Long-term memory is stored in cell structures and molecules within the brain. 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.
Cryonics goes further than the mainstream consensus that the brain does not have to be continuously active to survive or retain memory. Cryonics controversially asserts that a human person survives even within an inactive brain that has been badly damaged provided that original encoding of memory and personality can, in theory, be adequately inferred and reconstituted from structure that remains. 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. The cryonics argument that death does not 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.
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. Biological repair methods or mind uploading have also been proposed.