Machinability assessment of titanium alloy Ti-6Al-4V for biomedical applications

Titanium alloy (Ti-6Al-4V) has a wide range of application in various fields of engineering. Titanium is mainly used to manufacture aerospace components like landing gear, fuselage, wings, engines etc. and biomedical components like hip joint, knee joint, dental implants etc. Titanium has outstanding material properties such as corrosion resistance, fatigue strength, tensile strength and a very good biocompatibility which makes this material very alluring for biomedical applications. Contrary, the machinability of the material is problematic because of the phase transformations and thus, titanium alloy is a challenge for machining operation. This research is a comparative analysis between the implants manufactured by traditional method of casting and machining. The femoral stem of the hip joint replacement is designed and the component is machined using a five-axis CNC machine.The machined component was subjected to surface roughness testing, tensile testing and bulk hardness testing. The values were compared with the values of titanium implants manufactured by casting. © (2014) Trans Tech Publications, Switzerland.

Effect of heat-treatment atmosphere on the bond strength of apatite layer on Ti substrate

Objectives
The purpose of this study was to investigate the bond strength of apatite layer on titanium (Ti) substrate coated by biomimetic method and to improve the bonding of apatite layer to Ti substrate by optimizing the alkali heat-treatment process.

Methods
Ti plates pre-treated with an alkali solution of 10 M sodium hydroxide (NaOH) were heat-treated at 600 °C for 1 h at different atmospheres: in air and in vacuum. A dense apatite layer formed on top of the sodium titanate layer after soaking the alkali and heat-treated Ti samples in simulated body fluid (SBF) for up to 3 weeks. The bond strengths of the sodium titanate layer on Ti substrate, and apatite layer on the sodium titanate layer, were measured, respectively, by applying a tensile load. The fracture sites were observed with a scanning electron microscope (SEM).

Results
The apatite layer on the substrate after alkali heat-treatment in air achieved higher bond strength than that on the substrate after alkali heat-treatment in vacuum. It was found that the interfacial structure between the sodium titanate and Ti substrate has a significant influence on the bond strength of the apatite layer.

Significance
It is advised that titanium implants can achieve better osseointegration under load-bearing conditions by depositing an apatite layer in vivo on a Ti surface subjected to alkali and heat-treated in air.

Particulate-reinforced titanium composites for orthopaedic implants

In the present study, to enhance the strength of porous pure titanium scaffolds with high porosity, new particulate-reinforced Ti-based composites with the addition of biocompatible oxide particles such as TiO₂, SiO₂, ZrO₂ and Nb₂O₅ were prepared using a powder metallurgical method. The strengths of the new particulate-reinforced titanium composites were found to be significantly higher than that of pure titanium with an excellent biocompatibility. SaOS-2 osteoblast-like cells grew and spread well on the surfaces of the new particulate-reinforced titanium composites. The present study illustrated the feasibility of using the particulate-reinforced titanium composites as an orthopaedic implant material.

Bending fatigue testing of commercial purity titanium for dental implants

High fatigue strength is one of the major requirements for dental implant materials. It was previously shown that the fatigue strength under conventional stress-control tension–compression testing can be doubled for commercially pure (CP) titanium processed by equal channel angular pressing. However, the fatigue endurance of an implant exposed to cyclic loading in corrosive media (bodily fluids) may potentially be compromised. In this work, non-conventional bending fatigue testing in air and in simulated body fluid (SBF) has been carried out for coarse-grained and ultrafine-grained CP titanium.

Preparation and characterisation of new titanium based alloys for orthopaedic and dental applications

Various types of titanium alloys with high strength and low elastic modulus and, at the same time, vanadium and aluminium free have been developed as surgical biomaterials in recent years. Moreover, porous metals are promising hard tissue implants in orthopaedic and dentistry, where they mimic the porous structure and the low elastic modulus of natural bone. In the present study, new biocompatible Ti-based alloy foams with approximate relative densities of 0.4, in which Sn and Nb were added as alloying metals, were synthesised through powder metallurgy method.
The new alloys were prepared by mechanical alloying and subsequently sintered at high temperature using a vacuum furnace. The characteristics and the processability of the ball milled powders and the new porous titanium-based alloys were characterised by X-ray diffraction, optical
microscopy and scanning electron microscopy .The mechanical properties of the new titanium alloys were examined by Vickers microhardness measurements and compression testing.

