Effect of thermomechanical processing on the mechanical biofunctionality of a low modulus Ti-40Nb alloy

Albumin suppresses oxidation of TiNb alloy in the simulated inflammatory environment

Literature data has shown that reactive oxygen species (ROS), generated by immune cells during post-operative inflammation, could induce corrosion of standard Ti-based biomaterials. For Ti6Al4V alloy, this process can be further accelerated by the presence of albumin. However, this phenomenon remains unexplored for Ti β-phase materials, such as TiNb alloys. These alloys are attractive due to their relatively low elastic modulus value. This study aims to address the question of how albumin influences the corrosion resistance of TiNb alloy under simulated inflammation. Electrochemical and ion release tests have revealed that albumin significantly enhances corrosion resistance over both short (2 and 24 h) and long (2 weeks) exposure periods. Furthermore, post-immersion XPS and cross-section TEM analysis have demonstrated that prolonged exposure to an albumin-rich inflammatory solution results in the complete coverage of the TiNb surface by a protein layer. Moreover, TEM studies revealed that H2O2-induced oxidation and further formation of a defective oxide film were suppressed in the solution enriched with albumin. Overall results indicate that contrary to Ti6Al4V, the addition of albumin to the PBS + H2O2 solution is not necessary to simulate the harsh inflammatory conditions as could possibly be found in the vicinity of a TiNb implant.

Gallium-Containing Materials and Their Potential within New-Generation Titanium Alloys for Biomedical Applications

With the rising demand for implantable orthopaedic medical devices and the dominance of device-associated infections, extensive research into the development of novel materials has been prompted. Among these, new-generation titanium alloys with biocompatible elements and improved stiffness levels have received much attention. Furthermore, the development of titanium-based materials that can impart antibacterial function has demonstrated promising results, where gallium has exhibited superior antimicrobial action. This has been evidenced by the addition of gallium to various biomaterials including titanium alloys. Therefore, this paper aims to review the antibacterial activity of gallium when incorporated into biomedical materials, with a focus on titanium-based alloys. First, discussion into the development of new-generation Ti alloys that possess biocompatible elements and reduced Young's moduli is presented. This includes a brief review of the influence of alloying elements, processing techniques and the resulting biocompatibilities of the materials found in the literature. The antibacterial effect of gallium added to various materials, including bioglasses, liquid metals, and bioceramics, is then reviewed and discussed. Finally, a key focus is given to the incorporation of gallium into titanium systems for which the inherent mechanical, biocompatible, and antibacterial effects are reviewed and discussed in more detail, leading to suggestions and directions for further research in this area.

Effects of Ga on the structural, mechanical and electronic properties of β-Ti-45Nb alloy by experiments and ab initio calculations

This work aims to investigate the structural, mechanical and electronic properties of four novel β-type (100-x)(Ti-45Nb)-xGa alloys (x = 2, 4, 6, 8 wt%) for implant applications by means of experimental and theoretical (ab initio) methods. All alloys retain the bcc β phase in the solution-treated and quenched state while the lattice parameter decreases with increase in Ga content. This is due to its smaller atomic radius compared to Ti and Nb, in line with the present density functional theory (DFT) calculations. Tensile and microhardness tests indicate a clear strengthening effect with increasing Ga content, with yield strengths in the range 551 ÷ 681 MPa and microhardness in the range 174 ÷ 232 HV_0.1, mainly attributed to grain refinement and solid solution strengthening. Ga also positively affects ductility, with a maximum value of tensile strain at fracture of 32%. Non-destructive ultrasonic measurements and DFT calculations reveal that the bulk modulus is unaffected by the Ga presence. This phenomenon might be due to the fact that Ga introduced bonding and anti-bonding electron low energy states which balance the average bond strength among the atoms in the metallic matrix. Nevertheless, the introduction of new Ga-Ti super sp-like bonding orbitals along the [110] and [-110] directions in the Ga neighborhood could explain the increase of the Young's modulus upon Ga addition (73 ÷ 82.5 GPa) that was found experimentally in the present work. Hence, Ga addition to Ti-45Nb leads to a suitable balance between increased strength and low Young's modulus.

