From Cryonics January 2013
By Aschwin de Wolf
Scientific and practical considerations strongly support cryopreservation rather than chemopreservation for the stabilization of critically ill patients. Technology for achieving solid state chemopreservation of brains larger than a mouse brain does not yet exist. Chemical fixation is irreversible without very advanced technologies. Chemical fixation permits no functional feedback or development pathway toward reversible suspended animation. By contrast, cryopreservation seeks to maintain viability of the brain as far downstream as our capabilities and resources permit — an approach that reflects our view of cryonics as an extension of contemporary medicine. Cryopreservation preserves more options in that a cryopreserved brain could be scanned in future, or later chemically fixed, but the process of chemical fixation cannot be reversed and replaced by just low temperature storage. The cost benefits of chemopreservation over cryopreservation are exaggerated, largely because the standby and treatment procedures for effective chemopreservation would be just as extensive as for cryopreservation, if not more so, even assuming that highly toxic chemicals could be worked with safely in the field. Chemopreservation is being inherently tied to mind uploading, an association that is likely to limit its acceptance as a form of experimental critical care medicine by apparently requiring acceptance of the idea of substrate independent minds.
The formation of the Brain Preservation Foundation and the recent publication of Sebastian Seung’s book Connectome  have given rise to a renewed interest in chemical preservation as a means of personal survival. Alcor welcomes these developments and has even attempted to donate to the Brain Preservation Technology Prize to stimulate validation of both cryopreservation and chemopreservation as preservation technologies . In fact, in 2008 Alcor received a grant to conduct a preliminary investigation into chemopreservation . In addition, Alcor staff member Mike Perry published an extensive article about lowcost alternatives for cryonics  and Aschwin de Wolf published the first technical review of chemopreservation as an alternative method of biostasis in Cryonics magazine .
A common denominator in our research and writings has been the recognition that chemical preservation may constitute a viable alternative to cryopreservation on a theoretical level but that scientific and practical considerations strongly support cryopreservation for the stabilization of critically ill patients. In this article we will further explore these issues and also respond to some of the recurrent arguments that have been made in favor of chemical brain preservation.
One seemingly paradoxical position that will be clarified in this article is that Alcor aims for better preservation technologies than can be offered through chemical preservation but is also more optimistic about the resuscitation of patients preserved under suboptimal conditions with older cryopreservation technologies.
What distinguishes the long-term objective of Alcor from chemopreservation proposals is that we are not satisfied with preservation of the ultrastructure of the brain alone. The aim of Alcor is to keep the patient viable by contemporary medical criteria as far into our procedures as possible . There are a number of reasons for this choice.
The most important of these reasons is that restoring function after reversal of our procedures is the most credible test of the efficacy of our procedures. We are reluctant to settle for preservation of ultrastructure alone because this goal can always trigger objections that we are failing to preserve crucial identity-encoding parts of the brain. This is not just a theoretical concern. Recent discussions about chemical preservation of the “connectome” (pattern of connections between brain cells) have made it quite evident that absent functional recovery of the brain, there is no shortage of arguments that seek to show that chemical preservation will fail to produce the desired outcome. Some of these arguments invoke rather unorthodox views about how memory is encoded in the brain (such as the necessity of locking neurotransmitters in place) . However, absent a test showing that memory is preserved after reversal of the preservation procedure, we will not be able to progress beyond a debate in which different perspectives compete without empirical resolution.
Another important reason why Alcor seeks to maintain viability of the brain as far downstream as our capabilities and resources permit is that we view cryonics as an extension of contemporary medicine and allowing unnecessary damage would contradict this perspective. Contemporary chemopreservation methods depend on extensive cross-linking of proteins and this cannot be reversed by contemporary medical technologies. In a sense, one could argue that chemopreservation has to “kill” the brain to preserve it. Although even in “ideal” cryonics cases we are not yet able to sustain viability throughout all parts our procedures, Alcor’s research efforts and resources are dedicated to attacking this limitation from all angles (rapid cooling during stabilization, development of low toxicity vitrification agents, intermediate temperature storage, etc.).
Yet another argument of seeking reversible preservation procedures is that we want to minimize the time the patient has to be retained in low temperature care. The shorter the period the patient has to be maintained in biostasis the less risk there is that social and financial challenges will force cryonics providers to discontinue care of their patients. Another benefit of minimizing injury prior to long term care is that earlier resuscitation may reduce the amount of alienation for the resuscitated patient.
