Laser and electron beam additive manufacturing methods of fabricating titanium bone implants

Additive Manufacturing (AM) methods are generally used to produce an early sample or near net-shape elements based on three-dimensional geometrical modules. To date, publications on AM of metal implants have mainly focused on knee and hip replacements or bone scaffolds for tissue engineering. The direct fabrication of metallic implants can be achieved by methods, such as Selective Laser Melting (SLM) or Electron Beam Melting (EBM). This work compares the SLM and EBM methods used in the fabrication of titanium bone implants by analyzing the microstructure, mechanical properties and cytotoxicity. The SLM process was conducted in an environmental chamber using 0.4–0.6 vol % of oxygen to enhance the mechanical properties of a Ti-6Al-4V alloy. SLM processed material had high anisotropy of mechanical properties and superior UTS (1246–1421 MPa) when compared to the EBM (972–976 MPa) and the wrought material (933–942 MPa). The microstructure and phase composition depended on the used fabrication method. The AM methods caused the formation of long epitaxial grains of the prior phase. The equilibrium phases ( + ) and non-equilibrium ’ martensite was obtained after EBM and SLM, respectively. Although it was found that the heat transfer that occurs during the layer by layer generation of the component caused aluminum content deviations, neither methods generated any cytotoxic effects. Furthermore, in contrast to SLM, the EBM fabricated material met the ASTMF136 standard for surgical implant applications.peer-reviewe

Biodegradation behaviour of Fe-based alloys in Hanks’ Balanced Salt Solutions : part II. The evolution of local pH and dissolved oxygen concentration at metal interface

Commercially pure Fe, Fe35Mn, and (Fe35Mn)5Ag alloys were prepared by uniaxial pressing of the mixture of individual powders, followed by sintering. The influence of the alloying elements Mn and Ag on the corrosion behaviour of these Fe-based alloys was investigated in Hanks’ Balanced Salt Solution (HBSS). Furthermore, the role of the components in HBSS, particularly Ca2+ ions during alloys degradation was studied. Distribution of local pH and dissolved oxygen concentration was measured 50 μm above the interface of the degrading alloys. The results revealed that 5 wt% Ag addition to Fe35Mn alloy triggered micro-galvanic corrosion, while uniform corrosion dominated in pure Fe and Fe35Mn. Fast precipitation of Ca–P-containing products on the surface of these Fe-based alloys buffered local pH at the metal interface, and blocked oxygen diffusion at the initial stages of immersion. In the (Fe35Mn)5Ag, the detachment or structural changes of Ca–P-containing products gradually diminished their barrier property. These findings provided valuable insights into the degradation mechanism of promising biodegradable Fe-based alloys.peer-reviewe

Biodegradation behaviour of Fe-based alloys in Hanks’ Balanced Salt Solutions : part I. Material characterisation and corrosion testing

Research on Fe-based biodegradable alloys for implant applications has increased considerably over the past decade. However, there is limited information on the influence of testing electrolytes on corrosion product formation and general corrosion progress. In this work, the effect of Hanks’ Balanced Salt Solution (HBSS) with or without Ca2+ on the corrosion of Fe, Fe35Mn and (Fe35Mn)5Ag powder-processed coupons has been studied using potentiodynamic polarisation, Electrochemical Impedance Spectroscopy (EIS), and preliminary localised measurement of pH and dissolved oxygen concentration in close proximity to the metal surface. Both Fe35Mn and (Fe35Mn)5Ag alloys showed accelerated corrosion when compared to pure Fe based on potentiodynamic testing results, with FeMnAg exhibiting the highest corrosion rate in Ca2+-containing HBSS. The results indicate that in Ca2+-containing HBSS, the formation of a partially protective Ca/P layer decelerates the corrosion progress, whereas the Fe- and Mn-phosphates formed in Ca2+-free HBSS do not have the same effect. The Ca/P layer on (Fe35Mn)5Ag experienced a reduction in resistance following several hours of testing, indicating partial loss of its protective effect.peer-reviewe

In situ alloying of NiTi : influence of laser powder bed fusion (LBPF) scanning strategy on chemical composition

NiTi alloys are widely used in different industrial and medical applications. Due to the inherent difficulty in the machining of these alloys, the use of Additive Manufacturing (AM) methods has become a popular method for their production. When working with NiTi alloys, there is a requirement on the precise control of their chemical composition, as this determines the phase transition temperatures which are responsible for their shape memory or superelastic behaviour. The high energies used in AM to melt the NiTi alloy leads to nickel evaporation, resulting in a chemical change between the batch powder and the additively manufactured part. Therefore, in AM techniques applied to different NiTi alloys, understanding the relationship between the melting strategy and nickel evaporation is crucial during the developing the desired chemical composition of the final-fabricated material. In this study, three NiTi alloys were fabricated using laser powder bed fusion (LPBF) starting from elementally blended Ni and Ti powders. Different melting strategies, including single and multiple melting, were studied in this work. Remelting improved the density and reduced cracking of the AM part. Microscopic observations, using a Scanning Electron Microscope (SEM) with a Backscattered Electron (BSE) detector, showed that the chemical homogeneity of the materials was enhanced by multiple remelting. Pure Ni and Ti were not found in any of the samples, proving that the applied melting strategies ensured good alloying of both powders. Regardless of the number of melting runs, X-ray diffraction (XRD) analysis showed the presence of NiTi (B2) and (B19′ ) phases, as well as NiTi2, Ni4Ti3 and Ni3Ti precipitates in all samples. The research demonstrated that, during the AM process, and depending on the melting strategy, 1.6–3.0 wt% of nickel evaporates from the material. It was demonstrated that the amount of evaporated nickel increased with the increasing number of melt cycles.peer-reviewe

