A Microfluidic Method to Evaluate Small Molecule Uptake and Elution In Hydrogels

Small molecule elution from a hydrogel is characterized using a microfluidic technique that allows for both image and effluent analyses. For elution experiments, the microfluidic device has channels defined by dye-loaded hydrogel.  Photolithography is used to cure a solution of poly(ethylene glycol) diacrylate (PEG-DA), photoinitiator, and dye inside a microfluidic device with a channel through the center of the gel. As an aqueous solution is pumped through the channel, dye diffuses out of the hydrogel and into the water. Channel sizes within the devices range from 300-1000 μm to simulate vessels within the body. The properties of the PEG hydrogel were characterized by the extent of PEG-DA crosslinking, the swelling ratio, and the mesh size of the gel. The structure of the hydrogel was related to the UV exposure dosage and the initial water and dye content in the PEG-DA solution.
Three analysis techniques for measuring diffusion coefficients were used to quantify the diffusion of a solute in a hydrogel, including optical microscopy, characterization of device effluent, and NMR analyses. The optical microscopy technique analyzes images of the dye diffusion captured by a stereomicroscope to generate dye concentration v. position profiles. The data was fit to a diffusion model to determine diffusion coefficients and the dye release profile. Analysis of the device effluent was performed using ultraviolet-visible (UV/Vis) spectroscopy and high performance liquid chromatography (HPLC) to determine a short-time diffusion coefficient. Nuclear magnetic resonance (NMR) was used to determine a free diffusion coefficient of molecules in hydrogel without the effect of a concentration gradient.

Diffusion coefficients for methylene blue dye in PEG hydrogel calculated using the three analysis methods all agree well. It was determined that utilizing a combination of the three techniques offers greater insight into molecular diffusion in hydrogels than employing each technique individually. The optical and effluent methods were determined to be very effective for visualizing the diffusion and screening solutes to confirm that the solute is not just able to move within the hydrogel, but actually elute as well.

Separately, to study uptake, the neat hydrogel is contacted by dye-laden fluid. Implications for using hydrogels for controlled drug delivery are presented.