Biomedical Alloy Research Articles

Current and friction transients during tribocorrosion of a biomedical titanium alloy sliding against zirconia

The tribocorrosion mechanisms of a Ti–Nb–Zr–Ta (TNZT) alloy rubbing against a zirconia ball in a Phosphate Buffered Solution (PBS) were analyzed by considering the mechanical and chemical interactions that lead to the material degradation. To understand the governing mechanisms, tribocorrosion tests were carried out in a ball on flat contact and reciprocating tribometer applying two normal loads (6.7 and 11.2 N) and two different anodic potentials (400 and 1500 mV). Worn surfaces were characterized by SEM and FIB cross sections as well as surface analysis. During the tribocorrosion tests, transients consisting in simultaneous abrupt current increase and a decrease in the COF were found at different instants of the tests. Subsurface cracking as well as third body build up were observed. These different phenomena and their temporal sequence could be described by initial sliding tribocorrosion, including metal particle removal and surface oxidation, followed by the build-up of a protective third body. After a certain number of sliding cycles subsurface cracking due to strain accumulation beyond a critical level lead to the detachment of a large particles that reactivated the initial tribocorrosion process. This mechanistic view could be substantiated by a model based on a quantitative analysis of the results.

A feasibility study of promoting osseointegration surface roughness by micro-milling of Ti-6Al-4V biomedical alloy

The reliability of a prosthetic implant needs durability, biocompatibility, and osseointegration capability. Accomplishing these characteristics, Ti-6Al-4V alloy is the main used material for implant fabrication. Moreover, it can be processed by additive manufacturing technique, permitting to meet the needs of patience-tailored, often complex shaped, prosthesis topologies. Once an implant is realized, it is finished by machining operations and its osseointegration capability is heavily influenced by the resulting surface roughness. Consequently, the assessment of this latter is mandatory to evaluate the prosthesis durability. This paper presents the analysis of surface roughness of Ti-6Al-4V micro-milled specimens produced by plastic deformation, selective laser melting, and electron beam melting processes. A central composite design was employed for planning the cutting tests. The comparison between surface roughness results and its values for enhancing osseointegration, firstly permitted to individuate the range of micro-milling suitable applications, which have been individuated as ball joints, bone plates, and screws. Next, the statistical analysis of the experimental measurements allowed the identification of the most influential micro-milling parameters together with the determination of the mathematical models of surface roughness by response surface methodology. The good comparison among calculated and experimental results revealed the reliability of the model, allowing the prediction of achievable surface roughness once micro-machining parameters are selected, or their optimization as a function of a desired surface roughness value.

Electrochemical and in vitro biological behaviors of a Ti-Mo-Fe alloy specifically designed for stent applications

There is a deep interest in developing new Ni-free Ti-based alloys to replace 316 L stainless steel and Co-Cr alloys for endovascular stent application, mainly because the release of Ni can generate toxicity and allergenicity. Interactions of Ti alloy biomaterials with bone cells and tissues have been widely investigated and reported, while interactions with vascular cells and tissues, such as endothelial cells (ECs) and smooth muscle cells (SMCs), are scarce. Therefore, this study focused on the relationship among the surface finishing features, corrosion behavior and in vitro biological performances regarding human ECs, SMCs and blood of a newly developed Ti-8Mo-2Fe (TMF) alloy, specifically designed for balloon-expandable stent applications. The alloy performances were compared to those of 316 L and pure Ti, prepared with the same surface finishing techniques, which are mechanical polishing and electropolishing. Surface properties were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle (CA) and x-ray photoelectron spectroscopy (XPS). The corrosion behavior was assessed with potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests in phosphate buffered saline (PBS) solution. No significant differences were observed regarding the corrosion rate measured with PDP analyses, which was of the order of 2 × 10−4 mm/y for all the studied materials. Moreover, similarly to pure Ti, TMF exhibited an advantage over 316 L for biomedical applications, namely remarkable resistance to pitting corrosion up to high potentials. The results evidenced a good cytocompatibility and hemocompatibility, making this group of alloy a potential candidate for cardiovascular implants. In fact, both ECs and SMCs proliferated on TMF surfaces showing a 7-day viability similar to that of pure Ti. Regarding hemocompatibility, TMF did not cause hemolysis, and blood coagulation was delayed on its surface in comparison to pure Ti. When compared to 316 L, TMF showed similar hemocompatibility.

