S.J. Li

Influence of cell shape on mechanical properties of Ti-6Al-4V meshes fabricated by electron beam melting method

Ti-6Al-4V reticulated meshes with different elements (cubic, G7 and rhombic dodecahedron) in Materialise software were fabricated by additive manufacturing using the electron beam melting (EBM) method, and the effects of cell shape on the mechanical properties of these samples were studied. The results showed that these cellular structures with porosities of 88-58% had compressive strength and elastic modulus in the range 10-300MPa and 0.5-15GPa, respectively. The compressive strength and deformation behavior of these meshes were determined by the coupling of the buckling and bending deformation of struts. Meshes that were dominated by buckling deformation showed relatively high collapse strength and were prone to exhibit brittle characteristics in their stress-strain curves. For meshes dominated by bending deformation, the elastic deformation corresponded well to the Gibson-Ashby model. By enhancing the effect of bending deformation, the stress-strain curve characteristics can change from brittle to ductile (the smooth plateau area). Therefore, Ti-6Al-4V cellular solids with high strength, low modulus and desirable deformation behavior could be fabricated through the cell shape design using the EBM technique.

The influence of cell morphology on the compressive fatigue behavior of Ti-6Al-4V meshes fabricated by electron beam melting.

Additive manufacturing technique is a promising approach for fabricating cellular bone substitutes such as trabecular and cortical bones because of the ability to adjust process parameters to fabricate different shapes and inner structures. Considering the long term safe application in human body, the metallic cellular implants are expected to exhibit superior fatigue property. The objective of the study was to study the influence of cell shape on the compressive fatigue behavior of Ti-6Al-4V mesh arrays fabricated by electron beam melting. The results indicated that the underlying fatigue mechanism for the three kinds of meshes (cubic, G7 and rhombic dodecahedron) is the interaction of cyclic ratcheting and fatigue crack growth on the struts, which is closely related to cumulative effect of buckling and bending deformation of the strut. By increasing the buckling deformation on the struts through cell shape design, the cyclic ratcheting rate of the meshes during cyclic deformation was decreased and accordingly, the compressive fatigue strength was increased. With increasing bending deformation of struts, fatigue crack growth in struts contributed more to the fatigue damage of meshes. Rough surface and pores contained in the struts significantly deteriorated the compressive fatigue strength of the struts. By optimizing the buckling and bending deformation through cell shape design, Ti-6Al-4V alloy cellular solids with high fatigue strength and low modulus can be fabricated by the EBM technique.

Ultrafine-grained β-type titanium alloy with nonlinear elasticity and high ductility

Although stress-induced martensitic transformation accelerates grain refinement during plastic deformation, it results in serious damage to recoverable elasticity and ductility of severely cold-rolled multifunctional titanium alloys. Here, large-scale nonlinear elasticity and high ductility were achieved simultaneously in an ultrafine-grained Ti–Nb–Zr–Sn alloy by severe warm rolling at 473K to suppress the martensitic transformation from the β to the α″ phase. These results suggest that high strength can be attained in nonlinear elastic titanium alloys by improving the stability of the β phase with body-centered-cubic crystal structure through both composition design and grain refinement.

Electrochemical and surface analyses of nanostructured Ti-24Nb-4Zr-8Sn alloys in simulated body solution.

