Protein Adhesion on Silicon-Supported Hyperbranched Poly(ethylene glycol) and Poly(allylamine) Thin Films Maureen A. Dyer, ² Kristy M. Ainslie, ² and Michael V. Pishko* ,²,‡,§ Department of Chemical Engineering, 204 Fenske Laboratory, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802-4420, Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802-4420, and Department of Materials Science and Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802-4420 ReceiVed February 20, 2007. In Final Form: April 4, 2007 Hyperbranching poly(allylamine) (PAAm) and poly(ethylene glycol) (PEG) on silicon and its effect on protein adhesion was investigated. Hyperbranching involves sequential grafting of polymers on a surface with one of the components having multiple reactive sites. In this research, PAAm provided multiple amines for grafting PEG diacrylate. Current methodologies for generating PEG surfaces include PEG-silane monolayers or polymerized PEG networks. Hyperbranching combines the nanoscale thickness of monolayers with the surface coverage afforded by polymerization. A multistep approach was used to generate the silicon-supported hyperbranched polymers. The silicon wafer surface was initially modified with a vinyl silane followed by oxidation of the terminal vinyl group to present an acid function. Carbodiimide activation of the surface carboxyl group allowed for coupling to PAAm amines to form the first polymer layer. The polymers were hyperbranched by grafting alternating PEG and PAAm layers to the surface using Michael addition chemistry. The alternating polymers were grafted up to six total layers. The substrates remained hydrophilic after each modification. Static contact angles for PAAm (32-44°) and PEG (33-37°) were characteristic of the corresponding individual polymer (30-50° for allylamine, 34-42° for PEG). Roughness values varied from ∼1 to 8 nm, but had no apparent affect on protein adhesion. Modifications terminating with a PEG layer reduced bovine serum albumin adhesion to the surface by ∼80% as determined by ELISA and radiolabel binding studies. The hyperbranched PAAm and PEG surfaces described in this paper are nanometer-scale, multilayer films capable of reducing protein adhesion. Introduction Modification of metal surfaces has application in a variety of areas ranging from anticorrosion 1 to antibiofouling treatments. 2,3 Much of the research into biofouling prevention focuses on using thin films to reduce adhesion of biological entities. 4-10 Poly- (ethylene glycol) (PEG) modifications are most widely employed to reduce protein adhesion while maintaining a hydrophilic, biocompatible surface. Using in vitro protein and cell assays, Zhang et al. showed that increasing the amount of PEG decreased protein and cell adhesion to chitosan-PEG blended materials. 11 In in vivo rabbit studies, Rogero et al. demonstrated biocom- patibility of hydrogels incorporating PEG. The hydrogels were not cytotoxic to cells and did not irritate the rabbit skin. 12 Commercially available products also utilize PEG modifications to reduce protein adhesion. Nektar Therapeutics (www.nek- tar.com) has collaborated with several pharmaceutical companies (such as Schering-Plough, Roche) to generate six PEGylated products currently FDA approved for therapeutic use. In a review by Vermette and Meagher, 2 the available data on what aspects of PEG are involved in reducing protein adhesion are analyzed. The main characteristics of PEG contributing to its protein resistance are (1) high polymer surface density, (2) the need to overcome a potential energy barrier to adhere to PEG as compared to no barrier to adhere to a bare surface, (3) the generation of a repulsive energy due to the approaching protein compressing the PEG chains, and (4) limitation of PEG chain movement by protein proximity. 2 To introduce protein-resistant PEG coatings on surface, there is a variety of methods available. An early method involved polymer adsorption to glass or silica substrates. 13 This method has more recently been used to adsorb PEG-containing copoly- mers to target surfaces. Silicon wafers have also been modified with PEG by treating surfaces with PEG-silane molecules. Such treatment results in a covalently grafted PEG monolayer that reduces protein attachment. 8-10,14,15 PEG-silane monolayers are useful for substrates of homogeneous composition. 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