Antibacterial Effects of Bimetallic Clusters Incorporated in Amorphous Carbon for Stent Application

The purpose of this work is the development of Ag/a:C and Ag-Au/a:C coatings for ureteral stents, to provide them with antimicrobial characteristics. Silver was selected because of its well-known antibacterial properties, while gold was included to assess its capacity to accelerate the silver ion release forming a galvanic couple between Au and Ag. Thus, the metallic (Ag) and bimetallic clusters (Ag-Au) were produced by three different configurations: (i) unbalanced magnetron sputtering (conventional sputtering), (ii) plasma gas condensation process, and by (iii) a combination between both previous approaches. Coatings with Ag-Au bimetallic clusters were characterized by transmission electron microscopy (TEM) in order to study the arrangement (alloy, core-shell, and galvanic couple) of these particles in the carbon matrix. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to quantify the Ag ions released through artificial urine from the different coatings deposited on thermoplastic polyurethane tape (one of the materials used in the manufacture of the ureteral stent ). Then, the antibacterial and cytotoxicity properties of Ag and Ag-Au/a:C coatings were evaluated. TEM shows that a biphasic structure was not detected, thus not allowing to anticipate the establishment of a galvanic couple. The ICP-OES results demonstrate that the silver ionization is mainly a function of the amount of silver incorporated in the amorphous carbon (a:C) matrix, and the formation of a bimetallic alloy has a detrimental effect on release of the silver ions. The antibacterial activity was regulated by the silver ionization mechanisms because the coatings with higher Ag release had a higher antibacterial activity.