Degradation Testing of Magnesium and its Alloys aiming at Biodegradable Implant Applications

Magnesium and its alloys are increasingly interesting materials for biodegradable implant applications. The advantage of these implants is that they gradually degrade in the human body after fulfilling the purpose of their implantation. For instance, biodegradable implants can be used in orthopaedic applications such as fracture stabilisation, eliminating the need of a second operation for non-degradable implant, e.g. titanium removal. One challenge of biodegradable Mg implants is that the properties required by the implants are application-specific. This means that the implant must be selected depending on both implantation site and patient characteristics in order to obtain the correct implant performance during the healing process.

Therefore, in this thesis, swift and slow degrading Mg alloys are studied, by adding silver (Ag) and gadolinium (Gd) as alloying elements, respectively. On the one hand, corrosion can be accelerated by adding Ag increasing the antibacterial properties of the Mg alloy. On the other hand, Mg-Gd binary alloys show a slow degradation due to the reduction of the influence of impurities such as Fe, Ni or Cu. Pure Mg and Mg-4Y-3RE are also included as references to other studies. The reason to choose Mg-4Y-3RE is because it is similar to the alloys currently applied as biodegradable implants. Thus, the degradation behaviour of these Mg based materials are studied by applying electrochemical and immersion testing techniques. In order to understand the Mg degradation, many factors have to be considered regarding testing conditions and material.

Firstly, the testing conditions have a large influence on the Mg alloy degradation behaviour. These conditions are mainly defined by electrolyte, buffering, atmosphere, electrolyte volume to sample surface (V/S) ratio and sample positioning. With the aim to investigate some of these factors different testing conditions have been applied in immersion tests in vitro of the Mg based materials introduced above. The media applied in vitro are: phosphate buffered saline (PBS), Hank’s balanced salt solution (HBSS) and Dulbecco’s modified eagle medium (DMEM). When DMEM is applied also 5 vol.% of CO2 is added to the atmosphere to increase the buffering capacity. Large differences in degradation rate are found between Mg-Gd and Mg-Ag in PBS and HBSS while in DMEM the difference in degradation rate is reduced. Basically the chlorine content in the medium and the degradation layer formation process have a crucial influence on the degradation behaviour.

Secondly, the degradation behaviour of the material also depends on the different alloying additions and processing routes which induce considerable alterations of the microstructure and impurity content. Therefore, the degradation behaviour of pure Mg, Mg-10Gd and Mg-2Ag, produced in disc and pin shape, are compared by in vitro and in vivo experiments, respectively. This analysis reveals the large influence of the impurity content of Fe and Ni as well as the grain size on the degradation performance in vivo, whereas the role of alloying additions such as Gd and Ag are found less important. In this study, the degradation rate and the surface layers determined in DMEM are found more comparable to those determined in vivo. This indicates that DMEM mimics more closely in vivo conditions. However, to achieve this mimicking when DMEM is applied, the need of sterilization measures has been identified. Otherwise, medium contamination will accelerate the Mg degradation behaviour by acidifying the electrolyte and dissolving Ca and P rich corrosion products present at the degradation layer.

Electrochemical methods, such as potentiodynamic polarization (PDP) experiments, give a quick comparison between the degradation profiles of different Mg alloys. This technique is commonly used as the first test for degradation characterization. However, the surface layer formation during polarization under non-dynamic conditions is considered to play a major role in the results. The use of the rotating disc electrode (RDE) can induce a dynamic flow at the Mg working electrode, increasing the transport of the dissolved Mg2+ ions away from the surface and reducing the surface layer formation. Parameters such as rotation speed and scan rate have been analysed finding that values higher than 1500 rpm for the rotation and 5 mV/s for the scan rate allow for more reproducible measurements without degradation layer deposition. Hence, PDP measurements with the RDE can show repeatedly characteristic curves of the material-electrolyte system and give what is considered a better estimation of the initial degradation rate.

Surface treatments are widely studied in order to tailor the surface properties and the degradation behaviour of Mg implants. Some coatings could allow the addition of a drug to the implant which could help during the first inflammatory reaction or during the healing process. In this thesis, the first steps towards a new biodegradable drug delivery system are shown. This system consists of metal organic frameworks (MOFs). MOFs are promising materials as drug nano-carriers for their nanoporous structure and, in literature, they are also considered “bio-friendly”. Mg can be used as the metal in the MOF structure, which is called Mg-MOF. Currently, Mg-MOF is produced hydrothermally, and in this thesis, Mg-MOF layers are electro-synthesised on a Mg substrate, which would add the drug delivery capacity.