Finally, a more general argument can be offered in favor of this approach. The conventional case for cryonics rests on the expectation that we (a) can cure the terminal disease the patient suffered from prior to cryopreservation; (b) will have available credible rejuvenation technologies to prevent the patient succumbing to another age-associated illness; and (c) will be able to repair the damage associated with the cryopreservation process itself. Since most of the scientific skepticism concerns the damage being done by biostasis methods themselves, eliminating this form of damage would further strengthen the case for human cryopreservation.
Reversible cryopreservation would constitute true suspended animation for humans. At Alcor we believe that a credible cryonics organization should aim for perfecting human suspended animation. If we can achieve reversible cryopreservation, the objection that our patients sustain too much damage in our procedures can be effectively countered and the remaining debate will be about the technical feasibility of rejuvenating the patient and restoring them to good health. As currently conceived chemopreservation is fundamentally incapable of securing viability of the brain and cannot be brought under the rubric of evidence-based medicine.
Our Friends in the Future
A common objection to cryonics has been that future generations may have little interest in resuscitating cryopreserved patients. At Alcor we do not want to rely solely on the goodwill of future generations and we have set a substantial amount of funding aside to deal with this issue ourselves. Still, the first thing we should recognize, as former Alcor President Michael Darwin has pointed out , that friendship should come from both sides. Preservation technologies that transfer many challenges and puzzles to people in the future may not make us many friends. If the term “friends in the future” has any meaning at all it should require minimizing the burden on future generations and even provide them an incentive for wanting to resuscitate us. This understanding informs Alcor’s decision to offer the best procedures possible and not to send off a compromised brain to an unknown future based on just a series of logical arguments.
Limits of connectome preservation
How do we know if our procedures are good enough? As discussed above, if we can demonstrate that a person (or relevant animal) can survive our procedures intact without loss of identity and memory, this will inspire confidence. But how can advocates of chemical preservation of the connectome know that what they are doing is good enough?
If one confines oneself to structural preservation of the connectome, it is always possible to object that “just” preserving the connectome is not enough. One could argue that we also would need to preserve detailed information about all different kinds of neurons, the molecular state of synapses (“synaptome”), ion channels, microtubules, neurotransmitters, extrasynaptic interactions and so forth. The most extreme position would be to argue that for meaningful brain preservation complete preservation of the brain (or a molecular brain scan) would be required. Now some of these objections can be countered by arguing that the biochemical basis for brain functioning and short-term memory does not need to be preserved to preserve the individual. But such arguments may not completely satisfy critics who believe there is more to identity preservation than the connectome. Without functional tests, biostasis proposals will remain a source of criticism for people who want more robust empirical corroboration for the efficacy of the proposed procedures.
The prevailing proposal is to subject an experimental animal brain to a series of procedures and then re-construct the brain through 3D imaging technologies. And here is where we think there is a formidable challenge for chemical preservation. Because functional tests are not possible in cross-linked brains, the only available reference for looking at the efficacy of chemopreservation is to compare the results of this procedure against images that have been obtained through chemical preservation as well! Granted, electron microscopy has taught us a lot about brain anatomy but we cannot say for sure whether the procedures employed to prepare specimens for electron microscopy (irreversibly) damage specific areas of the brain that are crucial for memory and identity. In neural cryobiology, on the other hand, it is possible to subject the cryopreserved brain tissue to both a viability test and (subsequently) to ultrastructural examination.
The response of people advocating chemopreservation as a means of personal survival is to supplement their arguments with a substantial amount of philosophy to make their point. But philosophical arguments are no substitute for empirical evidence and the only empirical evidence that will be persuasive to critical observers is to seek functional recovery. Absent that, cynics will continue to invoke the existence of some “platonic” fragile brain that no preservation technique can salvage.
