Retardation and self-repair of erosion pits by a two-stage barrier on bioactive-glass/layered double hydroxide coating of biomedical magnesium alloys

Abstract Magnesium alloys for orthopedic applications are mainly restricted by their rapid degradation and undesirable surface mineralization. A hierarchical layered double hydroxide (LDH)/bioactive-glass (Bg) coating was constructed via in-situ hydrothermal growth to obtain LDH-container layer on magnesium alloys, and then a sol-gel spin-coating method to cover an outer bioactive-glass layer. The ion-exchange capacity of LDH-container layer was studied by stimuli-triggered release experiment. The results show that alkaline environment is conducive to the ingestion of aggressive chlorides accompanied fluoride-releasing, and this ion-exchange process reached equilibrium after 40 h. Electrochemical corrosion tests manifested that the LDH-container layer delayed the corrosion onset, declined the corrosion current by two orders of magnitude and positively shifted the corrosion potential of 1.15 V. Furthermore, corrosion degradation and mineralization evolution in vitro revealed that a well-defined self-repairing effect and collaborative mineralization were obtained when the LDH-container pre-encapsulated with fluoride and coupled with bioactive-glass. Specifically, the self-repairing was achieved by a local re-deposition of the degradation products Mg2+ and dissolved Ca2+ with the displaced F− to form stable magnesium fluoride or fluorapatite in the erosion pits. This intelligent two-stage strategy, based on physical protective barrier and self-repairing the eroded pits by re-depositing of bone-like minerals, progressively retarded rapid degradation and simultaneously facilitated remineralization. The strategy described herein provides a versatile route for designing smart corrosion protection coatings or targeting drug delivery systems.