Stem cells are defined by their capacities for self-renewal and differentiation into one or more cell lineages. Without tight regulation or control of these properties; however, any derivative cell population will exhibit a range of heterogeneous phenotypes, yielding artifacts that may complicate the development of pharmaceuticals and cell therapies. Recent work demonstrates that biomaterials (i.e., matrices, scaffolds, culture substrates) can present key regulatory signals that combine with other environmental and genetic influences to create synthetic microenvironments that control stem cell fate. It can be argued that many of the promising therapeutic applications of stem cells will require instructive materials that exert active control over stem cell phenotype. Such materials may be designed for stem cell expansion and differentiation ex vivo, tissue regeneration via implantation with stem cells, or implantation alone to direct endogenous stem cell behavior. This work addresses the design of such synthetic materials, namely the design of material modulus and ligand density to control adult neural stem cell self-renewal and differentiation.

We exploit the physical and chemical properties of hydrogels (polymers containing a significant volume of water) to mimic the native extracellular matrix surrounding mammalian cells. Using a biomimetic hydrogel, we define a robust synthetic and fully mechanically and chemically defined platform to regulate stem cell number and differentiation for the culture of adult neural stem cells. The synthetic hydrogel material properties, such as ligand type, ligand surface density, and stiffness (i.e., complex modulus), are quantitatively controlled and characterized. Previous work indicated that hydrogels, modified with the cell-binding ligand CGGNGEPRGDTYRAY from bone sialoprotein [bsp-RGD(15)] and synthesized on top of a stiff polystyrene substrate, can be used to regulate stem cell self-renewal and differentiation in a dose-dependent manner. To mimic the mechanical properties of functional tissue, we have synthesized hydrogels with bsp-RGD(15) with elastic moduli (i.e., stiffness) of 10^-1 to 10² kPa. At low elastic moduli (~1 kPa), the elastic modulus significantly effects self-renewal and differentiation of adult hippocampal neural stem cells. Ultimately, one can define a phenotype or proliferation rate of a neural stem cell culture by tuning the material properties of the hydrogel culture material.