Synthesis and Characterization of Nanostructured Oxide Layers on Ti-Nb-Zr-Ta and Ti-Nb-Zr-Fe Biomedical Alloys.

Nanoporous/nanotubular complex oxide layers were developed on high-fraction β phase quaternary Ti-Nb-Zr-Ta and Ti-Nb-Zr-Fe promising biomedical alloys with a low elasticity modulus. Surface modification was achieved by electrochemical anodization aimed at the synthesis of the morphology of the nanostructures, which exhibited inner diameters of 15-100 nm. SEM, EDS, XRD, and current evolution analyses were performed for the characterization of the oxide layers. By optimizing the process parameters of electrochemical anodization, complex oxide layers with pore/tube openings of 18-92 nm on Ti-10Nb-10Zr-5Ta, 19-89 nm on Ti-20Nb-20Zr-4Ta, and 17-72 nm on Ti-29.3Nb-13.6Zr-1.9Fe alloys were synthesized using 1 M H3PO4 + 0.5 wt% HF aqueous electrolytes and 0.5 wt% NH4F + 2 wt% H20 + ethylene glycol organic electrolytes.

Amorphous/quasi-amorphous CoCrMo-C coatings for improved electrochemical properties and tribocorrosion resistance of biomedical alloys

Coatings based on CoCrMo cobalt alloy have been designed to increase corrosion resistance of AISI 316L austenitic steel in body fluids. The CoCrMo-C coatings, with different carbon content (from 0 to 65 at.%), were synthesised by reactive magnetron sputtering of the medical CoCrMo (ISO 5832-12) alloy in the argon-acetylene atmosphere. Evolution of their structure, crystalline phase and chemical composition with increasing carbon fraction were investigated by X-ray diffractometry and X-ray photoelectron spectroscopy. Evaluation of corrosion resistance of CoCrMo-C films on 316L steel substrates was performed in Hanks' Balanced Salt Solution (HBSS) by means of electrochemical impedance spectroscopy and potentiodynamic polarisation tests. The studies have revealed high chemical inertness of all coatings. Passive/blocking layers formed on their surfaces exhibit a capacitive character, while their resistivity is nearly two orders of magnitude higher compared to the corresponding values obtained for the CoCrMo and 316L alloys tested under similar conditions. The CoCrMo-C coatings provide pitting corrosion protection for 316L austenitic steel. This is expected to extend the residence time of stainless steel in the tissue environment. The 316L/CoCrMo-C systems were also subjected to tribocorrosion tests in HBSS at three electrochemical potentials, i.e. at open circuit, cathodic, and anodic potential. A model was developed to relate the electrical values recorded during tribocorrosion experiments with the phenomena that characterize the coating wear process. It was found that the friction coefficient systematically decreased from 0.7 to 0.1 with increasing the carbon content under all potentials applied. The wear rate observed for majority of the coatings was lower than that for CoCrMo and 316L alloys. For the samples with the highest carbon content, the wear rate under oxidative conditions became even lower than for the cathodic potential.

A comparative study on the machinability of β-type novel Ti29Nb13Ta4.6Zr (TNTZ) biomedical alloys under micro-milling operation

