Sannakaisa Virtanen

Biomedical coatings on magnesium alloys - a review.

This review comprehensively covers research carried out in the field of degradable coatings on Mg and Mg alloys for biomedical applications. Several coating methods are discussed, which can be divided, based on the specific processing techniques used, into conversion and deposition coatings. The literature review revealed that in most cases coatings increase the corrosion resistance of Mg and Mg alloys. The critical factors determining coating performance, such as corrosion rate, surface chemistry, adhesion and coating morphology, are identified and discussed. The analysis of the literature showed that many studies have focused on calcium phosphate coatings produced either using conversion or deposition methods which were developed for orthopaedic applications. However, the control of phases and the formation of cracks still appear unsatisfactory. More research and development is needed in the case of biodegradable organic based coatings to generate reproducible and relevant data. In addition to biocompatibility, the mechanical properties of the coatings are also relevant, and the development of appropriate methods to study the corrosion process in detail and in the long term remains an important area of research.

Special modes of corrosion under physiological and simulated physiological conditions

The aim of this article is to review those aspects of corrosion behaviour that are most relevant to the clinical application of implant alloys. The special modes of corrosion encountered by implant alloys are presented. The resistance of the different materials against the most typical corrosion modes (pitting corrosion, crevice corrosion and fretting corrosion) is compared, together with observations of metal ion release from different biomaterials. A short section is dedicated to possible galvanic effects in cases when different types of materials are combined in a biomedical device. The different topics covered are introduced from the viewpoint of materials science, and then placed into the context of medicine and clinical experience.

Composition of corrosion layers on a magnesium rare-earth alloy in simulated body fluids.

In this study, the composition of corrosion product layers on a magnesium rare-earth alloy in simulated body fluid (m-SBF) containing albumin in physiological concentration is examined. The time dependence of the composition of the layer is studied. The ions from the body fluid that participate in the corrosion layer formation were identified by analyzing layers formed in different solutions that contain only some of the ions of SBF. The layer composition was analyzed by different complementary methods. We used energy dispersive X-ray analysis, grazing incidence X-ray diffraction, and Fourier transform infrared spectroscopy. The morphology of the corrosion layers was studied using scanning electron microscopy and light microscopy. In m-SBF with and without albumin we found an amorphous layer of carbonated calcium phosphate with some calcium replaced by magnesium. It can be clearly shown that calcium is only deposited in the corrosion layer if phosphates are in the solution. The cross-sections reveal that there are some sharp crevices in the substrate. The work systematically explores the nature of surface layers on magnesium rare-earth alloys formed in complex SBFs, with the aim to elucidate the influence of specific electrolyte components on the morphology, structure, and composition of corrosion layers on Mg alloys.

Effect of surface pre-treatments on biocompatibility of magnesium.

This study reports the influence of Mg surface passivation on the survival rate of human HeLa cells and mouse fibroblasts in cell culture experiments. Polished samples of commercially pure Mg show high reactivity in the cell culture medium, leading to a pH shift in the alkaline direction, and therefore cell adhesion and survival is strongly impaired. Passivation of the Mg surface in 1M NaOH can strongly enhance cell survival. The best initial cell adhesion is observed for Mg samples incubated in simulated body fluid (M-SBF), which leads to the formation of a biomimetic, amorphous Ca/Mg-phosphate layer with high surface roughness. This surface layer, however, passivates and seals the Mg surface only partially. Subsequent Mg dissolution leads to a significantly stronger pH increase compared to NaOH-passivated samples, which prevents long-term cell survival. These results demonstrate that surface passivation with NaOH and M-SBF together with the associated changes of surface reactivity, chemistry and roughness provide a viable strategy to facilitate cell survival on otherwise non-biocompatible Mg surfaces.

Control of magnesium corrosion and biocompatibility with biomimetic coatings.

The use of magnesium and its alloys as biodegradable metallic implant materials requires that their corrosion behavior can be controlled. We tailored the Mg release kinetics and cell adhesion properties of commercially pure Mg by chemical surface treatments in simulated body fluid, in Dulbeccos Modified Eagles cell culture medium in the presence or absence of fetal bovine serum (FBS), or in 100% FBS. HeLa cells were cultured for 24 h on these Mg surfaces to characterize their biocompatibility. Cell density on all treated surfaces was significantly increased compared with a polished Mg surface, where almost no cells survived. This low biocompatibility of pure Mg was not caused by the high Mg ion release with concentrations of up to 300 mg/L in the cell culture medium after 24 h, as cells grown on a glass substrate showed no adverse reactions to high Mg ion concentrations. Rather, the most critical factor for cell adhesion was a sufficiently reduced initial dissolution rate of the surface. A comparison among all surface treatments showed that an incubation of the Mg samples in cell culture medium gave the lowest dissolution rate and resulted in the best cell adhesion and spreading behavior.

Grain character influences on corrosion of ECAPed pure magnesium

Abstract This study seeks to clarify the influence of grain character on the corrosion rate of Mg. There is a special need to understand the largely unknown corrosion–grain size relationship for Mg, which nominally displays poor corrosion resistance, since any efforts to reduce or control the rate of Mg corrosion are of large technological significance. In this work, the microstructure is modified by equal channel angular pressing (ECAP) with up to eight passes, to achieve a range of refined microstructures ranging in grain size from a few hundred micrometres to a few micrometres. Results reveal a significant variation in corrosion rate with the number of ECAP passes, and hence grain size, which is of key significance. The results are discussed in terms of grain size, misorientation, along with resultant surface state produced and electrochemical/polarisation signatures collected.

