Oxidized alginate beads for tunable release of osteogenically potent mesenchymal stromal cells

Bone defect repair can benefit from local delivery of mesenchymal stromal cells (MSCs). However, local harsh environmental conditions after injury may necessitate a cell therapy strategy that shields MSCs initially and releases them locally over time. This may be possible by using biomaterials that exhibit stimuli-responsive degradability, such as oxidized alginate hydrogels that undergo hydrolytic degradation. However, it remains unknown whether varying encapsulation periods compromise MSC osteogenic differentiation capacity after release. To address this, we cultured MSCs in 3D alginate beads with tunable degradability before characterizing the function of released cells. Alginates were oxidized to different degrees (2%, 3%, and 4%) to achieve distinct rates of degradation (days to weeks), then functionalized with RGD peptides to enable cell adhesion, and modified additionally with 6-aminofluorescin to enable fluorescence-based detection. Bead morphology, degradation kinetics, cell morphology, and cell release kinetics were monitored over time. Cells that were released from the beads were stimulated to differentiate into the osteogenic lineage. Our results indicate that MSCs released from all bead groups retained a strong ability to deposit mineralized matrix under osteogenic differentiation conditions. These findings provide the basis for designing and implementing biomaterial-based strategies for the in-situ temporal delivery of potent MSCs at bone defect sites.