The Brain Preservation Technology Prize
In 2010 the Brain Preservation Foundation established the Brain Preservation Technology Prize. The Prize seeks to validate chemical preservation and/or cryopreservation of the brain for personal identity preservation, and develop protocols to apply these technologies to large mammalian brains. Although the Prize is open to both chemical and cryobiological preservation methods, the endpoint for evaluating the quality of preservation involves advanced 3D electron microscopic imaging techniques. Obviously brain preservation technologies based on methods used to prepare tissue for electron microscopy (chemical fixation, staining and embedding) have a natural advantage when the evaluation method is electron microscopy. Cryopreservation methods are at a comparative disadvantage because they are designed to achieve different preservation objectives than preparation for electron microscopy. To succeed, Prize competitors using cryopreservation must successfully load cryoprotectant chemicals into a whole brain, cool to cryogenic temperatures, unload cryoprotectant chemicals, and then still perform the chemical preservation steps necessary to prepare tissue for electron microscopy. An advantage cryopreservation has is that Prize officials are permitting cryopreservation competitors to perform the chemical preservation steps on small tissue pieces after whole brain cryopreservation.
A specific concern for Brain Preservation Technology Prize competitors using cryopreservation is that cryopreserved brains are currently very dehydrated. Due to this dehydration, which typically persists even after cryoprotectant removal, it is not yet clear that cryopreserved brains can be effectively evaluated by the Prize organizers. To be specific, the criterion for success is preservation of the connectome, which requires two things: preservation of synapses and preservation of enough information to infer the pattern of connections between them. Neural cryobiology researchers believe that they can achieve good ultrastructural preservation of the brain but dehydration compactifies the neuropil, reduces space between structures, and makes the tissue so dark in the electron microscope that it is hard to actually observe the synapses. So if a quick scanning method doesn’t discern all synapses that are actually there, it will fail. There are techniques for doing electron microscopy at cryogenic temperatures in the vitrified state, but these depend on the tissue being sliced before vitrification. Making slices out of a whole vitrified brain while vitrified is a tough problem. It is easier to make thin slices out of a whole brain that’s been turned into solid plastic because the resin used is designed for being cut into thin slices for microscopy. So plastination has a natural advantage in this competition — in terms of processing for the tests rather than in actual results.
We have no doubt that the designers of the prize sought to design a neutral prize, but it is challenging to develop a prize that is truly neutral in term of evaluation. For example, if the prize used viability as a criterion, cryopreserved brains would be at a great advantage. In fact, the effects of using aldehydes and powerful oxidizers would render the chemopreserved brains dead by even the most charitable functional criteria. It is our belief that a prize that aims to corroborate the case for personal survival technologies should embrace both ultrastructure and viability.
What is plastination?
While the term chemopreservation has been used to describe the idea of chemical fixation as an alternative to cryopreservation, many proponents of the idea of chemical brain preservation use the more narrow term ‘plastination.’ Plastination is usually described as a technique first developed by Gunther von Hagens in 1977 to preserve body parts for anatomical or educational purposes. This is a rather “harsh” technique, which requires dehydration by alcohol and replacement of the lipids by a polymer. To our knowledge, there are no credible peer reviewed ultrastructural studies of brains plastinated in such a manner.
What most writers have in mind when they use the word “plastination” as a means of biostasis is a procedure in which chemical fixation with an aldehyde is followed by treatment with osmium tetroxide and resin embedding. While previous proposals for chemical brain preservation only discuss the use of fixatives such as formaldehyde to crosslink and immobilize proteins, the addition of osmium tetroxide and resin (plastic) embedding provide greater longterm stability. Osmium tetroxide stabilizes unsaturated lipids in the cell membrane, and replacement of cell water with a solid polymer resin stops diffusion of molecules in a manner similar to cryopreservation. While theoretically sufficient, the empirical sufficiency of these measures for preserving identity-critical information for centuries is not currently known, and may require complex accelerated aging studies. Another reason for including the two additional steps of osmium tetroxide fixation and resin embedding is to prepare the brain for slicing and scanning for resuscitation in the future.
Whatever “plastination” method is chosen, the consequence will be that the brain is rendered non-viable by contemporary medical criteria. In fact, chemical fixation and osmium tetroxide are routinely used with the explicit aim of killing life and irreversibly stopping biochemical activity.
The cost of chemical brain preservation
One of the proposed advantages of chemical preservation of the brain is to be its comparatively low cost compared to human cryopreservation. It can be admitted that an isolated chemically preserved brain reduces long term space requirements compared to a typical Alcor neuropatient. The space saving, however, is modest since the annual storage cost for a neuropatient is only a few hundred dollars per year. A chemically fixed brain can be removed from the skull and may not require a dedicated (low temperature) storage environment. In reality, however, we do not expect most people to be comfortable with the idea of long-term brain preservation without any kind of institutional structure. (Would you want your chemopreserved brain to be sitting unsecured on the shelf of a person who has no contractual obligation or means to protect you and eventually revive you?) So the real cost difference may more reflect reductions in storage space and long-term maintenance than elimination of organizations that protect these brains and initiate resuscitation.