Electrochemical Behavior of Cr2 O 3 / Fe2 O 3 Artificial Passive Films Studied by In Situ XANES

The electrochemical behavior of thin sputter-deposited mixed Cr 2 O 3 /Fe 2 O 3 oxide films with Cr 2 O 3 -contents of 10, 20, 50, and 90% was studied with in situ X-ray absorption near edge spectroscopy (XANES). These measurements gave information on the chemical states and dissolution rates during anodic and cathodic polarization in different electrolytes. At low Cr oxide concentrations, the films dissolve when cathodically polarized and are resistant to dissolution when polarized in the anodic direction. At high Cr 2 O 3 concentrations, dissolution occurs when the films are anodically polarized, but the films are stable against cathodic dissolution. In the intermediate Cr oxide concentration range, the oxides neither dissolve under anodic nor cathodic polarization. However, in all the cases, even when no dissolution takes place, the species show electroactivity in that the redox reactions Fe 3+ → Fe 2+ and Cr 3+ → Cr 6+ can take place under cathodic and anodic polarization, respectively. In the mixed oxides a solid-state conversion takes place in the iron oxide phase during reduction, whereas the oxidation of the chromium oxide phase converts only the outermost layer. An acidic environment accelerates both anodic and cathodic dissolution, associated with chemical dissolution of the iron oxide. The results further show that critical threshold values exist for the dissolution resistance of the oxide. These values are different for anodic and cathodic reactions and further strongly depend on the solution chemistry.

In vitro biocompatibility of CoCrMo dental alloys fabricated by selective laser melting

OBJECTIVE Selective laser melting (SLM) is increasingly used for the fabrication of customized dental components made of metal alloys such as CoCrMo. The main aim of the present study is to elucidate the influence of the non-equilibrium microstructure obtained by SLM on corrosion susceptibility and extent of metal release (measure of biocompatibility). METHODS A multi-analytical approach has been employed by combining microscopic and bulk compositional tools with electrochemical techniques and chemical analyses of metals in biologically relevant fluids for three differently SLM fabricated CoCrMo alloys and one cast CoCrMo alloy used for comparison. RESULTS Rapid cooling and strong temperature gradients during laser melting resulted in the formation of a fine cellular structure with cell boundaries enriched in Mo (Co depleted), and suppression of carbide precipitation and formation of a martensitic ɛ (hcp) phase at the surface. These features were shown to decrease the corrosion and metal release susceptibility of the SLM alloys compared with the cast alloy. Unique textures formed in the pattern of the melting pools of the three different laser melted CoCrMo alloys predominantly explain observed small, though significant, differences. The susceptibility for corrosion and metal release increased with an increased number (area) of laser melt pool boundaries. SIGNIFICANCE This study shows that integrative and interdisciplinary studies of microstructural characteristics, corrosion, and metal release are essential to assess and consider during the design and fabrication of CoCrMo dental components of optimal biocompatibility. The reason is that the extent of metal release from CoCrMo is dependent on fabrication procedures.

Bulk Metal Oxides as a Model for the Electronic Properties of Passive Films

The aim of the present work is to compare the electronic properties of natural passive films on iron and chromium with sintered bulk metal oxides. The characterization of the semiconductive properties of various bulk oxides shows that the passive film on iron can be simulated by highly doped Fe{sub 2}O{sub 3}. The doping concentration of Fe-oxides is determined by their fe{sup 2+} content. At a dopant level of {approx}5{times}10{sup 20} cm{sup {minus}3} in Fe{sub 2}O{sub 3} the flatband potential, bandgap energy, intensity of the photocurrent, and the doping concentration are in good agreement with the natural passive film. A major advantage in using sintered bulk oxides as models for natural passive films is that oxide properties can be separated from effects associated with the underlying substrate. Therefore it could be shown, e.g., that the photocurrent behavior in the case of passive film on iron and chromium is mechanistically determined by interband transitions and is not caused by a photoemission process from the underlying metal. Further, investigations of bulk Fe-oxides revealed that their chemical stability is determined by the Fe{sup 2+} content and can be significantly improved by Cr{sub 2}O{sub 3} addition.

Corrosion of Mg alloy AZ91D in the presence of living cells.

Mg and Mg alloys are of interest for biodegradable implants as they readily corrode in biological fluids, and dissolved Mg ions are nontoxic. Even though it is well known that Mg dissolution leads to pH increase in the surroundings, the effect of the corrosion-induced alkalization on the biological environment has not been studied in detail. We therefore explored the interactions between corrosion-induced pH increase and cell growth on Mg alloy AZ91D surface. Cell adhesion and spreading on the alloy surface is unimpeded initially. However, with time a large fraction of cells de-adhere. We attribute this to the observed increase of the pH in the cell culture medium in the process of alloy dissolution. Cytotoxicity tests with HeLa cells grown on glass surfaces confirm that cell death increases with increasing alkalinity of the cell culture medium. We also show that a the cells that adhere on the Mg alloy surface act as a corrosion-blocking surface layer. In consequence, a slower pH increase in the medium takes place when the alloy surface is covered with cells. Electrochemical impedance spectroscopy measurements (EIS) verify that a cell layer slows down the corrosion process.