Aqueous Solute Partitioning and Mesh Size in HEMA/MAA Hydrogels

Using two-photon confocal microscopy, equilibrium partition coefficients, k, were measured for aqueous Na-fluorescein, fluorescently labeled dextrans with molecular masses ranging from 4 to 20 kDa, two fluorescently labeled proteins with opposite charges, anionic bovine serum albumin (BSA), and cationic avidin in anionic 70 wt % hydroxyethyl methacrylate (HEMA)/30 wt % methacrylic acid (MAA) gels saturated with aqueous phosphate buffer solution. Cross-linking density with ethylene glycol–dimethacrylate (EGDMA) ranged from 0 to 1 wt %. All partition coefficients, except for avidin, were considerably less than unity and diminished strongly with increasing Stokes–Einstein diameter of the free aqueous solute. The average mesh size of the wet gels, obtained from the zero-frequency oscillatory shear-storage gel modulus, ranged from 3.6 to 8.3 nm over the cross-link ratios studied. Except for Na-fluorescein, solute hydrodynamic diameters were larger than the smallest average gel mesh size. Yet, all solutes permeated the gels but with small partition coefficients less than about 0.001 for the largest diameter solutes in the small mesh size gels. To express deviation from ideal partitioning, we define an enhancement (or exclusion) factor, E ≡ k/(1 – φ), where φ is the polymer volume fraction in the gel and E is unity for point solutes. A hard-sphere excluded-volume Ogston mesh size distribution is adopted to predict a priori the measured enhancement factors as a function of average gel mesh size for those solutes that do not interact specifically with the anionic gel (i.e., for solutes with E < 1). Agreement between the extended Ogston distribution and experiment is qualitative for both enhancement factors and water content of the gels. The cationic protein, Fl-avidin, exhibits a large enhancement factor in the anionic gels due to strong specific interaction with the charged carboxylate groups of MAA. In this case, consideration must be given to both hard-sphere size exclusion and specific complexation with the polymer strands.