A team of researchers from Poland and Germany has fabricated magnesium alloy foams for use in tissue engineering and producing bone implants that are bioresorbable. Their findings have been published in the Journal of Magnesium and Alloys.
Study: Development of open-porosity magnesium foam produced by investment casting. Image Credit: Mongkolchon Akesin/Shutterstock.com
Metal foams are a highly interesting class of metallic materials, possessing unique properties that have made them the focus of research in several fields. Their main applications thus far have been in the aerospace and automotive industries, and they have also been researched for use as silencers, lightweight building materials, shock-absorbing materials, heat exchangers, and catalysts.
Metal foams possess highly porous structures, which has led to research interest for these materials for the biomedical industry, especially for innovative implants. Magnesium foams have shown particular promise for biomedical applications due to their superior biocompatibility.
Additionally, magnesium foams have similar mechanical properties and density to bone tissue, which gives them advantages over conventional implant materials such as titanium and steel. This property is important for implants as it decreases the stress experienced by the organic tissue, helping to aid bone growth.
Another advantage of magnesium is its biodegradability. This facilitates the development of short-term implants that require no further surgical intervention to remove, significantly reducing the stress faced by the patient. Whilst magnesium does possess low resistance to corrosion, the products of this corrosion are not harmful to the patient as they are non-toxic, soluble, and the body can get rid of them in urine.
However, whilst the products of magnesium corrosion themselves are not dangerous compared to conventional implant materials such as stainless steel and titanium, there are issues with the generation of localized hydrogen bubbles. This can lead to delayed healing due to tissue necrosis and tissue layer segregation.
Special coatings can extend the life of magnesium foam implants. Plasma electrolytic oxidation has proven particularly effective for this purpose. The space-holder method is the primary method for fabricating magnesium foams. Powder metallurgy and investment casting are also used.
The researchers behind the study published in the Journal of Magnesium and Alloys have presented a novel method based on investment casting for fabricating magnesium foams for use in bone implants. Their approach involves using a polyurethane foam pattern that is coated with wax slurry. This improves the material’s mechanical properties and increases its density.
Using the novel method, magnesium foams with a highly controlled morphology can be fabricated, which overcomes the drawbacks of the conventional salt particle space holder method. The method presented by the researchers uses readily available materials. Equipment used in this fabrication method is less specialized than other strategies for producing magnesium foams. The researchers used AZ91 alloy to cast the magnesium foams.
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To prepare the molds, they were subjected to heat treatment, which removed the polyurethane foam and left behind the pattern. The AZ91 alloy was then cast in a protective gas atmosphere at 660-680 oC. Calculations followed by experimental trials aided the selection of the molds and temperature for pouring the metal.
It was noted by the authors that multiple factors, including the vacuum system, influence the process conditions and outcomes. The temperature of the liquid metal and the mold, alongside the vacuum value, are the most important parameters which influence the penetration of cavities and minimize adverse reactions between metal and ceramic materials during the process.
A protective coating was applied to the material during pouring by feeding a mixture of CO2 and SF6 gas and precisely controlling the temperature parameters during both pouring and cooling stages. This was applied to prevent rupturing of the mold by the build-up of sulfur dioxide caused by the decomposition of calcium sulfate and accelerated oxidation.
Comprehensive testing on both uncoated magnesium foams and those treated with plasma electrolytic oxidation processing was carried out. Analysis of compression tests demonstrated that there was a correlation between the foam’s porosity and mechanical properties. An in-depth categorization of several properties of the foams was performed in the study, including a fractography analysis and an analysis of the material’s corrosive behavior.
The authors have stated that adding ceramic surface layers improves the foam’s degradation behavior, provides opportunities for adding surface functionality such as antimicrobial properties, and affects the mechanical properties of the prepared foams. However, the authors noted that their research is a feasibility study for exploring the conditions of applying surface coatings to magnesium foams. Further work will be needed to explore the optimization of the process to achieve a uniform coating using plasma electrolysis oxidation.
Several conclusions were drawn by the researchers. For instance, thin coatings do not achieve the desired mechanical functions, whereas thicker coatings do, but they are unpredictable and prone to fracturing. Furthermore, whilst using particles can add functionality, the effect of additional coatings on properties such as mechanical integrity, biocompatibility, and degradation behavior requires further studies to properly elucidate. However, their research has provided some pertinent pointers for future research on developing magnesium foams for use in bioresorbable bone implants.
Kapłona, H et al. (2022) Development of open-porosity magnesium foam produced by investment casting [online] Journal of Magnesium and Alloys 10(1) | sciencedirect.com. Available at: https://www.sciencedirect.com/science/article/pii/S2213956722000391