Wettability of silicone-hydrogel contact lenses in the presence of tear-film components

Abstract

Introduction. Modern application of soft contact lenses demands safe and comfortable wear over extended time periods up to one month. Lenses that exhibit and sustain complete water wetting allow thick tear-film deposition, minimize film rupture, and permit smooth tear recovery upon lid closure. Water contact angles determined using an air bubble captive on a lens best gauge the in-vivo wetting state. To achieve highly water wetting lenses demands that contact-angle hysteresis be eliminated and that the advancing and receding angles both approach zero. Since lens wear exposes the anterior surface to tear proteins, lens wettability should be measured in the presence of tear-film components. Methods. A captive-bubble technique is applied to measure the advancing and receding contact angles of two commercial silicone-hydrogel lenses: PureVision (PV) and Focus Night & Day (CF) and a standard HEMA (hydroxethyl-methacrylate) hydrogel lens: Acuvue (AV). In the captive-bubble method, an air bubble immersed in aqueous solution is brought into contact with the contact lens. The contact angle through water during bubble expansion yields the receding angle. Bubble contraction gives the water advancing angle. Contact-angle hysteresis is the difference between the advancing and receding angles. Results. In isotonic solution, all three lenses display considerable contact-angle hysteresis with advancing angles of almost 90°. When lysozyme and/or mucin were added to the aqueous solution, hysteresis was eliminated, and equivalent and high water wetting was achieved for the three lenses. Only the advancing angle in isotonic solution provided discriminating evidence for differences in surface chemistry. Covalent attachment of polyethyleneglygol (PEG) to the PV lens surface achieved complete water wetting independent of the presence of tear protein in the solution. Conclusions. The captive-bubble technique provides contact angles that are relevant to on-eye lens wear. Both advancing and receding contact angles are important to lens wettability performance. When lysozyme and/or mucin are present in the solution, PV, CF, and AV lenses display low advancing and receding contact angles indicative of equivalent wettability performance. This result is due to molecular adsorption of the proteins onto the lens external surface. Covalently attached PEG on the PV lens not only provides complete water wetting but also minimizes or even eliminates protein adsorption.