The ability of hydrogels to uptake solutes and later release them in a controlled manner has led to their application in drug delivery [1-2], tissue engineering , and biosensing . Hydrogels are also employed as contact lenses to detect tear-film components (e.g., glucose)  and administer drugs and bioactive agents to the eye . Because both solutes and hydrogel materials vary significantly in charge and chemistry, solute uptake and release rates depend highly on the solute-hydrogel combination.
In all applications, solute uptake and release rates are governed by the solute diffusion coefficient in the hydrogel [1-4]. Solutes (e.g., sugars, drugs, polymers and proteins) diffuse primarily through the water-filled meshes of the hydrogel-polymer network [7-8] and, depending on the hydrogel material, specifically interact with hydrogel-polymer chains [8-10]. Solute-specific interactions with hydrogel-polymer fall within two classes: weak and reversible adsorption, characteristic of pharmaceutical drugs ; and strong and irreversible binding, characteristic of polymeric surfactants and proteins [8, 10]. Effective solute diffusion rates in hydrogels are significantly reduced by specific interactions [8, 10]. Thus, drug, nutrient, and analyte interactions with the hydrogel-polymer matrix can dictate the success of hydrogels in controlled release, tissue engineering and biosensing respectively.
We report experimental and theoretically predicted solute diffusion coefficients in hydrogels with weak specific solute adsorption to the polymer chains. The hydrogels studied are contact-lens-material copolymers of 2-hydroxyethyl methacrylate and anionic (for pH > 5.2) methacrylic acid over a large range of water content. Two-photon laser-scanning confocal microscopy and UV/Vis-absorption spectrophotometry detect transient fluorescence-intensity profiles and solute concentration histories respectively. Diffusion coefficients are obtained for prototypical water-soluble drugs as a function of hydrogel composition at pH 2 and 7.4. Sufficient aqueous indifferent electrolyte is used for all diffusivity measurements so that electrostatic fields are swamped. Diffusion coefficients are predicted for four solutes (theophylline, acetazolamide, sodium fluorescein, and riboflavin) in five different water-content hydrogels accounting for hydrodynamics, physical obstruction, and weak specific adsorption. A priori predictions according to Large-Pore-Effective-Medium Theory  extended for specific adsorption are determined from independently measured parameters . In all cases, predicted solute diffusivities are in excellent agreement with the new experimental findings.
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