Covalently grafted polymer brushes are of increasing technological interest for materials used in biomedical application, particularly for reducing protein adsorption when densely grafted. One of the most widely studied polymers for this application is poly(ethylene oxide), which is available in a wide variety of either homo or hetero-bifunctional chemistries. One way of characterizing these grafted layers is by direct interaction force measurements. Generally, it is assumed that the interactions between solid surfaces and such polymer layers will be repulsive due to confinement of the polymer layer by an opposing surface (either coated with an identical polymer surface or bare).

In this study, we have used a combination of X-ray Photoelectron Spectroscopy (XPS), protein adsorption/binding studies and AFM colloid probe interaction force measurements to characterize surfaces with covalently grafted PEO layers of different grafting density and functional end group. The surfaces were prepared using cloud point grafting1 of functionalised PEO molecules onto amine plasma polymers.

Where the PEO coupling density was low, long ranged attractive polymer bridging forces were obtained between the PEO coated surfaces and AFM tips modified with silica particles2 in 0.15 M NaCl solutions. The range and magnitude of the forces were correlated with the grafting density of the PEO molecules, lower densities giving longer ranged, more attractive forces, and the molecular weight of the covalently attached PEO molecules. Long ranged attractive forces were also observed between silica and densely grafted layers with a small proportion of incorporated larger molecular weight chains, in an analogous fashion that those obtained for low grafting density, mono-dispersed layers. The origin of the attractive forces was related to the adsorption of PEO molecules onto the silica surface on approach, with more molecules adsorbing at smaller separation distances. This mechanism was verified by modification of the opposing surface (polymer graft or protein layer) and by specific coupling of NeutrAvidinTM to the grafted PEO layer.

References

1. Kingshott, P., Thissen, H. and Griesser, H.J., Biomaterials, (2002), 23, 2043.

2. Ducker, W.A., Senden, T.J. and Pashley, R.M., Nature, (1991), 353, 239.