“It’s like baking a cake,” Atala said.

A few years ago, Atala figured out how to produce human tissue with a desktop inkjet printer, using cells as the printer ink. In a TED talk last year, he described printing heart valves and other tissues. This week at TED, he brought one of his patients on stage. When he was 10, Luke Massella was among the first people to receive a re-engineered organ — he was born with spina bifida and received a new bladder grown from his own tissue. Now he’s a healthy college student.