Electrochemical comparative study on corrosion behavior of conventional and powder metallurgy titanium alloys in physiological conditions

This article focuses on the electrochemical study of two titanium alloys employed in the manufacture of orthopedic implants – Ti-6Al-4V and Ti-6Al-7Nb – both of them obtained through powder metallurgy (PM). For comparative purposes, Ti-6Al-4V fabricated conventionally has also been investigated. Samples were immersed in a simulated body fluid (SBF) and incubated at 37 °C for different immersion time. Under these experimental conditions, we compared the influence of the processing method of alloys (PM or conventional) and their composition in the corrosion resistance. The corrosion resistance of these alloys in contact with SBF was evaluated with electrochemical impedance spectroscopy (EIS). The resulting impedance plots of all of them showed good reproducibility. For the lowest frequency tested (10 mHz) all of the samples showed high impedance modulus value approximately on the order of 106 Ω. This behavior is usually ascribed to a high corrosion protection performance. Although no significant differences in the evolution of the corrosion behavior for different immersion times has been found; the Ti-6Al-7Nb alloy processed by PM delivers a steady growth of corrosion resistance from day one until twelve weeks immersion. This sample showed the best performance between the two studied compositions. The resulting impedance plots show how powder metallurgy allows obtaining materials with similar or superior corrosion resistance in physiological conditions, than alloys obtained conventionally. Alloys characterization by scanning electron microscopy revealed no evidence of pitting corrosion phenomenon.Authors acknowledge funds provided by Spanish Government (programme MINECO, Ref. MAT2012-38650-C02-01) and regional government of Madrid (programme MULTIMAT-CHALLENGE, Ref. S2013/MIT-2862)

Design and characterization of modified powder metallurgy titanium surfaces obtained by ß-Stabilizing elements diffusion treatments for biomedical applications

