Polyethylene wear simulation models applied to a prosthetic hip joint based on unidirectional articulations

Ultra-high molecular weight polyethylene (UHMWPE) is commonly used as soft-bearing material in total joint replacements. However, the release of polymeric wear debris is still related to complications leading to aseptic loosening. Recently, a novel hip prosthesis showing reduced wear was developed by the authors of this study, consisting of unidirectional cylindrical articulations instead of the conventional multidirectional ball-and-socket design. This study evaluates four different theoretical wear models applied to this new design. The calculated volumetric wear was compared to experimental results. Although all models provided a good indication of the wear rates for the ball-and-socket prosthesis, they exhibited high discrepancies when predicting the amount of wear of the new unidirectional design. It was observed that the closest agreement with experimental results was obtained by the models that consider the friction-induced molecular orientation phenomenon exhibited by UHMWPE.

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.

The Effect of a Duplex Surface Treatment on the Corrosion and Tribocorrosion Characteristics of Additively Manufactured Ti-6Al-4V

The use of additively manufactured components specifically utilizing titanium alloys has seen rapid growth particularly in aerospace applications; however, the propensity for retained porosity, high(er) roughness finish, and detrimental tensile surface residual stresses are still a limiting factor curbing its expansion to other sectors such as maritime. The main aim of this investigation is to determine the effect of a duplex treatment, consisting of shot peening (SP) and a coating deposited by physical vapor deposition (PVD), to mitigate these issues and improve the surface characteristics of this material. In this study, the additive manufactured Ti-6Al-4V material was observed to have a tensile and yield strength comparable to its wrought counterpart. It also exhibited good impact performance undergoing mixed mode fracture. It was also observed that the SP and duplex treatments resulted in a 13% and 210% increase in hardness, respectively. Whilst the untreated and SP treated samples exhibited a similar tribocorrosion behavior, the duplex-treated sample exhibited the greatest resistance to corrosion-wear observed by the lack of damage on the surface and the diminished material loss rates. On the other hand, the surface treatments did not improve the corrosion performance of the Ti-6Al-4V substrate.

Additively Manufactured 316L Stainless Steel Subjected to a Duplex Peening-PVD Coating Treatment

This research studies the individual and combined effects of mechanical shot peening and the deposition of TiAlCuN coating on additively manufactured 316L stainless steel. Shot peening has been found to induce a 40% increase in surface hardness, while the combined effect of shot peening and the coating produced an approximately three-fold increase in surface hardness when compared to the as-printed coupons. Shot peening reduced the surface roughness of printed metal coupons by 50%, showing that shot peening can also serve to improve the surface finish of as-printed 316L stainless steel components. The peening process was found to induce a compressive residual stress of 589 MPa, with a maximum affected depth of approximately 200 μm. Scratch testing of the printed and coated specimens showed complete delamination failure at a normal load of 14 N, when compared to hybrid treated samples which failed at 10 N. On the other hand, from the corrosion tests, it was found that the hybrid treated samples provided the optimal results as opposed to the other variables.

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 Ni_55.7Ti_44.3 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.

Supplementation with rac-GR24 Facilitates the Accumulation of Biomass and Astaxanthin in Two Successive Stages of Haematococcus pluvialis Cultivation

The unicellular freshwater green alga Haematococcus pluvialis has attracted much research attention due to its biosynthetic ability for large amounts of astaxanthin, a blood-red ketocarotenoid that is used in cosmetics, nutraceuticals, and pharmaceuticals. Recently, numerous studies have investigated the functions of natural astaxanthin; however, the high cost of the production of astaxanthin from H. pluvialis cultures restricts its commercial viability. There is an urgent need to fulfill commercial demands by increasing astaxanthin accumulation from H. pluvialis cultures. In this study, we discovered that treatment of H. pluvialis cultures at the beginning of the macrozooid stage (day 0) with 1 μM rac-GR24, a synthetic analogue of strigolactones (a class of phytohormones), led to significant increases in biomass [up to a maximum dry cell weight (DCW) of 0.53 g/L] during the macrozooid stage and astaxanthin (from 0.63 to 5.32% of DCW) during the hematocyst stage. We elucidated that this enhancement of biomass accumulation during the macrozooid stage by rac-GR24 is due to its increasing CO₂ utilization efficiency in photosynthesis and carbohydrate biosynthesis. We also found that rac-GR24 stimulated the overproduction of nicotinamide adenine dinucleotide phosphate (NADPH) and antioxidant enzymes in H. pluvialis cultures, which alleviated the oxidative damage caused by reactive oxygen species generated during the hematocyst stage due to the exhaustion of nitrogen supplies. Moreover, rac-GR24 treatment of H. pluvialis synergistically altered the activity of the pathways of fatty acid biosynthesis and astaxanthin esterification, which resulted in larger amounts of astaxanthin being generated by rac-GR24-treated cultures than by controls. In summary, we have developed a feasible and economic rac-GR24-assisted strategy that increases the amounts of biomass and astaxanthin generated by H. pluvialis cultures, and have provided novel insights into the mechanistic roles of rac-GR24 to achieve these effects.

