Design of the double-layer biocompatible coating on AZ31 magnesium alloy for highly effective corrosion resistance

A double-layer phytic acid@cerium/polycaprolactone@cerium substituted hydroxyapatite (PA@Ce/PCL@CeHA) composite coating was prepared on the AZ31 Mg alloys combining the cyclic assembly and electrospinning techniques. SEM images showed that the out layer of PA@Ce/PCL@Ce-HA coating displayed a typical disorderly distributed nanofiber structure, and the average diameter of the PCL@Ce-HA fibers was 680 +/- 45 nm. ATR-FTIR and XPS analysis demonstrated the existence of typical characteristic peaks of PO43- groups in the composite coating, indicating PA molecular and Ce-HA nanoparticles were successfully anchored to the alkaline pretreated Mg substrate. The results showed that this composite coating was composed of an inner PA@Ce complex and an outer PCL@Ce-HA nanofiber coating. The cross-section images verified that the thickness of the composite coating was 74.94 +/- 2.14 mu m. The coating adhesion tests indicated that the bonding strength between outer fibrous PCL@Ce-HA and substrates were improved, owning to the mechanical interaction and entanglement between fibrous coatings and the rough PA@Ce coating. Electrochemical tests showed the corrosion current density of PA@Ce/PCL@Ce-HA coated sample significantly dropped by two orders of magnitude compared to uncoated sample, indicating this coating significantly reduced the corrosion rate of the substrates. In addition, the lowest amount of hydrogen evolution and the minimal variation of pH value in the in-vitro immersion tests further confirmed that the PA@Ce/PCL@Ce-HA coating had a long-term stable corrosion protection for the substrates. Furthermore, the cell experiments showed that the double-layer coating provided an interface similar to the extracellular matrix (ECM), which was conducive to the attachment and proliferation of human osteoblast MG-63 cells. The hemolysis rate (HR) of PA@Ce/PCL@Ce-HA coated sample was 1.69%, which met the requirements of biomedical material applications. Thus, the outcomes of this work provided a new sight for corrosion protection of the Mg alloys and this coating has great potential in orthopedic applications.