Low modulus Ti–Nb–Hf alloy for biomedical applications

Effects of Cold Rolling or Precipitation Hardening Treatment on the Microstructure, Mechanical Properties, and Corrosion Resistance of Ti-Rich Metastable Medium-Entropy Alloys

Titanium-rich metastable medium-entropy alloys, designed for low elastic moduli, sacrifice strength. However, enhancing their mechanical strength is crucial for bio-implant applications. This study aims to enhance the mechanical properties and corrosion resistance of a metastable Ti80-Nb10-Mo5-Sn5 medium-entropy alloy using various treatments, including cold rolling (at 50% and 75% reduction) and precipitation hardening (at room temperature, 150 °C, 350 °C, 550 °C, and 750 °C). The results showed that the alloy underwent a stress-induced martensitic transformation during the rolling process. Notably, the α phase was precipitated in the β grain boundaries after 30 days of precipitation hardening at room temperature. The yield strengths of the alloy increased by 51% and 281.9% after room-temperature precipitation and 75% cold rolling, respectively. In potentiodynamic corrosion tests conducted in phosphate-buffered saline solution, the pitting potentials of the alloy treated using various conditions were higher than 1.8 V, and no pitting holes were observed on the surface of the alloys. The surface oxide layer of the alloy was primarily composed of TiO2, Nb2O5, MoO3, and SnO2, contributing to the alloy's exceptional corrosion and pitting resistance. The 75% rolled Ti80-Nb10-Mo5-Sn5 demonstrates exceptional mechanical properties and high corrosion resistance, positioning it as a promising bio-implant candidate.

Recent Progress on Nanocrystalline Metallic Materials for Biomedical Applications

Nanocrystalline (NC) metallic materials have better mechanical properties, corrosion behavior and biocompatibility compared with their coarse-grained (CG) counterparts. Recently, nanocrystalline metallic materials are receiving increasing attention for biomedical applications. In this review, we have summarized the mechanical properties, corrosion behavior, biocompatibility, and clinical applications of different types of NC metallic materials. Nanocrystalline materials, such as Ti and Ti alloys, shape memory alloys (SMAs), stainless steels (SS), and biodegradable Fe and Mg alloys prepared by high-pressure torsion, equiangular extrusion techniques, etc., have better mechanical properties, superior corrosion resistance and biocompatibility properties due to their special nanostructures. Moreover, future research directions of NC metallic materials are elaborated. This review can provide guidance and reference for future research on nanocrystalline metallic materials for biomedical applications.

In-Situ Laser Directed Energy Deposition of Biomedical Ti-Nb and Ti-Zr-Nb Alloys from Elemental Powders

In order to achieve the required properties of titanium implants, more resources and research are needed to turn into reality the dream of developing the perfect implant material. The objective of this study was to evaluate the viability of the Laser Directed Energy Deposition to produce biomedical Ti-Nb and Ti-Zr-Nb alloys from elemental powders (Ti, Nb and Zr). The Laser Directed Energy Deposition is an additive manufacturing process used to build a component by delivering energy and material simultaneously. The material is supplied in the form of particles or wire and a laser beam is employed to melt material that is selectively deposited on a specified surface, where it solidifies. Samples with different compositions are characterized to analyze their morphology, microstructure, constituent phases, mechanical properties, corrosion resistance and cytocompatibility. Laser-deposited Ti-Nb and Ti-Zr-Nb alloys show no relevant defects, such as pores or cracks. Titanium alloys with lower elastic modulus and a significantly higher hardness than Ti grade 2 were generated, therefore a better wear resistance could be expected from them. Moreover, their corrosion resistance is excellent due to the formation of a stable passive protective oxide film on the surface of the material; in addition, they also possess outstanding cytocompatibility.

