Dynamics of water and xanthan chains in hydrogels studied by nmr relaxometry and their influence on drug release.

The dynamic properties of water and polymer molecules in xanthan hydrogels at different polymer mass fractions were investigated through the combination of conventional and fast-field cycling NMR relaxation to obtain the information about dynamics in different time scales. The results showed that water dynamics were faster in diluted than in concentrated hydrogels. However, the type of polymer-chain dynamics did not change for xanthan fractions from 0.1 to 0.5, although they slowed at higher xanthan fractions as the system approached transition to the glass state. The addition of the nonionic small drug molecules pentoxifylline did not change the dynamics in the hydrogels, but they were affected by the medium pH. The water and polymer-chain dynamics were faster in the hydrogels for the neutral than the acid medium. These differences resulted in slower swelling and thinner and more rigid hydrogel layer of the matrix tablet in the acid medium that was less susceptible to erosion. Consequently, pentoxifylline release from xanthan tablets in acid medium is dominated by drug diffusion. At neutral pH, the molecular mobility is greater, which resulted in rapid and extensive swelling of the hydrogel, leading to erosion-dominated drug release.