Raising the dead sounds like science fiction, but a team of medical scientists at Yale University have managed to achieve just that–at least on a cellular level. They successfully revived cells from pigs that were dead for an hour, as a Nature study published August 3 reports. While the study authors emphasize the technology is ages away from being used on people, the work could eventually help keep human tissues alive longer, increasing the supply of viable organs for transplants.

“These cells are functioning hours after they should not be,” said Nenad Sestan, a professor of neuroscience and comparative medicine at Yale and lead author of the study, in a news briefing per CNN. “And what this tells us is that the demise of cells can be halted. And their functionality restored in multiple vital organs. Even one hour after death.”

Sestan and his colleagues received 100 pigs from a local breeder. They placed the pigs on ventilators and shocked the animals’ hearts to induce cardiac arrest. An hour after confirmed death, the Yale scientists used two systems to pump blood back into the bodies—an ECMO machine removed carbon dioxide and added oxygenated blood to one group, while another device, called OrganEx, pumped artificial blood back into the other. That fluid entered the blood vessels of the dead pigs, where synthetic forms of hemoglobin and other molecules protected cells from degradation and stopped blood clots. 

After six hours, the researchers recorded signs of oxygen recirculating into the pigs’ tissues. A heart scan confirmed signs of electrical activity in the heart of pigs on the OrganEx machine, though those organs did not fully restart. Elsewhere, there were signs of business as usual, too: The livers of the deceased pigs resumed production of a protein called albumin. Additionally, the cells of other vital organs were responsive to glucose, suggesting the pigs’ metabolic processes were working again.

The experiment is not the first time scientists have tried to redefine life and death. In the early 20th century, there were attempts to reboot the brains of deceased monkeys. And in 2019, neuroscientists reanimated the brains of decapitated pigs four hours after they died in a slaughterhouse.

Studies such as these raise questions about what it means to be dead. “We presume death is a thing, it is a state of being,” Nita Farahany, a Duke law professor who studies ethical, legal and social implications of emerging technologies, told The New York Times. “Are there forms of death that are reversible? Or not?”

The findings also call into question who is considered legally dead, especially as medicine adapts to make cardiac death one day reversible. “People tend to focus on brain death, but there’s not much consensus on when cardiac death occurs,” Arthur Caplan, a bioethicist at New York University told Nature News. “This paper brings that home in an important way.”

Ethical challenges abound if technology such as this were applied to people. In 2016 India’s medical research council, citing ethical concerns, blocked a planned clinical trial that aimed to revive brain-dead people to a minimally conscious state using a mix of stem cells and other techniques.

While the current study showed no signs of brain activity in the pigs, the researchers observed the heads, necks, and torsos moved. If brain activity was restored, there is no telling how functional or conscious the pigs would be, making it one of a slew of ethical questions scientists will need to answer as they breach this murky area of science.