Plasma electrolytic oxidation (PEO) coatings and biodegradable polymers such as polylactic acid (PLA) present interest as drug-eluting systems for osseointegrated biomedical implants. Given the biodegradable nature of PLA, the coating system may induce crevices at one or more interfaces during degradation. Crevices are known to severely affect the corrosion performance of additively manufactured (AM) Ti alloys. Therefore, understanding the interactions between the PEO/PLA system and titanium alloys is crucial for enhancing the longevity of implants. This study investigates the corrosion of mill-annealed and AM Ti-6Al-4V alloys functionalized with porous hybrid hierarchical coatings comprising a ceramic layer produced by plasma electrolytic oxidation and one or more polylactic acid layers.
PEO coatings were formed using an AC power supply on wrought and DMSL-produced Ti6Al4V. PLA was applied on PEO-coated samples by dip-coating using the “breath figures” technique. Optical profilometry, scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS) were utilized to assess the topography, morphology, phase composition, and elemental distribution in the coated alloys. Corrosion performance was evaluated through DC and AC electrochemical tests, including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), conducted in a simulated body-fluid at 37 ºC to mimic real-world conditions, including the presence of a controlled-size crevice.
The results revealed ~3.5-5.5 µm thick anatase- and rutile-based coatings enriched in several bioactive elements, at.%: 7.6% Ca, 5.76% P, 0.23% Zn, 1.5% Mg, 3.52 %Si. The ~2 µm thick PLA layer featured a 1-6 µm pore size and ~75000 pores mm-2. The PEO coating helped in avoiding the localized corrosion of AM Ti6Al4V. The post-corrosion characterization also elucidates the role of biodegradable sealing layers in crevice formation.
In conclusion, the PEO/PLA system is a viable coating material for enhancing the durability of AM Ti6Al4V. This research provides a foundation for future studies aimed at exploring alternative biodegradable materials as drug-eluting media.