Comparative Investigation of the Influence of Ultrafine-Grained State on Deformation and Temperature Behavior and Microstructure Formed during Quasi-Static Tension of Pure Titanium and Ti-45Nb Alloy by Means of Infrared Thermography

A comprehensive study was performed of the deformation and temperature behavior during quasi-static tension, as well as the peculiarities of accumulation and dissipation of energy during plastic deformation. Microstructural analysis at the pre-fracture stage of pure titanium and Ti-45Nb alloy in the coarse grain (CG) and ultrafine-grained (UFG) states was also conducted. It was shown that substructural and dispersion hardening leads to a change in the regularities of dissipation and accumulation energies during deformation of the samples of the pure titanium and Ti-45Nb alloy in the UFG state. Some features of structural transformations during deformation of the pure titanium and Ti-45Nb alloy samples in the CG and UFG states were studied. A band and cellular-network and fragmented dislocation structure was formed in the case of the CG state, while large anisotropic fragments were formed in the UFG state, thus specifying a local softening of the material before fracture.

Characteristic Features of Ultrafine-Grained Ti-45 wt.% Nb Alloy under High Cycle Fatigue

The paper presents the results of fatigue-testing ultrafine-grained and coarse-grained Ti-45 wt.% Nb alloy samples under very high cycle fatigue (gigacycle regime), with the stress ratio R = -1. The ultrafine-grained (UFG) structure in the investigated alloy was formed by the two-stage SPD method, which included multidirectional forging (abc-forging) and multipass rolling in grooved rollers, with further recrystallization annealing. The UFG structure of the Ti-45 wt.% Nb alloy samples increased the fatigue limit under the high-cycle fatigue conditions up to 1.5 times compared with that of the coarse-grained (CG) samples. The infrared thermography method was applied to investigate the evolution of temperature fields in the samples under cyclic loading. Based on numerical morphology analysis, the scale invariance (the Hurst exponent) and qualitative differences for UFG and CG structures were determined. The latter resulted from the initiation and propagation of fatigue cracks in both ultra-fine grained and coarse-grained alloy samples under very high-cycle fatigue loading.

Catechol Containing Polyelectrolyte Complex Nanoparticles as Local Drug Delivery System for Bortezomib at Bone Substitute Materials

The proteasome inhibitor bortezomib (BZM) is one of the most potent anti-cancer drugs in the therapy of multiple myeloma. In this study, an adhesive drug delivery system (DDS) for BZM was developed. Therefore, we extended the present DDS concept of polyelectrolyte complex (PEC) nanoparticle (NP) based on electrostatic interactions between charged drug and polyelectrolyte (PEL) to a DDS concept involving covalent bonding between PEL and uncharged drugs. For this purpose, 3,4-dihydroxyphenyl acetic acid (DOPAC) was polymerized via an oxidatively induced coupling reaction. This novel chemo-reactive polyanion PDOPAC is able to temporarily bind boronic acid groups of BZM via its catechol groups, through esterification. PDOPAC was admixed to poly(l-glutamic acid) (PLG) and poly(l-lysine) (PLL) forming a redispersible PEC NP system after centrifugation, which is advantageous for further colloid and BZM loading processing. It was found that the loading capacity (LC) strongly depends on the PDOPAC and catechol content in the PEC NP. Furthermore, the type of loading and the net charge of the PEC NP affect LC and the residual content (RC) after release. Release experiments of PDOPAC/PEC coatings were performed at medically relevant bone substitute materials (calcium phosphate cement and titanium niobium alloy) whereby the DDS worked independently of the surface properties. Additionally, in contrast to electrostatically based drug loading the release behavior of covalently bound, uncharged BZM is independent of the ionic strength (salt content) in the release medium.

