Cryonics

Technicians prepare a body for cryopreservation in 1985.

Cryonics (from Greek κρύος kryos meaning 'cold') is the low-temperature preservation (usually at −196°C) of people who cannot be sustained by contemporary medicine, with the hope that resuscitation and restoration to full health may be possible in the far future.^[1] Cryopreservation of humans is not reversible with present technology; cryonicists hope that medical advances will someday allow cryopreserved people to be revived.^[2]

Cryonics is regarded with skepticism within the mainstream scientific community and is not part of normal medical practice. It is not known if it will ever be possible to revive a cryopreserved human being. Cryonics depends on beliefs that the cryonics patient has not experienced information-theoretic death.^[3] Such views are at the speculative edge of medicine.^[4]

Cryonics procedures can only begin after legal death, and cryonics "patients" are considered legally dead. Cryonics procedures ideally begin within minutes of legal death,^[5] and use cryoprotectants to prevent ice formation during cryopreservation.^[6] The first corpse to be cryopreserved was that of Dr. James Bedford in 1967.^[7] As of 2014, about 250 bodies were cryopreserved in the United States, and 1,500 people had made arrangements for cryopreservation after their legal death.^[8]

Theory

Long-term memory is stored in cell structures and molecules within the brain.^[9] In surgeries on the aortic arch, hypothermia is used to cool the body while the heart is stopped; this is done primarily to spare the brain by slowing its metabolic rate, reducing the need for oxygen, and thus reducing damage from lack of oxygen. The metabolic rate can be reduced by around 50% at 28 °C, and by around 80% at 18 °C or profound hypothermia. By keeping the brain at around 25 °C (considered deep hypothermia), surgeries can stretch to be around a half-hour with very good neurological recovery rates; stretching that to 40 minutes increases the risk of short term and long term neurological damage.^[10]

Cryonics goes further than the mainstream consensus that the brain doesn't have to be continuously active to survive or retain memory. Cryonics controversially asserts that a human person survives even within an inactive brain that's been badly damaged provided that original encoding of memory and personality can, in theory, be adequately inferred and reconstituted from structure that remains.^[3]^[8]^[11] Cryonicists argue that as long as brain structure remains intact, there is no fundamental barrier, given our current understanding of physical law, to recovering its information content. Cryonicists argue that true "death" should be defined as irreversible loss of brain information critical to personal identity, rather than inability to resuscitate using current technology.^[3] The cryonics argument that death doesn't occur as long as brain structure remains intact and theoretically repairable has received some mainstream medical discussion in the context of the ethical concept of brain death and organ donation.^[12]^[13]^[14]

Cryonics uses temperatures below −130°C, called cryopreservation, in an attempt to preserve enough brain information to permit future revival of the cryopreserved person. Cryopreservation may be accomplished by freezing, freezing with cryoprotectant to reduce ice damage, or by vitrification to avoid ice damage. Even using the best methods, cryopreservation of whole bodies or brains is very damaging and irreversible with current technology.

Cryonics requires future technology to repair or regenerate tissue that is diseased, damaged, or missing. Brain repairs in particular will require analysis at the molecular level. This far-future technology is usually assumed to be nanomedicine based on molecular nanotechnology.^[15]^[16]^[17] Biological repair methods^[18] or mind uploading^[19] have also been proposed.