Surface Modification of Biomedical Polyurethanes

Abstract: The aim of the this thesis was surface modification of a biomedical polyurethane (PUR), Pellethane 2363-80AE', to improve the biocompatibility. Surfaces modified by poly(ethylene oxide) (PEO) were prepared either by grafting PEO covalently at the PUR surface by a two-step reaction, or by using amphiphilic polymers containing PEO as the hydrophilic part. The amphiphilic polymers consisted of one commercial ABA block copolymer, Pluronic 9400 (PE94), and three graft copolymers. The block copolymer was composed of PEO (A-blocks) and poly(propylene oxide) (B- block). Of the graft copolymers, two had backbones of poly(styrene-co-acrylamide) (STY) and one a backbone of poly(methyl methacrylate-co-ethylhexyl acrylate) (ACRY). The backbones carried grafts of PEO. All modified materials had different surface properties compared with the unmodified PUR, hydrophilic or amphiphilic. In the grafting process, one fraction of PEO was entrapped into the matrix as an interpenetrating network. PEO present in a PUR matrix phase separates, changing the mechanical and thermal properties of the material. Due to the phase separation, entrapped PEO also migrated to the surface and changed the surface properties. This was specially pronounced for the short PEO chains. On blending of the amphiphilic polymers into a PUR matrix, large changes in the surface chemistry were observed. The hydrophobic/hydrophilic balance of the amphiphiles, the additive/matrix compatibility, and the molecular mobility of the matrix, were shown to govern the surface accumulation. The adsorption of amphiphilic polymers from aqueous solutions at PUR surfaces, as well as at highly hydrophobic silica (HMS) surfaces, was studied. All amphiphilic polymers adsorbed at the HMS surface, as single molecules and micellar aggregates. At the more hydrophilic PUR surface, the graft copolymers having the most hydrophobic backbone (ACRY) did not adsorb, presumably due to formation of unimeric micelles. Modified surfaces generally had lower adsorption of human serum albumin (HSA) and fibrinogen (Fg), compared with unmodified surfaces. The amphiphilic polymers also reduced the adsorption of immunoglobulin G (IgG). ACRY modified PUR (blend) had the lowest adsorption of proteins, and factor XII, and had the longest blood plasma clotting time.