Whether the cost of resuscitation of chemically preserved brains will exceed that of cryopreserved patients will depend on the method of resuscitation. If biological or mechanical cell repair machines are used to restore function, the costs of chemopreservation may actually be higher because the informational and logistical requirements of restoring a brain to its pre-cross-linked state may be even more daunting than that of a “straight frozen” brain. An alternative for brain repair is to slice the brain, scan it, and upload it to a computer. Such a revival scenario may be substantially less expensive than repairing a cryopreserved brain but it cannot be taken for granted that such revival attempts will constitute meaningful resuscitation of the individual. In addition, this method, destructive mind uploading, is possible for cryopreserved brains as well.
The expected cost of preparing the brain for long term chemical preservation cannot be separated from the issue of acceptance of the procedure. If chemical brain preservation is not accepted by mainstream medicine it will not be available as an elective hospital-based procedure. Like cryonics, chemopreservation should be practiced as a form of emergency medicine. As such, it will require the same kinds of “standby” and “stabilization” procedures to prevent post-arrest deterioration of the brain. In cryonics, professional teams capable of performing stabilization procedures rapidly and effectively cost tens of thousands of dollars to bring to the bedside. The cost to deploy teams to restore circulation and perfuse solutions after clinical death would be no different for chemical preservation, and they would be even more critical for preservation to be successful.
An additional complication for chemical brain preservation is the toxicity of the necessary chemicals to the team and surrounding personnel. In simple terms, chemicals powerful enough to bind and inactivate biological molecules must by their nature be very reactive and toxic to living people. (This is in contradistinction to chemicals used for cryopreservation, which are practically innocuous by comparison.) The initial steps of chemical preservation require perfusion with aldehyde fixative chemicals such as formaldehyde, glutaraldehyde, or acrolein. Even fumes of these chemicals at low concentration are powerful irritants to eyes and lungs. They could not be used in an ordinary hospital room or hospice setting. (Being similar to embalming fluid, aldehyde fixatives could possibly be used in a mortuary.) After initial stabilization with aldehyde fixatives, a chemopreservation patient would have to be transported to a dedicated facility for treatment with even more toxic chemicals such as osmium tetroxide and plastic resin monomers. Osmium tetroxide is a volatile and extremely powerful oxidizer, and epoxy resin monomers are mutagenic carcinogens. In addition to being very dangerous, these chemicals are also expensive and would bring the costs of chemical brain preservation closer to the costs associated with vitrification solutions in cryonics.
If chemical brain preservation were to be accepted as a routine hospital-based procedure, costs would be reduced because of economies of scale and the reduced need to deploy standbys and stabilize patients in the field. However, it is doubtful that one form of preservation would be accepted and the other would be rejected. As a consequence, if acceptance would reduce costs, this would happen to both chemical preservation and low temperature preservation of the brain.
The no-reflow phenomenon
One of our biggest concerns about offering chemopreservation as a practical means of stabilizing critically ill patients is that if the procedure is practiced in non-ideal circumstances, the effects could include progressive decomposition of brain tissue despite chemical fixation. In terms of tolerance of warm and cold ischemic delays, chemopreservation is a lot more demanding. Since the 1960s it has been recognized by many biomedical researchers that even short periods of warm circulatory arrest can produce perfusion impairment in the brain . Any credible chemopreservation proposal requires access to the vessels of the patient. This means that in the case of delays due to warm and cold ischemia, there will be incomplete distribution of the fixatives. In fact, the recognition of this challenge is a standard part of textbooks on preparing specimens for electron microscopy.
Ischemia-induced “no-reflow” is a problem for both chemopreservation and cryopreservation, but even more so for chemopreservation. In the case of cryopreservation, incomplete distribution and equilibration of a cryoprotectant can produce ice formation, but long term care at cryogenic temperatures will stabilize the tissue with no further degradation. In the case of chemopreservation, the absence of low temperatures could permit ongoing degradation of poorly fixed and embedded tissue.
While it is possible that resin embedding (solidification) would halt autolysis, the ischemia- induced perfusion impairment that prevents complete distribution of aldehydes would also prevent adequate perfusion of the organic solvents and monomers for resin embedding. (Whether resin embedding could be achieved by perfusion even under ideal conditions is still an open question.)
