Cryogenic sleep, a concept often depicted in science fiction, involves placing individuals into suspended animation for extended periods without aging or deterioration. This concept frequently appears in narratives involving interstellar travel or as a means to preserve people until future medical cures become available. While the idea of pausing life for long durations is captivating, its current reality is far more intricate and limited than fictional portrayals suggest.
Principles of Suspended Animation
The concept of suspended animation centers on significantly reducing an organism’s metabolic activity, thereby slowing or pausing life processes. This reduction minimizes the demand for oxygen and nutrients, allowing cells to survive longer under otherwise damaging conditions. This state is naturally observed in various animals that employ strategies to endure harsh environmental conditions.
Animal hibernation provides a real-world example, where mammals like bears and ground squirrels enter a state of metabolic depression. During hibernation, their body temperature drops significantly, heart rates decrease dramatically, and respiration slows. Another example is anhydrobiosis, exhibited by organisms such as tardigrades, also known as water bears. These microscopic creatures can survive extreme dehydration by forming a “tun” state, reducing their metabolic activity to less than 0.01% of normal levels and enduring conditions like extreme temperatures, radiation, and even the vacuum of space.
Existing Cryopreservation Methods
While cryogenic sleep for humans remains science fiction, cryopreservation technology allows for the successful preservation and revival of various biological materials. This technology prevents biological degradation by maintaining samples at ultra-low temperatures, typically -196°C (-320.8°F) in liquid nitrogen, where cellular activities virtually cease.
Successful cryopreservation has been achieved for individual cells like sperm, eggs, and embryos, and some tissues such as blood cells and skin. Vitrification is a common method, cooling samples rapidly so water solidifies into a glass-like state, avoiding damaging ice crystals. Cryoprotective agents (CPAs) are crucial, penetrating cells to prevent intracellular ice formation, which can disrupt cellular structures. Despite advancements, successful cryopreservation becomes more complex with larger biological structures due to challenges in uniform cooling and cryoprotectant distribution.
Current whole-body human cryopreservation, known as cryonics, is an experimental procedure undertaken after legal death, with the speculative hope of future revival. This process does not involve reanimation with current technology. Challenges include the toxicity of high cryoprotectant concentrations and difficulty preventing ice crystal formation in large tissue volumes.
Hurdles to Human Cryogenic Sleep
Achieving true cryogenic sleep for humans, as depicted in fiction, faces several substantial and currently insurmountable scientific and technological hurdles. A primary challenge involves the uniform cooling and rewarming of a complex human body without irreparable damage. Ice crystal formation within cells and tissues during freezing is particularly destructive, as ice expands and can rupture cell membranes and disrupt cellular architecture. Even with vitrification, which aims to avoid ice, achieving a glass-like state throughout a large, heterogeneous body remains extremely difficult.
Preserving the intricate functions of the brain, including memory and consciousness, during and after cryopreservation presents another significant obstacle. While some studies show promising results in preserving brain tissue structural integrity and neuronal activity in small samples, fully maintaining the complex neural networks of a whole human brain through such extreme conditions is not yet possible. Upon rewarming, tissues are also susceptible to ischemia-reperfusion injury, a type of damage occurring when blood flow is restored to oxygen-deprived tissues. This injury can lead to inflammation and oxidative damage, further complicating revival efforts. The lack of known methods to repair the extensive cellular and tissue damage from current cryopreservation processes represents a fundamental barrier to human revival.