The use of nanostructuring to improve the stability of passive thin films on biomaterials can enhance their effectiveness in corrosion resistance and reduce the release of ions. The thickness of the ultrathin films that cover Ti and Ti alloys (only several nanometers) has prevented researchers from establishing systematic methods for their characterization. This study employed a multifunctional biomedical titanium alloy Ti-24Nb-4Zr-8Sn (wt.%) as a model material. Coarse-grained (CG) and nanostructured (NS) alloys were analyzed in 0.9% NaCl solution at 37°C. To reveal the details of the passive film, a method of sample preparation producing a passive layer suitable for transmission electron microscope analysis was developed. Electrochemical corrosion behavior was evaluated by potentiodynamic polarization tests and Mott-Schottky measurements. Surface depth chemical profile and morphology evolution were performed by X-ray photoelectron spectroscopy and in situ atomic force microscopy, respectively. A mechanism was proposed on the basis of the point defect model to compare the corrosion resistance of the passive film on NS and CG alloys. Results showed that the protective amorphous film on NS alloy is thicker, denser and more homogeneous with fewer defects than that on CG alloy. The film on NS alloy contains more oxygen and corrosion-resistant elements (Ti and Nb), as well as their suboxides, compared with the film on CG alloy. These characteristics can be attributed to the rapid, uniform growth of the passive film facilitated by nanostructuring.

Corrosion behavior of biomedical Ti-24Nb-4Zr-8Sn alloy in different simulated body solutions

Corrosion behavior of a multifunctional biomedical titanium alloy Ti-24Nb-4Zr-8Sn (wt.%) in 0.9% NaCl, Hanks solution and artificial saliva at 37 °C was investigated using open circuit potential, impedance spectroscopy and potentiodynamic polarization techniques, and some results were compared with pure titanium and Ti-6Al-4V alloy. The results showed that the alloy exhibited good corrosion resistance due to the formation of a protective passive film consisting mainly of TiO2 and Nb2O5, and a little of ZrO2 and SnO2. Ca ions were detected in the passive film as the alloy immersed in Hanks and artificial saliva solutions and they have negative effect on corrosion resistance. The EIS results indicated that either a duplex film with an inner barrier layer and an outer porous layer or a single passive layer was formed on the surface, and they all transformed into stable bilayer structure as the immersion time increased up to 24h. The polarization curves demonstrated that the alloy had a wider passive region than pure titanium and Ti-6Al-4V alloy and its corrosion current density (less than 0.1 μA/cm(2)) is comparable to that of pure titanium.

Osteoblast cellular activity on low elastic modulus Ti–24Nb–4Zr–8Sn alloy

OBJECTIVESnLow modulus β-titanium alloys with non-toxic alloying elements are envisaged to provide good biocompatibility and alleviate the undesired stress shielding effect. The objective of this study is to fundamentally elucidate the biological response of novel high strength-low elastic modulus Ti2448 alloy through the study of bioactivity and osteoblast cell functions.nnnMETHODSnCharacterization techniques such as SEM, EDX, XRD, and fluorescence microscopy were utilized to analyze the microstructure, morphology, chemical composition, and cell adhesion. The cellular activity was explored in terms of cell-to-cell communication involving proliferation, spreading, synthesis of extracellular and intracellular proteins, differentiation, and mineralization.nnnRESULTSnThe formation of fine apatite-like crystals on the surface during immersion test in simulated body fluid confirmed the bioactivity of the surface, which provided the favorable osteogenic microenvironment for cell-material interaction. The proliferation and differentiation of pre-osteoblasts and their ability to form a well mineralized bone-like extracellular matrix (ECM) by secreting bone markers (ALP, calcium, etc.) over the surface point toward the determining role of unique surface chemistry and surface properties of the Ti-24Nb-4Zr-8Sn (Ti2448) alloy in modulating osteoblasts functions.nnnSIGNIFICANCEnThese results demonstrated that the low modulus (∼49GPa) Ti2448 alloy with non-toxic alloying elements can be used as a potential dental or orthopedic load-bearing implant material.

