Hydrogels are water-rich polymer networks made from natural or synthetic polymers, which are widely used in biomaterials, medicine and antifouling. We explore PEG-based hydrogels grafted through a UV-initiated free-radical polymerization, primarily for antifouling applications. Their preparation can be tuned to produce a given polymer mass per surface area, but growth rates vary between substrate types, and is 2-4 times faster on gold than on silicon; two substrate types which are important for biosensors. Previous data point at variations in grafting densities as a reason for this, and to further investigate this, we compare the structures of polymers grafted from surfaces with controlled surface densities of initiators, using dilution of initiators in mixed monolayers of alkylthiols on gold, and organosilanes on silicon.