Strontium- and zinc-alginate hydrogels for bone tissue engineering

The development of bone replacement materials is an important healthcare objective due to the drawbacks of treating defects with bone autografts. In this work we propose a bone tissue engineering approach in which arginine-glycine-aspartic acid (RGD)-modified alginate hydrogels are crosslinked with bioactive strontium and zinc ions as well as calcium. Strontium was chosen for its ability to stimulate bone formation, and zinc is essential for alkaline phosphatase (ALP) activity. Calcium and strontium gels had similar stiffnesses but different stabilities over time. Strontium gels made with alginate with a high percentage of guluronic acid residues (high G) were slow to degrade, whereas those made with alginate rich in mannuronic acid (high M) degraded more quickly, and supported proliferation of Saos-2 osteoblast-like cells. After an initial burst, strontium release from alginate gels was steady and sustained, and the magnitude of release from high M gels was biologically relevant. Saos-2 cultured within alginate gels upregulated the osteoblast phenotypic marker genes RUNX2, collagen I (COL1A1) and bone sialoprotein (BSP), and ALP protein activity was highest in alginate gels cast with strontium ions. This strategy has the potential to be combined with other alginate-based systems for bone tissue engineering, or adapted to other tissue engineering applications.