Cryonics is the low temperature preservation of humans and other animals that can no longer be sustained by contemporary medicine until resuscitation may be possible in the future.
Human cryopreservation is not currently reversible. In the United States, cryonics can only be legally performed on humans after pronounced legally dead. 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 the information-theoretic definition of death.
Cryonics faces many obstacles, and 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 studies showing substantial preservation of brain cell structure by current methods, and 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. Many ethical questions revolve around the issue of whether cryonics can work.
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.
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 theoretical death until future repairs might be possible.