Hip joint replacement based on linear cylindrical articulations for reduced wear : a radical change in design

Hip replacement surgery is a common procedure for patients suffering from hip joint degeneration. However, wear of the implant components, particularly Ultra-High-Molecular- Weight Polyethylene (UHMWPE), remains a significant concern, often leading to complications such as osteolysis and implant loosening. This invention disclosure introduces a novel tri-axial hip replacement design aimed at addressing these issues. The design incorporates three orthogonal cylindrical articulations, each providing one rotational degree of freedom to replicate the natural movements of the hip joint. The prosthesis comprises two components made from UHMWPE (cup and rotator) and two components made from high-N stainless steel (flexor and abductor). Each articulation consists of metal-on-polyethylene bearing couples. Unlike traditional ball-and-socket implants, the novel design limits motion within each articulation to a single direction, taking advantage of friction-induced UHMWPE strain hardening. Moreover, cylindrical joints offer a larger contact surface area than their spherical counterparts, thereby reducing contact stresses. Mid-sized high-fidelity prototypes underwent wear resistance testing, demonstrating significantly superior performance compared to a commercial ball-and-socket implant of similar size tested in the same conditions. Moreover, a cadaveric implantation performed by experienced orthopaedic surgeons showed the implant has good stability even for postures requiring a wide range of motion. This innovative design represents a promising advancement in hip replacement technology, offering improved wear resistance and longevity, thus potentially reducing the need for revision surgeries and enhancing patient outcomes.peer-reviewe

Genetic structure of Tetraclinis articulata, an endangered conifer of the western Mediterranean basin

Tetraclinis articulata (Vahl) Masters is a tree distributed throughout the western Mediterranean basin. It is included in the IUCN (International Union for Conservation of Nature) red list, and protected by law in several of the countries where it grows. In this study we examined the genetic diversity and genetic structure of 14 populations of T. articulata in its whole geographic range using ISSR (inter simple sequence repeat) markers. T. articulata showed moderate genetic diversity at intrapopulation level and high genetic differentiation. The distribution of genetic diversity among populations did not exhibit a linear pattern related to geographic distances, since all analyses (principal coordinate analysis, Unweighted pair group method with arithmetic mean dendrogram and Bayesian structure analysis) revealed that spanish population grouped with Malta and Tunisia populations. Although it is possible that T. articulata earlier was natural in Southeast Spain, results suggest that the current population has been reintroduced into the Iberian Peninsula in historical times, due to its utility in mining and building. In addition, results could be used to propose management guidelines for the conservation of T. articulata.peer-reviewe

Corrosion degradation of AZ31 magnesium alloy coated by plasma electrolytic oxidation

Influence of plasma electrolytic oxidation (PEO) coating prepared using DC current technique on corrosion resistance of AZ31 magnesium alloy was studied. Morphology and chemical composition of the PEO coating was evaluated by scanning electron microscopy and EDS analysis respectively. Corrosion characteristics of ground and PEO coated samples were measured using electrochemical impedance spectroscopy (EIS) in 0.1 M NaCl. Scanning Kelvin Probe (SKP) technique was employed in order to investigate degradation mechanism of PEO samples after 24 h of salt spray test (NSS acc. to STN ISO 9227). Results showed that except of improved corrosion resistance of PEO coating, change of degradation mechanism was noticed after salt spray test.peer-reviewe

Heat treatment of NiTi alloys fabricated using laser powder bed fusion (LPBF) from elementally blended powders

The use of elemental metallic powders and in situ alloying in additive manufacturing (AM) is of industrial relevance as it offers the required flexibility to tailor the batch powder composition. This solution has been applied to the AM manufacturing of nickel-titanium (NiTi) shape memory alloy components. In this work, we show that laser powder bed fusion (LPBF) can be used to create a Ni55.7Ti44.3 alloyed component, but that the chemical composition of the build has a large heterogeneity. To solve this problem three different annealing heat treatments were designed, and the resulting porosity, microstructural homogeneity, and phase formation was investigated. The heat treatments were found to improve the alloy’s chemical and phase homogeneity, but the brittle NiTi2 phase was found to be stabilized by the 0.54 wt.% of oxygen present in all fabricated samples. As a consequence, a Ni2Ti4O phase was formed and was confirmed by transmission electron microscopy (TEM) observation. This study showed that pore formation in in situ alloyed NiTi can be controlled via heat treatment. Moreover, we have shown that the two-step heat treatment is a promising method to homogenise the chemical and phase composition of in situ alloyed NiTi powder fabricated by LPBF.peer-reviewe

FeMn and FeMnAg biodegradable alloys : an in vitro and in vivo investigation

Iron-based biodegradable metal bone graft substitutes are in their infancy but promise to fill bone defects that arise after incidents such as trauma and revision arthroplasty surgery. Before clinical use however, a better understanding of their in vivo biodegradability, potential cytotoxicity and biocompatibility is required. In addition, these implants must ideally be able to resist infection, a complication of any implant surgery. In this study there was significant in vitro cytotoxicity caused by pure Fe, FeMn, FeMn1Ag and FeMn5Ag on both human foetal osteoblast (hFOB) and mouse pre-osteoblast (MC3T3-E1) cell lines. In vivo experiments on the other hand showed no signs of ill-effect on GAERS rats with the implanted FeMn, FeMn1Ag and FeMn5Ag pins being removed largely uncorroded. All Fe-alloys showed anti-bacterial performance but most markedly so in the Ag-containing alloys, there is significant bacterial resistance in vitro.peer-reviewe