Permanently implanted biomaterials may cause problems to the host body associated with long term chronic inflammation which would eventually require revision surgery. The development of biodegradable materials which can be absorbed, consumed and excreted by the patient is therefore of interest. Magnesium alloys have for a long time been considered as potential biomaterials for load-bearing applications due to their excellent biological properties including superior biochemical and biomechanical compatibility compared to other alternatives such as biodegradable polymers and bioceramics.
However, the application of magnesium material in the biological area is still limited due to its intrinsically poor corrosion performance in the biological environments. Therefore, various methods have been explored to control the degradation rate of magnesium in biological fluid, of which plasma electrolytic oxidation (PEO) is the most promising method. PEO is a plasma-assisted anodising process that can convert the surface of magnesium into a ceramic layer, thus preventing the corrosive medium contacting the substrate; therefore, the degradation rate can be reduced. Furthermore, highly biocompatible coatings can be produced when appropriate electrolytes are used in the PEO process.
Motivated by the beneficial properties of magnesium and corrosion protection provided by the PEO technique, considerable efforts have been devoted towards the development of magnesium implants based on PEO protection. Nevertheless, the corrosion rate of magnesium has not been reduced to an acceptable level and a universal PEO process appropriate for magnesium has not yet been established.
In the present study, PEO processes on commercially pure (cp) magnesium and the resulting coating characteristics have been systematically studied. Through this progressive study, a biologically friendly electrolyte containing Ca and P compounds have been developed. An appropriate current regime for this electrolyte has also been studied. Finally, a hydroxyapatite layer, intended to enhance the sample bioactivity, was deposited on the PEO coated cp magnesium. The PEO process was studied according to key electrical characteristics including voltage transient, and voltage/current waveforms. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were employed to study the surface and cross-sectional morphology, elemental composition, phase composition of the coatings. Residual stress induced by the PEO process is also studied using XRD method. The corrosion properties of the coated samples in simulated body fluid (SBF) were studied using electrochemical methods including open circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS) measurement, and potentiodynamic polarisation scans. The mechanical properties, including static tensile properties and cyclic fatigue performance of the coated samples were also studied to verify the applicability of magnesium in biological areas from the mechanical point of view.
The results indicated that the combination of a pulsed unipolar (PUP) current regime of 3000 Hz and an electrolyte composed of 12 g/l Na3PO4•12H2O and 2g/l Ca(OH)2 provides the best process stability and success of Ca and P incorporation. Moreover, the corrosion resistance of cp magnesium in the SBF could be improved by more than 10 times. Nevertheless, such protection is very limited as the coating was degraded rapidly in the simulated body fluid, which is due to the chemical instability of MgO at the pH of SBF. Tensile and cyclic fatigue tests demonstrated that the PEO coated cp magnesium possesses sufficient mechanical properties for general load-bearing biomedical applications even though the fatigue strength is significantly deteriorated by the surface modification. Further work required to achieve better control over the biodegradation process of Mg implants can be outlined as follows: (i) robustness of the developed PEO process should be explored on other corrosion resistant magnesium alloys containing biologically friendly elements (like Ca, Zn, Mn); (ii) addition of F-, SiO32- in the electrolyte to facilitate the formation of stable compounds besides MgO in the PEO coating, thus reducing the degradation rate of magnesium based implant