β-type Ti29Nb13Ta4.6Zr (TNTZ) alloy is a new-generation alloy that does not contain toxic/allergic elements. The alloys have low Young's modulus (⁓60 GPa) close to the cortical bone, providing excellent biocompatibility over familiar commercial CP Ti or Ti6Al4V ELI. Different macro/micro-machining methods can be used to obtain the final shape of Ti alloys and impact on the implant's bio ecosystem and performance. This study compared the micro-mill machining performance of the TNTZ alloys with commercial CP Ti and Ti6Al4V ELI. Machining experiments were performed under similar and different cutting speeds and constant feed rate conditions. In the study, cutting forces, surface roughness, burr size and tool wear were obtained. The results showed that the Fx cutting forces for TNTZ are about 3 and 2 times higher than for CP Ti and Ti6Al4V alloys, respectively. Depending on the increasing cutting distance, the surface roughness of CP Ti and Ti6Al4V materials followed a similar trend. However, the surface roughness values for TNTZ alloy are higher and decrease with increasing cutting distance. Burr widths for CP Ti and Ti6Al4V are similar, but the milling direction at which maximum burr occurs varies. In the TNTZ alloy, the burr width is about 3 times higher, and the burr occurred inside the slot, not at the edge of the machined slot. The predominant type of damage in milling Ti6Al4V and CP Ti materials is abrasive and adhesive. The dominant damage type in TNTZ alloy is chipping. Increasing the number of revolutions in TNTZ causes the cutting forces and burr width to increase.

Quasicrystal-strengthened biomedical magnesium alloy fabricated by laser additive manufacturing

Magnesium (Mg) alloy, particular Mg-zinc (Zn) series alloy, owns great potential as implant materials, despite it shows uncontrollable corrosion rate and inadequate mechanical performance. Herein, alloying treatment with rare earth yttrium (Y) was adopted to improve the overall performance of Mg-Zn-Zr alloy. In detail, Mg-Zn-Y-Zr (ZW61) parts were fabricated using laser additive manufacturing. Results showed that Y addition had contributed to the formation of quasicrystal I-phase (Mg3Zn6Y), which could serve as weak cathode and alleviate the occurrence of micro-galvanic corrosion within Mg matrix. Meanwhile, Y addition also promoted the development of compact corrosion film, which further delayed the corrosion of Mg matrix. Thus, laser-processed ZW61 parts showed a considerably reduced corrosion rate of 0.67 mm/year. More importantly, the quasicrystal I-phase contributed to a more significant Orowan strengthening effect, as it exhibited a near-coherent interface with α-Mg. Besides, rare earth Y decreased the energy of stacking faults, and thereby activated numerous non-basal slip systems during deformation, as proved by TEM observation. Thus, as-built ZW61 part showed simultaneously improved strength and plasticity. Furthermore, in vitro cell testing proved its favorable biocompatibility. Our research exhibited that laser-processed ZW61 was a competitive candidate for orthopedics device application.

Poly(lactic acid) coating with a silane transition layer on MgAl LDH-coated biomedical Mg alloys for enhanced corrosion and cytocompatibility

Poly(lactic acid) coating with a silane transition layer on MgAl LDH-coated biomedical Mg alloys for enhanced corrosion and cytocompatibility

Discovery of novel low modulus Nb–Ti–Zr biomedical alloys via combined machine learning and first principles approach

The β-phase stabilized ternary Nb–Ti–Zr alloys with low Young's modulus similar to that of the human bones (30 GPã40 GPa) are highly desirable for biomedical application. However, owing to the extensive compositions and available solute contents, searching low-modulus ternary Nb–Ti–Zr alloys is still challenging and has attracted intense attention recently. Herein, based on first principles calculations and artificial neural networks (ANN) model, we discover a potential region of interest in the compositional map with a small dataset containing merely 85 data. The results obtained by the first principles calculations and machine learning (ML) approaches are extremely close, suggesting high predictive accuracy and good feature validity of the well-trained ANN model. The random forest classifier is also applied to search the features which have more significant influence on mechanical stability. The valence electron concentration, niobium content and average bulk modulus are the three key features strongly correlated with the elastic properties of Nb–Ti–Zr random solid solution. Then the feature importance was analyzed, which can help to deepen our understanding of the relationship between the solute effects and mechanical properties. Finally, the electronic structure was computed to investigate the bonding behavior, and the bonding force can account for the difference of the modulus.

