Cellular mechanotransduction studies typically utilize purely elastic materials,but tissues are typically viscoelastic.We have been studying the impact of biomaterial stiffness and stress-relaxation on cell behavior,and developed materials that allow changes in mechanical properties to be decoupled from changes in gel architecture and adhesion ligand presentation.Cell spreading,proliferation and stem cell fate have all been found to be highly dependent on the rate of stress relaxation,suggesting this as a new variable for biomaterial design.