Chemopreservation as emergency medicine?
Even if chemical brain preservation would be accepted as a routine hospital procedure there will still be many cases in which this procedure will have to be applied on short notice outside of the hospital or after long delays. For example, people can experience sudden cardiac arrest in the street, die in their sleep, or be involved in a traumatic accident in a remote area. In these circumstances chemical preservation will have to be conducted after a (prolonged) period of circulatory arrest. As discussed above, delayed chemical fixation will most likely fail to completely fix all areas of the brain as a consequence of perfusion impairment. This major inadequacy of chemopreservation leaves cryopreservation as an irreplaceable biostasis technology for cases of unexpected cardiac arrest. Cold is the only biostasis-inducing agent that can rapidly penetrate tissue regardless of its state of injury.
Practicing chemical fixation as emergency medicine raises another complex logistical issue. One part of the procedures is to perfuse the brain with the dangerous chemical osmium tetroxide (or any other oxidizing agent that can stabilize lipids). We wonder whether it is possible to establish a protocol that would permit a safe environment to conduct this procedure in the field. While it is true that osmium tetroxide does not necessarily need to be administered in the field, and aldehyde fixation would buy enough time to transport to dedicated facilities, even the practice of emergency aldehyde fixation would create much greater health hazards than the practice of remote blood substitution in cryonics, or even field cryopreservation. As far as we are aware, even the most “toxic” solution used in cryonics (the vitrification agent) is less dangerous than the least toxic solution (formaldehyde and/or glutaraldehyde) envisioned for chemopreservation.
Solid state chemopreservation is not applicable to human brains at present
The clinical application of chemopreservation is still hypothetical because technology for fixing and plastic embedding whole human brains doesn’t exist yet. At the time of writing, the chemopreservation technology competing for the Brain Preservation Technology Prize uses external diffusion to introduce osmium tetroxide and resin into a mouse brain by soaking it in various solutions for more than 250 hours. Since diffusion time varies as the square of distance, a similar soaking protocol applied to a human brain would require six years. As a practical matter, such a protocol would almost certainly fail because of resin polymerization during the long soaking time. Rather than diffusion, perfusion protocols that circulate all chemicals through the vascular system appear essential for solid state chemopreservation of large mammalian brains. Such protocols have yet to be developed, and face considerable obstacles of viscosity and blood-brain barrier penetration.
The “Prehoda fallacy”
The impossibility of conducting functional assays in chemically preserved brains is one of our concerns and reflects our aim to develop technologies that are reversible with contemporary technologies. On the other hand, a dominant perspective in the advocacy of chemical brain preservation is that perfect preservation is a necessary condition for medical acceptance of cryonics or chemopreservation.
One of the most prevalent objections to cryonics among the educated public and scientists is that absent proof of reversible cryopreservation cryonics should not be offered to the public. One of the most outspoken representatives of this kind of reasoning was the author Robert Prehoda. In 1969 Prehoda published the book Suspended Animation: The Research Possibility That May Allow Man to Conquer the Limiting Chains of Time . In this visionary book, he covered a variety of means to extend the maximum human life span including, but not limited to, chemical anabiosis, human hibernation, suspended animation, and controlling the aging process. Despite his participation in the 1967 cryopreservation of James Bedford (who is still a patient at Alcor) he was opposed to offering cryopreservation before the technology was perfected. He reiterated this stance in a 1969 interview in which he said: “I am still opposed, as I was before Dr. Bedford’s death, to freezing people at the present time because this money should be spent on research. Any human freezing is premature and without scientific basis until a mammal can be revived from the frozen state” .
Prehoda’s objection to offering cryopreservation continues to be made in either a strong or a weak version. In its strongest form it is argued that it is not “scientific” to offer cryonics services as long as reversible cryopreservation of a whole mammalian organism has not been demonstrated. Such claims are often presented in the form that there is no scientific “proof ” that cryonics will work. A weaker version of the argument also exists in which it is claimed that without evidence of reversible cryopreservation the general public and scientists have good reason to reject it.
