A method for modifying silk polymer by coupling a chemical moiety to a tyrosine residue of a silk polymer is described herein for the purpose of altering the physical properties of the silk protein. Thus, silk proteins with desired physical properties can be produced by the methods described herein. These methods are particularly useful when the introduction of cells to a mammal is desired, since modifications to the silk protein affect the physical properties and thus the adhesion, metabolic activity and cell morphology of the desired cells. The silk protein can be modified to produce, or modify, a structure that provides an optimal environment for the desired cells.