From Depressed Metabolism: "I think in the next 20 years more small animal organs, and perhaps some human organs, may be reversibly cryopreserved. The best scenario for cryonics would be improved, and possibly demonstrably reversible, cryopreservation of animal brains. It has been long observed that if reversible solid-state brain preservation could be demonstrated, then cryonics revival becomes a purely technical problem (albeit very complex one) of tissue regeneration. There would be no remaining doubt about whether the preservation itself was viably preserving human beings ... Reversible solid-state cryopreservation of whole mammals is a very difficult problem with existing technology. This is why when asked about it people will often defer to nanotechnology. References to nanotechnology as a solution to a medical problem basically say, 'We have no idea how to solve this problem with existing tools, but future abilities to completely analyze and repair tissue at the molecular level will be implicitly sufficient.' It's a valid argument, but saying that a medical problem will be solved when someday technology exists to solve every medical problem is not very illuminating about time lines or nature of the problem."