A novel hydrogel delivery systems useful for encapsulating and releasing pharmaceuticals or chemicals is disclosed where water soluble polymers containing crosslinker repeating units that associate or dissociate with complementary crosslinking repeating units or separate linkers to reversibly crosslink the hydrogel. In an exemplary embodiment, a DNA crosslinked hydrogel displays photoreversibility. An exemplary hydrogel delivery system comprises DNA polymer conjugates, wherein complementary DNA sequences are crosslinked with polymer chains and hybridization of the DNA sequences is controlled by photoresponsive moieties. Such hydrogels can be used to release drug molecules and/or other therapeutic reagents. The exemplary hydrogel employs photosensitive azobenzene moieties that are incorporated into the DNA crosslinker units. The azobenzene moieties respond to different wavelengths of light so that the state of azobenzene isomerization is induced by the proportion of visible and UV light irradiated. The isomer state of the azobenzene dictates whether the complementary DNA sequences hybridize to cross link the DNA polymer conjugates. Thus, irradiation of light (visible or UV) can transform the hydrogel network between a sol and any of multiple gel states to regulate the degree of crosslinking between complementary DNA sequences and, therefore, provide a profile of release of a hydrogel encapsulated pharmaceutical or other chemical.