A novel hip joint prosthesis with uni-directional articulations for reduced wear

A novel polymer-on-metal hip joint prosthesis design that makes use of uni-directional articulations was developed and tested in this work. The new implant was tested using two polymer variants, virgin ultra-high molecular weight polyethylene (UHMWPE), and Vitamin E-infused highly crosslinked polyethylene (VEHXPE). The degrees of freedom of the ball-and-socket are reproduced by three cylindrical orthogonally-aligned articulations. This unconventional design leverages on the molecular orientation hardening mechanisms of the polyethylene and increased contact area to minimize wear. An experimental hip joint simulator was used to compare the gravimetric wear of the conventional ball-on-socket and the new implant. The new prosthesis including UHMWPE components produced a 78% reduction in wear, whereas the new prosthesis with VEHXPE components produced a 100% reduction in wear, as no measurable wear was detected. Machining marks on the acetabular cups of the new prosthesis were retained for both polyethylene variants, further demonstrating the low levels of wear exhibited by the new implants. Both polyethylene materials produced particles in the range of 0.1-1.0 μm, which are the most biologically active. Nonetheless, the extremely low wear rates are likely to induce minimal osteolysis effects. Furthermore, the novel design also offers an increase of more than 24% in the range of motion in flexion/extension when compared to a dual-mobility hip implant. A prototype of the prosthesis was implanted into a Thiel-embalmed human cadaver during a mock-surgery, which demonstrated high resistance to dislocation and the possibility of performing a figure of four position.

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.

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.

Chemical Composition and in Vitro Cytotoxic Screening of Sixteen Commercial Essential Oils on Five Cancer Cell Lines

The in vitro cytotoxic activity on human cancer cell lines of sixteen commercial EOs such as Aloysia citriodora, Boswellia sacra, Boswellia serrata, Cinnamomum zeylanicum, Cistus ladanifer, Citrus × aurantium, Citrus limon, Citrus sinensis, Cymbopogon citratus, Foeniculum vulgare, Illicium verum, Litsea cubeba, Satureja montana, Syzygium aromaticum, Thymus capitatus and Thymus vulgaris was performed using the MTT reduction assay. The screening was carried out on human cancer cells of breast adenocarcinoma (MCF7, T47D and MDA-MB-231), chronic myelogenous erythroleukemia (K562) and neuroblastoma cell lines (SH-SY5Y). C. zeylanicum and L. cubeba EOs were the most active on almost all the cell lines studied and thus could be promising as an anticancer agent. These two species showed a difference in their composition even though they belong to the Lauraceae family. Almost 57 % of the true cinnamon composition was made of (E)-cinnamaldehyde, while L. cubeba showed citral as the major compound (68.9 %). The K562 cells were the most sensitive to these oils with an IC₅₀ ranging from 5.2 parts-per million (ppm) (C. zeylanicum) to 11.1 ppm (L. cubeba). The latter oil also showed an important cytotoxicity on MDA-MB-231 (13.4 ppm).

