Alginate hydrogels containing cell-interactive beads for bone formation

Alginate hydrogels containing cell-instructive cues are the subject of intense interest for their use as cell carriers in bone tissue engineering. Peptides and proteins are chemically grafted onto these hydrophilic materials to facilitate adhesion and direct phenotype of entrapped cells. However, the presentation of a single or small number of peptides does not represent the complexity of the native extracellular matrix (ECM) of bony tissues. Mesenchymal stem cells (MSCs) secrete ECM that can be harvested and deposited on various substrata to promote osteogenic differentiation. In this study, we hypothesized that the presentation of engineered cell-secreted ECM on microbeads suspended in alginate hydrogels would promote cell adhesion and enhance osteogenic differentiation of undifferentiated MSCs without chemical incorporation of cell-adhesive peptides. Human MSCs entrapped in alginate hydrogels loaded with ECM-coated beads showed increased interaction with beads, when compared with cells suspended in hydrogels containing uncoated blank (BLK) beads. MSCs entrapped in ECM gels exhibited increased alkaline phosphatase (ALP) activity and expression of osteogenic genes in vitro compared with hydrogels modified with arginine-glycine-aspartic acid (RGD)-containing peptides. Transplantation of MSCs into an ectopic site resulted in significant increases in blood vessel density for ECM hydrogels when compared with the BLK or RGD gels. Furthermore, we observed comparable levels of bone formation at 6 wk with ECM and RGD hydrogels. These findings demonstrate that engineered ECM can be deployed in a minimally invasive manner to direct the formation of bony tissue. This strategy may provide an alternative to the engraftment of proteins or peptides onto the polymer backbone of hydrogels for directing cellular behavior.