Yaqiong Li: Ph.D. candidate, West China School of Pharmacy, Sichuan University. Her research focuses on the area of stimuli-responsive hydrogels and drug delivery. She has extensively studied the water mobility and water-polymer interaction of temperature-sensitive and pH-sensitive hydrogels, and has established new methods for studying sol-gel transition, swelling behavior and microstructure variation of smart hydrogels based on LF-NMR. These new methods realize in situ measurements of gel point and swelling ratio without any damage. She has authored or co-authored three scientific journal publications that have appeared in Carbohydrate Polymers, Journal of Polymer Research and Journal of Porous Materials.
Intelligent hydrogels have attracted much attention in drug delivery due to the biocompatibility, low toxicity and biodegradability. Hydrogels that show stimuli-responsive swelling behavior has been designed to achieve smart drug delivery, and pH-sensitive hydrogels have been extensively studied for this purpose. Most of related research mainly focuses on the effects of structure variation caused by swelling without considering the water transport. In the present study, water transport and network change during drug release of chitosan/glutaraldehyde (CS/GA) hydrogels loaded with 5-FU were characterized by low-field NMR (LF-NMR), and It was proved that water transport played a critical role in drug delivery of pH-sensitive hydrogels. We found that water trapped in the hydrogel and water outside correspond to different component of transvers relation time (T2) measured by LF-NMR. Therefore, swelling behavior and water transport was easily characterized based on the proportional relationship between peak area T2 of and water content, and the network structure was analyzed by the proportional relationship between T2 value and pore size. The result of pH-responsive experiment showed that CS/GA exhibited swelling and pore expanding at pH 1.2, but de-swelling and pore shrinking at pH 7.4. The drug release at pH 7.4 was more rapidly than that of pH 1.2, which was contradicted the conclusion in other studies that larger pore size would facilitated the drug release. Moreover, similar results were obtained when drug release occurred in phosphate buffer with different pH. After excluding the drug-polymer interaction, we thought that the water transport that occurred in the opposite direction as drug diffusion when CS/GA swelled hindered the drug release, and the hinderer was eliminated when CS/GA de-swelled. Furthermore, the effects of water transport was dominant for drug release of CS/GA. Based on these, we further studied drug release of CS/GA with different GA concentrate and drug release in solution with different NaCl concentration, and all of the results proved the dominant role of water transport. Given these results, the effects of water transport should be taken into account when studying drug release of hydrogel shows stimuli-responsive swelling behavior.