A study on factors affecting the degradation of magnesium and a magnesium-yttrium alloy for biomedical applications.

Controlling degradation of magnesium or its alloys in physiological saline solutions is essential for their potential applications in clinically viable implants. Rapid degradation of magnesium-based materials reduces the mechanical properties of implants prematurely and severely increases alkalinity of the local environment. Therefore, the objective of this study is to investigate the effects of three interactive factors on magnesium degradation, specifically, the addition of yttrium to form a magnesium-yttrium alloy versus pure magnesium, the metallic versus oxide surfaces, and the presence versus absence of physiological salt ions in the immersion solution. In the immersion solution of phosphate buffered saline (PBS), the magnesium-yttrium alloy with metallic surface degraded the slowest, followed by pure magnesium with metallic or oxide surfaces, and the magnesium-yttrium alloy with oxide surface degraded the fastest. However, in deionized (DI) water, the degradation rate showed a different trend. Specifically, pure magnesium with metallic or oxide surfaces degraded the slowest, followed by the magnesium-yttrium alloy with oxide surface, and the magnesium-yttrium alloy with metallic surface degraded the fastest. Interestingly, only magnesium-yttrium alloy with metallic surface degraded slower in PBS than in DI water, while all the other samples degraded faster in PBS than in DI water. Clearly, the results showed that the alloy composition, presence or absence of surface oxide layer, and presence or absence of physiological salt ions in the immersion solution all influenced the degradation rate and mode. Moreover, these three factors showed statistically significant interactions. This study revealed the complex interrelationships among these factors and their respective contributions to degradation for the first time. The results of this study not only improved our understanding of magnesium degradation in physiological environment, but also presented the key factors to consider in order to satisfy the degradation requirements for next-generation biodegradable implants and devices

Modification of Biocorrosion and Cellular Response of Magnesium Alloy WE43 by Multiaxial Deformation

The study shows that multiaxial deformation (MAD) treatment leads to grain refinement in magnesium alloy WE43. Compared to the initial state, the MAD-processed alloy exhibited smoother biocorrosion dynamics in a fetal bovine serum and in a complete cell growth medium. Examination by microCT demonstrated retardation of the decline in the alloy volume and the Hounsfield unit values. An attendant reduction in the rate of accumulation of the biodegradation products in the immersion medium, a less pronounced alkalization, and inhibited sedimentation of biodegradation products on the surface of the alloy were observed after MAD. These effects were accompanied with an increase in the osteogenic mesenchymal stromal cell viability on the alloy surface and in a medium containing their extracts. It is expected that the more orderly dynamics of biodegradation of the WE43 alloy after MAD and the stimulation of cell colonization will effectively promote stable osteosynthesis, making repeat implant extraction surgeries unnecessary

Albumin Protein Impact on Early-Stage In Vitro Biodegradation of Magnesium Alloy (WE43)

Mg and its alloys are promising biodegradable materials for orthopedic implants and cardiovascular stents. The first interactions of protein molecules with Mg alloy surfaces have a substantial impact on their biocompatibility and biodegradation. We investigate the early-stage electrochemical, chemical, morphological, and electrical surface potential changes of alloy WE43 in either 154 mM NaCl or Hanks’ simulated physiological solutions in the absence or presence of bovine serum albumin (BSA) protein. WE43 had the lowest electrochemical current noise (ECN) fluctuations, the highest noise resistance (Zn = 1774 Ω·cm2), and the highest total impedance (|Z| = 332 Ω·cm2) when immersed for 30 min in Hanks’ solution. The highest ECN, lowest Zn (1430 Ω·cm2), and |Z| (49 Ω·cm2) were observed in the NaCl solution. In the solutions containing BSA, a unique dual-mode biodegradation was observed. Adding BSA to a NaCl solution increased |Z| from 49 to 97 Ω·cm2 and decreased the ECN signal of the alloy, i.e., the BSA inhibited corrosion. On the other hand, the presence of BSA in Hanks’ solution increased the rate of biodegradation by decreasing both Zn and |Z| while increasing ECN. Finally, using scanning Kelvin probe force microscopy (SKPFM), we observed an adsorbed nanolayer of BSA with aggregated and fibrillar morphology only in Hanks’ solution, where the electrical surface potential was 52 mV lower than that of the Mg oxide layer

Beurteilung des Abbauverhaltens von Magnesiumlegierungen mit elektrochemischen Methoden