Esta tesis contiene artículos de investigación en anexoTitanium (Ti) is a metal highly recognized by its employment in the biomedical sector since 1950. Nevertheless, in these last two decades from 1990 till date, a second generation of Ti biomaterials has received high attention. They are known as β-Ti alloys and its strong interest comes from their excellent combination of properties for biomaterials. Currently, due to the great demand to develop new biomaterials, the surface modification is one of the main alternatives for the design of new Ti biomaterials with improved properties for the biomedical sector. In this PhD Thesis, the design of new Ti surfaces modified by diffusion of niobium (Nb) and molybdenum (Mo) as β-stabilizing elements is presented as alternative to Cp-Ti or fully β-Ti alloys; preserving their lightness of Ti in the core. These diffusion elements share their capacity of decreasing the elastic modulus of titanium as well as their biocompatibility character. Thus, the global idea of this work is the design and study of these modified Ti surfaces produced by powder metallurgy with their evaluation on mechanical performance, tribological properties, corrosion and biocompatibility character to be considered as possible candidates for biomedical applications. Regarding this matter, different modified Ti surfaces were designed with several conditions: i) titanium substrate (green or sintered), ii) diffusion element (Nb or Mo), and iii) diffusion treatments (co-sintering plus diffusion, diffusion or thermo-reactive diffusion). These systems were compared with the behaviour of the commonly employed Ti biomaterial, the commercially pure titanium (Cp-Ti) obtained through powder metallurgy (PM). The β-stabilizing elements were deposited by means of aqueous suspensions through spraying. The gradients in microstructure and composition were analyzed by spectroscopy and diffraction techniques, and their differences were related to the designing parameters. The final surface conditions were investigated to obtain the most suitable ones in function of the final properties measured: mechanical properties according to hardness and elastic modulus, wear, corrosion and tribocorrosion behaviour and biocompatibility features. Therefore, this doctoral work covers the whole process from the design of the materials employing preparation and deposition of suspensions, diffusion treatments, surface conditions selection and microstructure investigations to their final characterization of hardness, modulus of elasticity, wear, corrosion and their synergistic effect (tribocorrosion), and bioactivity and cell-material response. Although some questions were found during different stages that should be further investigated, these new Ti surfaces have demonstrated some suitable characteristics for biomaterials, providing improvements with respect to titanium with a promising character for bone replacements.El titanio (Ti) es un metal altamente reconocido y empleado en el sector biomédico desde 1950. Sin embargo, una nueva generación de biomateriales de titanio ha recibido gran atención en estas dos últimas décadas. Esta se conoce con el nombre de segunda generación de biomateriales de titanio y su alto interés deriva de la excelente combinación de propiedades que presentan para biomateriales. Actualmente, debido a la gran demanda del desarrollo de nuevos biomateriales, la modificación superficial es una de las principales alternativas para el diseño de nuevos biomateriales de Ti con mejores propiedades para aplicaciones del sector biomédico. En esta tesis doctoral, se presenta una alternativa al titanio comercialmente puro (Cp-Ti) y a las aleaciones completamente beta (β-Ti) mediante el diseño de nuevas superficies de titanio modificadas con elementos betágenos, niobio (Nb) y molibdeno (Mo), mediante procesos de difusión. Estos elementos de difusión comparten la capacidad de disminuir el módulo elástico del titanio, mientras que son elementos biocompatibles. Por tanto, la idea global de este trabajo se centra en el diseño y estudio de superficies modificadas de titanio pulvimetalúrgico, junto con la evaluación de las propiedades mecánicas y comportamiento a desgaste, corrosión y tribocorrosión. Además, de la evaluación de la biocompatibilidad de estos para considerarlos como posibles candidatos para aplicaciones biomédicas. Para ello se diseñaron diferentes superficies modificadas de Ti basadas en las siguientes condiciones: i) substrato de titanio (prensado o sinterizado), ii) elemento de difusión (Nb o Mo), y iii) tratamiento de difusión (co-sinterización + difusión, difusión o difusión termo-reactiva). Todos los materiales se compararon con el material de titanio obtenido mediante pulvimetalurgia (PM). Los elementos betágenos se depositaron mediante pulverización de suspensiones acuosas. Los gradientes de microestructura y composición se analizaron con técnicas de espectroscopia y difracción. Se estudiaron las condiciones superficiales más idóneas para la caracterización mecánica (dureza y módulo elástico), comportamiento a desgaste, corrosión, tribocorrosión y biocompatibilidad. Por tanto, esta tesis doctoral abarca todo el proceso desde el diseño de los materiales, preparación y pulverización de las suspensiones, tratamientos de difusión, caracterización microestructural y superficial, hasta la evaluación de dureza, módulo de elasticidad, desgaste, corrosión, tribocorrosión y biocompatibilidad de las superficies modificadas de Ti. Aunque se necesita la evaluación más profunda de algunos aspectos, estas superficies modificadas de Ti han demostrado que presentan características adecuadas para biomateriales con mejoras con respecto al titanio, por tanto con propiedades prometedoras para reemplazo óseo.Programa Oficial de Doctorado en Ciencia e Ingeniería de MaterialesPresidente: José Luis González Carrasco.- Secretario: Sandra Carolina Cifuentes Cuéllar.- Vocal: Yaiza González Garcí

Ti-6Al-4V β Phase Selective Dissolution: In Vitro Mechanism and Prediction

Retrieval studies document Ti-6Al-4V β phase dissolution within total hip replacement systems. A gap persists in our mechanistic understanding and existing standards fail to reproduce this damage. This thesis aims to (1) elucidate the Ti-6Al-4V selective dissolution mechanism as functions of solution chemistry, electrode potential and temperature; (2) investigate the effects of adverse electrochemical conditions on additively manufactured (AM) titanium alloys and (3) apply machine learning to predict the Ti-6Al-4V dissolution state. We hypothesized that (1) cathodic activation and inflammatory species (H2O2) would degrade the Ti-6Al-4V oxide, promoting dissolution; (2) AM Ti-6Al-4V selective dissolution would occur and (3) near field electrochemical impedance spectra (nEIS) would distinguish between dissolved and polished Ti-6Al-4V, allowing for deep neural network prediction. First, we show a combinatorial effect of cathodic activation and inflammatory species, degrading the oxide film’s polarization resistance (Rp) by a factor of 105 Ωcm2 (p = 0.000) and inducing selective dissolution. Next, we establish a potential range (-0.3 V to –1 V) where inflammatory species, cathodic activation and increasing solution temperatures (24 oC to 55 oC) synergistically affect the oxide film. Then, we evaluate the effect of solution temperature on the dissolution rate, documenting a logarithmic dependence. In our second aim, we show decreased AM Ti-6Al-4V Rp when compared with AM Ti-29Nb-21Zr in H2O2. AM Ti-6Al-4V oxide degradation preceded pit nucleation in the β phase. Finally, in our third aim, we identified gaps in the application of artificial intelligence to metallic biomaterial corrosion. With an input of nEIS spectra, a deep neural network predicted the surface dissolution state with 96% accuracy. In total, these results support the inclusion of inflammatory species and cathodic activation in pre-clinical titanium devices and biomaterial testing

