MUCIN INTERACTIONS WITH BIOMATERIAL SURFACES

6Da (BSM-a). Recently, we have performed a more elaborate purification protocol with the aim of removing some of the protein components, mainly albumin (BSA), that appears strongly bound to the proteoglycan fraction. The resulting protein-depleted mucin (BSM-b) displays a surface behaviour that differs significantly from BSM-a, although the molecular weights of the two fractions are similar. In an effort to understand how a mucin coated biomaterial surface might interact with its biological environment it is worth noting that early rheological observations on solutions of mucin indicated large increases in viscosity upon additions of albumin and other lower molecular weight additives of low intrinsic viscosity (2). This fact suggests that also a mucin surface film might be strongly influenced by the composition of its surroundings. Specifically, one might envision contaminant effects on its structure and thereby on such physical properties as lubricity, close-packing, and steric shielding against bacterial adsorption. Our physical characterization of mucin surface films has mainly involved model surfaces of polystyrene to which BSM-b has been adsorbed, either alone or in mixture with BSA. The adsorption has been performed, either with the components in a 1:1 mixture or sequentially, with BSA first and then BSM-b or vice versa. Using a Quartz Crystal Microbalance with the ability to measure energy dissipation (QCM-D) (3) we find that there are indeed large differences between the various surface films, with the BSM-b alone forming a diffuse and highly viscoelastic layer that compacts significantly upon the addition of BSA. By contrast, BSA as the first layer yields a compact film, relatively resistant to mucin adsorption. These differences are also evident in the surface colonization by S. aureus, which appears significantly more suppressed by a mucin coating than by a coating of BSA. In a separate study similar results were also noted on silanized (APTES) titania, where the sequential treatment of the surface with BSM-b and bone morphogenetic protein (BMP-2) provided an effective protection against colonization with S. aureus. Surface lubricity is a property of particular importance to contact lenses. Recently, a technique has been developed to measure lubricity by means of Atomic Force Microscopy (AFM), in an arrangement where the AFM tip is equipped with a colloid probe that is modified to express the surface chemistry under study (4). When this technique was applied to the variously coated polystyrene surfaces described above it became apparent that while adsorbed layers of BSM-b provided some lubrication, these layers were not robust and sheared easily. By contrast, layers formed through sequential adsorption of BSM-b and BSA provided the best lubrication of all surfaces studied. This observation is of interest as it suggests the creation of contact lens materials that preferentially adsorb mucin from the tear film (5), where the proteoglycan is in abundant contact with albumin and other proteins of potentially stabilizing influence on the mucin surface layer.