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Brushes and proteins

2011

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Brushes and Proteins Wouter T. E. Bosker Protein adsorption at solid surfaces can be prevented by applying a polymer brush at the surface. A polymer brush consists of polymer chains end-grafted to the surface at such a grafting density that the polymer chains stretch out into the solution. This is schematically shown in figure 1. Fig. 1. Cartoons of a polymer brush. Two ways of preparation: (a) chemical grafting and (b) grafting through adsorption of block copolymers, for instance by Langmuir-Blodgett deposition (LB). The main parameters determining the protein resistance of a brush are the grafting density (σ), the chain length (N) and the solvent quality. The thickness of the brush is a function of these parameters: H ~ N σ1/3. This research is related to biofouling: 'the undesirable accumulation of proteins and cells at a surface', which starts by adsorption of proteins at the surface. Prevention of biofouling is of vital interest in medicine, where bacterial adhesion may cause severe infections on biomaterials used for implants. Treatment with antibiotics has hardly any effect. The only promising remedy against infections in this case is the prevention of a bacterial film. Because protein adsorption is the first step in this process, the research in this thesis is focused on prevention of protein adsorption by polymer brushes. Numerous studies over the past decades revealed that neutral polymer brushes, especially from poly(ethylene oxide) (PEO), can minimize protein adsorption. Mindful of the parameters determining the adsorbed amount mentioned above, the following three mechanisms can be identified, displayed in figure 2. Primary adsorption occurs when the diameter of the protein is (much) smaller than the distance between the polymer chains. In case of secondary adsorption, the protein is (much) bigger than the distance between the polymer chains. Ternary adsorption results from an attraction between the proteins and the polymer chains in the brush and was first discovered by Currie et al. In 1999. For a considerable time researchers have assumed a repulsion between the proteins and the polymer chains, thereby neglecting the possible ternary adsorption. However, there is increasing evidence that this attraction occurs, especially with PEO brushes. This is highlighted in this research, by adsorption studies at bimodal PEO brushes, consisting of a dense PEO brush of short chains with a varying PEO brush of long chains. Figure 2. Different mechanisms for protein adsorption at polymer brushes: primary, secondary and ternary adsorption. The main objective of this research was to investigate whether polysaccharide brushes, in particular dextran brushes, could be prepared at a solid surface and to study their protein repellency. It was suggested that brushes from these natural polymers would be more successful to prepare nonfouling surfaces with. Dextran brushes were prepared using Langmuir-Blodgett deposition (LB) and PS-dextran diblock copolymers, illustrated in figure 1. With the LB method it is possible to control both σ and N. The synthesis of the PS-dextran diblock copolymers is described in the thesis as well as the interfacial behavior. Quasi-2D aggregation occurred at the air-water interface during preparation (compression of the PS-dextran monolayer, see figure 1), resulting in inhomogeneous dextran layers at low grafting density. At higher grafting density these aggregates were pushed together to form a homogeneous dextran brush, as illustrated by AFM images. This transition from inhomogeneous to homogeneous results in non-continuous adsorption behavior at dextran brushes, in contrast to PEO brushes, as demonstrated in figure 3. Figure 3. Normalized adsorption of BSA (Γ/ Γ0) at dextran brushes (■) and PEO brushes (○). In case of dextran brushes the adsorption of BSA is constant up to a specific σ, followed by a drastic decrease, while PEO brushes show a gradual reduction.Figure 3 also demonstrates that dextran brushes are as efficient as PEO brushes in preventing protein adsorption, at high σ. This is the main conclusion of this research. It is expected that at even higher σ dextran brushes will completely suppress protein adsorption.

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