The appropriate selection and use of biodegradable magnesium Mg alloys

It is extremely important to know the requirements of the implant regarding its strength, whether it has to be degradable or not, what is the required degradation rate and under which environmental conditions the implant will be used. In fact, the apphcation in the cardiovascular or musculoskeletal system of the human body determines the entire property profile for the intended implant. These requirements are the benchmarks which will be used to decide if a development is ready for application or if additional work is necessary. 

2 The cycle of development to achieve a suitable biodegradable Mg implant for the required properties of the implant. 

After determining the boundary conditions the material and the according manufacturing processes can be selected. Special care has to be given to the selection of the alloying elements which must be neither toxic nor harmful. All intermetallic phases and reaction products that occur during processing must also be neither harmful nor toxic. 

For some elements such as calcium (Ca) and a number of rare earth elements (REE) their suitability in binary Mg alloys has already been proven. But further work still needs to be undertaken to assure that the intermetallic compound of Mg-Ca and Mg-REE are also not harmful to the human body. In the case that ternary or even more complex alloys will be developed the same tests have to be applied to make sme that all compounds that may form are safe. The experimental work in this area has to be accompanied by thermodynamic calculations. While binary phase diagrams are available for most elemental combinations, there are also reliable phase diagrams for a number of ternary systems. A major challenge is still the calculation of phase formation in more complex systems. Moreover, it might be the case that Mg alloy systems which contain more than three alloy components will be better solutions for biodegradable implants than those ones which are actually presented in literature. 

It is not only the toxicity of alloying elements that is of importance their influence on the property profile and the processability of the alloy is also relevant. Not all biocompatible alloying elements can be used due to restrictions in solubility, interactions with the processes and process parameters, etc. In fact, this is a complex system which, at the moment, has to be investigated in detail with a special regard to any alloying element and their influence on the chosen processing route. 

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