Introduction: Magnesium, a biodegradable biomaterial, has the potential to be applied in dental implantology as a bone augmentation material. The electrochemical characterization of the surface allows us to trace the deterioration process in vivo. The difficulty is to estimate the deterioration rate from the measurement data in order to predict the component's "lifetime". The formation of local elements should be avoided, ideally evenly distributed, so that a degradation takes place from the outside to the inside. This should ensure that the degradation of the Mg structure takes place as far as possible at the rate at which the bone is newly formed. The aim of the work was to carry out a more detailed evaluation of the degradation behavior on different Mg samples by microscopic observation of the surface during the electrochemical measurement. Methods: Setting up an optical-electrochemical cell chamber with magnesium and magnesium alloys in electrochemical corrosion measurements. Observing and recording the surface degradation process under microscope. At 37°C, specimens were examined in several circulating electrolytes, including MEM, HBSS, and MEM+ (MEM supplemented with NaHCO3). CV was measured followed by cycle polarization, which was then repeated previous steps after 30mins. Thus, it is possible to observe and correlate surface processes such as hydrogen evolution and oxide deposition in real time with electrochemical data. The electrochemistry data, which included the OCP exchange current density(i0) and corrosion potential, were compared to the potential changes that occurred over time throughout these treatments. They were computed in this instance using RP obtained from linear polarization and an estimate of the Stern–Geary constant. Additionally, the linear polarization curve and EC exchange current corrosion value are used to determine how magnesium and magnesium alloys evolve throughout the corrosion process. Result: The corrosion behavior of pure Mg and MgXAg alloys differs significantly. Adding Ag to magnesium standardizes electrochemical activity and hence deterioration independent of test medium. After the tests, the video microscopic movies indicate comparable corrosion and deterioration in CO2-containing electrolytes. Throughout OCP measurements, the surface develops distinctive patches that remain stable even during CV. Conclusion: Video microscopic observation of the degradation of Mg and Mg alloys raises a variety of new questions for assessing the degradation. The question of how the Ag additives act in the Mg alloy and act as possible hydrogen development spots can currently only be suspected. Further investigations are necessary to find an answer to these and other questions. For this purpose, strategies for image evaluation still have to be developed, which allow, among other things, an effective determination of the electrochemically active surface.Einleitung: Magnesium, ein biologisch abbaubares Biomaterial, hat das Potenzial, in der dentalen Implantologie als Knochenaufbaumaterial verwendet zu werden. Mit Hilfe der elektrochemischen Charakterisierung der Oberfläche ist es möglich, den Degradationsprozess so zu verfolgen, wie er auch in-vivo abläuft. Die Herausforderung besteht darin, aus den Messdaten die Degradationsgeschwindigkeit so zu bestimmen, dass eine verlässliche Prognose für die „Lebenszeit“ des Bauteils möglich wird. Es sollte die Bildung von Lokalelementen vermieden werden, idealerweise also gleichmäßig verteilt, so dass ein Abbau von außen nach innen erfolgt. So sollte sichergestellt werden, dass der Abbau der Mg-Struktur möglichst mit der Geschwindigkeit erfolgt, mit der der Knochen neu gebildet wird. Methoden: Aufbau einer optisch-elektrochemischen Zelle für elektrochemische Korrosionsmessungen an Magnesium und Magnesiumlegierungen. Beobachtung und Aufzeichnung des Oberflächenveränderungen unter dem Videomikroskop. Bei 37 °C wurden die Proben in verschiedenen, zirkulierenden Elektrolyten untersucht, darunter MEM, HBSS und MEM+ (MEM ergänzt mit NaHCO3). Zuerst wurde der OCP gemessen, dann ein CV und dann nach 30 Minuten eine Wiederholung der gleichen Schritte. Somit war es möglich, Oberflächenprozesse wie Wasserstoffentwicklung und Oxidabscheidung in Echtzeit zu beobachten und mit elektrochemischen Daten zu korrelieren. Die elektrochemischen Daten, OCP, Austauschstromdichte i0 und das Korrosionspotential, wurden für die Bewertung der Degradation herangezogen. I0 als Maß für die Degradationsgeschwindigkeit wurden in diesem Fall unter Verwendung von RP und der Stern-Geary-Konstante aus den verschiedenen CV berechnet. Der Mittelwert und die Standardabweichung sind in den Tabellen im Ergebnisteil zusammengefasst. Ergebnisse: Die Ergebnisse zeigen, dass es deutliche Unterschiede im Korrosionsverhalten zwischen den reinen Mg Proben und den MgXAg Legierungen gibt. Die Zugabe von Ag zu Magnesium führt zu einer Vereinheitlichung der elektrochemischen Aktivität und somit des Abbaus unabhängig vom gewählten Testmedium. Die videomikroskopischen Aufnahmen zeigen am Ende der Messungen in den CO2 haltigen Elektrolyten ähnliche Korrosions- respektive Degradationsspuren. Während der Zeit der OCP Messungen zeigen sich schon erste charakteristische Spots auf der Oberfläche, die auch während der CV sich nicht verändern. Schlussfolgerung: Die videomikroskopische Beobachtung der Degradation von Mg und Mg-Legierungen wirft eine Vielzahl neuer Fragen zur Beurteilung der Degradation von Mg- und Mg-Legierungen auf. Die Frage wie die Ag Zusätze in der Mg Legierung agieren und als mögliche Wasserstoffentwicklungsspots fungieren kann momentan nur vermutet werden. Hier sind weiter Untersuchungen notwendig, um auf diese und andere Fragen eine Antwort zu finden. Hierzu müssen noch Strategien zur Bildauswertung entwickelt werden, die u.a. eine effektive Bestimmung der elektrochemisch aktiven Oberfläche erlauben