In vitro bioactivity evaluation of titanium and niobium metals with different surface morphologies

Current orthopaedic biomaterials research mainly focuses on designing implants that could induce controlled, guided and rapid healing. In the present study, the surface morphologies of titanium (Ti) and niobium (Nb) metals were tailored to form nanoporous, nanoplate and nanofibre-like structures through adjustment of the temperature in the alkali-heat treatment. The in vitro bioactivity of these structures was then evaluated by soaking the treated samples in simulated body fluid (SBF). It was found that the morphology of the modified surface significantly influenced the apatite-inducing ability. The Ti surface with a nanofibre-like structure showed better apatite-inducing ability than the nanoporous or nanoplate surface structures. A thick dense apatite layer formed on the Ti surface with nanofibre-like structure after 1 week of soaking in SBF. It is expected that the nanofibre-like surface could achieve good apatite formation in vivo and subsequently enhance osteoblast cell adhesion and bone formation.

Mechanical properties of titanium foam for biomedical applications

Understanding the mechanical behaviour of pure titanium (Ti) foam is crucial for the design and development of Ti foam-based load-bearing implants. In this work, pure titanium foam is fabricated by a powder metallurgical process using the space-holder technique with a spacer size of 500 to 800 µm. Experimental data from static compression testing on the Ti foam are presented. The application of theoretical formulae to predict Young's modulus and yield strength of titanium foams is also discussed. A foam with 63% porosity, 87 ± 5 MPa yield strength, and 6.5 ± 1.3 GPa Young's modulus is found to be appropriate for a number of dental and orthopaedic applications.

In vitro cytotoxicity of binary TI alloys for bone implants

Interest in using titanium (Ti) alloys as load-bearing implant materials has increased due to their high strength to weight ratio, lower elastic modulus, and superior biocompatibility and enhanced corrosion resistance compared to conventional metals such as stainless steel and Co-Cr alloys. In the present study, the in vitro cytotoxicity of five binary titanium alloys, Ti15Ta, Ti15Nb, Ti15Zr, Ti15Sn and Ti15Mo, was assessed using human osteosarcoma cell line, SaOS-2 cells. The Cell proliferation and viability were determined, and cell adhesion and morphology on the surfaces of the binary Ti alloys after cell culture were observed by SEM. Results indicated that the Ti binary alloys of Ti15Ta, Ti15Nb and Ti15Zr exhibited the same level of excellent biocompatibility; Ti15Sn alloy exhibited a moderate biocompatibility while Ti15Mo alloy exhibited a moderate cytotoxicity. The SaOS-2 osteoblast-like cells had flattened and spread across the surfaces of the Ti15Ta, Ti15Nb, Ti15Zr and Ti15Sn groups; however, the cell shapes on the Ti15Mo alloy was shrinking and unhealthy. These results indicated that the Mo contents should be limited to a certain level in the design and development of new Ti alloys for implant material applications.

Apatite formation on nano-structured titanium and niobium surface

Current orthopaedic biomaterials research mainly focuses on developing implants that could induce controlled, guided and rapid healing. In the present study, the surface morphologies of titanium (Ti) and niobium (Nb) metals were tailored to form nanoporous, nanoplate and nanofibrelike structures through adjustment of the temperature in the alkali treatment. The in vitro bioactivity of these structures was then evaluated by soaking in simulated body fluid (SBF). It was found that the morphology of the modified surface significantly influenced the apatite inducing ability. The Ti surface with a nanofiber-like structure showed better apatite inducing ability, than the nanoporous or nanoplate surface structures. A thick dense apatite layer formed on the Ti surface with nanofiberlike structure after 1 week soaking in SBF. It is expected that the anofibre-like surface could achieve good apatite formation in vivo and subsequently enhance osteoblast cell adhesion and bone formation in vivo.

