Characterizing multilaminated hydrogels with spatially varying network structure and solute loading using confocal laser scanning microscopy

Multilaminated controlled release devices were formed through photopolymerization techniques to produce hydrogels with spatially varying solute loadings and network structures composed of poly(hydroxyl ethyl methacrylate) (PHEMA) and poly(ethylene glycol) (PEG). Using low molecular weight fluorescent dyes as model drugs, the distribution profiles were characterized non-invasively in pseudo-real-time with confocal laser scanning microscopy (CLSM) during release studies. For comparison, theoretical modeling based on Fickian diffusion theory was performed in conjunction with experimental work to identify any deviations from expected behavior and to guide in the development of future devices. In multilaminates composed of only PHEMA, the evolution of dye distribution during release and cumulative release profiles agreed well with theoretically predicted data, indicating continuity of diffusion and insignificant interfacial hindrance between layers. However, in devices composed of alternating layers of PHEMA and PEG, differences from predicted behavior were experimentally observed in both concentration profiles and release rates, suggesting interfacial obstruction of diffusion, possibly due to the formation of interpenetrating networks. Finally, the simultaneous release of two dyes at different rates from a PEG/PHEMA multilaminate was monitored to demonstrate the usefulness of CLSM in understanding the complex temporal changes in solute distributions in gel devices.