 If a functional group is not available for attaching the cross-linker, photochemical immobilization may be used.In addition to modifying the surface, biomaterials can be modified in bulk, meaning that the cell signaling peptides and recognition sites are present not just on the surface but also throughout the bulk of the material.The strength of cell attachment, cell migration rate, and extent of cytoskeletal organization formation is determined by the receptor binding to the ligand bound to the material; thus, receptor-ligand affinity, the density of the ligand, and the spatial distribution of the ligand must be carefully considered when designing a biomimetic material.