Corrosion assessment and enhanced biocompatibility analysis of biodegradable magnesium-based alloys

Magnesium alloys have raised immense interest to many researchers because of its evolution as a new third generation material. Due to their biocompatibility, density, and mechanical properties, magnesium alloys are frequently reported as prospective biodegradable implant materials. Moreover, magnesium based alloys experience a natural phenomena to biodegrade in aqueous solutions due to its corrosive activity, which is excellent for orthopedic and cardiovascular applications. However, major concerns with such alloys are fast and non-uniform corrosion degradation. Controlling the degradation rate in the physiological environment determines the success of an implant. In this investigation, three grades of magnesium alloys: AZ31B, AZ91E and ZK60A were studied for their corrosion resistance and biocompatibility. Scanning electron microscopy, energy dispersive spectroscopy, atomic force microscopy and contact angle meter are used to study surface morphology, chemistry, roughness and wettability, respectively. Additionally, the cytotoxicity of the leached metal ions was evaluated by a tetrazolium based bio-assay, MTS

CORROSION BEHAVIOUR OF THE AZ31 MAGNESIUM ALLOY AND SURFACE TREATMENTS FOR ITS CORROSION PROTECTION

Nowadays environmental conservation forces the transportation industry to manufacture lighter and low emissions transport vehicles. In this contest, magnesium alloys have found increasing attention by the automotive industry because of their low density associated with good mechanical properties. However the low corrosion resistance of magnesium alloys in wet environments is still a limiting factor against their widespread diffusion. The aim of this research is both studying the correlation between microstructure and corrosion behaviour of a AZ31 magnesium alloy and developing eco-friendly protective technologies. In particular inhibitors and surface pretreatments with different organic compounds have been investigated. The effect of microstructure on corrosion resistance of AZ31 alloy has been analyzed through a comparative study between the electrochemical behaviour of as-cast and hot rolled AZ31. Environmentally friendly sodium salts of mono-carboxylic acids have been studied as inhibitors of AZ31 alloy corrosion in a standard saline solution. Moreover, long chain sodium mono-carboxylates have been tested as promoters of conversion coatings for this alloy. Finally, significant improvements have been achieved by modifying the protective coatings obtained by 3-mercapto-propyl-trimethoxysilane through cerium nitrate or oxide nanoparticle additions

Native Oxide Films on AZ31 and AZ61 Commercial Magnesium Alloys – Corrosion Behaviour, Effect on Isothermal Oxidation and Sol–gel Thin Film Formation

The authors present a review of their recent research work in an endeavour to interpret the influence of native oxide films on the corrosion behaviour of commercial AZ31 and AZ61 magnesium alloys or on the oxidation kinetics in air at 200°C. The tendency of some of these thin films to be sufficiently protective in mild or weak corrosive environments is examined. For obtaining oxide films with different protective properties, some of the specimens are tested with the surface in the as-received condition, while others are tested immediately after mechanical polishing. The technique applied to characterise thin (thickness of just a few nanometres) oxide films present on the surface of alloys has basically been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Oxidation resistance of the alloys is quantified by thermo gravimetric (TG) curves and their corrosion rate is evaluated by Electrochemical Impedance Spectroscopy (EIS) and hydrogen evolution measurement in chloride solutions with different aggressivity. Emphasis is placed on the possible effects of: (a) the different thickness of the native oxide films formed on the polished surfaces on the corrosion behaviour of the alloys; and (b) the different film homogeneity and uniformity on the oxidation results. Finally, an attempt will be made to learn more about the influence of the native oxide films that cover the substrate on the subsequent growth and protective behaviour of the sol–gel coatings

Modification of biocorrosion and cellular response of magnesium alloy WE43 by multiaxial deformation

The study shows that multiaxial deformation (MAD) treatment leads to grain refinement in magnesium alloy WE43. Compared to the initial state, the MAD-processed alloy exhibited smoother biocorrosion dynamics in a fetal bovine serum and in a complete cell growth medium. Examination by microCT demonstrated retardation of the decline in the alloy volume and the Hounsfield unit value