Visualization of in situ hydrogels by MRI in vivo

It is of great interest to monitor implants in real-time using non-invasive detecting techniques in interventional therapies or tissue regeneration. In this study, MRI-visible hydrogels that are pH sensitive, injectable and self-healing were prepared based on chitosan and PEG. The macromolecular chelator of Gd(III), Ch-DTPA, was synthesized by functionalization of chitosan with diethylene triamine pentaacetic acid (DTPA). The PEG end-capped with aryl aldehyde, PEG-DA, was prepared as a crosslinker. Based on the Schiff's base reaction, the hydrogels can be obtained quickly just by mixing the aqueous solutions of Ch-DTPA and PEG-DA at ambient temperature. The gelating procedure is pH sensitive, and the hydrogels exhibit self-healing in an environment with a pH value of 4.5-5.3. The morphology of the lyophilized gel was observed by SEM. In vitro release behavior of the hydrogel was tested also using rhodamine B as a model drug. The MRI contrast enhancement effect of Gd(III)-binding hydrogels was investigated both in vitro and in vivo on rats. The results revealed that the hydrogels showed strong signals on T1-weighted MR images, and the hydrogels could always be detected before complete biodegradation. Combining the histology observation, the developed in situ hydrogels are of potential in the application of long-term reporting of implants by MRI.