A review on alloy design, biological response, and strengthening of β-titanium alloys as biomaterials

From the past few years, developments of β-Ti alloys have been the subject of active research in the medical domain. The current paper highlights significant findings in the area of β-Ti alloy design, biological responses, strengthening mechanisms, and developing low-cost implants with a high degree of biocompatibility. It is evident that an astonishing demand for developing the low modulus-high strength implants can be fulfilled by synchronizing β stabilizer content and incorporating tailored thermo-mechanical techniques. Furthermore, the biological response of the implants is as important as the physical properties that regulate healing response; hence, the optimum selection of alloying elements plays a curial role for clinical success. The paper also presents the evolution of patents in this field from the year 2010 to 2020 showing the relevant innovations that may benefit a wide range of researchers.

Effect of Pore Size on the Physicochemical Properties and Osteogenesis of Ti6Al4V Porous Scaffolds with Bionic Structure

Ti6Al4V is widely used in implants in the fields of orthopedics and dentistry due to its high compressive strength and good biocompatibility. Nevertheless, Ti6Al4V has a certain degree of biological inertness and the elastic modulus of Ti6Al4V is much higher than the cortex and trabecular bone. In this study, we designed and printed a new type of pore size Ti6Al4V with like-trabecular structure scaffold (the pore size is 800/900/1000 μm, named P8/P9/P10, respectively) with electron beam melting (EBM). Its elastic modulus, compressive strength, and other physical and chemical properties, as well as cell adhesion, proliferation, and differentiation ability and in vitro biological properties were studied. The physical and chemical performance test results showed that as the pore size increased, the surface wettability increased and the elastic modulus decreased. As the pore size increased, F-actin and alkaline phosphatase (ALP) increased significantly, and osteogenesis-related genes including BMP2, OCN, RUNX2, and ALP were upregulated significantly. The reason may be that the components on the Ti6Al4V pore size may have an influence on intracellular signal conversion and then change the mode of cell proliferation and diffusion. In summary, the like-trabecular porous structure can effectively reduce the elastic modulus of metal materials, thereby avoiding stress concentration and promoting the adhesion and proliferation of osteoblasts. Porous materials with larger pores are more conducive to the proliferation and differentiation of osteoblasts. The irregular porous Ti6Al4V scaffold prepared by the EBM technology has good mechanical properties and the potential to promote adhesion, proliferation, and differentiation of osteoblasts, and has the possibility of application in the field of implantation.

Titanium Scaffolds by Direct Ink Writing: Fabrication and Functionalization to Guide Osteoblast Behavior

Titanium (Ti) and Ti alloys have been used for decades for bone prostheses due to its mechanical reliability and good biocompatibility. However, the high stiffness of Ti implants and the lack of bioactivity are pending issues that should be improved to minimize implant failure. The stress shielding effect, a result of the stiffness mismatch between titanium and bone, can be reduced by introducing a tailored structural porosity in the implant. In this work, porous titanium structures were produced by direct ink writing (DIW), using a new Ti ink formulation containing a thermosensitive hydrogel. A thermal treatment was optimized to ensure the complete elimination of the binder before the sintering process, in order to avoid contamination of the titanium structures. The samples were sintered in argon atmosphere at 1200 °C, 1300 °C or 1400 °C, resulting in total porosities ranging between 72.3% and 77.7%. A correlation was found between the total porosity and the elastic modulus of the scaffolds. The stiffness and yield strength were similar to those of cancellous bone. The functionalization of the scaffold surface with a cell adhesion fibronectin recombinant fragment resulted in enhanced adhesion and spreading of osteoblastic-like cells, together with increased alkaline phosphatase expression and mineralization.

Design of high-ductile medium entropy alloys for dental implants

Developing new materials with high strength and ductility, low modulus and high biocompatibility is a continuing demand in the field of surgical implants. Inspired by the high-entropy design philosophy, two medium entropy alloys (MEAs), i.e. equiatomic TiZrHf and equi-weight Ti₄₀Zr₂₀Hf₁₀Nb₂₀Ta₁₀ were designed and their mechanical properties and biocompatibility were assessed. Both the single-phase hexagonal close-packed (HCP) structured TiZrHf alloy and the single-phase body-centered cubic (BCC) structured Ti₄₀Zr₂₀Hf₁₀Nb₂₀Ta₁₀ alloy show high strength-ductility combinations close to commercial Ti-6Al-4V wrought alloy and remarkably lower young's modulus than commercial pure titanium (CP-Ti) and Ti-6Al-4V. From the aspects of adhesion, proliferation, toxicity and related gene expression of human gingival fibroblasts (HGFs), the Ti₄₀Zr₂₀Hf₁₀Nb₂₀Ta₁₀ alloy exhibits distinctively better biocompatibility than that of CP-Ti while the TiZrHf shows only slightly better biocompatibility as compared with CP-Ti. These results indicate that these two ductile MEAs are potential candidates for dental application.

