Figure 1: Examples of the tools of tissue engineering. Tools that manipulate the timing and appearance of chemical and mechanical signals offer opportunities to organize and direct the differentiation of developing tissue. Chemical signals can be immobilized, (a) in the form of covalently bonded growth factors that direct cell migration, or mobile in a hydrogel, (b) to create a chemotactic signal, through diffusion, for cells to respond to. Mechanical signals can be presented as a constant force, such as substrate stiffness, (c) to modulate cell spreading or as an inducible force, (d) such as shear flow, to organize cells in the direction parallel to flow.