The effect of alloying elements on the properties of pressed and non-pressed biodegradable Fe-Mn-Ag powder metallurgy alloys

Current trends in the biodegradable scaffold industry call for powder metallurgy methods in which compression cannot be applied due to the nature of the scaffold template itself and the need to retain the shape of an underlying template throughout the fabrication process. Iron alloys have been shown to be good candidates for biomedical applications where load support is required. Fe–Mn alloys were researched extensively for this purpose. Current research shows that all metallurgical characterisation and corrosion test on Fe–Mn and Fe–Mn–Ag non pre-alloyed powder alloys are performed on alloys which are initially pressed into greens and subsequently sintered. In order to combine the cutting-edge field of biodegradable metallic alloys with scaffold production, metallurgical characterisation of pressed and non-pressed Fe, Fe–Mn and Fe–Mn–Ag sintered elemental powder compacts was carried out in this study. This was performed along with determination of the corrosion rate of the same alloys in in vitro mimicking solutions. These solutions were synthesised to mimic the osteo environment in which the final scaffolds are to be used.

Both pressed and non-pressed alloys formed an austenite phase under the right sintering conditions. The corrosion rate of the non-pressed alloy was greater than that of its pressed counterpart. In a potentiodynamic testing scenario, addition of silver to the alloy formed a separate silver phase which galvanically increased the corrosion rate of the pressed alloy. This result wasn't replicated in the non-pressed alloys in which the corrosion rate was seen to remain similar to the non-silver-bearing alloy counterparts.

Influence of cold rolling on in vitro cytotoxicity and electrochemical behaviour of an Fe-Mn-C biodegradable alloy in physiological solutions

The properties of cold-worked Fe-13Mn-1.2C steel, as candidate material for scaffolding and stenting applications, have been investigated. The study of the electrochemical corrosion susceptibility of Fe-13Mn-1.2C alloy in protein bearing and non-protein bearing physiological solutions, revealed that there were no differences between the as-received, 10% and 20% cold worked Fe-13Mn-1.2C samples. Although protein addition reduces the overall corrosion rate in static immersion degradation tests for both the cold worked and non-cold worked alloys, there were no discernible differences in the corrosion rates of samples with different percentages of cold work deformations. Similarly, potentiodynamic testing showed no differences between the corrosion rates in solutions with and without protein addition. Atomic absorption spectroscopy (AAS) results—post static immersion—showed similar values of Fe and Mn concentrations in the electrolyte for all the investigated conditions. Cold working was found to increase Grain Average Misorientation (GAM) and deformation twins within the steel, and, consequently, this led to an increase in the elastic modulus. Hence, cold-rolling may be used to achieve smaller sections (volumes) in order to support the equivalent load of the non-cold worked counterpart, giving a larger surface area to the volume ratio, thereby increasing the corrosion rate, and, in turn, rendering the degradation process shorter. When considering cytocompatibility in vitro, the collected supernatant particulate free Fe-13Mn-1.2C steel electrolytes were seen to be equally cytocompatible with no differences being observed between the different percentage cold work conditions. The presence of solid 80 μm size particles in the seeded elutions were seen to change the results and render the Fe-13Mn-1.2C steel non-cytocompatible.

The effect of sliding onto the metal-electrolyte interface: Studying model parameter modifications by means of EIS

Several problems are associated with corrosion-wear occurring on metal-on-metal hip implants made out of cobalt-chromium based alloys. Low temperature carburizing, a process that creates a hard and corrosion resistant diffused layer in Cobalt-Chromium-Molybdenum (CoCrMo) alloys, known as S-phase, may be a possible solution towards mitigating these problems. In this work, static- and tribo-corrosion testing involving an alumina versus CoCrMo (untreated and carburized) were conducted in Ringer's solution. Electrochemical impedance spectroscopy was used to compare impedance plots attained before and after sliding so as to understand how the metal-electrolyte interface is affected by rubbing. Both untreated and carburized CoCrMo experienced extensive reduction in corrosion resistance following sliding wear damage such that one should expect a considerably deteriorated performance of both surfaces in a tribocorrosion application. The structure of the interface was relatively unaffected after sliding at the equilibrium and passive potentials. This implies that the layers making up the interface before sliding were still present after sliding. However, their properties changed - the interface's real resistance dropped while its capacitance increased. The former was linked to a weaker, damaged passive film while the latter was linked to accumulation of wear debris and corrosion products.

