Hydrogels are water-rich polymer networks made from natural or synthetic polymers, which are widely used in biomaterials, medicine and antifouling. We have developed PEG-based hydrogels which are grafted through UV-initiated free-radical polymerization, suitable for use in biosensing applications since they have low non-specific protein adsorption and permit controlled modification by ligands. The processing 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. We will investigate how this affects the structure of the swollen hydrogels. The low polymer/water contrast in the wet polymer makes structural characterization difficult, but via H-D-substitution, neutron reflectometry can be used to determine the swelling and density.