Preparation of Low-Leachable and Nonhemolytic Antibacterial Thermoplastic Polyurethanes from Synthetic Cationic Polyesters Containing Quaternary Ammonium Salts

The synthesis and optimization of antibacterial agents for thermoplastic polyurethane (TPU) matrices is highly imperative for the development of antibacterial TPUs, especially for applications in biomedical devices. Macromolecular or polymer-based antimicrobials are extremely appealing for their environmental compatibility, low toxicity toward host cells, and low-leachable characteristic, making them ideal additives for antibacterial materials. Herein, quaternary ammonium salt (QAS)-containing copolyesters of P(MBL-N⁺-CL) were designed and synthesized as polymeric antimicrobials via ring-opening copolymerization (ROCP) of α-methylene-γ-butyrolactone (MBL) and ε-caprolactone (ε-CL) followed by selective post-functionalization via the thiol-ene "click" reaction to introduce cationic moieties. Owing to the flexibility of the synthetic routes, the density and distribution of QAS groups can be conveniently tuned by adjusting the content and functionality of pendent vinyl groups, giving an efficient immobilization strategy for the preparation of antibacterial modifiers. Commercial TPU Estane 58277 was then physically blended with the cationic P(MBL-N⁺-CL)s (∼9 wt %) to prepare a series of P(MBL-N⁺-CL)/TPU composite films via the solvent casting method. It was found that the addition of cationic P(MBL-N⁺-CL) did not alter the basic physical properties of TPU but showed much lower leachability and hemolysis ratio than that of TPU blends with a small molecular QAS additive. Particularly, P(MBL-N⁺-CL)-3/TPU with 1.5 wt % QAS units showed almost 100% contact-bactericidal rate against <i>Staphylococcus aureus</i> (<i>S. aureus</i>), and such effects were still maintained after 10 cycles of surface washing. The influence of different P(MBL-N⁺-CL) additives on surface, thermal, and mechanical properties and cytotoxicity was systematically investigated. The results revealed that the hydrophilicity of the TPU films improved after blending with P(MBL-N⁺-CL) due to the hydration of QAS units. The thermal stability of the composite films was almost maintained with a degradation temperature of 5% mass loss (<i>T</i>_d,5%) above 310 °C and the elasticity was improved, but the tensile strength slightly decreased. The high antibacterial activity, low leachability, negligible hemolytic activities, and low cytotoxicity endow these optimized antibacterial TPU composite films with great potential applications in various medical devices.