This work examines the interfacial structure and interaction between water and polymer chains in the hydrogel-hydrogel composites with the goal of establishing foundations for further investigation of drug diffusion from one hydrogel to another in the soft contact lens. This is based on the ability of the hydrogel-hydrogel composites to release ophthalmic drugs in a sustained manner. The hydrogel-hydrogel composites were synthesized by immersing the glycerol-swollen particles of crosslinked N-vinyl-2-pyrrolidone (NVP) into the monomer of hydroxyethylmethacrylate (HEMA) containing initiator benzoylperoxicle (BPO) that polymerizes to form a matrix in the presence of the first networks. The hydrogel-hydrogel composites were characterized by LJV/Vis spectrophotometer, scanning electronic micrography (SEM), and differential scanning calorimetry (DSC). The results showed that the samples of hydrogel-hydrogel composites of the particles of crossfinked NVP and poly-HEMA were transparent and glassy and suitable for soft contact lens. Three types of the interfacial structure, no interpenetrating interface, partly interpenetrating interface, and fully interpenetrating interface, of the hydrogel-hydrogel composites existed, and the type of the interfacial structure was determined by the degree to which the monomer of HEMA penetrated into the first networks before formation of the matrix. Different from poly-HEMA hydrogels, the peaks near 0 degrees C on DSC curves of the hydrogel-hydrogel composites did not split while they were kept acute, and the amount of freezable-bound water was less. This shows that the water incorporated in the hydrogel-hydrogel composites does not strongly interact with polymer matrix, so the hydrogel-hydrogel composites cannot keep their shape during the phase transition of water. (C) 2008 Wiley Periodicals, Inc.