The use of zerovalent iron (Fe 0 )-coated plates, which act both as a source of catalyst and as a reducing agent during surface-initiated atom transfer radical polymerization (SI-ATRP), enables the controlled growth of a wide range of polymer brushes under ambient conditions, and utilizing either organic or aqueous reaction media. Thanks to its cytocompatibility, Fe 0 SI-ATRP can be applied within cell cultures, providing a tool that can broadly and dynamically modify the substrate's affinity towards cells, without influencing their viability. Upon systematically assessing the application of Fe-based catalytic systems in the controlled grafting of polymers, Fe 0 SI-ATRP emerges as an extremely versatile technique that could be applied to tune the physicochemical properties of cell's microenvironments on biomaterials or within tissue engineering constructs.Experimental details and further characterization are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.