Sweet brushes and dirty proteins

We studied the protein repellency of dextran brushes. Dextran was grafted to a polystyrene surface in a broad range of grafting densities using polystyrene-dextran block copolymers and the Langmuir-Blodgett deposition technique. Ellipsometry measurements confirmed a successful transfer of the dextran brush from the air-water interface to the polystyrene surface. Water contact-angle measurements validated the presence of the dextran layer at the surface. At grafting densities <0.20 nm, a heterogeneous dextran coating is detected with tapping mode AFM, consisting of aggregates of polystyrene-dextran and relatively large interstitial areas without dextran chains. This is probably due to surface micellization of the block copolymer in the Langmuir-Blodgett procedure. At grafting densities ≥0.20 nm, a homogeneous dextran brush is observed. Adsorption studies of BSA and trypsin, using optical reflectometry, showed that adsorbed amounts at the heterogeneous coating (<0.20 nm) is only slightly lower, if at all, than at the bare polystyrene surface. Beyond 0.20 nm, a drastic decrease in adsorbed amount was observed, due to excluded volume interactions between the protein and the homogeneous dextran brush. Almost complete protein repellency could be reached at high grafting densities. Comparison with adsorption studies of PEO brushes indicated that dextran brushes do not outperform PEO brushes in suppressing protein adsorption.