Effect of Zr Content on Phase Stability, Deformation Behavior, and Young's Modulus in Ti-Nb-Zr Alloys

Ti alloys have attracted continuing research attention as promising biomaterials due to their superior corrosion resistance and biocompatibility and excellent mechanical properties. Metastable β-type Ti alloys also provide several unique properties such as low Young’s modulus, shape memory effect, and superelasticity. Such unique properties are predominantly attributed to the phase stability and reversible martensitic transformation. In this study, the effects of the Nb and Zr contents on phase constitution, transformation temperature, deformation behavior, and Young’s modulus were investigated. Ti–Nb and Ti–Nb–Zr alloys over a wide composition range, i.e., Ti–(18–40)Nb, Ti–(15–40)Nb–4Zr, Ti–(16–40)Nb–8Zr, Ti–(15–40)Nb–12Zr, Ti–(12–17)Nb–18Zr, were fabricated and their properties were characterized. The phase boundary between the β phase and the α′′ martensite phase was clarified. The lower limit content of Nb to suppress the martensitic transformation and to obtain a single β phase at room temperature decreased with increasing Zr content. The Ti–25Nb, Ti–22Nb–4Zr, Ti–19Nb–8Zr, Ti–17Nb–12Zr and Ti–14Nb–18Zr alloys exhibit the lowest Young’s modulus among Ti–Nb–Zr alloys with Zr content of 0, 4, 8, 12, and 18 at.%, respectively. Particularly, the Ti–14Nb–18Zr alloy exhibits a very low Young’s modulus less than 40 GPa. Correlation among alloy composition, phase stability, and Young’s modulus was discussed.

Novel α + β Type Ti-Fe-Cu Alloys Containing Sn with Pertinent Mechanical Properties

Rising demand for bone implants has led to the focus on future alternatives of alloys with better biocompatibility and mechanical strength. Thus, this research is dedicated to the synthesis and investigation of new compositions for low-alloyed Ti-based compounds, which conjoin relatively acceptable mechanical properties and low elastic moduli. In this regard, the structural and mechanical properties of α + β Ti-Fe-Cu-Sn alloys are described in the present paper. The alloys were fabricated by arc-melting and tilt-casting techniques which followed subsequent thermo-mechanical treatment aided by dual-axial forging and rolling procedures. The effect of the concentrations of the alloying elements, and other parameters, such as regimes of rolling and dual-axial forging operation, on the microstructure and mechanical properties were thoroughly investigated. The Ti94Fe1Cu1Sn4 alloy with the most promising mechanical properties was subjected to thermo-mechanical treatment. After a single rolling procedure at 750 °C, the alloy exhibited tensile strength and tensile plasticity of 1300 MPa and 6%, respectively, with an elastic modulus of 70 GPa. Such good tensile mechanical properties are explained by the optimal volume fraction balance between α and β phases and the texture alignment obtained, providing superior alternatives in comparison to pure α- titanium alloys.

On the Corrosion Behaviour of Low Modulus Titanium Alloys for Medical Implant Applications: A Review

The corrosion behaviour of new generation titanium alloys (β-type with low modulus) for medical implant applications is of paramount importance due to their possible detrimental effects in the human body such as release of toxic metal ions and corrosion products. In spite of remarkable advances in improving the mechanical properties and reducing the elastic modulus, limited studies have been done on the electrochemical corrosion behaviour of various types of low modulus titanium alloys including the effect of different beta-stabilizer alloying elements. This development should aim for a good balance between mechanical properties, design features, metallurgical aspects and, importantly, corrosion resistance. In this article, we review several significant factors that can influence the corrosion resistance of new-generation titanium alloys such as fabrication process, body electrolyte properties, mechanical treatments, alloying composition, surface passive layer, and constituent phases. The essential factors and their critical features are discussed. The impact of various amounts of α and β phases in the microstructure, their interactions, and their dissolution rates on the surface passive layer and bulk corrosion behaviour are reviewed and discussed in detail. In addition, the importance of different corrosion types for various medical implant applications is addressed in order to specify the significance of every corrosion phenomenon in medical implants.

