Cryonics (from Greek kryos – meaning icy cold) is the low-temperature preservation of humans and animals who can no longer be sustained by contemporary medicine, with the hope that healing and resuscitation may be possible in the future. Cryopreservation of people or large animals is not reversible with current technology. The stated rationale for cryonics is that people who are considered dead by current legal or medical definitions may not necessarily be dead according to the more stringent information-theoretic definition of death. It is proposed that cryopreserved people might someday be recovered by using highly advanced future technology. The future repair technologies assumed by cryonics are still hypothetical and not widely known or recognized. Responding to skepticism from scientists such as Steve Jones, an open letter supporting cryonics was written and signed by currently 62 scientists. As of 2010, only around 200 people have undergone the procedure since it was first proposed in 1962. In the United States, cryonics can only be legally performed on humans after they have been pronounced legally dead as otherwise it would count as murder or assisted suicide. Cryonics procedures ideally begin within minutes of cardiac arrest, and use cryoprotectants to prevent ice formation during cryopreservation. However, the idea of cryonics also includes preservation of people after longer post-mortem delays because of the possibility that brain structures encoding memory and personality may still persist or be inferable. Whether sufficient brain information still exists for cryonics to work under some preservation conditions may be intrinsically unprovable by present knowledge. Therefore, most proponents of cryonics see it as an intervention with prospects for success that vary widely depending on circumstances. A central premise of cryonics is that long-term memory, personality, and identity are stored in durable cell structures and patterns within the brain that
do not require continuous brain activity to survive. This premise is generally accepted in medicine; it is known that under certain conditions the brain can stop functioning and still later recover with retention of long-term memory. Additional scientific premises of cryonics are that brain structures encoding personality and long-term memory persist for some time after clinical death, these structures are preserved by cryopreservation, and future technologies that could restore encoded memories to functional expression in a healed person are theoretically possible. Cryonics is controversial because the technologies of premise are so advanced that premises and are considered irrelevant by most scientists. Whether biological traces of memory or personhood might persist after clinical death is obviously a question of interest. Similarly, outside of cryonics there is no interest in the question of whether memory encoding might survive cryopreservation because the question is regarded as meaningless until cryopreservation can be reversed. At present only cells, tissues, and some small organs can be reversibly cryopreserved. Medical science is primarily concerned with what is demonstrably achievable, not what is theoretically possible. There are therefore no established scientific specialties or journals directly concerned with the scientific questions posed by cryonics. Cryonics advocates claim that it is possible to preserve the fine cell structures of the brain in which memory and identity reside with present technology.