Surface modification of biodegradable magnesium implant materials for controlled biodegradation

Abstract

Magnesium and its alloys are promising candidates to be employed as a new generation of biodegradable metals in orthopaedic applications. However, the rapid degradation rate of magnesium alloys in the physiological environment has prevented their widespread application in medicine. The main objective of this thesis was to develop surface modification strategies that control the degradation rate of magnesium alloys in physiological environments and to provide an accurate assessment and evaluation of their biocompatibility in vitro. The overall thesis is composed of three individual projects. The first project was to develop an accurate method to test the in vitro biocompatibility of magnesium alloys. In this study, the CyQUANT assay was used to quantitatively evaluate the in vitro biocompatibility of Mg AZ31 alloy by both direct and indirect methods. The results demonstrated that the CyQUANT assay provides a more complete assessment of the overall in vitro biocompatibility of biodegradable metals by combining both direct and indirect analyses. In the second project, a multilayer coating consisting of a sol-gel silica layer followed by a mesoporous silica layer and finally a layer of calcium phosphate was developed. Surface characterization showed that a uniform and stable multilayer coating was successfully deposited on the Mg AZ31 alloy. In vitro characterization of the coatings confirmed this surface modification strategy significantly decreases the degradation rate of the magnesium alloy and that it is not cytotoxic. Superhydrophobic surfaces decrease the corrosion rate of magnesium alloys, however, cell adhesion is inhibited. In the third project, a superhydrophobic magnesium alloy surface was modified with the cell adhesive molecule, MAPTrix-F-RGD and the influence of this surface modification on cell adhesion was studied. The results demonstrate that although the MAPTrixF-RGD molecule was successfully immobilized to the superhydrophobic magnesium alloy surface, cell adhesion was not improved. The complex surface topography of the superhydrophobic Mg AZ31 surface may be responsible for the observed cell behavior. This thesis demonstrates that surface modification can be used to simultaneously control both the biodegradation rate and the biocompatibility of magnesium and its alloys, making these materials promising candidates for orthopaedic applications. In addition, it has been demonstrated that cell quantification assays based on the fluorescence of cyanine dyes are an excellent method for in vitro testing of these materials in direct contact with cells.