Biomimetic porous titanium scaffolds for orthopaedic and dental applications

The development of artificial organs and implants for replacement of injured and diseased hard tissues such as bones, teeth and joints is highly desired in orthopedic surgery. Orthopedic prostheses have shown an enormous success in restoring the function and offering high quality of life to millions of individuals each year. Therefore, it is pertinent for an engineer to set out new approaches to restore the normal function of impaired hard tissues.

Over the last few decades, a large number of metals and applied materials have been developed with significant improvement in various properties in a wide range of medical applications. However, the traditional metallic bone implants are dense and often suffer from the problems of adverse reaction, biomechanical mismatch and lack of adequate space for new bone tissue to grow into the implant. Scientific advancements have been made to fabricate porous scaffolds that mimic the architecture and mechanical properties of natural bone. The porous structure provides necessary framework for the bone cells to grow into the pores and integrate with host tissue, known as osteointegration. The appropriate mechanical properties, in particular, the low elastic modulus mimicking that of bone may minimize or eliminate the stress-shielding problem. Another important approach is to develop biocompatible and corrosion resistant metallic materials to diminish or avoid adverse body reaction. Although numerous types of materials can be involved in this fast developing field, some of them are more widely used in medical applications. Amongst them, titanium and some of its alloys provide many advantages such as excellent biocompatibility, high strength-to-weight ratio, lower elastic modulus, and superior corrosion resistance, required for dental and orthopedic implants. Alloying elements, i.e. Zr, Nb, Ta, Sn, Mo and Si, would lead to superior improvement in properties of titanium for biomedical applications.

New processes have recently been developed to synthesize biomimetic porous titanium scaffolds for bone replacement through powder metallurgy. In particular, the space holder sintering method is capable of adjusting the pore shape, the porosity, and the pore size distribution, notably within the range of 200 to 500 m as required for osteoconductive applications. The present chapter provides a review on the characteristics of porous metal scaffolds used as bone replacement as well as fabrication processes of porous titanium (Ti) scaffolds through a space holder sintering method. Finally, surface modification of the resultant porous Ti scaffolds through a biomimetic chemical technique is reviewed, in order to ensure that the surfaces of the scaffolds fulfill the requirements for biomedical applications.

Novel sphene coatings on Ti–6Al–4V for orthopedic implants using sol–gel method

Hydroxyapatite (HAp) is commonly used to coat titanium alloys (Ti–6Al–4V) for orthopedic implants. However, their poor adhesion strength and insufficient long-term stability limit their application. Novel sphene (CaTiSiO₅) ceramics possess excellent chemical stability and cytocompatibility. The aim of this study is to use the novel sphene ceramics as coatings for Ti–6Al–4V. The sol–gel method was used to produce the coatings and the thermal properties, phase composition, microstructure, thickness, surface roughness and adhesion strength of sphene coatings were analyzed by differential thermal analysis–thermal gravity (DTA–TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), atom force microscopy (AFM) and scratch test, respectively. DTA analysis confirmed that the temperature of the sphene phase formation is 875 °C and XRD analysis indicated pure sphene coatings were obtained. A uniform structure of the sphene coating was found across the Ti–6Al–4V surface, with a thickness and surface roughness of the coating of about 0.5–1 μm and 0.38 μm, respectively. Sphene-coated Ti–6Al–4V possessed a significantly improved adhesion strength compared to that for HAp coating and their chemical stability was evaluated by testing the profile element distribution and the dissolution kinetics of calcium (Ca) ions after soaking the sphene-coated Ti–6Al–4V in Tris–HCl solution. Sphene coatings had a significantly improved chemical stability compared to the HAp coatings. A layer of apatite formed on the sphene-coated Ti–6Al–4V after they were soaked in simulated body fluids (SBF). Our results indicate that sol–gel coating of novel sphene onto Ti–6Al–4V possessed improved adhesion strength and chemical stability, compared to HAp-coated Ti–6Al–4V prepared under the same conditions, suggesting their potential application as coatings for orthopedic implants.