These views rest on a fundamental misunderstanding of the rationale of cryonics and do not recognize the distinction between the objective of science and the objective of medicine. The objective of science is to generate knowledge about the physical world by testing hypotheses. The objective of medicine is to treat people (or non-human animals) by using the best knowledge from science and practical experience available. Medicine is inherently “messy” because it cannot avoid acting on incomplete information in conjunction with a (subjective) assessment of risk. For example, if a person is in overall good health most people would not support subjecting this person to an experimental treatment with potential severe adverse effects for a minor illness. On the other hand, if a person is born with a highly lethal single gene mutation, more risky experimental treatments could be justified. What distinguishes cryonics from conventional medicine is not decision making under uncertainty but the temporal separation of stabilization and treatment.
Evidence-based medicine is inherently conservative and the idea of cryonics extends this conservatism to end-oflife decisions. The fact that society has exhausted all means of curing critically ill patients does not mean that future medicine will not be able to treat this patient. The objective of cryonics is to ensure that a patient is stabilized to reach that future with as little additional damage as possible. The fact that current cryopreservation methods are not reversible and cause (additional) damage cannot be used as an argument against this reasoning because the argument that treatments may be available for presently terminal illnesses can also be extended to cover the damage associated with the cryopreservation process. The “Prehoda fallacy” consists of not recognizing the point that a procedure that aims to take advantage of future developments in science by definition cannot be experimentally demonstrated by contemporary science. Exercising our best judgment in this matter is neither “scientific” nor “unscientific” although one can question whether the reasoning involved is coherent or not.
This of course does not mean that science should not play a role in making such decisions. Certainly it should. The cryonics proposal can be submitted to the test of whether it contradicts known laws of physics or exceeds realistic computational abilities required for cell repair. More specifically, reasonable expectations about future medicine can be strengthened by improvements in cryopreservation or cell repair technologies. And, of course, we can generate experimental evidence to choose between alternative biostasis methods such as the use of cold temperatures or chemical fixation. But ultimately, cryonics cannot be “proven” in the conventional sense of the word because if all components of the proposal (curing the terminal disease, reversing cryopreservation, and rejuvenation) could be demonstrated now, cryonics would be redundant. We can make efforts to minimize this element of uncertainty but eliminating it completely may never be possible as there may always be diseases and traumatic insults that contemporary technologies cannot treat. In this sense, the acceptance of uncertainty in conjunction with reasonable expectations about future technological development is an intrinsic element of cryonics.
The reason why we highlight this fallacy is that we have observed a milder form among advocates of chemical brain preservation. Although lip service is being paid to the rationale of cryonics, the argument seems to be that technical feasibility is an important reason for scientists to reject cryonics. Such a perspective seems quite reasonable but it fails a basic reality check. Most scientists who comment on cryonics in public have made little effort to educate themselves about the procedure and often make uninformed statements about cryobiology and the ultrastructural effects of cerebral ischemia that even contradict the established knowledge in those fields of research. And when cryonics organizations introduce new procedures (such as vitrification) that aim to eliminate a scientific objection, the criticism simply moves to another part of the procedure. The residual element of uncertainty that characterizes cryonics can always be exploited to claim that the procedure lacks scientific proof. Eliminating ice formation or fracturing, or demonstrating preservation of the connectome will not satisfy critics who use these kinds of arguments to shield more subjective psychological and social objections to cryonics. Successful preservation of the connectome may win over some doubters but it is not likely that it will move chemopreservation and/or cryonics into the mainstream until these psychological and social objections can be effectively countered.
What constitutes preservation?
Insistence on demonstrated preservation of the connectome as a condition for offering a bio-preservation method to the terminally ill could backfire. Actually, we don’t know if the connectome is either necessary or sufficient. As long as it, and whatever else that is essential, if anything, can be inferred from the preserved brain (and the rest of the body) restoring the original healthy state should be possible . This argument does not just apply to biostasis procedures that introduce known and predictable forms of damage but also applies to any patient who suffers some degree of ischemia prior to preservation. In fact, a perfect preservation of an ischemic brain might be classified as not being successful if it does not conform to the preservation of the connectome of a control brain. But whether this dooms such preservations to failure depends on whether the original state can be inferred from what was preserved, which itself is a function of the degree and duration of ischemia.
We believe that a research program aimed at demonstrating under which conditions the original structure can be inferred from the injured brain could be at least as persuasive as a program to demonstrate successful preservation of the connectome of noncompromised brains. Demonstrating the scope and limits of such reconstructions will also corroborate the premise of cryonics that using a preservation technique that itself adds damage is not necessarily a dead end provided there is systematic knowledge of how this preservation method alters the structural and functional properties of the brain.
