Composite Coating-driven Regulation of In Vivo Magnesium Degradation and Tissue Response

Magnesium-based alloys are promising materials for fabrication of bioresorbable medical devices. The application is limited by rapid degradation and associated adverse tissue responses. In this study, nanometer-thin composite polyelectrolyte/wax (PEM/W) coatings were fabricated on magnesium-based implant prototypes to delay their degradation.

Coatings were applied using a layer-by-layer technique. Cytocompatibility was assessed according to DIN ISO 10993-5 using NIH/3T3 fibroblasts and human umbilical vein endothelial cells (HUVECs). Degradation behavior was monitored by high-resolution micro-computed tomography (μ-CT), while tissue compatibility and host responses were evaluated histologically following DIN EN ISO 10993-6.

The coatings were continuous, hydrophobic (water contact angles exceeding 100°), with sub-micrometer thicknesses and were found to improve in vitro cytocompatibility of magnesium biomaterials. In vivo evaluation using a rat subcutaneous implantation model demonstrated that the effectiveness of magnesium degradation modulation depends on the type of employed PEM. Micro-computed tomography analyses revealed that the hyaluronic acid/chitosan/wax coating provided the most robust protection over 60 days, exhibiting the lowest volume loss and superior preservation of implant geometry compared to the uncoated samples. Correspondingly, hydrogen-related gas cavity formation was reduced and temporally delayed in coated implants, indicating a more controlled degradation process. Histopathological analysis shows a moderate inflammatory response characteristic of biodegradable metallic implants, dominated by macrophages and lymphocytes. Importantly, coated implants were associated with reduced late-stage fibrosis and necrosis compared to uncoated magnesium.

Overall, composite PEM/W coatings, especially those based on natural polyelectrolytes, represent a promising surface-engineering strategy for improving the safety and performance of resorbable magnesium implants.