To survive the 90-year-long journey between Earth and the far-off planet Homestead II, humans on the ship in the 2016 sci-fi bomb Passengers were cryogenically frozen, suspending them in the throes of youth. The assumption is that thawing will bring their bodies and minds back to their normal, living states, and then life will resume where it left off. It’s wishful thinking: While scientists have figured out how to thaw and unthaw individual cells, whether it’s possible to reanimate life has remained a mystery.

But a new discovery puts them well on their way to finding out.

In a recent article in the journal ACS Nano, researchers reported that they’d successfully frozen zebrafish embryos, thawed them, and brought them back to life. Reanimation efforts on the tiny tropical fish, prized among scientists because their translucent embryos are easy to study, have been ongoing for 60 years, but none have been successful because of issues during the thawing process. This time around, the American team tweaked their defrosting strategy in order to speed it up.

When scientists preserve an embryo using cryogenic freezing, they drain some of its regular fluids, replacing them with an antifreeze-like substance to prevent the formation of damaging ice crystals, which can puncture a cell from the inside out. For the most part, scientists have figured out this process of “vitrification” — making fluids ice-free and thus glass-like — using a cryoprotectant solution of various sugar and alcohol molecules, like glycerol and propylene glycol (the major ingredient in car antifreeze). Previous studies have shown that dropping cryoprotected zebrafish embryos into a liquid nitrogen tank that cools them at 90,000 degrees Celsius per minute to a final temperature of −196 degrees Celsius results in perfectly frozen embryos. The problem is what happens as they are thawing.

“[The] large size of the yolk still impedes rapid cooling and warming, thereby yielding lethal ice crystal formation during cryopreservation,” the researchers write. When the defrosting process is too gradual, sometimes an event called devitrification occurs, in which the liquid “[reverts] from glass to ice.” For these scientists, thawing frozen embryos involved shining a 1064-nm laser pulse on them, which raised their temperature to 1.4 × 107 degrees Celsius per minute, but even this was too slow to create the perfect thaw.

zebrafish embryo
Once thawed, 10 percent of the sample zebrafish embryos survived for 24 hours.

But they finally managed to achieve it by adding a substance to the cryoprotectant solution that would heat up quickly and move the thawing process along faster — namely, tiny bits of metal. Adding gold nanorods, which conducted the laser’s heat efficiently, sped up the warming process so that there was no chance for damaging devitrification to take place.

In this way, zebrafish embryos that had been flash-frozen for a few minutes were rapidly unfrozen, and about 10 percent of those embryos stayed alive and developed for the next 24 hours.

These aren’t survival odds that any freezer-happy human would bank on, but they’re a start — and proof that a developing embryo’s mechanisms can continue after being briefly frozen, at least in a zebrafish. Future research will probably focus on figuring out how to extend the amount of time those embryos can stay frozen before they’re thawed, as well as pinpointing what causes them to die so soon after they’re unfrozen.

cryogenic freezing passengers
In theory, living cells can be unfrozen, but there's no guarantee that what's left will constitute life. 

It’s too early to say how this research could be applied to long-term human hibernation, but the scientists behind the study are hoping it will lead to a way to freeze the embryos of endangered water-bound species so they can repopulate future lifeless seas. In a way, their objectives aren’t too different from those laid out in Passengers, in which the spacefaring ship was laden with libraries of frozen human embryos waiting for a new habitat to populate.

As cryogenics science hurtles forward, it’s likely that the film’s promise that bodies can be viably frozen and thawed will increasingly seem less like a fantasy and more of a possibility. Whether those defrosted bodies will constitute conscious life, however, will be up to future humans to decide.

Photos via Khosla et al./ACS Nano, Passengers