Properties of small molecular drug loading and diffusion in a fluorinated PEG hydrogel studied by ^1H molecular diffusion NMR and ^(19)F spin diffusion NMR

R_f-PEG (fluoroalkyl double-ended poly(ethylene glycol)) hydrogel is potentially useful as a drug delivery depot due to its advanced properties of sol–gel two-phase coexistence and low surface erosion. In this study, ^1H molecular diffusion nuclear magnetic resonance (NMR) and ^(19)F spin diffusion NMR were used to probe the drug loading and diffusion properties of the R_f-PEG hydrogel for small anticancer drugs, 5-fluorouracil (FU) and its hydrophobic analog, 1,3-dimethyl-5-fluorouracil (DMFU). It was found that FU has a larger apparent diffusion coefficient than that of DMFU, and the diffusion of the latter was more hindered. The result of ^(19)F spin diffusion NMR for the corresponding freeze-dried samples indicates that a larger portion of DMFU resided in the R_f core/IPDU intermediate-layer region (where IPDU refers to isophorone diurethane, as a linker to interconnect the R_f group and the PEG chain) than that of FU while the opposite is true in the PEG–water phase. To understand the experimental data, a diffusion model was proposed to include: (1) hindered diffusion of the drug molecules in the R_f core/IPDU-intermediate-layer region; (2) relatively free diffusion of the drug molecules in the PEG-water phase (or region); and (3) diffusive exchange of the probe molecules between the above two regions. This study also shows that molecular diffusion NMR combined with spin diffusion NMR is useful in studying the drug loading and diffusion properties in hydrogels for the purpose of drug delivery applications

NMR self-diffusion studies on PDMS oil-in-water emulsion

In this paper, standard spin-echo pulsed field gradient (SE-PFG) and stimulated spin-echo pulsed field gradient (STE-PFG) H-1 NMR methods have been used to study the translational diffusion of polymeric-oil-in-water (O/W) emulsions, made with poly(dimethylsiloxane) PDMS fluids and two nonionic surfactants polyoxethylene (4) lauryl ether (Brij-30) and polyoxethylene (23) lauryl ether (Brij-35p). A detailed analysis of the free (Gaussian) diffusion of the PDMS fluid, its restricted diffusion inside the emulsion droplets, and the Brownian diffusion of the droplets themselves are presented. A graphical representation, which relates the square root of the mean-squared displacement of the fluid and emulsion droplets with the experimental diffusion times and the radius of the droplets, helps distinguish the different diffusion regimes. The Brownian diffusion of the emulsion particles becomes the dominant diffusion process with dilution of the system and increase in the molecular weight (viscosity) of the PDMS. These different processes which contribute to diffusion in the system need to be considered if a correct size distribution of the emulsion droplets is to be determined. For a concentrated emulsion system, the condition for restricted diffusion is reached by increasing the diffusion time. Under this condition, the Murday and Cotts model for diffusion inside a spherical droplet can be used to determine the size distribution of the emulsion. In very dilute systems by increasing the diffusion time, the Brownian diffusion becomes the dominant process so that the Stokes-Einstein model can be applied to obtain the size distribution