Block and Graft Copolymers of Poly(ethylene glycol) and Poly(dimethylsiloxane) for Blood Contacting Biomedical Materials Applications

Abstract

The incorporation of both hydrophobic and hydrophilic groups into a synthetic polymer is a potent way of controlling its surface and interfacial properties. With this end result in mind, we describe herein the synthesis and characterization of poly(dimethylsiloxane-b-ethyleneoxide) block copolymers (PDMS-b-PEO) and poly(dimethylsiloxane-g-ethyleneoxide) grafted copolymers (PDMS-g-PEO). These amphiphillic copolymers were also investigated as surface modifying agents for passifying hydrophobic polymer surfaces in blood contacting applications. Specifically, the various (PDMS-b-PEO) and (PDMS-g-PEO) copolymers were coated onto poly(styrene-divinylbenzene) microspheres by their physical adsorption from solution. These surfaces were then evaluated for blood contacting applications utilizing a fibrinogen and thrombin protocol. In particular, the binding of fibrinogen and the functionality of the surface bound fibrinogen on an otherwise hydrophobic surface (polystyrene) was investigated. As the conversion of fibrinogen to fibrin is facilitated by thrombin, the aggregation of the copolymer-coated hydrophobic microspheres was followed using an optical method after the sequential exposure of the microspheres to fibrinogen and then to thrombin. We were able to determine how the adsorbed copolymers affected the functionality of the bound fibrinogen at an interface. Our hypothesis is that the hydrophobic siloxane units of the copolymers will be in close proximity to the polystyrene surface and that the PEO will extend out from the surface and therefore render the synthetic polymer system hemocompatible. Following the fibrinogen and thrombin protocol and determining the fibrinogen-dependent aggregation, the results show that the PDMS-b-PEO copolymers (having a PEO content from 4.6 to 11.5 weight %) were similar in terms of particle aggregation when compared to the pure polystyrene microspheres (blank) or to the microspheres that were coated with a linear PDMS homopolymer. By comparison, the PDMS-g-PEO copolymers (having a PEO content from 58 to 80 weight %) were seen to reduce the fibrinogen functionality on the microsphere system surface. Thus the data indicate that the PDMS-g-PEO copolymers can behave like molecular brushes that are able to pacify the surface of the hydrophobic polystyrene microspheres. A somewhat unexpected observation was that for the copolymer system having a low PEO content the fibrinogen-dependent aggregation of the otherwise hydrophobic microspheres was observed to increase relative to the pure microspheres (blank). It is clear from the findings of this investigation that the surface packing and molecular orientation of both the adsorbed copolymer and also of the fibrinogen are important factors that govern the properties and applications of blood contacting biomaterials.