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Cryonics is the practice of preserving organisms, or at least their brains, for possible future revival by storing them at cryogenic temperatures where metabolism and decay almost completely stop. A person held in such a state (either frozen or vitrified) is said to be cryopreserved. Barring social disruptions of their cryopreservation, a perfectly vitrified person is expected to remain physically viable for a period of about 10,000 years, after which time cosmic ray damage has been thought to be irreparable. Many scientists in the field, most notably Ralph Merkle and Brian Wowk, hold that Molecular Nanotechnology has the potential to extend even this limit many times over.

Probably the most famous cryopreserved patient is Ted Williams. The popular urban legend that Walt Disney was cryopreserved is false; he was cremated, and interred at Forest Lawn Memorial Park Cemetery. Robert Heinlein, who wrote enthusiastically of the concept, was cremated and his ashes distibuted over the Pacific ocean.

Cryonics has traditionally been dismissed by mainstream cryobiology, of which it is arguably a part. The reason generally given for this dismissal is that the freezing process creates ice crystals, which damage cells and cellular structures--a condition known as whole body freezer burn--so as to render any future repair impossible. Cryonicists have long argued, however, that the extent of this damage was greatly exaggerated by the critics, provided that some reasonable attempt is made to perfuse the body with cryoprotectant chemicals (traditionally glycerol) that inhibit ice crystal formation.

The freezer burn objection became moot, however, in 2000, when cryobiologists Greg Fahy and Brian Wowk, of Twenty-First Century Medicine, virtually eliminated ice crystal formation, and hence the major source of freezing damage, using new cryoprotectants that allow vitrification, causing the cooled flesh to form a glass. In a cooled substance that is vitrified, rather than frozen, the molecules remain in a disordered state, as in a fluid, rather than forming an ordered crystalline structure. Alcor Life Extension Foundation, the world's largest cryonics organization, has since been using these cryoprotectants, along with a new, faster cooling method, to vitrify whole human brains. They continue to use the less effective glycerol-based freezing for patients that opt to have their entire bodies preserved, since vitrification of an entire body is beyond current technical capabilities.

The biggest drawback to current vitrification technology is the large-scale fracturing of the brain that occurs, due to cooling to a temperature significantly below the glass transition temperature of about -125C. Currently, brains are cooled to -196C, the temperature of liquid nitrogen, for long-term storage. According to Alcor, all that is needed to vitrify without fracturing is a long-term storage system that maintains the brain at a temperature somewhat below the glass transition temperature, but high enough to avoid the fracturing, at around -150C. 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 without fracturing, since the fractures are very clean breaks, and the loss of neurological structure is still less than that caused by ice formation, by orders of magnitude.

Some critics have speculated that because a cryonics patient has been declared legally dead, their organs are dead, and thus unable to allow cryoprotectants to reach the majority of cells. Cryonicists have empirically demonstrated, however, that so long as the cryopreservation process begins immediately after legal death is declared, the individual organs (and perhaps even the patient as a whole) remain biologically alive, and vitrification (particularly of the brain) is quite feasible.

Critics have often quipped that it is easier to revive a corpse than a cryonically frozen body. Many cryonicists might actually agree with this, provided that the "corpse" were fresh, but would argue that such a "corpse" may actually be biologically alive, under optimal conditions. A declaration of legal death does not mean that life has suddenly ended--death is a gradual process, not a sudden event. Rather, legal death is a declaration by medical personnel that there is nothing more they can do to save the patient. But if the body is clearly biologically dead, having been sitting at room temperature for a period of time, or having been traditionally embalmed, then cryonicists would hold that such a body is far less revivable than a cryonically preserved patient, since any process of resuscitation will depend on the quality of the structural and molecular preservation of the brain, which is largely destroyed by ischemic damage (due to lack of blood flow) within minutes or hours of cardiac arrest if the body is left to sit at room temperature. Traditional embalming also largely destroys this crucial neurological structure.

Cryonicists would also point out that the definitions of "death" and "corpse" currently in use may change with future medical advances, just as they have changed in the past, and so they generally reject the idea that they are trying to "raise the dead", viewing their procedures instead as highly experimental medical procedures, whose efficacy is yet to be either demonstrated or refuted. 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. 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.

While cryopreservation arrangements can be expensive (currently ranging from $28,000 to $120,000), most cryonicists pay for it with life insurance, making it quite affordable for the vast majority of people in the industrialized world, provided they make arrangements while they are still relatively young.

Historically, cryonics began in 1962 with the publication of The Prospect of Immortality by Robert Ettinger. In the 1970s, the damage caused by crystallization was not well understood. Two early organizations went bankrupt, allowing their patients to thaw out, bringing the matter to the public eye, at which point the problem with cellular damage became more well known and the practice gained something of the reputation of a scam. During the 1980s, as the problems associated with crystallization became better appreciated, the goal shifted from whole body to brain-only or "neuropreservation", on the assumption that the rest of the body could be regrown, perhaps by cloning of the person's DNA. 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. Today, cryonicists point to nanotechnology, claiming that its development will enable the repair of cellular damage 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 there may be no insurmountable problems. And the cryopreserved patient can wait a long time.

Some critics, and even some cryonicists, still question this emphasis on the brain, arguing that there is no particular reason to suggest that the mechanical structure of the brain is wholly responsible for personal identity and memories. Partly for this reason, the Cryonics Institute (the second-largest cryonics organization) preserves only whole bodies.

See also: Interstellar travel, Biostasis, Immortality Institute

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