Collagen type-I leads to in vivo matrix mineralization and secondary stabilization of Mg-Zr-Ca alloy implants

Biodegradable magnesium-zirconia-calcium (Mg-Zr-Ca) alloy implants were coated with Collagen type-I (Coll-I) and assessed for their rate and efficacy of bone mineralization and implant stabilization. The phases, microstructure and mechanical properties of these alloys were analyzed using X-ray diffraction (XRD), optical microscopy and compression test, respectively, and the corrosion behavior was established by their hydrogen production rate in simulated body fluid (SBF). Coll-I extracted from rat tail, and characterized using fourier transform infrared (FT-IR) spectroscopy, was used for dip-coating the Mg-based alloys. The coated alloys were implanted into the femur bones of male New Zealand white rabbits. In vivo bone formation around the implants was quantified by measuring the bone mineral content/density (BMC/BMD) using dual-energy X-ray absorptiometry (DXA). Osseointegration of the implant and new bone mineralization was visualized by histological and immunohistochemical analysis. Upon surface coating with Coll-I, these alloys demonstrated high surface energy showing enhanced performance as an implant material that is suitable for rapid and efficient new bone tissue induction with optimal mineral content and cellular properties. The results demonstrate that Coll-I coated Mg-Zr-Ca alloys have a tendency to form superior trabecular bone structure with better osteoinduction around the implants and higher implant secondary stabilization, through the phenomenon of contact osteogenesis, compared to the control and uncoated ones in shorter periods of implantation. Hence, Coll-I surface coating of Mg-Zr-Ca alloys is a promising method for expediting new bone formation in vivo and enhancing osseointegration in load bearing implant applications.

Titanium in biomedical applications—properties and fabrication: a review

Ti and Ti-based alloys have unique properties such as high strength, low density and excellent corrosion resistance. These properties are essential for the manufacture of lightweight and high strength components for biomedical applications. In this paper, Ti properties such as metallurgy, mechanical properties, surface modification, corrosion resistance, biocompatibility and osseointegration in biomedical applications have been discussed. This paper also analyses the advantages and disadvantages of various Ti manufacturing processes for biomedical applications such as casting, powder metallurgy, cold and hot working, machining, laser engineering net shaping (LEN), superplastic forming, forging and ring rolling. The contributions of this research are twofold, firstly scrutinizing the behaviour of Ti and Ti-based alloys in-vivo and in-vitro experiments in biomedical applications to determine the factors leading to failure, and secondly strategies to achieve desired properties essential to improving the quality of patient outcomes after receiving surgical implants. Future research will be directed toward manufacturing of Ti for medical applications by improving the production process, for example using optimal design approaches in additive manufacturing and investigating alloys containing other materials in order to obtain better medical and mechanical characteristics.

Accelerated growth of preosteoblastic cells on ultrafine grained titanium

This work is part of a general effort to demonstrate the effect of the bulk microstructure of titanium as a model bone implant material on viability of osteoblasts (bone-forming cells). The objective of this work was to study the proliferation of preosteoblastic MC3T3-E1 cells extracted from mice embryos on commercial purity titanium substrates. Two distinct states of titanium were considered: as-received material with an average grain size of 4.5 microm and that processed by equal channel angular pressing (ECAP), with an average grain size of 200 nm. We report the first results of an in vitro study into the effect of this extreme grain refinement on viability and proliferation of MC3T3-E1 cells. By means of MTT assays it was demonstrated that ECAP processing of titanium enhances MC3T3-E1 culture proliferation in a spectacular way. This finding suggests that bone implants made from ECAP processed titanium may promote bone tissue growth.