Fig. 1: The hierarchy of physical and mechanical properties of the extracellular matrix.

Simple and complex confinement is the basis of 3D environments. Confinement can be induced by 2 or more surfaces and by the pores formed in a fibrillar matrix. Spatial organization, or the physical properties, of a fibrillar matrix adds more complexity for migrating cells. Alignment of fibers, thickness, and pore size are all important factors that affect migration in a fibrillar environment. The most elusive properties of a 3D environment are the mechanical properties. Mechanical properties are present at all levels of 3D environments.