A method of determining toxicity of a test agent in a tissue including A) incubating a test agent within a three-dimensional tissue engineered structure comprising a first mold or polymer scaffold, a semi-permeable membrane, and a second mold or polymer scaffold, wherein the semi-permeable membrane is disposed between the first and second molds or polymer scaffolds, and wherein the first mold or polymer scaffold has microchannels comprising vessels that bifurcate, and B) obtaining information from step A) to assess toxicity.