Poly(l-lactic acid) (PLLA) Coatings with Controllable Hierarchical Porous Structures on Magnesium Substrate: An Evaluation of Corrosion Behavior and Cytocompatibility.

Magnesium (Mg) and its alloys have been intensively explored as the next generation of metallic bone substitutes in past decades, but their rapid corrosion rate in physiological environments is still a great hindrance for further therapeutic applications. In the present study, we attempt to design biodegradable poly(l-lactic acid) (PLLA) coatings on pure Mg substrates (99.99 wt %) with tunable surface morphologies through dip-coating in combination with mixed nonsolvent induced phase separation (Dip-coating-mNIPS) method to regulate their corrosion behavior and biocompatibility. We applied the mixtures of ethanol and hexane as the coagulation baths, and changed the composition of mixed nonsolvent and the concentration of polymer solution to obtain PLLA coatings with different pore sizes and morphologies. Standard electrochemical measurements and immersion tests demonstrated that all PLLA coatings could effectively enhance the corrosion resistance of Mg substrates but that the corrosion behaviors varied among coatings with different surface and inner structures. A systematic investigation of cellular response through MTT assay, LIVE/DEAD staining, cell distribution, and cell attachment indicated that PLLA-coated Mg substrates could enhance cytocompatibility in comparison to pure Mg. In addition, the cellular behaviors were affected by the corrosion activity as well as the surface properties of different PLLA coatings. Our findings illustrated that through the Dip-coating-mNIPS method, the structure of the PLLA membrane on Mg substrates could easily be controlled to regulate the corrosion behaviors and further improve the biocompatibility. This presents great potential in designing functional polymer coatings on Mg-based orthopedic implants to meet specific clinical requirements.