Hydrogel-based drug delivery systems: comparison of drug diffusivity and release kinetics

Hydrogels are extensively studied as matrices for the controlled release of macromolecules. To evaluate the mobility of embedded molecules, these drug delivery systems are usually characterized by release studies. However, these experiments are time-consuming and their reliability is often poor. In this study, gels were prepared by step-growth polymerization of poly(ethylene glycol) (PEG) and loaded with fluoresceine isothiocyanate (FITC) labeled dextrans. Mechanical testing and swelling studies allowed prediction of the expected FITC-dextran diffusivity. The translational diffusion coefficients (D) of the incorporated FITC-dextrans were measured by fluorescence recovery after photobleaching (FRAP) and pulsed field gradient NMR spectroscopy. Because the determined values of D agreed well with those obtained from release studies, mechanical testing, FRAP, and pulsed field gradient NMR spectroscopy are proposed as alternatives to release experiments. The applied methods complemented each other and represented the relative differences between the tested samples correctly. Measuring D can therefore be used to rapidly evaluate the potential of newly developed drug delivery systems.