Cryonics is not a finished recipe of cool, store and wait. It is an active research field, and the distance between a preservation performed in 1970 and one performed today is large and measurable. Every improvement in the fidelity of a brain's ultrastructural preservation directly raises the odds that future medical technology will enable revival with intact memories, personality, and identity. This article covers how the field advances, what it has demonstrated, what it has not, and who is doing the work.
A field, not a single technique
The decisive shift was from freezing to vitrification, the transition of a cryoprotectant-loaded tissue into a vitreous state without ice, proposed by cryobiologist Greg Fahy in 1984 and adopted for human cases around 2000.
Vitrification did not arrive finished, and the reason it did not is the subject of the next section. The cryoprotectant concentrations required to prevent ice are high enough to be toxic, and every vitrification solution since has been an attempt to widen the gap between the concentration that vitrifies and the concentration that kills.
The four problems the field is working on
1. Cryoprotectant toxicity. This is the central problem, and the standard reference states it without hedging. Human Cryopreservation Procedures (de Wolf and Platt, fact-checked by Brian Wowk) puts it as follows: the actual challenge in cryobiology is not to design solutions that resist ice formation at slow cooling rates, but to design vitrification solutions with no toxicity.
Progress here has been a sequence of specific design results rather than a single breakthrough. Toxicity of some cryoprotectants falls when they are combined with others; Fahy's discovery that DMSO neutralises the toxicity of formamide is the best known case. A compositional variable called qv* predicts the general toxicity of a vitrification solution, which allowed formulation to become rational rather than empirical. Applying it produced a counterintuitive result: weaker glass formers favour higher viability, so substituting a higher concentration of ethylene glycol for propylene glycol raised cellular viability. Synthetic ice blockers attack the problem from the other side. In 2000 Brian Wowk and colleagues showed that polymers could inhibit heterogeneous ice nucleation, and the resulting polyvinyl alcohol copolymer and polyglycerol, sold as X-1000 and Z-1000, lower the minimum cryoprotectant concentration needed to vitrify at all. Less cryoprotectant required means less toxicity delivered.
The current state of this problem is documented in the same reference. In cases conducted under good conditions, viability of the brain is believed to be lost during the early to mid stages of cryoprotective perfusion, through cryoprotectant toxicity and cryoprotectant-induced dehydration. Preserving ultrastructure and preserving viability are not currently the same achievement, and only the first is presently within reach.
2. Getting the cryoprotectant into a whole brain. German et al. are explicit about why a whole brain is harder than a slice. Cryoprotectant must arrive through the vasculature, and permeability and hydraulic conductivity are mismatched at the blood-brain barrier because of the high concentration of aquaporin-4 in the perivascular endfeet of astrocytes. Their whole-brain protocol succeeded in one attempt of three. Aquaporins are named as a target worth attacking directly.
3. Getting the heat back in. Rewarming is the harder half of the cycle. Cooling has to outrun ice once; on rewarming the tissue crosses the same range more slowly, so ice that never formed on the way down can form then. Conductive rewarming works inward from the surface and therefore has a size ceiling, which the German group place near the largest tissue their method can handle. Volumetric methods are required above that. In 2023 a University of Minnesota group vitrified rat kidneys, stored them for up to 100 days and rewarmed them at a mean 63.7°C per minute by exciting silica-coated iron oxide nanoparticles in a radiofrequency field; the kidneys were transplanted and sustained life. The same year Ramon Risco's group in Seville reached 217.8°C per minute using high-intensity focused ultrasound, which deposits heat through a volume and requires no nanoparticles in the tissue. A 2025 paper extended vitrification and nanowarming to litre-scale volumes. Neither method has been run on anything the size of a human brain.
4. Fracturing below the glass transition. Below the glass transition the solid continues to contract and can no longer relax, and relieves the stress by fracturing. A 2025 study showed that cracking tracks thermal contraction and the temperature gradients produced by fast cooling. The mitigations are a slow cooldown to cryogenic temperatures and Intermediate Temperature Storage.
What has been demonstrated, and in what
The claims below are ordered by what each experiment actually established, and in which system.
A whole organ, vitrified and transplanted. Fahy's group at 21st Century Medicine reported in 2009 that a rabbit kidney loaded with the vitrification solution M22, cooled to -135°C without ice, rewarmed and transplanted, functioned as the animal's only kidney. This remains the clearest demonstration that a complex organ can survive the complete cycle.
Ultrastructure preserved across a whole brain, by fixation. Aldehyde-stabilized cryopreservation, published in Cryobiology in 2015, took the Brain Preservation Foundation's large mammal prize in 2018 by preserving synaptic connectivity across an entire pig brain, verified by three-dimensional electron microscopy after rewarming. Glutaraldehyde fixes ultrastructure first, which rules out biological revival of that tissue; what it buys and what it costs are covered in preservation without freezing.
Ultrastructure preserved across a whole brain, by vitrification alone. A 2026 preprint from Fahy, Ralf Spindler, Brian Wowk and colleagues reports ultrastructural and histological cryopreservation of mammalian brains by vitrification, without prior aldehyde fixation. It has not yet completed peer review.
