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K. A. Copley, C. Wu, L. Chen, C. J. Radke; Polymeric-Surfactant Adsorption onto and Absorption into Soft Contact Lenses. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5415.
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Many clinicians and researchers believe that reducing the water-contact angle (CA) of soft contact lenses (SCLs) improves pre-lens tear-film stability leading to better wearing comfort. Accordingly, some contact-lens care solutions contain poly(ethylene oxide)-poly(propylene oxide) block-copolymer surfactants that act as wetting agents. These wetting agents significantly reduce the CA of some SCLs. We hypothesize that the efficacy of these wetting agents is dependent upon their ability to absorb into the polymer matrix of the SCLs and slowly release during wear. We examine the kinetics of desorption of two such wetting agents, Tetronic 1304 (T1304) and Pluronic F127 (PF127), from two SCL brands, Acuvue 2 (AV2) and O2 Optix (O2O).
SCLs were saturated with surfactant and then placed in PBS. Tensiometry was used to detect the amount of leached surfactant in the PBS solution. A captive-bubble apparatus was used to measure CAs of the SCLs with and without surfactant exposure. Adhesion tension was measured after CA measurement. A diffusion model is used to model the leaching kinetics.
A significant amount of T1304 absorbs into the AV2 lenses. Several days are required for complete desorption. Comparatively less PF127 absorbs into O2O lenses, and most PF127 desorbed in under 24 hours. Adhesion-tension measurements show that T1304 significantly changes the surface of the AV2 lenses, but PF127 does not. A simple diffusion model proves inadequate to model accurately the leaching kinetics. However, a better fit is observed when reversible adsorption onto the internal matrix is included in the model.
We conclude that successful wetting agents increase wettability through two mechanisms: 1. continual slow desorption of polymeric-surfactant from the lens matrix which reduces the liquid/vapor surface tension and, in turn, reduces the liquid contact angle; and 2. adsorption of wetting agent onto the external surface of lens, which changes the adhesion tension. The desorption process is not governed by simple diffusion alone. There is also interaction of the wetting agent with the interior surface of the lens matrix.
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