Degradation of Mg-Sr Alloys and Their Cytocompatibility with BMSCs for Orthopedics

Magnesium (Mg) can be absorbed into the human body as it degrades and possesses a high strength-to-weight ratio; these properties make Mg an ideal candidate for orthopedic implant applications. However, in order to employ Mg as an effective biodegradable implant, the rate of corrosion must be controlled.  One solution is to develop Mg alloys that enhance the degradation resistance while also improving mechanical performance and biocompatibility.  The objective of this study was to evaluate the degradation resistance and cytocompatibility of binary Mg-Strontium (Sr) alloys in direct culture with bone marrow derived mesenchymal stromal cells (BMSC) during a 24 hour incubation period. The alloying elements were chosen because Sr is found naturally in the human body, salts of this element are therapeutic agents to treat osteoporosis, and it improves the mechanical properties of Mg alloys. The compositions of the alloys investigated were Mg-xSr (x = 0.2, 0.5, 1.0, 2.0 wt. %). The degradation of the alloys was quantified by measuring the pH and Mg²⁺ ion concentration of the cell culture media after incubation. Cell viability was evaluated by counting adhered DAPI-stained nuclei on the sample surface and on the culture plate surrounding each sample after incubation. Cellular F-actin was stained with Alexa Fluor 488^® and used to qualitatively assess cell morphology. The cell-alloy interface was evaluated using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy. Although the degradation rate of the Mg-xSr alloys was faster than that of the Mg control, the cell viability was higher for the alloys than for the Mg. The SEM images of the cell-alloy interface provided characterization information of surface degradation and suggest that the relative surface integrity of the biomaterials may play a role in the BMSC adhesion and viability.