Influence of the Elastic Modulus on the Osseointegration of Dental Implants

The load transfer from metallic prosthesis to tissue plays an important role in the success of a designed device. From a mechanical behavior point of view, the load transfer will be favored when the elastic modulus between the metallic implant and the bone tissue are similar. Titanium and Ti-6Al-4V are the most commonly used metals and alloys in the field of dental implants, although they present high elastic moduli and hence trigger bone resorption. We propose the use of low-modulus β-type titanium alloys that can improve the growth of new bone surrounding the implant. We designed dental implants with identical morphology and micro-roughness composed of: Ti-15Zr, Ti-19.1Nb-8.8Zr, Ti-41.2Nb-6.1Zr, and Ti-25Hf-25Ta. The commercially pure Ti cp and Ti-6Al-4V were used as control samples. The alloys were initially mechanically characterized with a tensile test using a universal testing machine. The results showed the lowest elastic modulus for the Ti-25Hf-25Ta alloy. We implanted a total of six implants in the mandible (3) and maxilla (3) for each titanium alloy in six minipigs and evaluated their bone index contact (i.e., the percentage of new bone in contact with the metal—BIC%) after 3 and 6 weeks of implantation. The results showed higher BIC% for the dental implants with lowest elastic modulus, showing the importance of decreasing the elastic modulus of alloys for the successful osseointegration of dental implants.

Bone Homeostasis and Repair: Forced Into Shape

Mechanical loading is a potent anabolic regulator of bone mass, and the first line of defense for bone loss is weight-bearing exercise. Likewise, protected weight bearing is the first prescribed physical therapy following orthopedic reconstructive surgery. In both cases, enhancement of new bone formation is the goal. Our understanding of the physical cues, mechanisms of force sensation, and the subsequent cellular response will help identify novel physical and therapeutic treatments for age- and disuse-related bone loss, delayed- and nonunion fractures, and significant bony defects. This review highlights important new insights into the principles and mechanisms governing mechanical adaptation of the skeleton during homeostasis and repair and ends with a summary of clinical implications stemming from our current understanding of how bone adapts to biophysical force.

Characterization, corrosion behavior, cellular response and in vivo bone tissue compatibility of titanium-niobium alloy with low Young's modulus

β-Type titanium alloys with a low elastic modulus are a potential strategy to enhance bone remodeling and to mitigate the concern over the risks of osteanabrosis and bone resorption caused by stress shielding, when used to substitute irreversibly impaired hard tissue. Hence, in this study, a Ti-45Nb alloy with low Young's modulus and high strength was developed, and microstructure, mechanical properties, corrosion behaviors, cytocompatibility and in vivo osteo-compatibility of the alloy were systematically investigated for the first time. The results of mechanical tests showed that Young's modulus of the Ti-Nb alloy was reduced to about 64.3GPa (close to human cortical bone) accompanied with higher tensile strength and hardness compared with those of pure Ti. Importantly, the Ti-Nb alloy exhibited superior corrosion resistance to Ti in different solutions including SBF, MAS and FAAS (MAS containing NaF) media. In addition, the Ti-Nb alloy produced no deleterious effect to L929 and MG-63 cells, and cells performed excellent cell attachment onto Ti-Nb surface, indicating a good in vitro cytocompatibility. In vivo evaluations indicated that Ti-Nb had comparable bone tissue compatibility to Ti determined from micro-CT and histological evaluations. The Ti-Nb alloy with an elasticity close to human bone, thus, could be suitable for orthopedic/dental applications.

