(713c) Effect of Composition On Ion Permeability of Silicone Hydrogels as Extend-Wear Contact Lens

Silicone-Hydrogel materials have a bicontinuous morphology with inder-dispersed silicone and hydrogel phases. Such structure allows these materials to have high ion and ion permeabilities. The high ion permeability is required to ensure that the salt concentration in the tear film in between the lens and the cornea is maintained at the physiological value. If the ion permeability of the lenses is small, the tear film thins, eventually leading to hydrophobic binding between cornea surface and silicone contact lens. The lens material needs to provide sufficient ion transport to maintain a stable hydrodynamic boundary layer between the lens and the cornea[1]. Nicolson et al claimed that the ionoflux diffusion coefficient (Dion) should be larger than 1.5*10-6 mm2/min for sufficient on-eye movement of lens [2]. In this talk we focus on establishing the relationship between the composition of silicone hydrogels and the ion permeability.

Two different experimental approaches were devised to explore the ion transport mechanism of silicone hydrogels. Monomer mixture that comprises of hydrophobic monomer (3-Methacryloxypropyltris(trimethylsiloxy)silane, TRIS), hydrophilic monomer (N, N-Dimethylcarylamide, DMA; methacrylic acid, MAA; 1-vinyl-2 pyrrolidone, NVP), co-blocked macromer (acryloxy(polyethyleneoxy)-propylether terminated poly(dimethylsiloxane), DBE-U12), crosslinker (Ethylene glycol dimethacrylate, EGDMA) and photo initiator (2, 4, 6-trimethylbenzoyl-diphenyl-phosphineoxide, Darocur® TPO) with various compositions was cured under UVB irradiation in a glass mold of desired thickness. Ion permeability measurement of these synthesized hydrogels were conducted in a horizontal diffusion cell and under perfect sink conditions to explore the effects of gel composition, salt concentration and gel thickness on transport.

Salt release models based on one-dimensional diffusion were proposed and the good fit between the experiment results and model prediction proved that ion transport in these systems is diffusion-controlled. Silicone hydrogels of several different compositions and microstructures were prepared by manipulating the ratios of various components. The salt partition coefficient and diffusivity were determined for each composition, and results were correlated to the water content and microstructure of the material. The Dion of the majority of our synthesized silicone hydrogel are one to two orders higher than the minimum requirement, suggesting a promising potential application for extended wear contact lens.

The results of this study can be used for tailoring the composition of the silicone-hydrogel materials to obtain the properties suitable for various applications including use as extended-wear contact lenses.

[1] Tighe B, Silicone hydrogel materials ? how do they work? In: Sweeney D, editor,. Silicone hydrogels: the rebirth of continuous wear contact lenses. Oxford; Betterworth Heinemann, 2000. p.1-21 [2] Nicolson P, Baron RC, Chabrecek P, Court J, Domscheke A, Griesser HJ, et al. Extended wear ophthalmic lens. US Patent No. 5760100, 1998.