Biodegradable metals are breaking the current paradigm in biomaterial science to develop only corrosion resistant metals. In particular, metals which consist of trace elements existing in the human body are promising candidates for temporary implant materials. These implants would be temporarily needed to provide mechanical support during the healing process of the injured or pathological tissue. Magnesium and its alloys have been investigated recently by many authors as a suitable biodegradable biomaterial. In this investigative review we would like to summarize the latest achievements and comment on the selection and use, test methods and the approaches to develop and produce magnesium alloys that are intended to perform clinically with an appropriate host response. 1. Introduction 1.1 General: Major recent advances 2. Magnesium and its alloys 2.1. Chemical composition and production process of magnesium 2.1.1. Magnesium alloys 2.1.2. The alloying elements 2.1.3. The production process 2.1.4. The effect of alloying elements 2.2. Experimental test system used in in vivo and in vitro studies 2.2.1. In vivo testing of magnesium alloys 2.2.2. In vitro testing of magnesium alloys 3. Environmental conditions influencing Mg corrosion-in vitro and in vivo 3.1. Effect of the solution and organic content 3.2. Effect of flow and temperature 3.3. Effect of hydrogen diffusion coefficient 4. How to choose the right magnesium alloy? * separate references Click here to view linked References

Magnesium alloys are very biocompatiable and show promise for use in orthopaedic implant. Significant progress of research on bioabsorbable magnesium stents and orthopaedic bones has been achieved in recent years. The issues on degradation, hydrogen evolution, and corrosion fatigue and erosion corrosion of magnesium alloys and various influencing factors in simulated body fluid (SBF) are discussed. The research progress on magnesium and its alloys as biomaterials and miscellaneous approaches to enhancement in corrosion resistance is reviewed. Finally the challenges and strategy for their application as orthopaedic biomaterials are also proposed.

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This review aims to provide a foundation for the safe and effective use of magnesium (Mg) alloys, including practical guidelines for the service use of Mg alloys in the atmosphere and/or in contact with aqueous solutions. This is to provide support for the rapidly increasing use of Mg in industrial applications, particularly in the automobile industry. These guidelines should be firmly based on a critical analysis of our knowledge of SCC based on (1) service experience, (2) laboratory testing and (3) understanding of the mechanism of SCC, as well as based on an understanding of the Mg corrosion mechanism.

Degradable magnesium alloys for biomedical application are on the verge of being used clinically. Rare earth elements (REE) are used to improve the mechanical properties of the alloys, however in more or less undefined mixtures. Therefore in this study the in vitro cytotoxicity of the elements yttrium (Y), neodymium (Nd), dysprosium (Dy), praseodymium (Pr), gadolinium (Gd), lanthanum (La), cerium (Ce), europium (Eu), lithium (Li), zirconium (Zr), was evaluated by incubation with the chlorides (10-2000 µM), magnesium (Mg) and calcium (Ca) were tested at higher concentrations (200 and 50 mM, respectively). The influence on viability of human osteosarcoma cell line MG-63, human umbilical cord perivascular (HUCPV) cells and mouse macrophages (RAW 264.7) was determined, as well as the induction of apoptosis and the expression of inflammatory factors (TNF-α, IL-1α). Significant differences between the applied cells could be observed. RAW exhibited the highest and HUCPV the lowest sensitivity. La and Ce showed the highest cytotoxicity of the analysed elements. From the elements with high solubility in magnesium alloys Gd and Dy seem to be more suitable than Y. The focus of magnesium alloy development for biomedical applications should include most defined alloy compositions with well known tissue specific and systemic effects.

Magnesium alloys have gained increasing interest in the past years due to their potential as implant materials. This interest is based on the fact that magnesium and its alloys are degradable during their time of service in the human body. Moreover magnesium alloys offer a property profile that is very close or even similar to that of human bone. The chemical composition triggers the resulting microstructure and features of degradation. In addition the entire manufacturing route is having an 2 influence on the morphology of the microstructure after processing. Therefore composition and manufacturing route have to be chosen carefully with regard to the requirements of an application. This paper will discuss the influence of composition and heat treatments on microstructure, mechanical properties and corrosion behaviour of cast Mg-Gd alloys. Recommendations will be given for the design of future degradable magnesium based implant materials.

Magnesium materials are of increasing interest in the development of biodegradable implants as they exhibit properties that make them promising candidates. However, the formation of gas cavities after implantation of magnesium alloys has been widely reported in the literature. The composition of the gas and the concentration of its components in these cavities are not known as only a few studies using non-specific techniques were done about 60 years ago. Currently many researchers assume that these cavities contain primarily hydrogen because it is a product of magnesium corrosion in aqueous media. In order to clearly answer this question we implanted rare earth-containing magnesium alloy disks in mice and determined the concentration of hydrogen gas for up to 10 days using an amperometric hydrogen sensor and mass spectrometric measurements. We were able to directly monitor the hydrogen concentration over a period of 10 days and show that the gas cavities contained only a low concentration of hydrogen gas, even shortly after formation of the cavities. This means that hydrogen must be exchanged very quickly after implantation. To confirm these results hydrogen gas was directly injected subcutaneously. Most of the hydrogen gas was found to exchange within 1h after injection. Overall, our results disprove the common misbelief that these cavities mainly contain hydrogen and show how quickly this gas is exchanged with the surrounding tissue.

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