(265g) Hindered Diffusion in Contact-Lens-Material Hydrogels | AIChE

(265g) Hindered Diffusion in Contact-Lens-Material Hydrogels

Authors 

Liu, D. - Presenter, University of California Berkeley
Radke, C., University of California-Berkeley
Taylor, N., University of California Berkeley
Dursch, T. J., Massachusetts Institute of Technology



Solute diffusion in soft contact lenses (SCLs) is critical in designing optimal care-packaging solutions for wetting and disinfection, controlling ocular drug delivery, and mitigating lens fouling. In all applications, the extent of solute partitioning and rate of diffusion is regulated by solute and hydrogel charge, solute size, and mesh size.  Solute uptake and release in hydrogels is governed by hindered diffusion and occurs primarily through the water-filled meshes. Diffusivities of aqueous solutes in hydrogels (D) are reduced relative to their bulk values (D0) due to interaction with the polymer chains including hydrodynamic drag and physical obstruction.1-3 We investigate relative diffusivities in SCL-material hydrogels experimentally and theoretically, and develop an a priori extended Brinkman-Effective-Medium model utilizing the distribution of meshes available for solute transport.

Characteristic of SCL material hydrogels is their relatively high polymer volume fraction (>15%) leading to smaller meshes and more pronounced effects on gel diffusivities. In comparison, previous studies have focused almost exclusively on low polymer content gels (<10%).2-7 The polymer network contains a distribution of polymer-strand molecular weights between cross-links, and correspondingly a distribution of meshes.8, 9  Since solutes are excluded from meshes smaller than their size, only a subset of the mesh-size-distribution is available to the solute as it diffuses. Thus, in addition to solute-chain interactions, significant size-exclusion can affect reduced diffusivities when solute size and gel mesh size are comparable.

We study diffusion of macromolecular solutes in representative SCL- material hydrogels (hydroxethyl methacrylate-co-methacrylic acid) of varying composition. The Stokes Diameters of the macromolecules tested (3.1-6.7nm) are of comparable size to the average gel mesh size (3.6-8.4nm) determined by oscillatory shear rheometry and elastic theory.  Uptake and release experiments under perfect sink conditions are performed using fluorescence confocal microscopy and diffusivities are fit to resultant transient concentration profiles. Independent measurement of the average gel mesh size and equilibrium water volume content allows for computation of an approximate polymer fiber radius assuming an Ogston mesh-size-distribution.9

We interpret our data with an extended Brinkman-effective-medium theory using no adjustable parameters. The relative diffusivity is expressed as a product of a hydrodynamic factor, F,  and a steric factor, S, related inversely to tortuosity or D/D0 = FS.10 Each factor is modified for the phenomena that solutes cannot access meshes smaller than their size and, thus, only experience drag and obstruction within accessible pores.  In this way, we construct a Large-Pore-Effective-Medium (LPEM) only taking into account the distribution of meshes available for transport. Predicted diffusivities from this effective-medium model are in excellent agreement with those measured.

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