Laura Niklason is one of those pushing this boundary. During a visit, I followed one of her postdocs into a refrigerated closet in her Yale University laboratories. He reached out to a shelf and took down a jar. Unlike the amorphous piece of heart muscle Gordana Vunjak-Novakovic had showed me, there was no mistaking what was floating inside this container. It was a perfectly preserved pair of rat lungs, taken from an actual animal and "decellularized."

Like those who are engineering simpler tissue, Niklason relies on physical forces and a chemical soup to replicate the native environment of the organ and coax stem cells to mature into the kind of tissue she desires when manufacturing lungs. But she came to believe early in her efforts that science did not yet offer the technology to construct an artificial scaffolding detailed enough to emulate the shape and architecture of a real lung, a complex structure as labyrinthine as a Minotaur's maze.