Neural archeology and suspended animation
There is a wide gap between the aim of moving toward reversible human cryopreservation and the state of the brain of many cryopreservation patients. It might be tempting to conclude that a commitment to developing true human suspended animation implies a pessimistic outlook on the prospects of resuscitating patients that were preserved under suboptimal conditions with older technologies. In our view, such a perspective ignores the important point that one can aim for the best preservation technologies possible but at the same time hold that advanced “neural archeology” might be able to infer the original state from a brain with severe damage . What makes Alcor’s perspective unique is that we share both the belief that our procedures should be subjected to the most rigorous testing possible with the goal of perfecting preservation technologies but that we also recognize that our understanding of the limits of “inferability” will remain incomplete as long as our scanning, computational, and repair technologies evolve. There is no question that providing the best technologies that we can offers the best prospects of resuscitating our patients in the future but this argument cannot be used to categorically claim which patients are beyond repair and which are not . In our opinion, the perspective that informs many advocates of chemopreservation sets the bar too low and too high.
Are there advantages to chemical brain preservation?
One of the envisioned advantages of chemopreservation over cryopreservation is that plastinated brains do not require continued maintenance or even organizational continuity. This may be true but there are a number of qualifications that need to be discussed. As discussed above, this advantage only applies to brains that were preserved under ideal conditions. In non-ideal conditions, the brain will most likely experience regional or global autolysis over time. Strictly speaking, we do not even know anything about the fate of wellpreserved brains after very long periods of time and it might still be the case that even these brains benefit from storage at low (non-freezing) temperatures.
While it is technically feasible that such brains do not need a permanent storage facility like cryonics patients, it is hard to imagine chemopreservation being offered without the existence of an organization that is committed to the fate of such patients and maintains sufficient funding for future resuscitation attempts.
It cannot be denied that cryopreservation patients require ongoing replenishment of liquid nitrogen to keep them at low temperatures but this does not mean that cryonics patients would be adversely affected by short interruptions of liquid nitrogen deliveries. Calculations at Alcor predict that it will take at least three months of non-delivery of liquid nitrogen before the brains of patients would start dangerous warming. If such a scenario is due to supplier unavailability (such as a refusal to deliver to Alcor) Alcor could purchase and transport liquid nitrogen from elsewhere, start producing liquid nitrogen itself, or (temporarily) switch to other means of maintaining cryogenic temperatures.
In case cryonics patients cannot be maintained in dewars at all, emergency chemopreservation will be an option. This can be achieved by either perfusing the formerly cryopreserved patients or by slicing the brains and using passive diffusion to chemically fix them.
The most negative scenario would be a prohibition of cryonics and forced burial of patients. While not impossible, it is doubtful that in such an environment chemically fixed brains will be permitted to exist. Both cryopreservation and chemopreservation would have to continue as underground operations.
Chemopreservation and mind uploading
One of the lessons that we have learned in cryonics is that it is not helpful to make the idea more controversial than necessary. Cryonics (or chemical brain preservation) is already controversial enough on its own and we do not see the benefit of associating it with ideas such as immortalism, transhumanism, mind uploading, or any political ideologies. This is not just a strategic or public relations consideration but reflects our view of offering cryonics as a form of experimental critical care medicine.
In many ways the promotion of chemical brain preservation has been characterized by many of the PR mistakes that characterized the beginning of cryonics. In particular, we are concerned that, instead of remaining agnostic about resuscitation methods including mind uploading, chemical brain preservation is now closely associated with this one method.
There is something decidedly ad hoc about this association. One could just as well imagine a campaign for chemical brain preservation that identified mechanical or biological cell repair technologies as the means of resuscitation. What is unfortunate about the almost exclusive focus on mind uploading is that it not only requires potential supporters to take seriously the idea of chemical preservation of the brain but also commit to the idea of substrate independent minds.
It is no surprise that the defense of mind uploading depends on mainly philosophical arguments because at this point these are the only possible arguments to defend it. While the arguments in favor of mind uploading deserve critical scrutiny, we think that ultimately the feasibility of this approach is an empirical matter and cannot be settled by thought experiments or analogies  For example, both proponents and skeptics of mind uploading accuse each other of not being consistent “materialists.”