Cellular response of osteoblasts to low modulus Ti-24Nb-4Zr-8Sn alloy mesh structure

Titanium alloys (Ti-6Al-4V and Ti-6Al-7Nb) are widely used for implants, which are characterized by high elastic modulus (∼110 GPa) with (αu2009+u2009β) structure and that may induce undesirable stress shielding effect and immune responses associated with the presence of toxic elements. In this regard, we have combined the attributes of a new alloy design and the concept of additive manufacturing to fabricate 3D scaffolds with an interconnected porous structure. The new alloy is a β-type Ti-24Nb-4Zr-8Sn (Ti2448) alloy with significantly reduced modulus. In the present study, we explore the biological response of electron beam melted low modulus Ti2448 alloy porous mesh structure through the elucidation of bioactivity and osteoblast functions. The cellular activity was explored in terms of cell-to-cell communication involving proliferation, spreading, synthesis of extracellular and intracellular proteins, differentiation, and mineralization. The formation of fine apatite-like crystals on the surface during immersion test in simulated body fluid confirmed the bioactivity of the scaffold surface, which provided the favorable osteogenic microenvironment for cell-material interaction. The combination of unique surface chemistry and interconnected porous architecture provided the desired pathway for supply of nutrients and oxygen to cells and a favorable osteogenic micro-environment for incorporation (on-growth and in-growth) of osteoblasts. The proliferation and differentiation of pre-osteoblasts and their ability to form a well mineralized bone-like extracellular matrix (ECM) by secreting bone markers (ALP, calcium, etc.) over the struts of the scaffold point toward the determining role of unique surface chemistry and 3D architecture of the Ti2448 alloy mesh structure in modulating osteoblasts functions.

Functional response of osteoblasts in functionally gradient titanium alloy mesh arrays processed by 3D additive manufacturing.

We elucidate here the osteoblasts functions and cellular activity in 3D printed interconnected porous architecture of functionally gradient Ti-6Al-4V alloy mesh structures in terms of cell proliferation and growth, distribution of cell nuclei, synthesis of proteins (actin, vinculin, and fibronectin), and calcium deposition. Cell culture studies with pre-osteoblasts indicated that the interconnected porous architecture of functionally gradient mesh arrays was conducive to osteoblast functions. However, there were statistically significant differences in the cellular response depending on the pore size in the functionally gradient structure. The interconnected porous architecture contributed to the distribution of cells from the large pore size (G1) to the small pore size (G3), with consequent synthesis of extracellular matrix and calcium precipitation. The gradient mesh structure significantly impacted cell adhesion and influenced the proliferation stage, such that there was high distribution of cells on struts of the gradient mesh structure. Actin and vinculin showed a significant difference in normalized expression level of protein per cell, which was absent in the case of fibronectin. Osteoblasts present on mesh struts formed a confluent sheet, bridging the pores through numerous cytoplasmic extensions. The gradient mesh structure fabricated by electron beam melting was explored to obtain fundamental insights on cellular activity with respect to osteoblast functions.

Microstructure and mechanical properties of open cellular Ti–6Al–4V prototypes fabricated by electron beam melting for biomedical applications

Titanium and titanium alloys with open cellular structures and foams possess low Youngs modulus matching to human bone and the capability to provide space for bone tissue ingrowth to reach a better fixation, which have been thought as a good choice for the replacement of commercial dense implants. However, currently developed fabrication methods of titanium cellular alloys, such as powder sintering, reveal shortcomings like the low porosity and poor control over the size, shape and distribution of the pores. Recently, additive manufacturing using the electron beam melting method has been applied successfully to fabricate titanium cellular meshes and foams. Compared to other reported methods, this technique has the advantages of accurate control of internal pore architectures and complex cell shapes. In the present paper, the authors briefly review the fabrication, Youngs modulus, mechanical properties and biocompatibility of Ti–6Al–4V cellular structures fabricated by electron beam melting techniques, along with future development trends.

A synthetic-aperture mode filtering for source localization by autonomous underwater vehicles

In this paper, a new technique of null-steering mode filtering for a synthetic aperture array is applied for the passive source localization on an AUV mounted array. The proposed method utilizes modal spatiotemporal consistency on a moving array in the stratified waveguide, and overcomes physical aperture limitation for source localization by the AUV mounted short horizontal array. The simulated results indicate that performance of source localization is improved and with faster moving speed, the beam side-lobe level of bearing estimation can be reduced.