Cryonics (often mistakenly called "cryogenics") is the practice of cryopreserving humans or animals that can no longer be sustained by contemporary medicine until resuscitation may be possible in the future.

Cryonicists do not believe that legal death is real death (irreversible destruction of the anatomical basis of mind) any more than conventional medicine now accepts that cessation of heartbeat is "real death", when the heart can be restarted with a defibrillator.

The basis of cryonics is that at cryogenic temperatures there will be no alteration in biological tissue for thousands of years, which allows plenty of time for future medicine to achieve the required capabilities. The process is not currently reversible, and by law can only be performed on humans after legal death. Cryonics is viewed with skepticism by most scientists and doctors today. However, there is a high representation of scientists among cryonics supporters.

Scientific support for cryonics is based on projections of future technology, especially molecular nanotechnology and nanomedicine. Some scientists believe that future medicine will enable molecular-level repair and regeneration of damaged tissues and organs decades or centuries in the future. Disease and aging are also assumed to be reversible.

The process

Cryonics is not freezing of humans or pets. Ice is very damaging to body tissues, so all cryonics organizations use cryoprotectants to prevent ice formation, ie, anti-freeze substances that can reduce or prevent ice formation. Formerly cryonics organizations used glycerol as their cryoprotectant, which resulted in about 80% ice elimination (vitrification) and about 20% freezing. Cryonicists believed that damage that was being caused by disease, by aging and by the freezing would someday be repaired by nanotechnology. With vitrification the burden on future technology has been greatly reduced. With cells and tissues mainly preserved by cooling, future technology should be able to repair damage resulting if the cooling process is not too delayed. 

Since the 1990s vitrification solutions have been developed that have virtually eliminated ice formation (reduced to less than 0.2%). In fact, it was announced in July 2005 that one such solution had been used to vitify rabbit kidney at -135°C, and was later transplanted into a rabbit with full viability. 

Stoppage of heartbeat and breathing, the usual criteria for legal death, do not correspond to the death of cells and tissues of the body. The cells and tissues are still very much alive when death is pronounced. Even at room temperature cells and tissues take hours to die, and days to decompose. Although neurological damage is the usual consequence of cessation of heartbeat for more than 4-6 minutes, the irreversible neurodegenerative processes do not manifest for hours. 

Cryonics rescue procedures cannot begin until legal pronouncement of death has occurred, and pronouncement is usually based on cessation of heartbeat (only very rarely on brain activity measurements). When the heart stops beating and blood flow ceases, ischemic damage begins. Deprived of oxygen and nutrient, cells, tissues and organs begin to deteriorate. If the heart is restarted after too many minutes have passed, the reintroduced oxygen can cause even more damage due to oxidative stress, a phenomenon known as reperfusion injury. Cryonicists try to minimize ischemic and reperfusion injury by beginning cardio-pulmonary support (much like CPR) and cooling as soon as possible after pronouncement of death. Anti-clotting agents like heparin and antioxidants may be administered. Suspended Animation, Inc is a Florida company that specializes in research into, and implementation of, optimal procedures for minimizing ischemic injury in cryonics rescue. People, especially children, have survived up to an hour without heartbeat after having fallen into ice water.

The central premise of cryonics is that memory, personality, and identity are stored in the structure and chemistry of the brain. While this view is widely accepted in medicine, and brain activity is known to stop and later resume under certain conditions, it is not generally accepted that current methods preserve the brain well enough to permit revival in the future. Cryonics advocates point to studies showing that high concentrations of cryoprotectant circulated through the brain before cooling can mostly prevent freezing injury, preserving the fine cell structures of the brain in which memory and identity presumably reside. 

To its detractors, the justification for the actual practice of cryonics is unclear, given present limitations of preservation technology. Currently cells, tissues, blood vessels, and some small animal organs can be reversibly cryopreserved. Some frogs can survive for a few months in a partially frozen state a few degrees below freezing, but this is not true cryopreservation. Cryonics advocates counter that demonstrably reversible preservation is not necessary to achieve the present-day goal of cryonics, which is preservation of basic brain information that encodes memory and personal identity. Preservation of this information is said to be sufficient to prevent information theoretic death until future repairs are possible.

During the 1980s, the problems associated with crystallization were becoming better appreciated, but the publication of the book Engines of Creation by K. Eric Drexler in 1986 aroused a great deal of interest in the idea that nanotechnology would be able to repair freezing damage. Alcor and the American Cryonics Society shifted emphasis from whole body to "neuropreservation" ("neuro", head-only cryopreservation), on the assumption that the rest of the body could be regrown, perhaps by cloning of the person's DNA or reconstructed with nanotechnology.

The main goal now seems to be to preserve the information contained in the structure of the brain, on which memory and personal identity depends. Available scientific and medical evidence suggests that the mechanical structure of the brain is wholly responsible for personal identity and memories (for instance, spinal cord injury victims, organ transplant patients, and amputees appear to retain their personal identity and memories).

