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Csaba Kotsmar, Teresa Nadolski, Clayton J. Radke; Mesh Size of Soft-Contact-Lens Hydrogels. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6489.
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Transport of solute molecules through soft-contact-lens (SCL) hydrogels, such as salts, wetting agents, nutrients, and drugs is important to on-eye behavior. Solute molecules larger than the sieve or mesh size, however, can not penetrate the gel. This work measure the mesh size of contact lens materials and demonstrate that solute diffusion coefficients increase with increasing mesh size.
To determine the mesh size of HEMA-based hydrogels with various crosslink densities, we utilize oscillatory shear rheometry. The zero-frequency storage modulus of the lens material is inversely proportional to the square root of the mesh size. We synthesize hydrogels from HEMA and MAA with varying crosslink densities and measure the storage modulus for MAA contents varying between 0 and 100 %. We use two-photon confocal microscopy to measure solute diffusion coefficients in the hydrogels. Transient concentration profiles are fit to classical theory to give the solute diffusion coefficients (D).
For crosslink densities ranging from 0.1 to 1% in 30% MAA/70% HEMA, the mesh size decreases linearly from 9±1.3 to 2±0.5 nm. A typical fluorescein-dextran (4000 g/mol) concentration profile is shown in Figure 1. D increases from 2.1±0.1 x 10-8 cm2/s to 8.3 ± 0.1 x 10-7 as the mesh size increases from 2 to 9 nm.
Using shear rheometry and polymer physics we successfully measure mesh sizes of SCL materials. As mesh size increases, solute diffusion rates increase, all else being equal. Mesh size now becomes a tool for designing contact lens materials for specific applications.
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