Effects of pH and thermally sensitive hybrid gels on osteogenic differentiation of mesenchymal stem cells

Osteoblastic differentiation of mesenchymal stem cells from bone marrow is an essential step for bone formation. The osteogenesis is normally induced by chemical mediators. Recent laboratory studies have revealed that mechanical properties of an extracellular matrix, typically hydrogels with different modules, also affect the fate of stem cells. The question is how to adjust their mechanical properties inside the body in biomedical applications. In this study, we designed/used a novel extracellular matrix, namely, a hybrid gel made of billions of injectable small thermally and pH-sensitive poly(N-isopropylacrylamide-co-acrylic acid) microgels whose swelling at the body pH and temperature physically jammed them and mesenchymal stem cells together, which enabled us to in situ apply an adjustable mechanical stress on those embedded stem cells. By treating the cell layer with the microgels, we found that an earlier incorporation of the microgels significantly increases the alkaline phosphatase activity, while a later addition of the microgels after the primary calcium deposition enhances the extracellular matrix mineralization in the mesenchymal stem cells cultures accompanied by up-regulation of osteogenic marker genes expression, presumably due to the calcium fixation by the carboxyl groups inside the microgels and the physical contact between the microgels and mesenchymal stem cells layers. These microgels provide an extracellular matrix microenvironment to affect the fate and biological behavior of mesenchymal stem cells, facilitating their potential applications in regenerative therapies.