Yulin Hao

Super-elastic titanium alloy with unstable plastic deformation

Here we report a non-toxic β-type titanium alloy exhibiting unstable elastic and plastic deformation behavior. Elastic instability leads to remarkable elastic softening, i.e., the decrease of incipient Young’s modulus with slight pre-straining. In spite of partial recovery during room-temperature aging, a stable modulus of 33GPa matching that of human bone can be maintained. Plastic instability causes highly-localized deformation which is very effective in grain refinement but contributes little to strength. We thus obtain soft nanostructured metallic materials (NMMs): The flow stress increases by only ∼5.5% as coarse grains are reduced to below 50nm, in contrast with several times increase for previously-reported NMMs.

The site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1-5 at.%) in TiAl alloys with different compositions, and in Ti3Al with the compositions of Ti-26 at.%Al-(1-2 at.%)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms occupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in particular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti-Al-V-Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti3Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti3Al. The experimental results are interpreted in terms of a Bragg-Williams-type model and bond-order data obtained from electronic structure calculation. Qualitative agreement between the model and measurements is reached

Phase stability and elastic modulus of Ti alloys containing Nb, Zr, and/or Sn from first-principles calculations

The alloying effects of Nb, Zr, and/or Sn on the phase stability and elastic properties of Ti are investigated by using a first-principles method. Our calculation results indicate that a carefully designed Ti-Nb-Zr-Sn system can be a good candidate for low modulus biomedical materials. We find that the well-known correlation between the e/a ratio and both elastic and phase stabilities for Ti alloyed with transition metal elements breaks down for the Ti-Sn alloy.

Porous titanium-6 aluminum-4 vanadium cage has better osseointegration and less micromotion than a poly-ether-ether-ketone cage in sheep vertebral fusion.

Interbody fusion cages made of poly-ether-ether-ketone (PEEK) have been widely used in clinics for spinal disorders treatment; however, they do not integrate well with surrounding bone tissue. Ti-6Al-4V (Ti) has demonstrated greater osteoconductivity than PEEK, but the traditional Ti cage is generally limited by its much greater elastic modulus (110 GPa) than natural bone (0.05-30 GPa). In this study, we developed a porous Ti cage using electron beam melting (EBM) technique to reduce its elastic modulus and compared its spinal fusion efficacy with a PEEK cage in a preclinical sheep anterior cervical fusion model. A porous Ti cage possesses a fully interconnected porous structure (porosity: 68 ± 5.3%; pore size: 710 ± 42 μm) and a similar Youngs modulus as natural bone (2.5 ± 0.2 GPa). When implanted in vivo, the porous Ti cage promoted fast bone ingrowth, achieving similar bone volume fraction at 6 months as the PEEK cage without autograft transplantation. Moreover, it promoted better osteointegration with higher degree (2-10x) of bone-material binding, demonstrated by histomorphometrical analysis, and significantly higher mechanical stability (P < 0.01), shown by biomechanical testing. The porous Ti cage fabricated by EBM could achieve fast bone ingrowth. In addition, it had better osseointegration and superior mechanical stability than the conventional PEEK cage, demonstrating great potential for clinical application.

New intrinsic mechanism on gum-like superelasticity of multifunctional alloys

Ti-Nb-based Gum Metals exhibit extraordinary superelasticity with ultralow elastic modulus, superior strength and ductility, and a peculiar dislocation-free deformation behavior, most of which challenge existing theories of crystal strength. Additionally, this kind of alloys actually displays even more anomalous mechanical properties, such as the non-linear superelastic behavior, accompanied by a pronounced tension-to-compression asymmetry, and large ductility with a low Poissons ratio. Two main contradictory arguments exist concerning the deformation mechanisms of those alloys, i.e., formation of reversible nanodisturbance and reversible martensitic transformation. Herein we used the in-situ synchrotron high-energy X-ray scattering technique to reveal the novel intrinsic physical origin of all anomalous mechanical properties of the Ti-24Nb-4Zr-8Sn-0.10O alloy, a typical gum-like metal. Our experiments provide direct evidence on two different kinds of interesting, stress-induced, reversible nanoscale martensitic transitions, i.e., the austenitic regions with B2 structure transform to α″ martensite and those with BCC structure transform to δ martensite.

Biomedical titanium alloys and their additive manufacturing

Titanium and its alloys have been widely used for biomedical applications due to their better biomechanical and biochemical compatibility than other metallic materials such as stainless steels and Co-based alloys. A brief review on the development of the β-type titanium alloys with high strength and low elastic modulus is given, and the use of additive manufacturing technologies to produce porous titanium alloy parts, using Ti–6Al–4V as a reference, and its potential in fabricating biomedical replacements are discussed in this paper.

