Abstract: Compositions for tissue engineering and drug delivery have been developed based on solutions of two or more polymers which form semi-interpenetrating or interpenetrating polymer networks upon exposure to active species following injection at a site in a patient in need thereof. The polymers crosslink to themselves but not to each other; semi-interpenetrating networks are formed when only one of the polymers crosslink. The resulting viscous solutions retain the biologically active molecules or cells at the site of injection until release or tissue formation, respectfully, occurs. As a result of studies conducted with polymer-cell suspensions forming interpenetrating polymer networks, it has been determined that polymer solutions can be formulated wherein the active species is provided by exposure of the polymer solution to an exogenous souce of active species, typically electromagnetic radiation, preferably light.