Interfaces

Interfaces, as well as the interactions that take place in interfacial regions, can be complex. In fact, the interface has been described as a fourth state of matter (6). The properties of atoms or atomic groups at a material surface are different than those of the bulk material. The first layer of atoms, in contact with the fluid phase, is particularly unique. Chemical composition, molecular orientation, and properties relevant to crystallinity differ at the surface. In addition, surfaces have different electrical and optical properties and can be characterized by atomic- or molecular-level textures and roughnesses. Surfaces have wettabilities or hydrophobic/hydrophilic balances related to the factors named above. Further, surfaces are generally energetically heterogeneous. For example, although a surface may be assigned a particular wettability, it would most likely be the result of a distribution of surface regions of varying wettabilities.

In spite of this complexity, many researchers have met with success in describing some aspect of protein adsorption in terms of one or several surface properties. The effects of charge distribution, surface energy (i.e., whether it is high or low), and surface hydrophobicity, for example, have received much attention (1-5). From a purely thermodynamic standpoint, the extent of protein adsorption or biological adhesion in general could be determined purely by surface energetics, that is, the surface energies of the synthetic material, liquid medium, and adsorbates involved. Such an approach would imply that the free energy of adsorption is minimized at equilibrium. Adsorption would be favored if it caused the free energy function to decrease and would not be favored if it caused the function to increase. In the absence of electrostatic and specific receptor-ligand interactions, the change in free energy upon adsorption could be written

where Fads(J/m2) is the free energy of adsorption per unit of surface area, and yAS, yAL, and ySL (J/m2) are the adsorbate-solid, adsorbate-liquid, and solid-liquid interfacial energies, respectively.

If all the required interfacial energies of equation 1 could be estimated, one could predict the relative extent of adsorption among different surfaces. This would lead to a distinction between two situations (7,8), depending on whether adsorbate surface energy is greater than or less than the surface energy of the suspending liquid. Concerning protein adsorption from aqueous media, equation 1 would predict increasing adsorption with decreasing surface energy. In other words, a given protein would be expected to adsorb with greater affinity to hydrophobic as opposed to hydrophilic surfaces.

The importance of hydrophobic/hydrophilic balance in protein adsorption has prompted numerous investigators to develop techniques for measurement of this property at solid surfaces. Contact angle methods have been prominent in this regard (9). Contact angle analysis is inexpensive, rapid, and fairly sensitive. However, contact angle data can be difficult to interpret, and the technique is subject to artifacts caused by macroscopic, energetic heterogeneities in the surface, hysteresis, and drop-volume effects, among others. Still, useful conclusions regarding biological interactions with surfaces have been based on the results of contact angle analysis in areas of red blood cell adhesion, platelet adhesion, bacterial adhesion, and protein adsorption (9,10). Surface properties have been correlated to biological responses using other methods as well, including electron spectroscopy for chemical analysis (ESCA), secondary ion mass spectroscopy (SIMS), infrared and vibrational methods, and scanning probe microscopies. These methods and their relevance to biomedical technology were reviewed by Ratner and Porter (9).

The properties of a synthetic material's surface play a large role in dictating any biological response the material may evoke. But although much is known about selected surface property effects on protein adsorption, in a quantitative sense we know very little about how the molecular properties of protein influence its adsorption. Interfacial behavior is a cumulative property of a protein, influenced by many factors; among these are its size, shape, charge, and thermodynamic (thermal, structural, or conforma-tional) stability. Experimentally observed differences in interfacial behavior among different protein molecules have been very difficult to quantify in terms of these factors because proteins usually vary substantially from one another in each category. The following discussion is an attempt to summarize the salient results from a wide range of experiments, focused on study of surface, solution, and protein effects on adsorption. Note that many observations have been explained in terms of a protein's charge, its tendency to unfold, and contact surface hydrophobicity.

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