The concept of freezing a human body and later bringing it back to life is often explored in science fiction. Currently, however, it is not possible to freeze a human and successfully revive them. While cryopreservation, the preservation of biological material at extremely low temperatures, is a well-established scientific practice, applying it to a human body presents unresolved challenges. Despite this, approximately 500 individuals worldwide have undergone cryopreservation after legal death, and over 4,000 more are on waiting lists, indicating continued hope for future advancements.
The Science of Cryopreservation
Cryopreservation involves cooling biological samples to ultra-low temperatures, typically around -196°C, the temperature of liquid nitrogen. At these temperatures, biological activity effectively ceases, allowing for long-term storage of cells and tissues. The primary goal is to prevent ice crystal formation, which can severely damage cellular structures.
To mitigate ice damage, cryoprotective agents (CPAs) are introduced. These substances, such as dimethyl sulfoxide (DMSO) or glycerol, reduce the freezing point of water and promote vitrification. Vitrification is a process where the solution solidifies into a glass-like state without forming ice crystals, minimizing mechanical injury to cells. This method is favored over slow freezing for preserving delicate biological structures. Proper CPA concentration and controlled cooling rates are essential for successful vitrification, balancing ice prevention with potential CPA toxicity.
Biological Hurdles to Human Revival
Applying cryopreservation to a human body introduces several biological hurdles that current science cannot overcome. A major obstacle is ice crystal formation. Despite CPAs, completely preventing damaging ice crystals throughout all tissues of a human remains difficult, leading to widespread cellular and structural damage.
Another challenge stems from cryoprotectant toxicity. The high concentrations of CPAs required for vitrification can be harmful to cells and tissues, particularly sensitive organs like the brain. The time to cool a large body also contributes to damage; lack of blood flow during cooling can lead to oxygen deprivation and ischemic injury before cryogenic temperatures are reached.
Uniformly rewarming a vitrified human body without causing additional damage is a major technological hurdle. Rapid and even rewarming is necessary to prevent ice recrystallization, where small ice crystals grow larger and cause further cellular destruction. Uneven rewarming can also lead to thermal stress, cracking, or localized overheating. Even if structural integrity could be maintained, restoring the complex neurological and physiological functions of an entire human after such extreme preservation is currently beyond scientific capability.
Current Research and Future Outlook
While whole-body human revival remains a future possibility, significant progress has been made in cryopreservation for smaller biological entities. Techniques like vitrification are routinely used for successful preservation and revival of human embryos and oocytes in reproductive medicine. Ovarian tissue can also be cryopreserved, with live births following its retransplantation.
Research is advancing in the cryopreservation of more complex structures, such as embryonic kidneys and whole rat livers, with some successful transplantations in animal models. Even small organisms like C. elegans (a type of nematode) can be frozen and revived with high survival rates under optimized conditions. Emerging technologies such as nanotechnology are being explored for their potential to address current limitations. Nanoparticles could potentially be used for uniform rewarming or for repairing cellular and molecular damage from cryopreservation. These advancements, though not yet applicable to whole humans, provide insights and hope for long-term goals in the field.