In Vivo Imaging of Composite Hydrogel Scaffold Degradation Using CEST MRI and Two-Color NIR Imaging.

Hydrogel scaffolding of stem cells is a promising strategy to overcome initial cell loss and manipulate cell function post-transplantation. Matrix degradation is a requirement for downstream cell differentiation and functional tissue integration, which determines therapeutic outcome. Therefore, monitoring of hydrogel degradation is essential for scaffolded cell replacement therapies. We show here that chemical exchange saturation transfer magnetic resonance imaging (CEST MRI) can be used as a label-free imaging platform for monitoring the degradation of crosslinked hydrogels containing gelatin (Gel) and hyaluronic acid (HA), of which the stiffness can be fine-tuned by varying the ratio of the Gel:HA. By labeling Gel and HA with two different NIR dyes having distinct emission excitation frequencies, we show here that the HA signal remains stable for 42 days, while the Gel signal gradually decreases to <25% of its initial value at this time point. Both imaging modalities were in excellent agreement for both the time course and relative value of CEST MRI and NIR signals (R2=0.94). These findings support the further use of CEST MRI for monitoring biodegradation and optimizing of gelatin-containing hydrogels in a label-free manner.