Routes to control diffusive pathways and thermal expansion in Ti-alloys

β-stabilized Ti-alloys present several unexplored and intriguing surprises in relation to orthorhombic α″ phases. Among them are (i) the diffusion-controlled formation of transitional α″_iso, α″_lean and α″_rich phases and ii) the highly anisotropic thermal expansion of martensitic α″. Using the prototypical Ti-Nb system, we demonstrate that the thermodynamic energy landscape reveals formation pathways for the diffusional forms of α″ and may lead to a stable β-phase miscibility gap. In this way, we derive temperature-composition criteria for the occurrence of α″_iso and resolve reaction sequences during thermal cycling. Moreover, we show that the thermal expansion anisotropy of martensitic α″ gives rise to directions of zero thermal strain depending on Nb content. Utilizing this knowledge, we propose processing routes to achieve null linear expansion in α″ containing Ti-alloys. These concepts are expected to be transferable to other Ti-alloys and offer new avenues for their tailoring and technological exploitation.

Review on titanium and titanium based alloys as biomaterials for orthopaedic applications

Variety of implant materials have been employed in various disciplines of medical science depending on the requirement of a particular application. Metals, alloys, ceramics, and polymers are the commonly used biomaterials. The main focus of this study is to review the various structural and microstructural properties of titanium and titanium based alloys used as orthopaedic implants. Orthopaedic implants need to possess certain important qualities to ensure their safe and effective use. These properties like the biocompatibility, relevant mechanical properties, high corrosion and wear resistance and osseointegration are summarized in this review. Various attempts to improve upon these properties like different processing routes, surface modifications have also been inculcated in the paper to provide an insight into the extent of research and effort that has been put into developing a highly superior titanium orthopaedic implant.

Effects of new beta-type Ti-40Nb implant materials, brain-derived neurotrophic factor, acetylcholine and nicotine on human mesenchymal stem cells of osteoporotic and non osteoporotic donors

Introduction

Treatment of osteoporotic fractures is still challenging and an urgent need exists for new materials, better adapted to osteoporotic bone by adjusted Young’s modulus, appropriate surface modification and pharmaceuticals.

Materials and methods

Titanium-40-niobium alloys, mechanically ground or additionally etched and titanium-6-aluminium-4-vanadium were analyzed in combination with brain-derived neurotrophic factor, acetylcholine and nicotine to determine their effects on human mesenchymal stem cells in vitro over 21 days using lactate dehydrogenase and alkaline phosphatase assays, live cell imaging and immunofluorescence microscopy.

Results

Cell number of human mesenchymal stem cells of osteoporotic donors was increased after 14 d in presence of ground titanium-40-niobium or titanium-6-aluminium-4-vanadium, together with brain-derived neurotrophic factor. Cell number of human mesenchymal stem cells of non osteoporotic donors increased after 21 d in presence of titanium-6-aluminium-4-vanadium without pharmaceuticals. No significant increase was measured for ground or etched titanium-40-niobium after 21 d. Osteoblast differentiation of osteoporotic donors was significantly higher than in non osteoporotic donors after 21 d in presence of etched, ground titanium-40-niobium or titanium-6-aluminium-4-vanadium accompanied by all pharmaceuticals tested. In presence of all alloys tested brain-derived neurotrophic factor, acetylcholine and nicotine increased differentiation of cells of osteoporotic donors and accelerated it in non osteoporotic donors.

Conclusion

We conclude that ground titanium-40-niobium and brain-derived neurotrophic factor might be most suitable for subsequent in vivo testing.

Functionalization of Ti-40Nb implant material with strontium by reactive sputtering

BACKGROUND

Surface functionalization of orthopedic implants with pharmaceutically active agents is a modern approach to enhance osseointegration in systemically altered bone. A local release of strontium, a verified bone building therapeutic agent, at the fracture site would diminish side effects, which could occur otherwise by oral administration. Strontium surface functionalization of specially designed titanium-niobium (Ti-40Nb) implant alloy would provide an advanced implant system that is mechanically adapted to altered bone with the ability to stimulate bone formation.

