Xipeng Tan

Metallic powder-bed based 3D printing of cellular scaffolds for orthopaedic implants: A state-of-the-art review on manufacturing, topological design, mechanical properties and biocompatibility

Metallic cellular scaffold is one of the best choices for orthopaedic implants as a replacement of human body parts, which could improve life quality and increase longevity for the people needed. Unlike conventional methods of making cellular scaffolds, three-dimensional (3D) printing or additive manufacturing opens up new possibilities to fabricate those customisable intricate designs with highly interconnected pores. In the past decade, metallic powder-bed based 3D printing methods emerged and the techniques are becoming increasingly mature recently, where selective laser melting (SLM) and selective electron beam melting (SEBM) are the two representatives. Due to the advantages of good dimensional accuracy, high build resolution, clean build environment, saving materials, high customisability, etc., SLM and SEBM show huge potential in direct customisable manufacturing of metallic cellular scaffolds for orthopaedic implants. Ti-6Al-4V to date is still considered to be the optimal materials for producing orthopaedic implants due to its best combination of biocompatibility, corrosion resistance and mechanical properties. This paper presents a state-of-the-art overview mainly on manufacturing, topological design, mechanical properties and biocompatibility of cellular Ti-6Al-4V scaffolds via SLM and SEBM methods. Current manufacturing limitations, topological shortcomings, uncertainty of biocompatible test were sufficiently discussed herein. Future perspectives and recommendations were given at the end.

Revealing martensitic transformation and α/β interface evolution in electron beam melting three-dimensional-printed Ti-6Al-4V.

As an important metal three-dimensional printing technology, electron beam melting (EBM) is gaining increasing attention due to its huge potential applications in aerospace and biomedical fields. EBM processing of Ti-6Al-4V as well as its microstructure and mechanical properties were extensively investigated. However, it is still lack of quantitative studies regarding its microstructural evolution, indicative of EBM thermal process. Here, we report α′ martensitic transformation and α/β interface evolution in varied printing thicknesses of EBM-printed Ti-6Al-4V block samples by means of atom probe tomography. Quantitative chemical composition analysis suggests a general phase transformation sequence. By increasing in-fill hatched thickness, elemental partitioning ratios arise and β volume fraction is increased. Furthermore, we observe kinetic vanadium segregation and aluminum depletion at interface front and the resultant α/β interface widening phenomenon. It may give rise to an increased α/β lattice mismatch and weakened α/β interfaces, which could account for the degraded strength as printing thickness increases.

Fabrication and microstructural characterisation of additive manufactured Ti-6Al-4V parts by electron beam melting

An industrial impeller has been successfully fabricated by using Ti-6Al-4V extra low interstitial (ELI) powder via an Arcam A2XX electron beam melting (EBM®) machine. With the introduction of the A2XX, Arcam allowed the production of complex large-scale parts for industrial applications. However, the implementation of as-built metallic additive manufacturing (AM) parts in the industry is still far from straightforward and requires a sound understanding of the processing and the material properties of as-built part. This study examines the EBM thermal process influencing the microstructure evolution of Ti-6Al-4V by the EBM process. Ti-6Al-4V blocks with varying thicknesses were fabricated to simulate the different thickness of complex parts. The microstructure and Vickers hardness were evaluated. This paper mainly contributes to the EBM manufacturing of the industrial impeller and the build thickness dependence of microstructure and mechanical properties in EBM additive manufactured Ti-6Al-4V.

Hybrid microscaffold-based 3D bioprinting of multi-cellular constructs with high compressive strength: A new biofabrication strategy

A hybrid 3D bioprinting approach using porous microscaffolds and extrusion-based printing method is presented. Bioink constitutes of cell-laden poly(D,L-lactic-co-glycolic acid) (PLGA) porous microspheres with thin encapsulation of agarose-collagen composite hydrogel (AC hydrogel). Highly porous microspheres enable cells to adhere and proliferate before printing. Meanwhile, AC hydrogel allows a smooth delivery of cell-laden microspheres (CLMs), with immediate gelation of construct upon printing on cold build platform. Collagen fibrils were formed in the AC hydrogel during culture at body temperature, improving the cell affinity and spreading compared to pure agarose hydrogel. Cells were proven to proliferate in the bioink and the bioprinted construct. High cell viability up to 14 days was observed. The compressive strength of the bioink is more than 100 times superior to those of pure AC hydrogel. A potential alternative in tissue engineering of tissue replacements and biological models is made possible by combining the advantages of the conventional solid scaffolds with the new 3D bioprinting technology.

Characterization, mechanical behavior and in vitro evaluation of a melt-drawn scaffold for esophageal tissue engineering.

Tubular esophageal scaffolds with fiber diameter ranging from 13.9±1.7μm to 65.7±6.2μm were fabricated from the highly elastic poly(l-lactide-co-ε-caprolactone) (PLC) via a melt-drawing method. The morphology, crystallinity, thermal and mechanical properties of the PLC fibers were investigated. They were highly aligned and have a uniform diameter. PLC is found to be semicrystalline consisting of α- and β- lactide (LA) crystals. The crystallinity increases up to 16.8% with increasing melt-drawing speeds due to strain-induced crystallization. Modulus and strength increases while ductility decreases with an increase in crystallinity of the PLC samples. Moisture will not degrade the overall tensile properties but affect its tangent modulus at the low strain. L929 cells are able to attach and proliferate on the scaffolds very well. The cells seeded on the scaffolds show normal morphology with >90% cell viability after 6 days of culture. These results demonstrate that the PLC fibrous scaffold has good potential for use in esophageal tissue engineering application.

