Poly-lactic acid coatings on the biomedical WE43 Mg alloy : Protection mechanism and ion permeation effects

Poly-lactic acid (PLA) coatings are deposited on the biomedical WE43 Mg alloy and the degradation behavior and mechanism are investigated during 504 h immersion in Hanks' solution. The protection mechanism is investigated by studying the ion permeation phenomenon and a custom in situ apparatus is constructed to evaluate the performance and elucidate the underlying principle. Na⁺ and Cl⁻ penetrate the PLA membrane gradually and the different permeation effects stem from the different hydrated radii. Ion permeation can be separated into three stages. Penetration of Cl⁻ is dominant in the first stage and Na⁺ dominates in the third stage. Penetration of Ca²⁺, PO₄³⁻ along with other ions also take part in the third stage leading to the formation of Ca−P on the WE43 substrate. Based on the experimental results, models are established to describe the ion permeation behavior through the PLA membrane and predict the protection efficiency of the PLA coating. The results enrich our understanding of ion permeation in polymeric coatings and provide insights into the development of Mg-based biomedical implants. © 2023 Elsevier B.V.