The interactions that are involved in protein adsorption to solid surfaces (Lee et al. 2001) are known to include van der Waals forces, electrostatic "double-layer" forces, solvation forces, and entropic forces (Israelachivili and McGuiggan 1988; Oscarsson 1997). The extent of protein adsorption is
Electrostatic interaction Hydrophobic interaction Specific interaction Van der Waais force
Electrostatic interaction Hydration forces Steric repulsion
Fig. 2. Types of attractive and repulsive forces determined by competition between attractive interactions and nonspecific repulsion (Fig. 2).
Among these forces, van der Waals and electrostatic forces are fundamental, while others are introduced by structural or conformational changes. The electrostatic attractions are not likely to play a significant role since most biomaterial surfaces are made of electrically neutral materials. In addition, the Debye length in physiological fluids is usually less than 10 A. Specific interactions are also expected to be absent, since biomaterial surfaces do not generally possess structures that can be recognized by molecules with specific tertiary structures, such as binding sites of antibodies and enzymes. In general, specific interactions are highly attractive. For example, the free energy of the binding of dinitrophenol by its specific antibodies ranges from -35 kJ/mol to -72 kJ/mol (Bongard 1988). In the absence of such interactions, however, van der Waals forces and hydrophobic interactions are most important. Between hydrophobic surfaces, hydropho-bic interactions generally operate over greater distances than does the van der Waals force (Pashley et al. 1985).
As mentioned earlier, ofthe repulsive interactions, electrostatic repulsion is not significant in physiological conditions such as in body fluids or blood. Repulsive hydration forces arise whenever water molecules bind to the surface containing hydrophilic groups (Israelachvili 1985). Steric repulsion can provide long-range repulsion; the long-range aspect is provided by the thickness of the grafted layer of hydrophilic molecules. Thus, it is mostly the long-range steric repulsion that effectively overcomes the attractive interactions in the physiological milieu. If the grafted layer is rather thin (i. e., if the range of the steric repulsion is rather short), the hydration repulsion becomes a significant portion of the overall repulsion. In that case it may not be easy to distinguish between the osmotic factor of the steric repulsion and the hydration forces (Israelachvili 1985).
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