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C.J. Radke, L.S. Cheng, S.J. Muller; Wettability of Soft Contact Lenses Exposed to Tear Film Components . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3684.
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Purpose: We study the surface wettability of commercial soft contact lenses and how that wettability is altered by selective components in human tears. Also, the possible improvement in surface wettability is studied by grafting polyethylene glycol (PEG) derivatives onto existing commercial lenses. Methods: We designed a new contact lens holder that permits reliable contact-angle determination with fragile, submerged, and curved lenses in aqueous solution. Because of its applicability to contact lens performance, we adopt the captive-bubble technique to measure advancing and receding contact angles of various lenses in solutions containing electrolyte, mucin, and lysozyme and their combinations. Contact angles are measured without exposure of the lenses to air thereby eliminating possible drying artifacts. The role of surface modification is accomplished by covalently attaching mPEG-silane to a PureVision lens in water and measuring the contact angles in isotonic solution and in combination with mucin and lysozyme. All angles are calculated using automated edge-detection and curve-fitting software, and verified independently with a graphics program. AFM characterizes the surface topography of the various lenses. Results: All lens types equilibrated in isotonic solution exhibit dramatic contact-angle hysteresis in that the advancing angles are larger than the corresponding receding angles. Receding angles in isotonic solution are relatively small (20 - 25 °) whereas advancing angles differ substantially and can approach nonwetting conditions (greater than 90 °) Addition of either mucin or lysozyme or their combination eliminates hysteresis, which indicates almost complete wetting. Advancing contact angles differ between lenses for a given solution and between solutions for a given lens. Likewise, no hysteresis is evident after modifying the PureVision lens with PEG. When we expose the PEG-modified lenses to isotonic solutions of mucin and lysozyme, contact angles are always lower than those of the unmodified control lens in the same solutions. Conclusions: We find that advancing contact angles provide a more discriminating gauge of soft contact lens wettability than do receding angles. Our results highlight the importance of tear-component adsorption onto the lens surface, allowing tears to re-wet (advance) over de-wetted (receding) patches at approximately the same angle, regardless of lens type. PEG modified lenses both improve wettability and reduce the adsorption of mucin and lysozyme.
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