The idea of freezing people and later bringing them back to life has long captured the imagination. However, reviving a whole human body after freezing is not currently possible. While cryopreservation involves freezing biological materials, its capabilities differ significantly from popular culture depictions. This article explores the scientific principles of freezing living matter, what can currently be preserved, and the speculative practice of cryonics.
The Science of Freezing Biological Material
Freezing living biological material presents significant challenges primarily due to the formation of ice crystals. As water within and around cells freezes, it expands, forming sharp crystals that puncture cell membranes and disrupt cellular structures. This mechanical damage hinders successful cryopreservation of complex tissues and organs.
The freezing process also causes dehydration. As extracellular water turns to ice, solute concentration outside cells increases, drawing water out through osmosis. This osmotic stress leads to severe cellular dehydration and chemical imbalance. Scientists explore vitrification, where a substance solidifies into a glass-like state without forming ice crystals. However, achieving this uniformly throughout large, complex structures remains a considerable challenge.
Current Realities: What Can Be Successfully Frozen?
Cryopreservation has achieved success with specific biological materials, primarily single cells and simple tissues. Sperm, eggs, and embryos are routinely cryopreserved for fertility treatments, allowing for long-term storage and later successful use. Blood cells, including red blood cells and stem cells, are also commonly frozen for transfusions and medical therapies.
Success with these materials stems from their relatively small size and simpler structure. Their small volume allows for rapid cooling rates, minimizing ice crystal formation, and facilitates even penetration of cryoprotective agents. These agents, like DMSO or glycerol, replace water within cells, reducing ice damage during the freezing process. However, these successes are limited to individual cells or very thin tissues. They do not extend to complex organs or whole organisms due to the difficulty of achieving uniform cooling and cryoprotectant distribution without toxicity.
Cryonics: The Attempt to Freeze Humans
Cryonics is a practice that involves preserving legally dead human bodies, or sometimes just the head, with the speculative hope of future reanimation. This differs significantly from established medical cryopreservation, as no proven scientific method exists to revive a cryopreserved human or even a complex organ. The process typically begins shortly after legal death, involving rapid cooling of the body to near-freezing temperatures.
Blood is then replaced with a cryoprotective solution intended to minimize ice formation, similar to the vitrification concept. The body is cooled further, to liquid nitrogen temperature, around -196 degrees Celsius (-321 degrees Fahrenheit), for long-term storage. Despite these elaborate procedures, a primary limitation remains the inability to prevent all cellular damage during freezing and the complete absence of technology for rewarming without causing further damage. There is also no current method to repair the extensive cellular and tissue damage that occurs, let alone reanimate a whole organism.
Scientific Hurdles and Future Prospects
Overcoming whole-body cryopreservation challenges requires immense scientific and technological advancements. A significant hurdle involves perfecting vitrification for large biological volumes, ensuring no damaging ice crystals form throughout the organism. Researchers also need methods to non-invasively assess cell and tissue integrity after preservation.
Perhaps the most formidable challenge is developing technologies for molecular and nanoscale repair of damaged cells and tissues. This necessitates breakthroughs in advanced nanotechnology and regenerative medicine. While human reanimation from a cryopreserved state remains a compelling prospect, it currently resides in the realm of future hypothetical science, requiring foundational discoveries not yet on the horizon.