The concept of “cryosleep,” as popularized in science fiction, involves placing a living human body into long-term, deep-cold suspended animation with the expectation of full, healthy revival years or centuries later. This differs fundamentally from cryonics, the current practice of preserving human remains or organs after legal death using extreme cold. While cryonics aims for future reanimation, current technology cannot prevent extensive cellular damage, making it an irreversible procedure today. Achieving true biological suspension requires overcoming immense scientific and engineering obstacles.
Biological Barriers to Full Body Freezing
The body’s high water content presents the first significant challenge to freezing whole, complex organisms. When water freezes, it expands and forms sharp, hexagonal ice crystals that puncture and destroy delicate cellular structures, including cell membranes and organelles. This mechanical damage irreparably damages most cells and tissues.
The second major destructive mechanism is osmotic stress, which occurs when water freezes outside the cells. As external water turns to ice, the remaining solution becomes highly concentrated with salts and other solutes, drawing water out of the cells through osmosis. This extreme dehydration causes cells to shrink and exposes them to damagingly high concentrations of electrolytes, changing protein structures and causing DNA damage.
To counteract ice formation, scientists use high concentrations of chemicals known as cryoprotective agents (CPAs), which act like medical-grade antifreeze. However, this introduces the third barrier: the inherent toxicity of these agents. Common CPAs, such as dimethyl sulfoxide (DMSO), must be used in high concentrations to effectively prevent ice crystallization. At these levels, they become toxic to the cells, which remains a major impediment to the cryopreservation of large organs.
The human brain and other complex organs are particularly vulnerable to all three forms of damage. Neurons are highly sensitive to both physical and chemical disruption. The billions of precise connections within the brain are easily wrecked by cellular damage caused by ice or toxic CPAs. The brain also has a low tolerance for oxygen deprivation and ion imbalances, making the entire organ difficult to preserve without losing the information it contains.
Successful Applications of Preservation Technology
Current medical applications of cold preservation demonstrate the boundary of what is successfully achievable today. One successful technique is Targeted Temperature Management (TTM), which involves temporary, moderate cooling to protect the brain and other organs. TTM is used for comatose survivors of cardiac arrest, where the body temperature is lowered to 32°C to 36°C for 12 to 24 hours. This temporary cooling slows the body’s metabolic rate, reducing the demand for oxygen and energy in the brain, thereby limiting damage after insufficient blood flow.
The most robust success in true long-term cryopreservation is limited to small, simple biological structures. Reproductive cells, such as human sperm and eggs, along with early-stage embryos, can be successfully stored for decades in liquid nitrogen at -196°C. This success is due to their small size, which allows cryoprotective agents to penetrate uniformly and water to be rapidly removed or cooled. Modern cryopreservation of these samples often uses vitrification, which rapidly cools the sample into a glass-like solid, avoiding the formation of destructive ice crystals entirely.
Research Paths Toward Suspended Animation
Active research is focused on overcoming the challenges of ice formation and cryoprotectant toxicity to achieve long-term preservation of larger structures. One promising avenue is the development of advanced vitrification techniques for whole organs, which could revolutionize transplant medicine by enabling organ banking. Scientists have successfully vitrified and rewarmed a rabbit kidney and demonstrated human organ-scale vitrification and rewarming in a porcine liver model.
This process requires highly concentrated CPAs to prevent ice formation and advanced heating methods. These methods, such as nanowarming using magnetic nanoparticles, ensure uniform rewarming and avoid damage from ice recrystallization.
Another research focus involves developing next-generation cryoprotectants with significantly lower toxicity. Current CPAs are toxic, limiting the concentration and exposure time that can be used for large organs. Researchers are exploring new formulations, such as multi-component osmolyte solutions and bio-inspired deep eutectic solvents. These have shown reduced toxicity and improved post-thaw viability in cell cultures, including brain cells.
A separate, non-freezing approach to suspended animation is the research into induced torpor or hibernation in non-hibernating mammals. Torpor is a state of reduced body temperature and metabolic rate that allows animals to survive periods of harsh conditions. Scientists are attempting to induce a torpor-like state in non-hibernating animals by stimulating specific brain regions. This metabolic suppression is being investigated for clinical applications in emergency medicine to reduce tissue damage after trauma or to stabilize patients during long surgeries.