Biomedical core–shell micro-nanocrystalline Ti6Al4V5Cu alloy with high fatigue properties

Fatigue strength is one of the most important indicators used to evaluate titanium alloy orthopedic implants. Sufficient fatigue strength can enhance the reliability of orthopedic implants. It is well known that grain refinement is beneficial to enhancing material strength but against fatigue resistance. This work has fabricated a triple-phase core–shell micro-nanostructure in a biomedical Ti6Al4V5Cu alloy with the ultrafine grained α phase stabilized by the nanoscale conjugated β/Ti2Cu shells, of which results in a high fatigue resistance. The results show that the average grain size of the α, β, and Ti2Cu phases are 585 nm, 421 nm and 135 nm, respectively. The soft β phases in the conjugated β/Ti2Cu shells can produce a plastic zone at the crack tip to hinder the propagation of fatigue cracks. The fatigue strength of the core–shell structured Ti6Al4V5Cu alloy is 750 MPa under the experimental condition of R = -1 andNf = 107, which is 36.3 % higher than the widely used Ti6Al4V alloy. The mechanisms for improving the fatigue properties have also been discussed. These are of great significance for the development of a new generation of high fatigue strength titanium alloys.

Electrochemical and biological characterization of Ti–Nb–Zr–Si alloy for orthopedic applications

The performance of current biomedical titanium alloys is limited by inflammatory and severe inflammatory conditions after implantation. In this study, a novel Ti–Nb–Zr–Si (TNZS) alloy was developed and compared with commercially pure titanium, and Ti–6Al–4V alloy. Electrochemical parameters of specimens were monitored during 1 h and 12 h immersion in phosphate buffered saline (PBS) as a normal, PBS/hydrogen peroxide (H2O2) as an inflammatory, and PBS/H2O2/albumin/lactate as a severe inflammatory media. The results showed an effect of the H2O2 in inflammatory condition and the synergistic behavior of H2O2, albumin, and lactate in severe inflammatory condition towards decreasing the corrosion resistance of titanium biomaterials. Electrochemical tests revealed a superior corrosion resistance of the TNZS in all conditions due to the presence of silicide phases. The developed TNZS was tested for subsequent cell culture investigation to understand its biocompatibility nature. It exhibited favorable cell-materials interactions in vitro compared with Ti–6Al–4V. The results suggest that TNZS alloy might be a competitive biomaterial for orthopedic applications.

In Vitro Bioelectrochemical Properties of Second-Generation Oxide Nanotubes on Ti–13Zr–13Nb Biomedical Alloy

Surface charge and in vitro corrosion resistance are some of the key parameters characterizing biomaterials in the interaction of the implant with the biological environment. Hence, this work investigates the in vitro bioelectrochemical behavior of newly developed oxide nanotubes (ONTs) layers of second-generation (2G) on a Ti-13Zr-13Nb alloy. The 2G ONTs were produced by anodization in 1 M (NH4)2SO4 solution with 2 wt.% of NH4F. The physical and chemical properties of the obtained bamboo-inspired 2G ONTs were characterized using scanning electron microscopy with field emission and energy dispersive spectroscopy. Zeta potential measurements for the examined materials were carried out using an electrokinetic analyzer in aqueous electrolytes of potassium chloride, phosphate-buffered saline and artificial blood. It was found that the electrolyte type and the ionic strength affect the bioelectrochemical properties of 2G ONTs layers. Open circuit potential and anodic polarization curve results proved the influence of anodizing on the improvement of in vitro corrosion resistance of the Ti-13Zr-13Nb alloy in PBS solution. The anodizing conditions used can be proposed for the production of long-term implants, which are not susceptible to pitting corrosion up to 9.4 V.

