Functional Surface Coating on Magnesium to Improve Corrosion Resistance and Biocompatibility for Biodegradable Medical Applications

Abstract

Biodegradable implants were suggested as an ideal biomedical implant due to the elimination of a second surgery to remove the devices after the healing of the surrounding tissues. Magnesium (Mg) and its alloys have gained considerable attention as a promising biomaterial for biodegradable implants due to the outstanding mechanical properties and biocompatibility, especially in dental, orthopedic and vascular stent applications. However, despite the significant advantages of Mg, the excessively high corrosion rate of Mg and its alloys is one of the major drawbacks for clinical use of Mg-based implants. Therefore, in order to not only decrease the degradation rate but also enhance the biological responses to improve the function of the implant, various functional surface treatments have been performed. In the first study, PEI-Silica hybrid coated biomimetic Mg was fabricated for dental and orthopedic application. By mimicking the structure and component of the bone, biodegradable Mg implant with high strength and pore interconnectivity and good osteoconductivity can be acquired. Bone has structure with combined dense and porous structure resulting in high strength/density ratio. Furthermore, it is composed with hydroxyapatite (HA) which has excellent osteoconductivity. By spark plasma sintering and space holder process Mg scaffold with combined dense/porous structure can be fabricated. Due to the bone-mimicking structure, Mg implant can have high strength and stiffness with high pore interconnectivity. Moreover, by controlling the ratio of dense structure, mechanical strength and stiffness can be controlled. By aqueous precipitation coating method HA can be coated on the Mg implant. This HA layer can enhance both corrosion resistance and biocompatibility with osteoblast cells. However, cracks form on HA coating layer due to the brittle property of HA. These cracks could be critical in corrosion and biological behavior on porous Mg due to complex shape and large surface area. Consequently, PEI-Silica hybrid layers were dual coated on the bone-mimetic Mg. Due to the high corrosion resistance of PEI and excellent bioactivity of silica, corrosion rate of bone-mimetic Mg decreased remarkably and both biocompatibility and bioactivity with bone tissue were enhanced. Thus, PEI-Silica hybrid coated biomimetic Mg implant could be a promising biodegradable scaffold for dental and orthopedic applications. In the second study, selective PEI/PLGA dual layer was coated on Mg for the biodegradable drug eluting stent application. Corrosion rate of Mg stent is need to be slowed down to maintain the scaffolding ability until the revascularization has been finished. Moreover, anti-proliferative drug is needed to be released to prohibit restenosis. To achieve these needs, Mg stent was selectively coated with PEI/PLGA dual layer by spray coating process. PEI was fully coated on the surface of the Mg stent to decrease the fast corrosion rate and improve the biocompatibility with endothelial cells. Subsequently, sirolimus loaded PLGA was selectively coated on abluminal side of the Mg stent to control the release of the drug to the wall of the blood vessel. This selective drug release can prohibit the proliferation of smooth muscle cells and prohibit the anti-proliferative effect to endothelial cell on luminal side of the Mg stent. Thus, selective PEI/PLGA dual coating on Mg stent offers a promising approach for the development of biodegradable drug eluting vascular stent application. In conclusion, these researches were about improving the functions of the Mg implant by enhancing the corrosion resistance and biocompatibility with stable surface coatings for various biomedical applications. The degradation tests and in-vitro cell tests showed the PEI-Silica hybrid coating and selective PEI/PLGA dual coating had effectively enhanced corrosion protection and cellular responses of Mg implant offering excellent function for dental/orthopedic application and vascular stent application respectively.