Ti6Ta4Sn alloy and subsequent scaffolding for bone tissue engineering

Porous titanium (Ti) and titanium alloys are promising scaffold biomaterials for bone tissue engineering, because they have the potential to provide new bone tissue ingrowth abilities and low elastic modulus to match that of
natural bone. In the present study, a new highly porous Ti6Ta4Sn alloy scaffold with the addition of biocompatible alloying elements (tantalum (Ta) and tin (Sn)) was prepared using a space-holder sintering method. The
strength of the Ti6Ta4Sn scaffold with a porosity of 75% was found to be significantly higher than that of a pure Ti scaffold with the same porosity. The elastic modulus of the porous alloy can be customized to match that of
human bone by adjusting its porosity. In addition, the porous Ti6Ta4Sn alloy exhibited an interconnected porous structure, which enabled the ingrowth of new bone tissues. Cell culture results revealed that human SaOS₂
osteoblast-like cells grew and spread well on the surfaces of the solid alloy, and throughout the porous scaffold. The surface roughness of the alloy showed a significant effect on the cell behavior, and the optimum surface
roughness range for the adhesion of the SaOS2 cell on the alloy was 0.15 to 0.35 mm. The present study illustrated the feasibility of using the porous Ti6Ta4Sn alloy scaffold as an orthopedic implant material with a special
emphasis on its excellent biomechanical properties and in vitro biocompatibility with a high preference by osteoblast-like cells.

Porous Ti-Nb-Zr alloy through powder metallurgy for biomedical applications

This work investigated the structure and properties relationship, surface modification, biocompatibility and bioactivity of a porous Ti-Nb-Zr alloy. The porous alloy exhibited inter-connected porous structure, good biocompatibility and high mechanical strength with an elastic modulus close to that of bone. Porous Ti-Nb-Zr alloys are thus promising biomaterials for hard tissue replacement.

Biomimetic modification of porous TiNbZr alloy scaffold for bone tissue engineering

Porous titanium (Ti) and Ti alloys are important scaffold materials for bone tissue engineering. In the present study, a new type of porous Ti alloy scaffold with biocompatible alloying elements, that is, niobium (Nb) and zirconium (Zr), was prepared by a space-holder sintering method. This porous TiNbZr scaffold with a porosity of 69% exhibits a mechanical strength of 67MPa and an elastic modulus of 3.9GPa, resembling the mechanical properties of cortical bone. To improve the osteoconductivity, a calcium phosphate (Ca/P) coating was applied to the surface of the scaffold using a biomimetic method. The biocompatibility of the porous TiNbZr alloy scaffold before and after the biomimetic modification was assessed using the SaOS2 osteoblast–like cells. Cell culture results indicated that the porous TiNbZr scaffold is more favorable for cell adhesion and proliferation than its solid counterpart. By applying a Ca/P coating, the cell proliferation rate on the Ca/P-coated scaffold was significantly improved. The results suggest that high-strength porous TiNbZr scaffolds with an appropriate osteoconductive coating could be potentially used for bone tissue engineering application.

Nanohydroxyapatite coating on a titanium–niobium alloy by a hydrothermal process

A novel one-step hydrothermal coating process was used to produce nanohydroxyapatite (nano-HA) coating on a titanium–niobium (TiNb) alloy substrate in a newly designed solution containing calcium and phosphate ions. The morphology of the coating was studied using scanning electron microscopy. The phase identification of the coating was carried out using X-ray diffraction, attenuated total reflectance Fourier transform infrared spectroscopy and transmission electron microscopy. The reaction between the surface of TiNb alloy and the solution during the hydrothermal process was studied by Xray photoelectron spectroscopy. Results show that the coating formed on the surface of TiNb alloy was composed of nano-HA particles. During the hydrothermal process, TiO₂ and Nb₂O₅ formed on the TiNb alloy surface and hydrated to Ti(OH)4 and Nb(OH)₅, respectively. Calcium phosphate nucleated and grew into a layer of nano-HA particles on the surface of TiNb alloy under the hydrothermal conditions. The crystallinity of the nano-HA coating was improved with the increase in hydrothermal treatment temperature and time duration. Nano-HA coating with good crystallinity was produced on the TiNb alloy via the hydrothermal process at a temperature of 200 ºC for 12 h.