Corrosion behaviour of Ti6Al4V ELI nanotubes for biomedical applications

[EN] Surfaces engineering on titanium biomedical alloys aiming for improving bone regeneration, healing periods and increasing lifetime needs fora fundamental understanding of the electrochemical reactions occurring at the interface biomaterial/human fluid. There, electrochemical corrosion plays an important role in implant-tissue interaction. The aim of this study is to investigate the effect of different TiO2 surfaces and nanotubes on a Ti6Al4V ELI in their electrochemical corrosion resistance by different electrochemical techniques (open circuit potential, electrochemical impedance spectroscopy, and potentiodynamic polarization). The electrochemical behaviour of native, anodized, nanotubular and annealed nanotubular surfaces were investigated in 1 M NaCl solution. The nanotubular topography was obtained by electrochemical oxidation and the annealing treatment allowed at changing the crystalline structure of the oxides. The nanotube morphology, chemical composition, and structure was studied by Field Emission Scanning Electron Microscopy, Energy Dispersive Spectroscopy, X-ray diffraction and Transmission Electron Microscopy respectively. The results show that the anodic oxidation treatment creates a nanotubular topography that increases the surface area and changes the surface chemical composition. The electrochemical corrosion resistance decreased on the as-formed TiO2 tubes compared to the native oxide layer, due to higher surface area and amorphous crystal structure of the passive film. After annealing treatment, the fluoride ions are eliminated, and nanotubular resistance is enhanced through anatase stabilization.The authors wish to thank the Spanish Ministry of Economy and Competitiveness for the financial support of Research Project MAT2014-53764-C3-1-R, the Generalitat Valenciana for support through PROMETEO 2016/040, and the European Commission via FEDER funds to purchase equipment for research purposes and the Microscopy Service at the Valencia Polytechnic University. Thanks to Alba Dalmau and Javier Navarro Laboulais from Instituto de Seguridad Industrial y Medio Ambiente, Valencia Polytechnic University for the technical assistance with preparation of the electrochemical tests. Thanks to Irene Llorente and Jose Antonio Jimenez from CENIM/CSIC for the technical assistance with XRD characterization.Lario, J.; Viera, M.; Vicente-Escuder, Á.; Igual Muñoz, AN.; Amigó, V. (2019). Corrosion behaviour of Ti6Al4V ELI nanotubes for biomedical applications. Journal of Materials Research and Technology. 8(6):5548-5556. https://doi.org/10.1016/j.jmrt.2019.09.023S554855568

Towards load-bearing biomedical titanium-based alloys: From essential requirements to future developments

The use of biomedical metallic materials in research and clinical applications has been an important focus and a significant area of interest, primarily owing to their role in enhancing human health and extending human lifespan. This article, particularly on titanium-based alloys, explores exceptional properties that can address bone health issues amid the growing challenges posed by an aging population. Although stainless steel, magnesium-based alloys, cobalt-based alloys, and other metallic materials are commonly employed in medical applications, limitations such as toxic elements, high elastic modulus, and rapid degradation rates limit their widespread biomedical applications. Therefore, titanium-based alloys have emerged as top-performing materials, gradually replacing their counterparts in various applications. This article extensively examines and highlights titanium-based alloys, along with an in-depth discussion of currently utilized metallic biomedical materials and their inherent limitations. To begin with, the essential requirements for load-bearing biomaterials are introduced. Then, the biomedical metallic materials are summarized and compared. Afterward, the microstructure, properties, and preparations of titanium-based alloys are explored. Furthermore, various surface modification methods are discussed to enhance biocompatibility, wear resistance, and corrosion resistance. Finally, the article proposes the development path for titanium-based alloys in conjunction with additive manufacturing and the novel alloy nitinol

