The team worked with stem cells from umbilical cord blood and gave them a slight shock, opening a gateway for a plasmid to insert seven genes into the cells. The plasmids caused the cells to turn into iPS cells. Then, Burridge fed them his specialized, simplified broth, designed to make them into heart cells. The researchers lowered oxygen levels to simulate the hypoxic environment these cells would experience when they grow inside an embryo. Nine days later, the nonviral iPS cells became functioning cardiac cells, according to Hopkins.