FRAP Measurements of Solute Diffusion Through Hydrogels
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The distribution of therapeutic agents such as proteins or drug-loaded nanoparticles within a hydrogel following release influences their rate of absorption at the desired site of action. For formulation design purposes, it is, therefore, important to be able to predict their diffusivity within vehicles like the hydrogel. For this purpose, the diffusivity of solute probes within a hydrogel can be estimated using a mathematical expression based on describing the diffusion of spheres within a macromolecular matrix. The assumptions of the obstruction-scaling model were assessed using fluorescence recovery after photobleaching. The obstruction model demonstrated excellent predictive capabilities for solute diffusion in polymer solutions using different molecular agents as shown through the work of Zhang and Amsden. To extend its applicability to hydrogels, the assumption must be valid that the correlation length of a hydrogel is the same as that of a polymer solution at equivalent concentrations. The results in this study confirm the validity of this assumption by comparing the predictions of the obstruction model to diffusion measurements of FITC-dextran (4, 10, 20, 40 kDa) in alginate-methacrylate gels. The model provided good predictions for small diffusing agents (4, 10 kDa) and underestimated the diffusion coefficients of dextrans exceeding 20 kDa. This difference was attributed to reptation effects that become more influential as the solute radius approaches the correlation length of a network.