Finishing the surface micro-layer of additively manufactured TiAl alloy using electro-thermal discharge assisted post-processing

Selective laser melting (SLM) of titanium–aluminium (TiAl) alloy components has gained significant attention in the modern industrial world. The flexibility of the SLM process in producing complex shapes with minimum utilization of material and energy makes it dominant over other manufacturing techniques. As aerospace and biomedical industries demand complex-shaped TiAl alloy components, part fabrication using SLM becomes the ultimate solution. However, the unacceptable level of surface integrity and anisotropic behavior of SLM components demand post processing operations such as laser polishing, chemical polishing, and conventional polishing methods. In this study, a recently developed polishing method called wire electrical discharge polishing (WEDP) is performed on TiAl alloys for obtaining a smooth and defect-free surface. This study aims to investigate the micro-layer modification occurring to the WEDP-processed surface in detail. The experimental results establish the effectiveness of WEDP method in terms of improved surface integrity. The surface finish (Sa) got enhanced by ~88% after WEDP processing. In addition, the thickness of recast layer formed by WEDP was found to be minimum. Moreover, post-processing of TiAl alloy resulted in better surface morphology specifically at lower settings of peak current. It is noteworthy that the migration of wire material was minimum with zinc-coated brass electrode compared to the normal brass electrode. Hence, coated wire electrodes are recommended for WEDP process. In short, an excellent surface integrity can be achieved using WEDP process through favorable surface modification aided by lower peak current and coated wire electrodes. Furthermore, less electrode wear observed in WEDP process enables the deployment of lower feed rates leading to minimal electrode consumption.

Surface, mechanical, and in vitro corrosion properties of arc-deposited TiAlN ceramic coating on biomedical Ti6Al4V alloy

The surface characteristics and in vitro corrosion properties of nanocrystalline TiAlN coating deposited on biomedical Ti6Al4V alloy by cathodic arc evaporation were studied. The structure and surface morphology were analyzed using XRD and FE-SEM. The surface microhardness was characterized using microindentation. The in vitro corrosion was carried out in simulated body fluid and artificial saliva. The results exhibited that a TiAlN film was deposited with an enhanced surface hardness of 44.4 GPa, a elastic modulus of 419.9 GPa, and enhanced resistance to plastic deformation compared to uncoated alloy. The wetting angle of TiAlN coated surfaces ((86.27±2.2)°) was larger than that of uncoated Ti6Al4V surfaces ((70.61±1.25)°). Biocorrosion examinations revealed that TiAlN-coated Ti6Al4V alloy exhibited a significant positive shift of about 150 mV in corrosion potential with the notable reduction of about one order of magnitude in corrosion current density and higher charge transfer resistance, confirming the improved barrier performance. The surface, mechanical and corrosion performance of TiAlN coating was enhanced compared to uncoated alloy.

Multi Wear Response Optimization of Ti-6Al-7Nb Biomedical Alloy

Titanium-Niobium (Ti-Nb) based alloys are predominantly used as an implant material within the Bio-medical field due to their unique characteristics such as non-toxicity, better Osseo-integration, high resistance to corrosion, high strength to weight quantitative relation and biocompatibility. This paper proposes to optimize the control parameters for multi-response optimization of Ti-6Al-7Nb bio-medical alloy based Grey Relational Analysis combined with the Taguchi approach. Wear rate (WR), coefficient of friction (COF), and frictional force were the response characteristics studied (FF). The Taguchi methodology is used in conjunction with the grey interpersonal evaluation as a performance index to determine the best set of control parameters. Applied Load, Rotational Speed, and Time were the control parameters evaluated. Experimentations are designed using L9 Taguchi's orthogonal array and carried out on a pin-on-disc setup in agreement by ASTM G99. The experimental outcomes display that the applied load has the greatest impact on the Ti-6Al-7Nb bio-medical alloy's various wear characteristics. This approach has been successfully rummage-sale to recover the wear response of Ti-6Al-7Nb bio-medical alloy.

