Investigating structure-dependent diffusion in hydrogels using spatially resolved NMR spectroscopy.

Hypothesis: Incorporation of the drug-loaded surfactant micelles into polymer hydrogels is a common method used to achieve controlled drug delivery. The characterization of the diffusion processes in drug delivery systems is critical in order to tune the drug loading and release.

Experiments: We present a simple and efficient NMR protocol to investigate the transport of the surfactant molecules in hydrogels on micro- and macroscale under non-equilibrium conditions. Our experimental protocol is based on a combination of H 1D NMR chemical shift imaging and slice-selective diffusion experiments, which enables determination of the mutual and self-diffusion coefficients of the surfactant in the non-equilibrium hydrogel-based system within the same short time frame.

Findings: Our results show that the self-diffusion coefficient of the positively charged surfactant in the hydrogel (D) decreases with the increasing surfactant concentration until it reaches a plateau value of 6.6±0.5×10ms. The surfactant self-diffusion in the solution (D) remains constant over the experiment with an average value of 6.7±0.3×10ms. The surfactant mutual diffusion coefficient obtained from 1D chemical shift imaging in this hydrogel system (D) is 7.7±0.5×10ms. Correlation of the localized D to the 1D chemical shift images gives insight into the structure-dependent diffusional behavior of surfactant molecules in the hydrogel. This NMR protocol will be of great value in studies of concentration dependent structures on the interfaces between two immiscible liquids.