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. The process is not currently reversible. Cryonics can only be performed on humans after clinical death, and a legal determination that further medical care is not appropriate (legal death). The rationale for cryonics is that the process may be reversible in the future if performed soon enough, and that cryopreserved people may not really be dead by standards of future medicine. 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 largely prevent freezing injury, preserving the fine cell structures of the brain in which memory and identity presumably reside. 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 might be possible. Damage from ice formation and ischemia
The freezing process creates ice crystals, which some scientists have claimed damage cells and cellular structures 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, presuming that some reasonable attempt is made to perfuse the body with cryoprotectant chemicals (traditionally glycerol) that inhibit ice crystal formation. Vitrification preserves tissue in a glassy rather than frozen state. In glass, molecules do not rearrange themselves into grainy crystals as they are cooled, but instead become locked together while still randomly arranged as in a fluid, forming a "solid liquid" as the temperature falls below the glass transition temperature. Vitrification is faster than other cooling Current solutions being used for vitrification are stable enough to avoid crystallization even when a vitrified brain is warmed up. However, if the complete circulation of the protectant in the brain is compromised, protective chemicals may not be able to reach all parts of the brain, and freezing may occur either during cooling or during rewarming. Cryonicists argue, however, that injury caused during cooling might, in the future, be repairable before the vitrified brain is warmed back up, and that damage during rewarming might be prevented by adding more cryoprotectant in the solid state, or by improving rewarming methods. But even given the best vitrification that current technology allows, rewarming still does not allow revival, even if crystallization is avoided, due to the toxic effects of the cryoprotectants. Some critics have speculated that because a cryonics patient has been declared legally dead, their organs must be dead, and thus unable to allow cryoprotectants to reach the majority of cells. Cryonicists respond that it has been empirically demonstrated 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...
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