Strategies for controlling egress of therapeutic cells from hydrogel microcapsules.

Endothelial progenitor cells (EPCs) and human mesenchymal stem cells (hMSCs) have shown great regenerative potential to repair damaged tissue; however, their injection in vivo results in low retention and poor cell survival. Early clinical research has focused on cell-encapsulation, to improve viability and integration of delivered cells. However, this strategy has been limited by the inability to reproduce large volumes of standardized microcapsules and the lack of information on cell-specific egress and timed release from hydrogel microcapsules. Here, we address both of these limitations. First, we use a droplet microfluidic platform to generate monodisperse, agarose microcapsules and second, we encapsulate and characterize egress of therapeutically relevant cells (human umbilical vein endothelial cells, EPCs, and hMSCs). With increased temporal resolution, we demonstrate distinct differences in egress between cell types. Importantly, therapeutic cells (hMSCs) egress quickly, in < 6 hours following encapsulation. Further, we examined potential escape mechanisms, and show that proliferation can be exploited by cells for microcapsule translocation. We also systematically characterized the egress of fibroblasts (as model cells) following alterations to the microcapsules. Specifically, we show that microcapsule size and hydrogel density impacts cell egress efficiency. Overall, our results demonstrate the need for characterization of cell-specific egress and tuning of the cocoon microenvironment, prior to delivery, for timely release and successful engraftment.