, we investigated the biodegradability, cytocompatibility and hemocompatibility of four kinds of WE Mg alloys (where “W” indicates the metallic element Y and “E” represents mixed rare earth [RE] elements; Y: 2.5, 5.0, 6.5, and 7.5 wt.%; Nd: 1.0, 2.5, 2.6, and 4.2 wt.%; Zr: 0.8 wt.%) for their application in intravascular stent fabrication. The content of alloying elements affected not only mechanical properties of materials, but also their biocompatibility. We found that addition of RE elements could reduce the corrosion rates. Human umbilical vein endothelial cells (HUVECs) and vascular smooth muscle cells (VSMCs) were cultured in different extracts of WE Mg alloys. MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used to examine the cytotoxicity. The nitric oxide (NO) and lactate dehydrogenase (LDH) release were used to evaluate effects of Mg alloys on HUVECs. The addition of Y, Nd and Zr increased the cell viability and improved the hemocompatibility. Different alloy elements affected the morphology of samples, Mg^2 + release, and pH values in the medium. The results of mechanical properties, biocompatibility and biodegradability showed that Mg–5.0Y–2.6Nd–0.8Zr might be used as alternative materials of stent. However, it still needs to be further modified for clinical use. These findings suggest that selecting suitable alloying elements is particularly important.     

In vitro Response of Human Mesenchymal Stromal Cells to Titanium Coated Peek Films and Their Suitability for Magnetic Resonance Imaging

Medical imaging is an important tool for the post-operative checkup of an accurate position of an implant as well as for monitoring the integration in the adjacent tissue that may influence the success of a medical device.Unfortunately,the possibility to use imaging methods is associated with the implant material and all the established metallic materials for surgery do not show a proper "imaging compatibility".The present study is a combined investigation of the in vitro response to human mesenchymal stromal cells(hMSC) and magnetic resonance imaging(MRI) compatibility of the potential material combination polyetheretherketone/titanium(PEEK/Ti) for medical devices.Because of the advantageous imaging properties and the mechanical and chemical stability,PEEK becomes more and more an alternative to common metallic implant materials like titanium or cobalt-chrome.However,PEEK is a bioinert material having a limited ability for direct bone incorporation.Due to its excellent biocompatibility,Ti was chosen as coating material to enhance the cellular response.The result is a combination with advantageous properties:the magnetic susceptibility and elastic modulus close to bone,corrosion resistance and mechanical flexibility of PEEK and the excellent biocompatibility of titanium.The appearance of metal-related artifact was discussed in electrical resistivity and magnetic susceptibility.Therefore,two titanium coatings have been investigated:a complete coating and a structured surface avoiding surface conductivity.To determine the in vitro biocompatibility,the cell responses were assessed in terms of the overall morphology of the hMSC and their cell area distribution,proliferation,osteogenic differentiation and mineral deposition.The cellular stress was evaluated by the prostaglandin E_2 level.The bonded materials both produced no disturbing artifacts in magnetic resonance imaging.Compared to the pure PEEK material,the titanium coated specimens showed an enhanced biocompatibility,which is indicated by a higher cell number,larger activity of the enzyme tissue non-specific alkaline phosphatase and therefore a greater amount of deposited calcium and phosphate.The results on bare PEEK are accompanied with a higher cellular stress level,which is indicated by prostaglandin E_2.