Protein adhesion on silicon-supported hyperbranched poly(ethylene glycol) and poly(allylamine) thin films

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 degrees) and PEG (33-37 degrees) were characteristic of the corresponding individual polymer (30-50 degrees for allylamine, 34-42 degrees for PEG). Roughness values varied from approximately 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 approximately 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.