Protein repellant silicone surfaces by covalent immobilization of poly(ethylene oxide)

Abstract

Polydimethylsiloxane elastomers were surface modified with passivating polyethylene oxide (PEO) polymers of different molecular weights, both monofunctional and bifunctional. Following the introduction of Si–H groups on the surfaces by acid-catalyzed equilibration in the presence of polymethylhydrosiloxane, the PEO was linked by platinum-catalyzed hydrosilylation. ATR-FTIR, X-ray photoelectron spectroscopy (XPS) and water contact angle results confirmed that the PEO was successfully grafted to the silicone rubber. Atomic force microscopy and XPS suggested that surface coverage with PEO was very high on the modified surfaces but not complete. The protein-resistant properties of the PEO-modified surfaces were demonstrated by measuring the adsorption of fibrinogen from both buffer and plasma. Fibrinogen adsorption from buffer to the PEO-modified surfaces was reduced by more than 90% compared with controls.

Introduction

Silicone polymers (poly dimethylsiloxane elastomers (PDMS)) have many attributes that make them excellent materials for biomedical and drug delivery applications [1]. For example, PDMS has been used as an ophthalmic [2], [3], [4], [5], [6], [7] and a blood-contacting [8], [9], [10], [11] biomaterial. However, their use in these applications and their future evolution as biomaterials is somewhat constrained by their extremely high surface hydrophobicity, which results in the adsorption of significant quantities of proteins from the surrounding biological environment [12], [13]. While potentially desirable in applications such as drug delivery [14], [15], [16], in most cases the non-specific adsorption of proteins is detrimental to the performance of the material. It has been widely demonstrated that the adsorption of proteins from biological solutions mediates subsequent biological reactions [17]. Thus, for the further development of PDMS in biomedical applications, it is desirable to alter the hydrophilicity of the surface to reduce protein adsorption and improve biocompatibility.

Polyethylene glycol (PEO), a water-soluble, non-toxic, and non-immunogenic polymer, has been widely shown to improve the biological compatibility of materials. The presence of a layer of PEO on a biomaterial surface is accompanied by reductions in protein adsorption, and cell and bacterial adhesion [18], [19], [20], [21]. While silicones do not normally possess appropriate surface functional groups that could be used to tether passivating polymers such as PEO, several approaches have been developed to introduce organic functionalities on silicone surfaces, including the use of a mercury lamp to create radicals [22] and oxidation by an O₂-based plasma to give alcohols and more highly oxidized species [23]. Alternative methods exploit plasma polymerization of various molecules to generate a functional surface for subsequent modification [5], [24]. However, these methods require several synthetic steps, are not always reproducible and often result in incomplete surface coverage with the functional molecule of interest [25]. Herein, we describe a effective method for functionalizing silicone elastomer surfaces by creating high-density surface Si–H groups through acid-catalyzed equilibration in the presence of polymethylhydrosiloxane (MeHSiO)_n and subsequent modification with PEO using a platinum-catalyzed hydrosilylation to create protein resistant surfaces.