Understanding and improving the degradation behaviour of magnesium-based biomaterials

In recent years, there has been a growing interest in the development of biodegradable implants for bone fracture repair. Magnesium (Mg) is a promising candidate for these applications, due to its good biocompatibility, biodegradability and favourable mechanical properties. However, pure Mg degrades too rapidly under physiological conditions. There has been a large amount of research done recently to mitigate the degradation rate of Mg in order to make it a viable biomaterial. This is done primarily through the use of either alloying with other metals, or through a protective coating that degrades at a more favourable rate. To date, there has been limited success in achieving acceptable degradation behaviour through either alloying or coatings. This work was concentrated on two areas: (i) mechanistic understanding of the degradation of Mg-based alloys in physiological conditions, and (ii) utilising partially protective coatings to improve the degradation behaviour of Mg-based alloys. This work culminated in further understanding of the effects of surface roughness and microgalvanic effects of degradation behaviour of Mg alloys, and a novel coating that provided a significantly improved degradation rate for Mg-based alloys. The influence of surface roughness on the passivation and pitting degradation behaviour of AZ91 Mg alloy in chloride-containing and simulated body fluid (SBF) environments was found to play a critical role on the initial breakdown of the passive layer and the subsequent localised pitting attack. In chloride environment, potentiodyanamic polarisation and electrochemical impedance spectroscopy tests suggested that the passivation behaviour of the alloy was affected by increasing the surface roughness. Similarly, in SBF the polarisation resistance (R(P)) of the rough surface alloy immersed in SBF for 3 h was ~30% lower as compared to that of the smooth surface alloy. After 12 h immersion in SBF, the R(P) values for both the surface finishes decreased and were also similar. However, localised degradation occurred sooner, and to a noticeably higher severity in the rough surface alloy as compared to the smooth surface alloy. Thus the study suggests that the surface roughness plays a critical role in the passivation behaviour of the alloy and hence the pitting tendency. Microgalvanic effects between the primary matrix and secondary phases are known to influence degradation behaviour. However, there had previously been some concern in regards to the perceived high stability of secondary precipitates under physiological conditions. This work was able to show that after galvanic coupling between the primary (α) and secondary (β) phases was removed, the long term degradation resistance of the β-phase approached that of pure magnesium. Galvanic coupling of β- phase (Mg₁₇Al₁₂) with pure Mg in simulated body fluid resulted in the formation of carbonated calcium phosphate on the β-phase. While the calcium phosphate layer initially increased the degradation resistance of the β-phase, the layer rapidly degraded once the galvanic coupling was removed. Within 48 h immersion in SBF, the degradation resistance of the β-phase began to approach that of pure Mg. The results suggest that under long-term immersion period in SBF, the degradation resistance of the β-phase will decrease and eventually the β-phase will dissolve in body fluid as the micro-galvanic effects are reduced due to complete dissolution of the Mg matrix around the β-phase. This work produced three coatings on Mg-based substrates: a single layer CaP, a dual layer PEO/CaP, and a triple layer PEO/CaP/PLLA. The single layer CaP coating was electrochemically deposited on an Mg-Ca substrate using a pulse potential waveform. EIS testing showed an initial increase in the R(P) of ~15 times when compared to the bare alloy. Following 72 h immersion in SBF, only ~70 % higher than that of the bare alloy. CaPs have a significantly lower dissolution rate than Mg alloys, which suggests that the rapid deterioration of the coating was instead caused by penetration of the electrolyte through the CaP layer. Open circuit potential measurements showed a constant ~100 mV difference between the coated and uncoated samples across the entire immersion period, which suggests that the coating remained largely unchanged. This work also investigated whether the cathodic activity of the underlying Mg alloy substrate plays a significant role on the electrochemical deposition of such a coating, calcium phosphate (CaP). CaP was deposited electrochemically on two Mg alloys, i.e., magnesiumcalcium (Mg-Ca) and magnesium-aluminium-zinc (AZ91), with different electrochemical degradation behaviour. The in vitro degradation behaviour of the CaP coated samples was evaluated using electrochemical impedance spectroscopy (EIS) in SBF. The R(P) of the CaP coated Mg-Ca alloy was ~85 % lower than that of the CaP coated AZ91 alloy. Fourier transform infrared (FTIR) analysis showed no difference in the chemical nature of the coatings. However, scanning electron microscopy (SEM) analysis revealed that the coating particles on Mg-Ca alloy were less densely packed than that on the AZ91 alloy. This can be attributed to the higher dissolution rate of Mg- Ca alloy as compared to AZ91 alloy. As a result, the former exhibited higher cathodic charge density which produced higher hydrogen evolution, and thereby affecting the coating process. In order to reduce the rate of penetration of electrolyte through the coating layer, a dual layer coating was employed. A porous PEO layer first produced on pure Mg as the first layer. This PEO layer acted as a scaffold for previously discussed CaP layer. This dual layer coating produced a much more tightly packed layer that showed a much higher degradation resistance than either layer did separately. The single PEO layer improved the R(P) of the pure Mg by an order of magnitude, and the addition of the second CaP layer added another order of magnitude. This improvement can be attributed to the PEO layer acting as a scaffold for the CaP, allowing for a more tightly packed structure and thereby reducing the porosity. A third, PLLA layer was then added to further improve the pore resistance of the coating. The PLLA layer was able to improve the R(P) by a third order of magnitude for low immersion times. Interestingly, by 72 h immersion, the triple layer coating displayed an average R(P) lower than that of the dual layer. During the immersion period, the electrolyte was able to penetrate the highly porous polymer layer, allowing for the bulk erosion to take place. The subsequent release of the acidic products damaged the underlying ceramic layer, which reduced the overall degradation resistance