Damage caused by freezing and fracturing is thought to be potentially repairable in the future, using nanotechnology, which will enable the manipulation of matter at the molecular level. To critics, this appears a kind of futuristic deus ex machina, but while the engineering details remain speculative, the rapidity of scientific advances over the past century, and more recently in the field of nanotechnology itself, suggest to some that there may be no insurmountable problems. And the cryopreserved patient can wait a long time. With the advent of vitrification, the importance of nanotechnology to the cryonics movement may begin to decrease.

Some critics, and even some cryonicists, question this emphasis on the brain, arguing that during neuropreservation some information about the body's phenotype will be lost and the new body may feel "unwanted," and that in case of brain damage the body may serve as a crude backup, helping restore indirectly some of the memories. The argument is also made that the body is personal memorabilia of life-history, much like diaries and photo albums. Partly for this reason (as well as for better public relations), the Cryonics Institute preserves only whole bodies. Some proponents of neuropreservation agree with these concerns, but still feel that lower costs and better brain preservation justify preserving only the brain. About three-quarters of the patients stored at Alcor are "neuros".

Financial issues

The biggest drawback to current vitrification practice is a cost issue. Because the most cost-effective means of storing a cryopreserved person is in liquid nitrogen, fracturing of the brain occurs, a result of thermal stresses that develop when cooling from −130°C to −196°C (the temperature of liquid nitrogen). Fracture-free vitrification would require inexpensive storage at a temperature significantly below the glass transition temperature of about −125°C, but high enough to avoid fracturing (−130°C is about right). Alcor is currently developing such a storage system. Alcor believes, however, that even before such a storage system is developed, the current vitrification method is far superior to traditional glycerol-based freezing. The fractures are very clean breaks that occur even with traditional glycerol cryoprotection, and the loss of neurological structure is much less than that caused by ice formation, by orders of magnitude. 

Cryopreservation arrangements can be expensive, currently ranging from $28,000 at the Cryonics Institute to $150,000 at Alcor and the American Cryonics Society. Most cryonicists fund the costs by making cryonics organizations the beneficiaries of life insurance policies. The elderly, and others who may be uninsurable for health reasons, will often pay for the procedure through their estate. Others simply invest their money over a period of years, accepting the risk that they might die in the meantime. All in all, cryonics is actually quite affordable for the vast majority of those in the industrialized world who really want it, especially if they make arrangements while still young. 

Even assuming perfect cryopreservation techniques, many cryonicists would still regard eventual revival as a long shot. In addition to the many technical hurdles that remain, the likelihood of obtaining a good cryopreservation is not very high because of logistical problems. The likelihood of the continuity of cryonics organizations as businesses, and the threat of legislative interference in the practice, don't help the odds either. Most cryonicists, therefore, regard their cryopreservation arrangements as a kind of medical insurance—not certain to keep them alive, but better than no chance at all and still a rational gamble to take.

Philosophy and Ethics

Cryonics is based on a view of dying as a process that can be stopped in the minutes, and perhaps hours, following clinical death. If death is not an event that happens suddenly when the heart stops, this raises philosophical questions about what exactly death is. In 2005 an ethics debate in the medical journal, Critical Care, noted “…few if any patients pronounced dead by today’s physicians are in fact truly dead by any scientifically rigorous criteria.” Cryonics proponent Thomas Donaldson has argued that “death” based on cardiac arrest or resuscitation failure is a purely social construction used to justify terminating care of dying patients. In this view, legal death and its aftermath are a form of euthanasia. Philosopher Max More suggested a distinction between death associated with circumstances and intention versus death that is absolutely irreversible. Absolutely irreversible death has also been called information-theoretic death. Bioethicist James Hughes has written that increasing rights will accrue to cryonics patients as prospects for revival become clearer, noting that recovery of legally dead persons has precedent in the discovery of missing persons. 

Ethical and theological opinions of cryonics tend to pivot on the issue of whether cryonics is regarded as interment or medicine. If cryonics is interment, then religious beliefs about death and afterlife come into consideration. Resuscitation is generally deemed impossible because the soul is gone, and only God can resurrect the dead. Expensive interment is seen as a waste of resources. If cryonics is regarded as medicine, with legal death as a mere enabling mechanism, then cryonics is a long-term coma with uncertain prognosis. It is continuing to care for sick people when others have given up, and a legitimate use of resources to sustain human life. Cryonics advocates complain that theological dismissal of cryonics because it is interment is a circular argument because calling cryonics interment presumes that cryonics cannot work. They believe future technical advances will validate their view that cryonics patients are recoverable, and therefore never really dead. 

Alcor has published a vigorous Christian defense of cryonics, including excerpts of a sermon by Lutheran Reverend Kay Glaesner. Noted Christian apologist John Warwick Montgomery has defended cryonics. In 1969, a Roman Catholic priest consecrated the cryonics capsule of Ann DeBlasio, one of the first cryonics patients. In 2002, a Muslim cleric indicated in a media interview that cryonics would be compatible with Islam if it were medicine.