In fact, by subjecting the preservation method to empirical scrutiny but using philosophical arguments to corroborate the resuscitation method, we think that the Brain Preservation Foundation conveys a mixed perspective about validation of personal survival technologies. The kinds of cell repair technologies that are envisioned for the resuscitation of cryonics patients are highly advanced, but do not require a shift in thinking about human biology and identity. Mind uploading, on the other hand, is neither conceptually necessary for resuscitation of chemically preserved brains nor does it constitute an appealing idea to gain more support for chemopreservation.
Toward a new definition of death
In this article we have critically investigated the claims in favor of chemopreservation, and its (envisioned) advantages over cryopreservation. While, everything carefully considered, we believe that cryopreservation is more suitable for robust scientific validation and presents a more versatile, practical, and safe option than chemical brain preservation, we strongly support any technologies that draw attention to the inadequacies of contemporary practices surrounding death. Throughout history the medical definition of death has been subject to continuous revision as medical and resuscitation technologies have advanced  There can be no doubt that many people who are written off by today’s medicine will simply be considered critically ill in the future  Both cryonics and chemical brain preservation constitute a means to stabilize patients to reach that future. In the coming years we may see additional proposals to stabilize critically ill patients such as “room temperature vitrification” or biostasis induced by advanced nanotechnology. Clearly, the idea that death should be defined relative to today’s medical capabilities is no longer adequate and needs to be replaced by a concept of death that recognizes that clinical death can only be considered irreversible if identity-critical information has been erased beyond recognition .
This article greatly benefited from the encouragement and contributions of Max More and Brian Wowk. I also want to thank the Alcor R&D Committee for carefully reviewing earlier versions of this article.
The original paper version of this article included the results of an experimental model to understand the effects of ischemia on perfusion fixation of the rat brain. Subsequent comments and questions prompted me to omit them from the current (online) version because these results raise complex methodological issues about modelling perfusion fixation of the ischemic human brain in a rat model and I believe that those cannot be done justice without changing the nature of the article. I wish to convey that these results are part of an ongoing research project and using them as an illustration of the potential consequences of conducting perfusion fixation in the ischemic human brain would be premature. Excluding these preliminary results does not affect the general arguments made in this article and restores its intended aim as an opinion piece. Omitting them should not be interpreted as an endorsement of the idea of perfusion fixation of ischemic human brains as a life extension strategy.
Appendix: Resin Embedding of Mouse Brains
In 2012 a group from the Max-Planck Institute for Medical Research in Germany published a method for resin embedding an entire intact mouse brain suitable for electron microscopy [S. Mikula, J. Binding, W. Denk, Staining and embedding the whole mouse brain for electron microscopy, Nature Methods, published online 21 October 2012; doi:10.1038/nmeth.2213]. This group is also one of the announced competitors for the Brain Preservation Technology Prize. Their recently-published method may qualify for the Stage 1 (small brain) portion of the Prize.
They achieved a technical tour-de-force because a mouse brain is much larger than the tiny milligram size previously required for tissue pieces to be prepared for electron microscopy. However their basic approach is still the same as traditional methods for preparing small tissue pieces. First, proteins are chemically fixed by perfusing an aldehyde solution through the whole animal. Second, the brain is removed and then soaked in solutions containing osmium tetroxide to fix and stain membranes. Third, the brain is soaked in organic solvents to replace water. Finally the brain is soaked in solutions of resin monomer molecules that eventually polymerize, turning the tissue completely solid.
Except for the first protein fixation step, this approach relies completely on passive diffusion (soaking) rather than perfusion. According to the paper, to resin embed a mouse brain, the time required for the soaking steps is:
5 x 8 hours (buffer rinses)
3 x 48 hours (wbPATCO osmium stain)
4 x 12 hours (acetone dehydration)
3 x 12 hours (resin monomer infiltration)
268 hours total
Calculation of the time that theoretically would be required to perform these steps on a human brain is sobering. A human brain is 1500 grams / 0.5 grams = 3000 times more massive than a mouse brain. Taking the cube root of 3000, that translates to 14 times greater diameter than a mouse brain. Using the rule that diffusion time scales quadratically with distance, the extrapolated preparation time for a human brain would be
14 x 14 x 268 hours = 52,528 hours
which is six years. In practice, the process would likely stop early in the resin soaking phase as monomers polymerized in the outer layers of the brain, increasing viscosity and preventing deeper infiltration.
Development of fundamentally new technology – technology using perfusion for all phases – is required before resin embedding can be seriously considered for biostasis of large mammalian brains.