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.