If you can build new living tissue to be implanted in patients, then why not also give it the capacity to perform additional useful tasks? This is a technology platform with some potential: "combining gene therapy with tissue engineering could avoid the need for frequent injections of recombinant drugs. Patients who rely on recombinant, protein-based drugs must often endure frequent injections, often several times a week, or intravenous therapy. Researchers [have demonstrated] the possibility that blood vessels, made from genetically engineered cells, could secrete the drug on demand directly into the bloodstream. ... Such drugs are currently made in bioreactors by engineered cells, and are very expensive to make in large amounts. ... The paradigm shift here is, 'why don't we instruct your own cells to be the factory?' ... [Researchers] provide proof-of-concept, reversing anemia in mice with engineered vessels secreting erythropoietin (EPO). ... The researchers created the drug-secreting vessels by isolating endothelial colony-forming cells from human blood and inserting a gene instructing the cells to produce EPO. They then added mesenchymal stem cells, suspended the cells in a gel, and injected this mixture into the mice, just under the skin. The cells spontaneously formed networks of blood vessels, lined with the engineered endothelial cells. Within a week, the vessels hooked up with the animals' own vessels, releasing EPO into the bloodstream. Tests showed that the drug circulated throughout the body and reversed anemia in the mice."