Adsorption of proteins onto poly(ether urethane) with a phosphorylcholine moiety and influence of preadsorbed phospholipid.

In a previous report we demonstrated that the blood compatibility of poly(ether urethane) (PEU) was improved by grafting phosphorylcholine (PC) groups on the surface. The improved blood compatibility was indicated by decreased platelet adsorption/activation and reduced thrombin formation at the polymer surface in experiments in which the surfaces were contacted with platelet-rich plasma in vitro. In the present study, we investigated the effect of grafted PC groups at a PEU surface on protein and phospholipid adsorption. Adsorption of human fibrinogen (Fg), human serum albumin (Alb), human high-molecular-weight kininogen (HMWK), and dioleoyl phosphatidylcholine (DOPC) vesicles was measured by ellipsometry. For this purpose, thin PEU films were cast on silicon wafers. The polymer film was photochemically modified with a PC-containing aryl azide. The presence of PC groups on the polymer surface was demonstrated by ESCA (Electron Spectroscopy for Chemical Analysis). The hydrophilicity of the polymer surface increased by the surface modification, as indicated by a decrease of the contact angle from 59 degrees before to 43 degrees after modification. Our data show that the presence of PC groups has little effect on the adsorption of proteins to a PEU surface. The highest adsorption was observed for Fg (0.49 microgram/cm2 on PC-modified PEU and 0.50 microgram/cm2 on PEU), followed by HMWK (0.28 microgram/cm2 on both PC-modified PEU and PEU), and Alb (0.16 microgram/cm2 on PC-modified PEU and 0.18 microgram/cm2 on PEU). Protein adsorption was further studied on a "biomembrane-like" DOPC bilayer formed on hydrophilic silicon. We found no protein adsorption on this DOPC bilayer. The adsorption of small unilamellar DOPC vesicles on the polymer surfaces amounted to about 0.06 microgram/cm2 (corresponding to circa 30% of monolayer coverage) and was similar for PC-modified PEU and PEU. Despite this partial surface coverage, preadsorbed DOPC on the polymer surface diminished the subsequent adsorption of proteins considerably. These results show that the mere presence of phosphorylcholine groups on a PEU surface is insufficient to suppress protein adsorption. The highly ordered structure of natural phospholipid bilayers seems to be required to suppress protein adsorption effectively.

A photochemical method for the surface modification of poly(etherurethanes) with phosphorylcholine-containing compounds to improve hemocompatibility.

Phosphorylcholine groups attached to polymer surfaces are known to improve hemocompatibility. A photochemical method is presented to couple phosphorylcholine-containing aryl azides to poly(etherurethane) surfaces (PEUs). Two aryl azides that consist of a photoactivatable 4-azidobenzoyl group, a short spacer chain, and a phosphorylcholine endgroup were synthesized. The two compounds differ only in the type of spacer used: triethylene glycol for compound 1 and hexanediol for compound 2. These compounds were physically adsorbed to PEU surfaces. Upon UV irradiation, reactive intermediates are formed that react with nucleophilic groups on the polymer surface. The modified surfaces showed decreased underwater contact angles, indicating that hydrophilic phosphorylcholine groups are present at the surface. ESCA measurements showed the presence of phosphorus and positively charged nitrogen atoms in the outermost polymer layers (analyzed depth about 50 A), which is a strong indication of the presence of phosphorylcholine groups. Hemocompatibility in vitro was tested with thrombin generation assays and platelet adhesion tests. In thrombin generation assays the clotting time of platelet-rich plasma in contact with the polymer surface is determined. Clotting times were clearly prolonged for the modified surfaces. Surfaces modified with compound 2 showed slightly higher clotting times than those modified with compound 1. Repeated surface modification with compound 2 further increased the clotting time. For the tested surfaces an increase in the clotting time corresponds to an increase in the concentration of phosphorylcholine groups at the surface (as measured by ESCA and contact angle). Platelet adhesion studies with scanning electron microscopy demonstrated that fewer platelets (showing less activation) adhered to the modified surfaces than to the unmodified polyurethane.