Quantifying Small Molecule Diffusion in a Hydrogel Using a Microfluidic Technique

Hydrogels are composed of cross-linked networks of hydrophilic polymers that are biocompatible due to their high water content. In this work, the rate of diffusion of a small dye molecule, sulforhodamine 101, in the hydrogel poly(ethylene glycol) diacrylate (PEG-DA) has been characterized. The diffusion was studied by performing both uptake and elution experiments in microfluidic devices fabricated with a 600μm diameter channel running lengthwise through the PEG-DA hydrogel. During experiments deionized water or dye loaded aqueous solution was pumped through the hydrogel channel allowing dye to diffuse into or out of the channel. With dimensions on the micro scale laminar flow occurs through the devices; thus diffusion (rather than convection) is the dominant method of mass transport.

Optical images of the dye in the hydrogel were captured continuously during experiments using a microscope. Dye concentration profiles (concentration v. position) were was extracted from these images for each time point using MATLAB. Diffusion coefficients were found from modeling these concentration profiles. For elution experiments ultra-violet visible spectroscopy was used for effluent analysis to find the concentration of dye released during the experiment. The diffusion coefficients determined in this work are comparable to diffusion coefficients in the same dye/hydrogel system found by other researchers.