Continuous Surface Polymerization via Fe(II)‐Mediated Redox Reaction for Thick Hydrogel Coatings on Versatile Substrates

at the solid-liquid interface, which shows controllable growth kinetics. Hydrogel modification is fast, controllable, and performed in mild conditions at room temperature. The chemical components, thickness, and network structure of the hydrogel coating can be well controlled. The surface catalytically initiated radical polymerization method allows reinitiation of the polymerization when the grafted hydrogel coating is polished away, and allows continuous surface polymerization to form multi-interpenetrating network hydrogel coatings. Interestingly, it is fully compatible with 3D-printing technology, and by using 3D-printed composites as the catalytic template, it demonstrates an extreme advantage for engineering 3D hollow hydrogel objects with various complex structures. The versatility of this method makes it generate potential applications in the field of surface/interface and biological engineering.