Development of Novel Low-Modulus β-Type Ga-/Cu-Bearing Ti–Nb Alloys for Antibacterial Bone Implant Applications

Commercially available titanium (Ti) alloys, such as Ti–6Al–4V and c.p. Ti, even though established in clinical use as load-bearing bone implant materials in orthopedics and dentistry, possess significant drawbacks that may lead to implant failure: i) presence of alloying elements with harmful health effects, ii) high Young’s modulus (E > 100 GPa) compared to human cortical bone (Ebone = 10 – 30 GPa), and iii) lack of antibacterial activity against multidrug-resistant bacteria, which may lead to implant-associated infections. To overcome the first two drawbacks, a new generation of biocompatible metastable β-type Ti alloys has been developed, in particular β-type Ti–Nb alloys, which are versatile candidates due to their low Young’s modulus, high strength-to-weight ratio and improved corrosion resistance. The present work aims to tackle all three aforementioned issues by developing novel β-type Ti–45Nb-based alloys with potential intrinsic antibacterial activity by adding antibacterial gallium (Ga) and copper (Cu) in minor amounts (up to 8 wt.%) via metallurgical route. Nine alloys with the following chemical compositions: (100-x)(Ti–45Nb)–xGa, (100-x)(Ti–45Nb)–xCu (where x = 2, 4, 6, 8 wt.%), and 96(Ti–45Nb)–2Ga–2Cu, based on alloy design approaches, were produced by controlled casting and homogenization treatment. The effect of antibacterial alloying additions on phase constitution, mechanical characteristics, corrosion, and tribocorrosion response in a simulated physiological environment has been investigated. All nine alloys in the homogenized state display a single-phase β (BCC) phase microstructure, whose lattice parameter is proved to be sensitive to Ga and Cu content, with an almost linear contraction. The mechanical characteristics are strongly influenced by Ga and Cu addition, with a general strengthening effect mainly attributed to substitutional solid solution strengthening, and to grain boundary strengthening in case of Ga. Deformation behavior indicates high mechanical stability of the β phase, suggesting dislocation slip as dominant deformation mechanism. The results demonstrate that strategic alloy design is an effective method to significantly increase strength without adversely affecting the Young’s modulus, which remains in the range of good biomechanical compatibility (E = 64 – 104 GPa). Evaluation of the corrosion response and metal ion release in simulated physiological environment demonstrates the high corrosion resistance of the nine alloys, whereas tribocorrosion wear resistance increases upon Ga addition. Further thermal (aging) treatments, carried out on a specific Cu-containing alloy, proved the feasibility of tailoring enhanced mechanical, chemical and potentially antibacterial properties by thermally-induced precipitation of Ti₂Cu intermetallic compound. These novel developed alloys are considered to be promising candidates for biomedical bone implant applications

Recent developments of metallic implants for biomedical applications

Medical implants have undoubtedly made an indelible mark on our world during the last century. More than 100 million humans carry at least one major internal medical device. The prosthesis industry has topped 50 billion US$ in annual sales, with approximately 150 universities throughout the world proposing an undergraduate program in bioengineering or biomedical engineering. Despite that, however, most medical devices have been constructed using a significantly restricted number of conventional metallic, ceramic, polymeric, and composite biomaterials. In this study, recent developments of metallic implants are summarized for biomedical applications. To do this, first desired properties for biomaterials are defined. Then, types of metallic biomaterials are classified as stainless steel, Mg, Co, Ti, nobble and biodegradable ones. After that, surface modifications are defined for corrugation, topographies and chemical modification. Finally, future perspective is outlined for the sake of development new materials as well as production point of view