Osteoblast functions in functionally graded Ti-6Al-4 V mesh structures

We describe here the combined efforts of engineering and biological sciences as a systemic approach to fundamentally elucidate osteoblast functions in functionally graded Ti-6Al-4 V mesh structures in relation to uniform/monolithic mesh arrays. First, the interconnecting porous architecture of functionally graded mesh arrays was conducive to cellular functions including attachment, proliferation, and mineralization. The underlying reason is that the graded fabricated structure with cells seeded from the large pore size side provided a channel for efficient transfer of nutrients to other end of the structure (small pore size), leading to the generation of mineralized extracellular matrix by differentiating pre-osteoblasts. Second, a comparative and parametric study indicated that gradient mesh structure had a pronounced effect on cell adhesion and mineralization, and strongly influenced the proliferation phase. High intensity and near-uniform distribution of proteins (actin and vinculin) on struts of the gradient mesh structure (cells seeded from large pore side) implied signal transduction during cell adhesion and was responsible for superior cellular activity, in comparison to the uniform mesh structure and non-porous titanium alloy. Cells adhered to the mesh struts by forming a sheet, bridging the pores through numerous cytoplasmic extensions, in the case of porous mesh structures. Intercellular interaction in porous structures provided a pathway for cells to communicate and mature to a differentiated phenotype. Furthermore, the capability of cells to migrate through the interconnecting porous architecture on mesh structures led to colonization of the entire structure. Cells were embedded layer-by-layer in the extracellular matrix as the matrix mineralized. The outcomes of the study are expected to address challenges associated with the treatment of segmental bone defects and bone-remodeling through favorable modulation of cellular response. Moreover, the study provides a foundation for a new branch of functionally graded materials with interconnected porous architecture.

The functional response of bioactive titania-modified three-dimensional Ti-6Al-4V mesh structure toward providing a favorable pathway for intercellular communication and osteoincorporation.

The objective of the study is to fundamentally elucidate the biological response of 3D printed mesh structures subjected to plasma electrolytic oxidation process through the study of osteoblast functions. The cellular activity of plasma electrolytic-oxidized mesh structure was explored in terms of cell-to-cell communication involving proliferation, synthesis of extracellular and intracellular proteins, and mineralization. Upon plasma electrolytic oxidation of the mesh structure, a thin layer of bioactive titania with pore size 1-3 µm was nucleated on the surface. The combination of microporous bioactive titania and interconnected porous architecture provided the desired pathway for supply of nutrients and oxygen to cells and tissue and a favorable osteogenic microenvironment for tissue on-growth and in-growth, in relation to the unmodified mesh structure. The formation of a confluent layer as envisaged via electron microscopy and quantitative assessment of the expression level of proteins (actin, vinculin, and fibronectin) point toward the determining role of surface-modified mesh structure in modulating osteoblasts functions.

Fabrication of open-cellular (porous) titanium alloy implants: osseointegration, vascularization and preliminary human trials

In this study we describe the fabrication of a variety of open-cellular titanium alloy (Ti-6Al-4V) implants, both reticular mesh and foam structures, using electron beam melting (EBM). These structures allow for the elimination of stress shielding by adjusting the porosity (or density) to produce an elastic modulus (or stiffness) to match that of both soft (trabecular) and hard (cortical) bone, as well as allowing for bone cell ingrowth, increased cell density, and all-matrix interactions; the latter involving the interplay between bone morphogenetic protein (BMP-2) and osteoblast functions. The early formation and characterization of elementary vascular structures in an aqueous hydrogel matrix are illustrated. Preliminary results for both animal (sheep) and human trials for a number of EBM-fabricated, and often patient-specific Tialloy implants are also presented and summarized. The results, while preliminary, support the concept and development of successful, porous, engineered “living” implants.摘要本文采用电子束增材(EBM)制造技术制备了多种具有开放孔隙结构的多孔钛合金(Ti-6Al-4V)植入物, 包括网状和泡沫状结构. 该多孔钛合金植入物可以通过调节孔隙率(或密度)降低其弹性模量(或刚度)以减轻 “应力屏蔽” 效应, 实现与软(小梁)和硬(皮质)骨的弹性模量(或刚度)匹配; 同时还可以促进骨组织长入, 增加细胞密度和细胞外基质间的相互作用, 后者涉及骨形态发生蛋白(BMP-2)和成骨细胞功能之间的相互影响. 总结了在水性水凝胶基质中初级血管结构的早期形成和特征, 报道了EBM技术制备的个性化钛合金植体在动物(羊)和人体临床试验的初步结果. 本文结果为钛合金多孔材料作为组织工程“活性”植入物的应用可行性研究提供了有力支持.

Surface nanotopography-induced favorable modulation of bioactivity and osteoconductive potential of anodized 3D printed Ti-6Al-4V alloy mesh structure

The objective of the study described here is to fundamentally elucidate the biological response of 3D printed Ti-6Al-4V alloy mesh structures that were surface modified to introduce titania nanotubes with an average pore size of ∼80 nm via an electrochemical anodization process from the perspective of enhancing bioactivity. The bioactivity of the mesh structures were analyzed through immersion test in simulated body fluid, which confirmed the nucleation and growth of fine globular nanoscale apatite on the nanoporous titania-modified (anodized) mesh structure surface, and agglomerated apatite with fine flakes of apatite crystals on as-fabricated mesh structure surface, that were rich in calcium and phosphorous. The cellular activity of bioactive anodized mesh structure was explored in terms of cell–material interactions involving adhesion, proliferation, synthesis of extracellular and intracellular proteins, differentiation, and mineralization. Cells adhered with a sheet-like morphology on as-fabricated mesh structure, whereas, on anodized mesh structure, numerous filopodia-like cellular extensions interacting with nanotube pores were observed. The formation of a bioactive nanoscale apatite, cell–nanotube interactions as imaged via electron microscopy, higher expression of proteins (actin, vinculin, fibronectin, and alkaline phosphatase (ALP)), and calcium content points toward the determining role of anodized mesh structure in modulating osteoblasts functions. The unique combination of nanoporous bioactive titania and interconnected porous architecture of anodized titanium alloy mesh structure provided a multimodal roughness surface ranging from nano to micro to macroscale, which helps in attaining strong primary and secondary fixation of the implant device along with the pathway for supply of nutrients and oxygen to cells and tissue.