Powder metallurgical low-modulus Ti-Mg alloys for biomedical applications

In this work, powder metallurgical (PM) Ti-Mg alloys were prepared using combined techniques of mechanical alloying and spark plasma sintering. The alloys mainly consist of super saturations of Mg in Ti matrix, and some laminar structured Ti- and Mg-rich phases. The PM Ti-Mg alloys contain a homogeneous mixtures of nanocrystalline Mg and Ti phases. The novel microstructures result in unconventional mechanical and biological properties. It has been shown that the PM Ti-Mg alloys have a much lower compression modulus (36-50GPa) compared to other Ti alloys, but still remain a very high compressive strength (1500-1800MPa). In addition, the PM Ti-Mg alloys show good biocompatibility and bioactivity. Mg can dissolve in the simulated body fluids, and induce the formation of the calcium phosphate layer. The compression modulus of PM Ti-Mg alloys decreases with the amount of Mg, while the bioactivity increases. Although the corrosion resistance of Ti-Mg alloys decreases with the content of Mg, the alloys still show good stability in simulated body fluid under electrochemical conditions. The indirect and direct cytotoxicity results show that PM Ti-Mg alloys have a good biocompatibility to NIH-3T3 cells. Therefore, the PM Ti-Mg alloys are promising candidates in biomedical applications.

Mechanical and physicochemical characterization along with biological interactions of a new Ti25Nb21Hf alloy for bone tissue engineering

Nowadays, one of the main challenges in metal implants for bone substitution is the achievement of an elastic modulus close to that of human cortical bone as well as to provide an adequate interaction with the surrounding tissue avoiding in vivo foreign body reaction. From this perspective, a new Ti-based alloy has been developed with Nb and Hf as alloying elements which are known as non-toxic and with good corrosion properties. The microstructure, mechanical behaviour and the physicochemical properties of this novel titanium alloy have been studied. Relationship of surface chemistry and surface electric charge with protein adsorption and cell adhesion has been evaluated due to its role for understanding the mechanism of biological interactions with tissues. The Ti25Nb21Hf alloy presented a lower elastic modulus than commercial alloys with a superior ultimate strength and yield strength than CP-Ti and very close to Ti6Al4V. It also exhibited good corrosion resistance. Furthermore, the results revealed that it had no cytotoxic effect on rat mesenchymal stem cells and allowed protein adsorption and cell adhesion. The experimental results make this alloy a promising material for bone substitution or for biomedical devices.

In vitro evaluation of biocompatibility of Ti-Mo-Sn-Zr superelastic alloy

Shape memory alloys (SMAs) including superelastic alloys have unique properties such as shape memory and superelasticity, thus they are recognized as very useful biomaterials. These properties are very advantageous for medical use, and actually the SMA wires have been widely used in medical field. However, biocompatibility of nickel-titanium (Ni-Ti) alloy, which is the only practical SMA at present, has been questioned because of its high nickel content. The aim of this study was to evaluate the biocompatibility of a newly developed Ni-free Ti-based SMA for medical use. The newly developed SMA made of Ti-Mo-Sn-Zr system was processed into a disk of 15.1 mm in diameter. Pure titanium of the same shape was prepared as control. All the disk surfaces were polished using emery papers, #120, #400, and #600. Scanning electron microscopy and a 3D optics profiler were used to evaluate the surface of the materials. In vitro evaluations included colony examination for evaluation of the cell cytotoxicity, DNA quantification for the cell proliferation, Alamar blue assay for metabolic activity, FDA staining for the live cell imaging, and cell cycle analysis, using Chinese hamster fibroblastic V-79 cells and mouse osteoblastic MC3T3-E1 cells. In colony examination and DNA quantification, there was no significant difference between the Ti-Mo-Sn-Zr and the pure titanium. In FDA staining, cultured cells on the Ti-Mo-Sn-Zr alloy showed the same biocompatibility as those on the pure titanium. The present results suggest that the newly developed Ti-Mo-Sn-Zr alloy showed the high biocompatibility comparable to pure titanium and can be used as efficient biomaterial for medical use.