Antibiofilm properties of functionalized polyurethanes adsorbed with metal ions (Ag+, Cu2+, Zn2+, Al3+ and Fe3+)

This research is focused on the development of intrinsically antimicrobial polyurethanes in which the carboxyl groups present in the polymer backbone were neutralized with Ag+, Cu2+, Zn2+, Al3+ and Fe3+. The antimicrobial activity of these metal ion-containing polyurethanes was studied alone or in combination with ciprofloxacin (CPF). Aims of the study were to evaluate the most efficacious ions in counteracting microbial growth on polymeric surfaces and to assess the possible synergistic role of ciprofloxacin. An acidic polyetherurethane (PEUA), containing one carboxyl group per repetitive unit, was turned into PEUA-Ag, PEUA-Cu, PEUA-Zn, PEUA-Al and PEUA-Fe by treatment with equimolar amounts of the salts AgNO3, Cu(CH3COO)2, ZnNO3, AlCl3 and Fe(NO3)3, respectively. The obtained polymers were characterized by mechanical and swelling tests. The antibacterial efficacy of the metal ion-containing polymers, alone or treated with ciprofloxacin, was evaluated against Staphylococcus epidermidis ATCC 35984 by: i) the Kirby Bauer test; ii) the count of colony forming units (CFUs) per polymer surface unit; iii) the observation of microbial biofilm formation on the polymer surface by scanning electron microscopy. Due to the interaction between the ionic groups leading to physical cross-linking, metal ion-containing polymers exhibited better mechanical properties in terms of hardness and strength at break than the pristine PEUA. With the exception of PEUA-Al, all the other polymers showed significant antimicrobial properties. The best antibacterial effect was obtained by the PEUA-Ag for its ability to inhibit the S. epidermidis biofilm formation up to 16 days. The ciprofloxacin adsorption to the polymers prolonged their antimicrobial activity, demonstrating a synergistic antibiofilm effect of PEUA-Ag-CPF up to 25 days. Our experiments suggest that: i) carboxylated polyurethanes can coordinate antimicrobial metal ions offering a simple methodology to obtain intrinsically antibiofilm polyurethanes; ii) the methodology employed in this study allows the direct incorporation of Ag ions, i.e. the active species of silver, differently from other coating techniques; iii) as expected, the adding of ciprofloxacin prolongs the antibacterial activity of PEUA-Ag to 25 days.