Double-layer calcium phosphate sandwiched siloxane composite coating to enhance corrosion resistance and biocompatibility of magnesium alloys for bone tissue engineering

As a potential new metal biomaterial implant, the problem of fast degradation of magnesium (Mg) alloy and the resulting hydrogen release and local alkalinization need to be solved urgently. To this end, a double-layer dicalcium phosphate dihydrate (DCPD) sandwiched siloxane composite coating was prepared on Mg alloy via chemical conversion, immersion treatment and biomimetic deposition methods. The morphologies and compositions of the samples were systematically analyzed, and the corrosion resistance, as well as the biocompatibility, was specifically explored. The electrochemical test result evidenced that the corrosion current density of the composite coated sample was 3.0 × 10−8 A/cm2, which was much smaller than that of Mg alloy (1.4 × 10−5 A/cm2). The immersion experiment showed that the coating provided continuous protection to the sample. The hydrogen evolution of the optimal coating was only 1/9 of that of Mg alloy, and the coated sample had good biomineralization ability. The coated samples also showed good biocompatibility in cell experiments (cell viability ≥116.5 %). This study proposes a novel idea for the construction of surface coatings of biomedical Mg alloys for bone tissue engineering.

Poly-lactic acid coatings on the biomedical WE43 Mg alloy: Protection mechanism and ion permeation effects

Poly-lactic acid (PLA) coatings are deposited on the biomedical WE43 Mg alloy and the degradation behavior and mechanism are investigated during 504 h immersion in Hanks' solution. The protection mechanism is investigated by studying the ion permeation phenomenon and a custom in situ apparatus is constructed to evaluate the performance and elucidate the underlying principle. Na+ and Cl− penetrate the PLA membrane gradually and the different permeation effects stem from the different hydrated radii. Ion permeation can be separated into three stages. Penetration of Cl− is dominant in the first stage and Na+ dominates in the third stage. Penetration of Ca2+, PO43− along with other ions also take part in the third stage leading to the formation of Ca−P on the WE43 substrate. Based on the experimental results, models are established to describe the ion permeation behavior through the PLA membrane and predict the protection efficiency of the PLA coating. The results enrich our understanding of ion permeation in polymeric coatings and provide insights into the development of Mg-based biomedical implants.

Optimizing mechanical properties and Ag ion release rate of silver coatings deposited on Ti-based high entropy alloys

This paper details the characterization of microstructure, texture, mechanical properties, and ion release behavior of antibacterial Ag thin films sputtered on two novel biomedical high entropy alloys (HEAs), namely the Ti23Ta10Hf27Nb12Zr28 (HEATi23) and Ti28Ta10Hf30Nb14Zr18 (HEATi28) alloys. Specifically, the influences of varying deposition time and Ar flow rate were investigated to reveal the mechanisms dictating the microstructure, texture, and mechanical properties of the coatings. In addition, static immersion experiments were carried out in simulated body fluid (SBF) for 28 days to establish the relationship between ion release from the coatings and the deposition parameters, microstructure, and surface texture. It was shown that texture evolution in Ag thin films depends on both film thickness and Ar flow rate, such that there exists a critical thickness at which the energy minimization mechanism is altered. A very good correlation was also observed between an increase in (111) peak intensity and a decrease in released Ag ion fraction. Overall, the findings of the work presented herein suggest that the alterations in Ag deposition parameters could be optimized to obtain the desired mechanical properties while enhancing the biocompatibility of the HEA substrates by coating them with antibacterial Ag films.

Effect of low dissolved oxygen concentration on the defects and composition of regenerated passive film of Ti-6Al-4V alloy under continuous wear.

Tribocorrosion is one of the most common forms of failure of biomedical titanium alloys. As the passive film of titanium alloys is highly dependent on oxygen conditions, the passivation behavior and the microstructure of the passive film of Ti-6Al-4V under tribocorrosion in 1 M HCl with a low dissolved oxygen concentration (DOC) were studied by means of electron probe microanalysis (EPMA), Ar-ion etched X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) milling and high resolution transmission electron microscopy (HRTEM). The results showed that the protective ability of the regenerated passive film decreased sharply under low DOC. Al and V ions dissolved in excess, and a large number of oxygen atoms entered the matrix, leading to internal oxidation. Structural characterization indicated that Ti atoms occupied more metal lattice points in the regenerated passive film and that the high dislocation density in the deformed layer caused by wear facilitated the diffusion of Al and V. Finally, the first-principles calculation showed that Al had the minimum vacancy formation energy.

Smart self-healing coatings on biomedical magnesium alloys: A review

Smart self-healing coatings on biomedical magnesium alloys: A review