Here’s what you’ll learn when you read this story…

• In a groundbreaking study, researchers found that frozen tissue from mouse brains could return to normal function after facing extremely cold conditions.
• The findings suggest that brain tissue is significantly more resilient than previously believed.
• According to experts, there is still a ways to go before we engineer any sort of “human hibernation”—but this may be a step in the right direction.

If we want to travel to distant stars like Alpha Centauri, we’ll have to drastically rethink how humans operate in space. How will we store all the food and water we’ll need? And could some kind of human hibernation bring astronauts to deep space—without killing them—as seen in the recent box office hit Project Hail Mary?

This kind of “cryosleep” has mostly been relegated to the world of science fiction—that is, until now. A new study published in the Proceedings of the National Academy of Sciences showed it might just be possible (at least in slices of brain tissue from mice). This could be groundbreaking for the prospect of human interstellar travel. Still, there is far more research that scientists must conduct before we can translate these findings from rodents to people.

In their study, the scientists were essentially aiming to solve a hypothesis: Can adult mammal brain tissue recover after a specific process puts the tissue into a cryogenic, or extremely low temperature, state? That requires a process called vitrification, in which you use water to cool something—like brain tissue—so rapidly that its molecules stop moving. Vitrification is already used to harvest human eggs for fertility treatments. This means scientists already have a working foundation to build off of when testing other parts of the body in cryogenic states.

So, the team of German scientists ran an experiment on mice’s hippocampal tissue—the part of the brain associated with memory and learning. The researchers wanted to see if the brain’s cells, called neurons, could resume normal activity after it was cooled to minus 196 degrees Celsius. They primarily experimented on slices of mouse brain, but also tested the resilience of the whole organ. The takeaway?

“Adult mouse hippocampal tissue can indeed recover after rewarming,” says Alexander German, MD, lead author of the study and a clinician scientist at Germany’s University Hospital Erlangen.

Slices of this part of the mouse brain not only had “structural integrity” after freezing, but the neurons and synapses needed for learning and memory also appeared to be functioning. German says there were also recovery signals in the whole mouse brain, but “that part is still much less mature than the slice work.”

The findings from the study were a massive leap forward, as they show that the tolerance of brain tissue now extends beyond hypothermic (very cold) to cryogenic (extreme cold) states. But to approach humans, we still need to close a major knowledge gap.

So, how close are we to cryosleep exactly? Sandy Martin, PhD, a professor emerita at the University of Colorado Anschutz, says she is unable to comment on the process of vitrification in humans specifically. But she claims scientists are making general progress in learning what mechanisms might induce hibernation in humans.

Martin, who has studied hibernation in mammals for decades, emphasizes that we don’t have the tools yet for humans. But in hibernating animals, researchers are learning more about how they slow down the body metabolically with the hypothalamus, the part of the brain that manages the body’s temperature.

Hibernation can vary by species. Martin says some hibernators, such as ground squirrels, rewarm—or wake up from hibernation from internal heating, rather than with the help of warm weather. This rewarming takes place even with no changes in the environment, so something internally must be driving it.

The rewarming and cooling down again, Martin says, is “energetically, incredibly demanding. It is a time where [squirrels] have little oxygen delivery, because their blood is slow and sluggish. It’s cold, and yet their tissues are demanding a great deal of oxygen.” What’s more, the animals successfully warm up and cool down without harming their tissues. “Whatever damage occurs, they’re able to fix it,” she continues.

There’s interest as well in studying hibernation in fat-tailed dwarf lemurs—which, like humans, are primates. It appears the dwarf lemurs don’t need to rewarm during hibernation. But studying the lemurs is a challenge, because the species is vulnerable (a classification referring to their risk of extinction in the wild) and thus presents ethical issues for study, Martin explains.

German, the vitrification study lead author, also pointed to the effort required to push forward vitrification research in humans. There is proof-of-concept that the human cortex (the outermost layer of the brain) could regain normal function after freezing, German says. But applied to the whole body? “The gap is still enormous,” he says. He points specifically to issues like creating better cooling and rewarming methods for large volumes of tissues, and “validation in larger animal models” over longer observation periods.

There are small signs of progress. A spinoff company German recently co-founded, called Hiber, aims to bring post-death brain preservation to human neural tissue, as a sort of “biological archive” for future research. The startup is in the early stages of cryopreserving the human heart, another complex organ, with the hope that one day that process could be used for organ transplants.

So could astronauts one day be frozen in stasis using vitrification? German says not to necessarily count on it, but the “first principles” of suspending a human organism’s normal processes for a while could be useful. “Our work supports [human hibernation] in a very limited sense,” he explains. “If something becomes practical sooner, it may well be milder, torpor-like states rather than whole-body vitrification.”

Martin adds that if hibernation could be induced, no matter the process, “it would be good for a number of uses, including space travel.” But to get there, she says we will need research funding over a sustained period. She brought up HIV research as an analogy: it took about 20 years and “tons of money” (at least billions from the National Institutes of Health alone) to find ways to manage the condition.

While we don’t yet know how to induce hibernation in people, there’s hope that related research could one day help astronauts travel long distances. But this will also bring up broader questions about whether it’s worth it, or whether it would just be simpler to send machines in our stead—allowing us to focus medical research dollars elsewhere.