METHODS

Strontium-containing coatings were prepared by reactive sputtering of strontium chloride (SrCl2) in a self-constructed capacitively coupled radio frequency (RF) plasma reactor. Film morphology, structure and composition were investigated by scanning electron microscopy (SEM), time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy (HR-TEM) was used for the investigation of thickness and growth direction of the product layer. TEM lamellae were prepared using the focused ion beam (FIB) technique. Bioactivity of the surface coatings was tested by cultivation of primary human osteoblasts and subsequent analysis of cell morphology, viability, proliferation and differentiation. The results are correlated with the amount of strontium that is released from the coating in biomedical buffer solution, quantified by inductively coupled plasma mass spectrometry (ICP-MS).

RESULTS

Dense coatings, consisting of SrOxCly, of more than 100 nm thickness and columnar structure, were prepared. TEM images of cross sections clearly show an incoherent but well-structured interface between coating and substrate without any cracks. Sr2+ is released from the SrOxCly coating into physiological solution as proven by ICP-MS analysis. Cell culture studies showed excellent biocompatibility of the functionalized alloy.

CONCLUSIONS

Ti-40Nb alloy, a potential orthopedic implant material for osteoporosis patients, could be successfully plasma coated with a dense SrOxCly film. The material performed well in in vitro tests. Nevertheless, the Sr2+ release must be optimized in future work to meet the requirements of an effective drug delivery system.

Metal release and cell biological compatibility of beta-type Ti-40Nb containing indium

Small indium (In) additions up to 5 wt % to the beta-type Ti-40Nb alloy effectively improve its mechanical biofunctionality. The impact on its biocompatibility is addressed in this work. Comparative electrochemical polarization studies and inductively coupled plasma optical emission spectrometry analyses were conducted in Tris-buffered saline (on the basis of 150 mM NaCl) with pH 7.6 and 2.0 at 310 ± 1 K with Ti-6Al-4V as reference. The metal ion releases from beta-type alloys were generally very low, for example, those of In³⁺ ions from (Ti-40Nb)-4In specimens were below 6 × 10^-7 mmol/cm² . X-ray photoelectron spectroscopy revealed the passivation mainly by Ti- and Nb-oxides with traces of In-oxides as the dominating surface process. In vitro studies demonstrate a better human bone marrow stromal cells (hBMSC) activity on the beta-type alloys in comparison to CP-Ti (grade 2), which is mainly due to their high Nb content. At 24 h after seeding on (Ti-40Nb)-4In the metabolic activity of hBMSC was 1.5-fold higher and after 11 days, the tissue non-specific alkaline phosphatase activity was 1.8-fold higher relative to values for CP-Ti. Surface treatments, like chemical etching or plasma oxidation, change the surface topography and the thickness and composition of the oxide layers, but they are not effective in further improving the cell response. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1686-1697, 2018.

The mechanical behavior of TiNbSn alloys according to alloying contents, cold rolling and aging

The present study is focused on mechanical properties that result from cold rolling and aging treatments applied to TiNbSn alloys comprising different Nb (35% and 42%) and Sn (0% and 2.5%) contents. The alloys were arc melted, homogenized, solubilized, cold rolled and aged at 400°C for different aging times. A set of characterization tests performed, included microstructural analysis, X-ray diffraction, microhardness, tensile tests and fracture analysis. The alloys contained all three β, α" and ω phases after cold rolling, regardless of the alloying content. The solid solution effect led to changes in the alloys' mechanical behavior. Furthermore, the alloys presented α phase precipitation, and it led to a peak-aged stage after different aging times due to the Nb content. The alloys containing 42% and 35% Nb content reached the peak-aged stage within 48 and 72h, respectively. The α phase precipitation in the alloys at peak-aged stage increased the hardness, tensile strength and elastic modulus of the alloys; however, it also caused ductility to decrease. The fine dispersed precipitates of the α phase generated small and shallow dimples, which are a characteristic fracture micromechanism of peak-aged alloys.