Strategically designed bioactive dual-layer coating of octacalcium phosphate and dicalcium phosphate dihydrate for enhancement of the corrosion resistance of pure magnesium for orthopaedic applications

The application of biodegradable implants manufactured from magnesium (Mg) has gained traction in clinical applications despite concerns about their corrosion behaviour hindering widespread acceptance. This study aims to advance the field by focussing on developing calcium phosphate (CaP) coatings on pure Mg, with the overall objective of modulating the corrosion rate of Mg-based orthopaedic implants to facilitate effective bone remodelling. In this work, different configurations of bioactive CaP-based coatings, including dicalcium phosphate dehydrate (DCPD), octacalcium phosphate (OCP), and dual-layer DCPD-OCP coatings, were applied to control the corrosion behaviour of Mg. XRD and SEM results confirmed the formation of hydroxyapatite (HA) after 14 days of immersion in Hank's solution. The adhesion force results demonstrated that the DCPD coating provided a higher adhesion force than the OCP coating, indicating its potential to enhance the interface stability between the implant and tissue. The results from the potentiodynamic polarisation tests demonstrated a five-fold reduction in corrosion rate for DCPD-OCP-coated Mg compared to uncoated Mg. The incorporation of dual-layer CaP coatings leverages the synergistic advantages of each constituent, enhancing coating adhesion via the compact structure of the DCPD base layer and providing a hydrophilic surface (contact angle ≤15°) via the OCP layer. This dual-layer architecture results in superior corrosion resistance for Mg-based orthopaedic implants, thereby potentially addressing a critical bottleneck in their clinical application.