Scientists revive brain activity in frozen mice for the first time

“Cryosleep” remains the preserve of science fiction, but researchers are getting closer to restoring brain function after freezing

By Although Thompson & Nature magazine

A familiar science fiction trope is that of the cryopreserved time traveler, whose body is frozen deep in suspended animation, then thawed and awakened in another decade or century with all mental and physical abilities intact.

Researchers attempting to cryogenically freeze and thaw brain tissue from humans and other animals – primarily young vertebrates – have already shown that neuronal tissue can survive freezing at the cellular level and, after thawing, function to some extent. But it was not possible to completely restore the processes necessary for proper brain function – neuronal activation, cellular metabolism and brain plasticity.

A German team has now demonstrated a method of cryopreserving and thawing mouse brains that leaves some of this functionality intact. The study, published March 3 in Proceedings of the National Academy of Sciencesdetails the authors’ use of a method called vitrification, which preserves tissue in a glass-like state, as well as a thawing process that preserves living tissue.

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“If brain function is an emergent property of its physical structure, how can we recover it from a complete shutdown? asks Alexander German, a neurologist at the University of Erlangen-Nuremberg in Germany and lead author of the study. The results, he says, suggest the possibility of one day protecting the brain in cases of illness or following serious injury, creating organ banks and even achieving whole-body cryopreservation in mammals.

Mrityunjay Kothari, who studies mechanical engineering at the University of New Hampshire in Durham, agrees that the study advances the state of the art in brain tissue cryopreservation. “This kind of progress is what gradually transforms science fiction into scientific possibility,” he says. However, he adds that applications such as long-term conservation of large organs or mammals remain well beyond the study’s capabilities.

Preserved for the future

The main reason the brain has trouble fully recovering from freezing is due to damage caused by the formation of ice crystals. These displace or puncture the delicate nanostructure of the tissue, disrupting key cellular processes. “Beyond ice, we need to take into account several considerations, including osmotic stress and toxicity from cryoprotectants,” says German.

German and his colleagues turned to an ice-free cryopreservation method called vitrification in an effort to preserve brain function. Vitrification cools liquids quickly enough to trap molecules in a disorganized, glass-like state before they have a chance to form ice crystals. “We wanted to see if function could restart after molecular mobility completely stops in the vitreous state,” says German.

They first tested their method on 350-micrometer-thick slices of mouse brains, including the hippocampus, a brain hub for memory and spatial navigation. The brain slices were pretreated in a solution containing cryopreservation chemicals before being rapidly cooled with liquid nitrogen to -196ºC. They were then stored in a freezer at −150 ºC in a glass-like state for ten minutes to seven days.

After thawing the brain slices in warm solutions, the team analyzed the tissue to see if it retained any functional activity. Microscopy showed that neuronal and synaptic membranes were intact, and mitochondrial activity tests revealed no metabolic damage. Electrical recordings from the neurons showed that, despite moderate deviations from control cells, the neurons’ responses to electrical stimuli were close to normal.

Hippocampal neural pathways still exhibited the synaptic strengthening or “long-term potentiation” that underlies learning and memory. However, as these slices degrade naturally, observations were limited to a few hours.

The team extended the method to the entire mouse brain, maintaining it in a vitreous state at –140ºC for up to eight days. However, the protocol required repeated adjustments to minimize brain shrinkage and cryoprotectant toxicity.

When the brains were thawed, brain slices were prepared and recordings from the hippocampus confirmed that the neural pathways – including hippocampal pathways involved in memory – had survived and could still undergo long-term potentiation. However, because the recordings were made from slices of brain tissue, the researchers could not measure whether the animals’ memories survived cryopreservation.

Still science fiction

German and his team are extending their method from mice to human brain tissue. “We already have preliminary data demonstrating the viability of human cortical tissue,” he says. The team is also investigating how the vitrification method could be used for cryopreservation of whole organs, including the heart.

However, Kothari points out that the success rate was low with the whole-brain protocol and the results might not translate directly to larger human organs, which presents other challenges. “Some of these challenges are related to heat transfer stresses and higher thermomechanical stresses that can cause cracking,” says Kothari.

German adds that “better vitrification solutions and cooling and warming technologies will be needed before these principles can be applied to large human organs.”

This article is reproduced with permission and has been published for the first time March 11, 2026.