ABSTRACT
Grafting of poly(ethylene glycol) (PEG) is an effective way of reducing adsorption of proteins and bacteria to hydrophobic surfaces. The paper discusses and compares two different routes of attaching PEG chains to surfaces: adsorption of block copolymers of ethylene oxide and propylene oxide (EO–PO block copolymers) and grafting via use of an anchoring polymer, poly(ethylene imine) (PEI). An overriding goal is to achieve a dense packing of PEG chains. The best effect in terms of protein and bacteria rejection, judged from short-term experiments, is obtained by adsorbing a pre-formed copolymer of PEG grafted to PEI on a negatively charged surface. Using PEGs of molecular weight 1500 g/mol or higher, protein adsorption is reduced to a few percent of the amount adsorbed at an untreated surface. The block copolymer adsorption route is less effective, mainly due to protein-induced desorption of the hydrophilizing agent. Bacterial adherence is also minimal when the PEI–PEG route is used. Branched PEGs are slightly less effective than linear PEGs of the same molecular weight. The difference in performance between linear and branched PEGs is discussed in terms of difference in entropy change when the hydrophilic surface-bound layer is compressed by an approaching protein. Branched PEGs, having smaller exclusion volumes and less freedom of motion, will lose less entropy on compression. The effects exerted on protein adsorption by PEG attached to a surface parallel its effect on particle mobility in electrophoresis. Similar molecular properties seem to be responsible for both protein and bacteria rejection and reduction of electrokinetic effects.