Influence of cross-rolling on the micro-texture and biodegradation of pure iron as biodegradable material for medical implants

Iron-based biodegradable metals have been shown to present high potential in cardiac, vascular, orthopaedic and dental in adults, as well as paediatric, applications. These require suitable mechanical properties, adequate biocompatibility while guaranteeing a low toxicity of degradation products. For example, in cardiac applications, stents need to be made by homogeneous and isotropic materials in order to prevent sudden failures which would impair the deployment site. Besides, the presence of precipitates and pores, chemical inhomogeneity or other anisotropic microstructural defects may trigger stress concentration phenomena responsible for the early collapse of the device. Metal manufacturing processes play a fundamental role towards the final microstructure and mechanical properties of the materials. The present work assesses the effect of mode of rolling on the micro-texture evolution, mechanical properties and biodegradation behaviour of polycrystalline pure iron. Results indicated that cross-rolled samples recrystallized with lower rates than the straight-rolled ones due to a reduction in dislocation density content and an increase in intensity of {100} crystallographic plane which stores less energy of deformation responsible for primary recrystallization. The degradation resulted to be more uniform for cross-rolled samples, while the corrosion rates of cross-rolled and straight-rolled samples did not show relevant differences in simulated body solution. Finally, this work shows that an adequate compromise between biodegradation rate, strength and ductility could be achieved by modulating the deformation mode during cold rolling.

Biocompatibility and characterization of a Kolsterised(®) medical grade cobalt-chromium-molybdenum alloy

High failure rates of cobalt-chromium-molybdenum (Co-Cr-Mo) metal-on-metal hip prosthesis were reported by various authors, probably due to the alloy's limited hardness and tribological properties. This thus caused the popularity of the alloy in metal-on-metal hip replacements to decrease due to its poor wear properties when compared with other systems such as ceramic-on-ceramic. S-phase surface engineering has become an industry standard when citing surface hardening of austenitic stainless steels. This hardening process allows the austenitic stainless steel to retain its corrosion resistance, while at the same time also improving its hardness and wear resistance. By coupling S-phase surface engineering, using the proprietary Kolsterising(®) treatment from Bodycote Hardiff GmbH, that is currently being used mainly on stainless steel, with Co-Cr-Mo alloys, an improvement in hardness and tribological characteristics is predicted. The objective of this paper is to analyze the biocompatibility of a Kolsterised(®) Co-Cr-Mo alloy, and to characterize the material surface in order to show the advantages gained by using the Kolsterised(®) material relative to the original untreated alloy, and other materials. This work has been performed on 3 fronts including; Material characterization, "In-vitro" corrosion testing, and Biological testing conforming to BS EN ISO 10993-18:2009 - Biological evaluation of medical devices. Using these techniques, the Kolsterised(®) cobalt-chromium-molybdenum alloys were found to have good biocompatibility and an augmented corrosion resistance when compared with the untreated alloy. The Kolsterised(®) samples also showed a 150% increase in surface hardness over the untreated material thus predicting better wear properties.

Corrosion properties of S-phase layers formed on medical grade austenitic stainless steel

The corrosion properties of S-phase surface layers formed in AISI 316LVM (ASTM F138) and High-N (ASTM F1586) medical grade austenitic stainless steels by plasma surface alloying with nitrogen (at 430°C), carbon (at 500°C) and both carbon and nitrogen (at 430°C) has been investigated. The corrosion behaviour of the S-phase layers in Ringer's solutions was evaluated using potentiodynamic and immersion corrosion tests. The corrosion damage was evaluated using microscopy, hardness testing, inductive coupled plasma mass spectroscopy and X-ray diffraction. The experimental results have demonstrated that low-temperature nitriding, carburising and carbonitriding can improve the localised corrosion resistance of both industrial and medical grade austenitic stainless steels as long as the threshold sensitisation temperature is not reached. Carburising at 500°C has proved to be the best hardening treatment with the least effect on the corrosion resistance of the parent alloy.

Evaluation of the biocompatibility of S-phase layers on medical grade austenitic stainless steels

S-phase surface layers were formed in AISI 316LVM (ASTM F138) and High-N (ASTM F1586) medical grade austenitic stainless steels by plasma surface alloying with nitrogen (at 430°C), carbon (at 500°C) and both carbon and nitrogen (at 430°C). The presence of the S-phase was confirmed by microscopy, hardness testing, depth-profile analysis of chemical composition and X-ray Diffraction. Attachment and proliferation of mouse osteoblast MC3T3-E1 cells were tested on S-phase and untreated controls and the results demonstrated that all the S-phase layers formed were biocompatible under the conditions used. Cells adhered equally well to all samples but proliferation was enhanced on the treated materials.