Self-healing poly(lactic acid) film incorporated with phytic acid on dicalcium phosphate dihydrate-coated magnesium for corrosion protection in orthopedic applications

Magnesium is considered a promising orthopedic implant material due to its excellent biocompatibility and biodegradability. However, its rapid degradation and the associated corrosive byproducts can trigger inflammatory responses, limiting clinical application. To address this issue, a self-healing Mg-based composite coating was developed, consisting of an inner layer of dicalcium phosphate dihydrate (DCPD), a middle layer of polylactic acid (PLA) containing the corrosion inhibitor phytic acid (PA), and an outer PLA layer. The dissolution of the DCPD layer releases Ca²⁺ and PO₄³⁻ ions, which react with Mg²⁺ to form a stable calcium-phosphate (Ca-P) precipitate. Simultaneously, PA molecules released from halloysite nanotubes (HNT) in the middle layer chelate with Mg²⁺ to form magnesium phytate, enhancing corrosion resistance through a synergistic mechanism. Scratch tests demonstrated that the coating effectively inhibited corrosion propagation after 8 h of immersion in 0.9 wt% NaCl solution. During immersion, the current density initially increased from 1.460 × 10^-⁸ A·cm⁻² to 8.374 × 10⁻⁷ A·cm⁻², followed by a slight decrease to 6.338 × 10⁻⁷ A·cm⁻², indicating a self-healing response. In vitro experiments with MC3T3-E1 cells confirmed good biocompatibility of the coated Mg. Furthermore, in a rat femoral implantation model, the coated Mg exhibited improved in vivo corrosion resistance and enhanced new bone formation, with elevated expression of osteocalcin (OCN) and osteopontin (OPN) around the implant.