Porous shape memory alloy scaffolds for biomedical applications : a review

The interest in using porous shape memory alloy (SMA) scaffolds as implant materials has been growing in recent years due to the combination of their unique mechanical and functional properties, i.e. shape memory effect and superelasticity, low elastic modulus combined with new bone tissue ingrowth ability and vasculariszation. These attractive properties are of great benefit to the healing process for implant applications. This paper reviews current state-of-the art on the processing, porous characteristics and mechanical properties of porous SMAs for biomedical applications, with special focus on the most widely used SMA nickel-titanium (NiTi), including (i) microstructural features, mechanical and functional properties of NiTi SMAs; (ii) main processing methods for the fabrication of porous NiTi SMAs and their mechanical properties and (iii) new-generation Ni-free, biocompatible porous SMA scaffolds.

Titanium-based shape memory alloys and scaffolds for biomedical applications

Porous Ti-50.5Ni shape memory alloys with different porosities were produced using a space-holder sintering method. A new Ni-free Ti-based shape memory alloy, Ti-18Nb-5Mo-5Sn, was developed for potential biomedical applications, and a novel one-step hydrothermal process was applied to produce hydroxyapatite coatings on the surface of Ti alloy.

Thermodynamics approach to the hydrogen diffusion and phase transformation in titanium particles

The hydrogen diffusion and phase transformation in a titanium particle were studied based on thermodynamic calculation. The mechanisms of hydrogen diffusion in different phases (alpha-Ti, beta-Ti and TiHx) were analyzed. A mobility database was developed for titanium– hydrogen system based on the experimental works on hydrogen diffusion coefficient reported in literature and the fundamental of diffusion. To implement the calculation, a commercial software package for the simulation of diffusion-controlled phase transformation was used. The hydrogen diffusion process, hydrogen distribution, phase transformation and phase growth rate during hydrogenization of a titanium particle at temperatures of 560 K, 800K and 1000K were discussed. The thermodynamics and kinetics analysis provided quantitative insight into the diffusion process and improved the understanding of diffusion mechanism and phase transformation.

Thermodynamic and kinetic modelling hydrogenation of titanium particles and sheets

Use of hydrogen as a temporary alloying element in Ti alloys is an attractive approach to improve the mechanical properties of the materials, enhance processability and thereby reduce manufacturing costs. In this paper, the hydrogen diffusion process and the phase transformation both between Ti particles and in Ti sheets were simulated to analyze the mechanism of hydrogen diffusion in different phases (α-Ti, β-Ti and TiHx). With the simulation based on the kinetics and thermodynamics, quantitative behaviors of the hydrogen diffusion and the phase transformation were analyzed. The simulation results provide an insight into the diffusion process and improve the fundamental understanding of the mechanism of diffusion and phase transformation.

Sensitivity of deformation twinning to grain size in titanium and magnesium

The impact of grain size on deformation twinning in commercial purity titanium and magnesium alloy Mg–3Al–1Zn (AZ31) is investigated. Tensile tests were carried out for the titanium samples; compression testing was employed for the magnesium specimens. Average values of the true twin length, true twin thickness and the number density of twins were determined using stereology. A key difference between these two materials is that twinning contributes little to the plastic strain in the titanium while it accounts for nearly all of the early plastic strain in the magnesium. In some respects (e.g. volume fraction and number density) the phenomenology of twinning differed between the two materials, while in others (e.g. twin shape and size) both materials showed a similar response. It is found that in both materials, twins span the entirety of their parent grains only for grain sizes less than ∼30 μm. Both the nucleation density per unit of nucleating interface (i.e. grain and twin boundaries) and the aspect ratio of twins scale with applied stress. The impact of grain size on twin volume fraction is modelled analytically.

Microstructure evolution of martensitic Ti-6Al-4V alloy during warm deformation

The microstructure evolution of martensitic Ti-6Al-4V alloy was investigated through uniaxial hot compression at 700°C and a strain rate of 10-3 s-1. A combination of scanning electron microscopy observation in conjunction with high resolution electron back scattered diffraction (EBSD) was used to characterize the microstructure in detail. The development of the microstructure displayed continuous fragmentation of martensitic laths with increasing strain (i.e. continuous dynamic recrystallization), concurrently with decomposition of supersaturated martensite resulting in the formation of equiaxed grains. At a strain of 0.8, an ultrafine equiaxed grained structure with mostly high angle grain boundaries was successfully obtained. The current work proposes a novel approach to produce equiaxed ultrafine grains in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. © (2014) Trans Tech Publications, Switzerland.

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.