Preparation And Characterization Of Thromboresistant Polyurethane Surfaces

Polyurethanes (PU) are widely used as biomaterials due to their excellent physical properties and relatively good biocompatibility. However, surface induced thrombosis remains a significant challenge for these unmodified PU when in contact with blood. Poly(ethylene glycol) (PEG) has gained recognition as a biocompatible materials since it appears to be highly effective in minimizing protein adsorption and platelet adhesion. Surfaces, containing high concentrations of lysine in which the s-amino groups are free, are able to adsorb significant quantities of plasminogen (Plg) from plasma. Upon treatment of the Plg-adsorbed surfaces with tissue plasminogen activator (t-PA) they could dissolve nascent clots formed around them.Hence, the hypothesis underlying the present work is that attachment of lysine to a polyurethane surface through PEG will lead to a surface possessing both resistance to nonspecific protein adsorption and potential fibrinolytic activity, which will improve the biocompatibility of PU. The present work involves surface modification of conventional polyurethanes by blending and covalent grafting.The blending method involves the combination of PU with amphiphilic Pluronic copolymer conjugated by s-lysine (Pluronic-Lys). The Pluronic-Lys polymers were synthesized by the following method: an NHS-terminated polymer (Pluronic-NHS) was reacted with amine of H-Lys(t-BOC)-OH to give Pluronic-Lys(P) polymer. The removal of the tBOC protecting group on lysine gave Pluronic-Lys polymer in which the 8-amino groups of the lysine are free. Ninhydrin, ~1H-NMR spectra data obtained on the products demonstrated that the synthesis of the copolymers was successful. Contact angle measurements and the protein adsorption experiment were used to investigate the surface properties of materials formed by blending the block copolymers with polyurethane. It has been concluded that it is possible to achieve strongly protein repellent PU surfaces by the simple process of blending with Pluronic-Lys block copolymers, but Plg can't be specifically absorbed to surface through the blending method because of the deficiency of lysine on the surface, which is probably due to the hydrogen bonding between lysine and PU.As for the grafting method via covalent bond, lysine was attached to a PU suface through a PEG spacer, leaving the s-amino group of the lysine free. More specifically, PEG was first induced to a PU surface to give a PEG-grafted PU(PU-PEG) film. Then the free -OH groups on PU-PEG were transformed to N-succinimidyl carbonate by reacting with N, N'-disuccinimidyl carbonate (DSC) to obtain PU-PEG-NHS surface; Finally, PU-PEG-NHS was coupled to amine of H-Lys(t-BOC)-OH to give PU-PEG-Lys(P) surface. The discs were immersed in a trifuoroacetic acid solution to remove the tBOC protecting group on lysine (PU-PEG-Lys). The result is a lysine-containing surface in which the e-amino groups of the lysine are free. XPS data provides evidence for the preparation of modified surfaces at different stages. The result of water contact angle measurement showed that all of the modified surfaces were much more hydrophilic than the control surface. In addition, the protein adsorption experiments showed that these surfaces reduced nonspecific protein adsorption efficiently and adsorbed significant quantities of Plg from plasma. Upon treatment of the Plg-adsorbed surfaces with t-PA they were able to dissolve nascent clots formed around them.