On the biodegradability, mechanical behavior, and cytocompatibility of amorphous Mg72Zn23Ca5and crystalline Mg70Zn23Ca5Pd2alloys as temporary implant materials

The evolution of microstructure and mechanical properties of almost fully amorphous Mg72Zn23Ca5 and crystalline Mg70Zn23Ca5Pd2 alloys during immersion in Hank's balanced salt solution (HBSS), as well as their cytocompatibility, are investigated in order to assess the feasibility of both materials as biodegradable implants. Though the crystalline Mg70Zn23Ca5Pd2 sample shows lower wettability and more positive corrosion potential, this sample degrades much faster upon incubation in HBSS as a consequence of the formation of micro-galvanic couples between the nobler Pd-rich dendrites and the surrounding phases. After 22-h immersion, the concentration of Mg ions in the HBSS medium containing the Mg70Zn23Ca5Pd2 sample is six times larger than for Mg72Zn23Ca5. Due to the Zn enrichment and the incipient porosity, the mechanical properties of the Mg72Zn23Ca5 sample improve within the first stages of biodegradation (i.e., hardness increases while the Young's modulus decreases, thus rendering an enhanced wear resistance). Cytocompatibility studies reveal that neither Mg72Zn23Ca5 nor Mg70Zn23Ca5Pd2 are cytotoxic, although preosteoblast cell adhesion is to some extent precluded, particularly onto the surface of Mg70Zn23Ca5Pd2, because of the relatively high hydrophobicity. Because of their outstanding properties and their time-evolution, the use of the Pd-free alloy in temporary implants such as screws, stents, and sutures is envisioned.