Geometry dependence of microstructure and microhardness for selective electron beam-melted Ti–6Al–4V parts

ABSTRACT In an additive-manufactured metallic part, distinct and different microstructure and mechanical properties may exist in different areas due to differences in shape and location. Two parts, one with straight-finned structure and the other with curve-finned structure, were fabricated by the selective electron beam melting method using pre-alloyed Ti–6Al–4V ELI powder. Microstructural characterisation of these two parts that have varying fin thickness and shape was carried out to investigate the synthetical influence of 2D planar build geometry and in-fill hatching strategy on selective electron beam melting. It was found that the β interspacing is larger in the curve-finned structure, leading to a lower microhardness as compared to the straight-finned structure. It suggests a slower cooling rate in the curve-finned structure due to the differences in build geometry and in-fill hatching strategy.

Anisotropic Mechanical Properties in a Big-Sized Ti-6Al-4V Plate Fabricated by Electron Beam Melting

In this study, in order to realize the application of the electron beam melting (EBM) technology for the printing of large components, the microstructure and mechanical properties of a big-sized Ti-6Al-4V plate (6 mm×180 mm×372 mm) additively manufactured by EBM were investigated. The paper focused on the graded microstructure and anisotropic mechanical properties by using x-ray diffraction, optical microscope, scanning electron microscope, microhardness and tensile test. A gradual change in microstructure with an increase in build height was observed. The formation of a graded microstructure was observed and discussed based on the thermal history experienced during printing. The mechanical properties were influenced accordingly by the graded microstructure. Moreover, the specimens which were printed parallel and perpendicular to the printing directions exhibited high elongation of ~18% and ~14%, respectively. The anisotropy in ductility was also observed and discussed according to the columnar prior β structure and grain boundary α phases present.

Simultaneously enhanced strength and ductility for 3D-printed stainless steel 316L by selective laser melting

Laser-based powder-bed fusion additive manufacturing or three-dimensional printing technology has gained tremendous attention due to its controllable, digital, and automated manufacturing process, which can afford a refined microstructure and superior strength. However, it is a major challenge to additively manufacture metal parts with satisfactory ductility and toughness. Here we report a novel selective laser melting process to simultaneously enhance the strength and ductility of stainless steel 316L by in-process engineering its microstructure into a <011> crystallographic texture. We find that the tensile strength and ductility of SLM-built stainless steel 316L samples could be enhanced by ~16% and ~40% respectively, with the engineered <011> textured microstructure compared to the common <001> textured microstructure. This is because the favorable nano-twinning mechanism was significantly more activated in the <011> textured stainless steel 316L samples during plastic deformation. In addition, kinetic simulations were performed to unveil the relationship between the melt pool geometry and crystallographic texture. The new additive manufacturing strategy of engineering the crystallographic texture can be applied to other metals and alloys with twinning-induced plasticity. This work paves the way to additively manufacture metal parts with high strength and high ductility.3D printing: adding strength and ductility to stainless steelA steel alloy with both high tensile strength and ductility has been three-dimensional (3D) printed by researchers in Singapore. Additive manufacturing builds 3D objects by adding materials layer by layer, a relatively simple process for plastics. However, this manufacturing process is much more difficult for metals, which are susceptible to defects and internal pores. This is particularly problematic when the final product needs excellent mechanical properties, such as hardness or strength. Xipeng Tan and co-workers from Nanyang Technological University used a specific laser scanning strategy to melt metallic powders and form a stainless steel alloy with a zig-zag crystallographic microstructure. The tensile strength and ductility of their stainless steel samples were increased by approximately 16% and 40%, respectively, compared to an alloy with the typical microstructure.A creative approach to substantially enhance both the strength and ductility of SLM-printed metal parts was successfully demonstrated on the ubiquitous marine-grade stainless steel 316L. The new discovery improves the strength and ductility of stainless steel parts by ~16% and 40% compared with the typical 3D printing process and conventional manufacturing methods. Control of the crystallographic texture is key for this breakthrough, which was achieved by tailoring the geometrical features of the melt pool involved in the laser-based 3D printing process. The desired <011> crystallographic texture favors the activation of the nano-twinning mechanism, which simultaneously enhances the strength and ductility.

Tribochemical Characterization and Tribocorrosive Behavior of CoCrMo Alloys: A Review

Orthopedic implants first started out as an all-metal hip joint replacement. However, poor design and machinability as well as unsatisfactory surface finish subjected the all-metal joint replacement to being superseded by a polyethylene bearing. Continued improvement in manufacturing techniques together with the reality that polyethylene wear debris can cause hazardous reactions in the human body has brought about the revival of metal-on-metal (MOM) hip joints in recent years. This has also led to a relatively new research area that links tribology and corrosion together. This article aims at reviewing the commonly used tribochemical methods adopted in the analysis of tribocorrosion and putting forward some of the models and environmental factors affecting the tribocorrosive behavior of CoCrMo alloys, a widely-used class of biomaterial for orthopedic implants.

Application of Electron Beam Melting (EBM) in Additive Manufacturing of an Impeller

An industrial impeller has been successfully fabricated by using Ti-6Al-4V ELI powder via a newly installed Arcam A2XX electron beam melting (EBM) machine. EBM was found to be preferable to build the circular or complex-shaped parts with thin walls. Several problems that frequently take place during the EBM fabrication are proposed based on the practical experience. It is found that metallization peeling-off, warpage, “swelling” and arc trips are the main reasons resulting in the failures of building jobs. It is suggested to pay close attention to the following aspects: a thorough cleanliness of the entire EBM system before start, a reasonable placement of builds on the start plate, an optimized design on geometries, etc., for a successfully built part.