In Vitro Corrosion of Polyester-Coated Magnesium Alloy under pH-Static Conditions

The resorption rate of bioresorbable implants requires tuning to match the desired field of application. The use of Mg as implant material is highly advantageous, as it provides sufficient mechanical strength combined with its biodegradability. Consequently, the implant vanishes after it has served its intended purpose, allowing the complete restoration of natural tissue and organ function. However, a biodegradable Mg implant requires a biodegradable coating to slow the rate of Mg corrosion, as a permanent coating would negate the benefits of using Mg as an implant material. Therefore, degradable polymers are the materials of choice, especially polyester-based coatings, such as PLLA, as they have been proven in clinical practice over the long term. Within this work, the degradation retarding effect of a physical barrier in form of four clinically relevant polyester-based coatings, poly-l-lactide (PLLA), poly-l-lactide-co-glycolide (PLGA), poly(l-lactide-co-PEG) triblock copolymer (PLLA-co-PEG), and polydioxanone (PDO), is investigated in vitro under pH-static conditions using CO₂ gas to compensate pH changes due to Mg corrosion. Coating thicknesses of 7.5 to 8.3 μm were comparable to commercially available stent systems. Quantitative analysis of magnesium concentration in buffered test medium by a photometric assay allows real-time monitoring. Shielding effect of different polyesters through polymer coating and formation of a protective passivation layer beneath the polymer coating was observed and characterized using SEM and EDX techniques. Our finding was that even imperfect polymer layers provide a considerable protective effect, and the used in vitro setup matches reported in vivo observations regarding elemental composition of corrosion products.