Function recovered in murine hippocampus. In March 2026 Alexander German and colleagues at Erlangen-Nuremberg published functional recovery of the adult murine hippocampus after cryopreservation by vitrification in PNAS. Slices, and whole brain in situ, were held below the glass transition for between ten minutes and seven days, rewarmed, and recorded from. Resting membrane potentials, synaptic transmission, metabolic responsiveness and long-term potentiation all returned. LTP measured 138.06 ± 6.90% after vitrification against 157.68 ± 7.14% in controls, P = 0.072, from seven and eight slices respectively across seven mice. That difference is not statistically significant, but at this sample size the comparison is not powered to exclude a real reduction either, and the authors describe the result as short-term recovery. The finding is that the cellular machinery of learning and memory remained operational through complete cessation of molecular mobility, in mouse hippocampus, over days. It is not a result about human brains, whole organs at human scale, or long storage times. PNAS published an independent commentary on it in June 2026.
Living human brain tissue banked and recovered. In 2024 a group at Fudan University published a protocol they call MEDY, based on methylcellulose, ethylene glycol, DMSO and Y27632. After thawing, human brain tissue and neural organoids retained structure, cell diversity and neural function.
Measuring preservation quality
Two different things get called quality measurement, and conflating them is how a field ends up reassuring itself. One measures the preservation actually achieved. The other measures the insult the patient went through on the way there. Both belong in a case record, and only the first is evidence about the result.
Outcome measures: what the preservation achieved
The standard reference sets three, each with a method attached. Elimination of ice formation is assessed by CT scanning, which distinguishes vitrified from frozen regions because ice and the surrounding vitrified solution differ in density. Preservation of ultrastructure is assessed by electron microscopy on microliter brain samples. Viability is assessed on those same samples by assays such as the potassium to sodium ratio. The reference states the standard plainly: each case report should contain CT scans and electron micrographs documenting the degree of vitrification and ultrastructural preservation achieved.
The same reference is candid about the third measure: high viability readings from patient brain samples are not currently achievable, for the toxicity reasons above. A good case today is one that demonstrates vitrification and ultrastructural preservation, not one that demonstrates viability.
Exposure measures: what the patient went through
S-MIX, a standardized measure of ischemic exposure, converts a case's temperature and circulation history into equivalent minutes of ischemia at normal body temperature, weighting warm ischemia more heavily than cold. Initial cooling rate normalised to patient weight is the other. These make cases comparable and let protocol changes be judged against something. They describe the conditions a preservation was performed under. They do not measure how it turned out.
What Tomorrow.bio measures
CT scanning is performed on every one of our cases, so the degree of vitrification is documented rather than assumed. We have CT scanned more patients, and published the results for more of them, than any other cryopreservation organisation; the individual case reports are public and can be read in full. Electron microscopy requires explicit consent to extract brain tissue samples, so it is performed only where a member granted that permission; to date that is one case.
The distinction matters when reading any organisation's quality claims: CT tells you whether ice formed, and only electron microscopy tells you what happened to the synapses.
Who is doing the work
21st Century Medicine produced the rabbit kidney result, aldehyde-stabilized cryopreservation, the qv* framework, M22 and the synthetic ice blockers. More of the cryoprotectant chemistry in current use originated there than anywhere else.
Alexander German's group at Erlangen-Nuremberg did the functional recovery work above. German is a clinician-scientist in molecular neurology and a co-founder of Hiber, which works on structural brain preservation.
Advanced Neural Biosciences, founded in 2008 by Aschwin de Wolf and Chana Phaedra, is one of the few institutes devoted specifically to brain cryobiology. It targets vitrification solutions with low toxicity, low viscosity and good penetration across the blood-brain barrier, and studies the no-reflow problem, in which tissue will not re-perfuse after a delay. De Wolf is a co-author of the standard procedures reference cited throughout this article.
The organ banking field supplies tools that transfer. The Minnesota nanowarming work came out of that community rather than from cryonics.
Connectomics bounds what any of this could mean. In 2025 the MICrONS consortium published a cubic millimetre of mouse visual cortex containing roughly 200,000 cells and 523 million synapses, the product of an international consortium working for years. That is a small volume of a small brain, and it is also more synapses than anyone could map a decade ago.
The Brain Preservation Foundation performs no preservation and much of the adjudication, setting prizes with published criteria and independent electron-microscope verification.
Tomorrow.bio and the European Biostasis Foundation work at the applied end: standby and stabilization protocols, perfusion hardware, the logistics of reaching a patient in time, and long-term storage at the EBF facility in Rafz, Switzerland, where the first human-size Intermediate Temperature Storage dewar is going into testing. A second laboratory is under construction in Berlin. Most of the results on this page came from other groups.
A 2024 review of structural brain preservation sets out the case and its uncertainties in a peer-reviewed venue. Its authors include Aschwin de Wolf.
What would count as evidence against
Three results would undermine the case, and each is the kind of thing the current research programme could produce.
If cryoprotectant delivery through an intact blood-brain barrier proved impossible at whole-brain scale, the slice results would remain interesting and stop being relevant. If synaptic ultrastructure were shown to be routinely destroyed rather than damaged by current protocols, the core claim would fail. If long-term memory turned out to depend substantially on ongoing electrical activity rather than on preserved structure, preservation would be preserving the wrong thing.
None has occurred, and evidence has moved against each. They remain the questions worth tracking.
Why the trajectory matters
None of this changes the position that revival is not currently possible. What it changes is the rate at which preservation fidelity is improving, and fidelity is the variable that determines what a future technology would have to work with. Four communities are each reducing a different part of the problem: cryobiology, organ banking, connectomics and clinical medicine. Results transfer between them. The candidate routes this feeds into are set out in how we might achieve revival.
TL;DR: Research is improving cryoprotectants, organ preservation, brain-structure measurement and rewarming. These advances can improve preservation quality, but they have not made human revival possible.
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