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Biodegredable Mg and Mg alloys: Tailoring the degradation rate and biocompatibility by surface modifications

Subject Area Biomaterials
Term from 2011 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 185367898
 
Final Report Year 2015

Final Report Abstract

Our results demonstrate for the first time that it is possible to culture cells on chemically pure magnesium over prolonged time periods of more than 20 days. We achieved this by a prepassivation of the Mg surface prior to cell coating, thereby avoiding the potentially toxic effects of alloy elements that are conventionally added to control the corrosion behavior of Mg. In our studies, we investigated the behavior of endothelial cells, osteoblasts, and fibroblasts. The detachment of an endothelial cell layer from the Mg matrix after culture times in excess of 20 days is also seen for cells grown on tissue culture treated plastic, and is attributable to a down-regulation of adhesion receptors as the cells become overcrowded. Thus, we conclude that a chemically pure Mg surface becomes over time to the cell undistinguishable from traditional matrix surfaces. Moreover, we showed that a cell layer acts as an additional passivation layer that slows down the corrosion of the Mg surface. The key for utilizing the passivation approach is therefore to limit the initial highly violent corrosion of Mg and the ensuing hydrogen gas evolution. We found that a coating of the Mg sample with stearic acid (SA) followed by a passivation in cell culture medium with 10% fetal calf blood serum was most effective on both, limiting the initial corrosion burst, and facilitating a rapid cell adhesion. In a next step, it will be important to test of such a long-term effect of surface passivation can be sustained in the environment of the living body, i.e. in the presence of immune cells such as macrophages that may attack the passivated surface. Moreover, the uncontrolled adhesion of stromal and other cells on the Mg implant surface cannot be tested in-vitro and thus must be explored under in-vivo conditions. Our results thus far, however, suggest that surface passivation of otherwise chemically pure Mg implants has long-term effects on slowing the corrosion rate if an adhesive cell layer can be formed before the passivated surface has been corroded away. A possible strategy to ensure a long-term effect of surface passivation may be the formation of a passivation cell layer prior to introducing the implant into the more complex environment of the living body. During the funding period of this project, we developed a number of technologies that allowed us to investigate cell behavior on Mg samples in the first place, in particular a spinning disk shear stress apparatus to measure the adhesion strength of cells on an the material surface, electrochemical impedance spectroscopy for measuring the corrosion behavior of metal samples overgrown with living cells, and a method to quantify the initial adhesion and spreading of cells on opaque materials utilizing an upright confocal microscope in combination with fluorescently stained cells. These technologies will form the basis for further research in the area of cell-matrix interactions, also beyond the immediate field of magnesium-based biomaterials.

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