By Barry Nguyen, Biochemistry & Molecular Biology ‘23

Author’s Note: I became interested in ways to bypass built-in lifespans after taking HDE 117, a longevity class with Dr. James Carey. During the course of the class, I was exposed to many different ways to extend the human lifespan. However, I was most interested in cryogenics and its prospects of human rejuvenation, prompting me to explore the possibilities of human cryopreservation. 

Summary

This paper is focused on exploring the prospects of human cryopreservation. The first section discusses cryogenics and its relevance in the discussion of human cryopreservation. The following section utilizes empirical modeling to support the relationship between temperature and reaction rates. Next, the paper discusses the cryopreservation procedure itself and explores how the definition of death can be reimagined. We then transition to discussing cryopreservation’s possibility for rejuvenation. Specifically, we redefine the definition of aging itself and discuss aging phenomena on the molecular scale and use both of these as a basis for the discussion of immortality. The succeeding section is concerned with the limitations of human cryopreservation. Finally, the paper concludes with a brief discussion of the possible future of cryogenic technology. 

Cryogenics

Cryogenics is a field of study focused on material behaviors at very low temperatures, ranging from -150°C to -273°C. At these extremely low temperatures, chemical properties are altered, and molecular interactions are halted [1]. By halted, it is not correct to say that all molecular interactions have been stopped. Rather, the molecular interactions have come as close to theoretically possible to ceasing and are at the lowest possible energy state. At these temperatures, chemical properties are also altered and unique phenomena emerge, allowing for extensive applications, most notably human cryopreservation. Because heat is related to the motion of particles, at these temperatures the biochemical activities within living systems are effectively reduced [9]. The prospects for preserving an individual at extremely cold temperatures have been increasing throughout the years as research within the field continues to develop. As of now, human cryopreservation seems more of a speculation than reality. Freezing an individual is one thing, but there is no guarantee that the individual will wake up from such an extensive period of suspension. Although extremely low temperatures serve as an appropriate basis for human cryopreservation, many more factors must be considered to avoid consequences that may occur during the procedure and after revival.

Empirical Modeling

The rates of biochemical processes at extremely low temperatures can be modeled mathematically [4]. The Arrhenius equation, proposed by Arrhenius in 1889, establishes a relationship between temperature and reaction rates. In figure 1, K is the reaction rate, Ea is the activation energy, A is the frequency factor (related to the orientations of molecules necessary to produce a favorable reaction), R is the universal gas constant, and T is the temperature. Manipulating the equation, we produce a form that directly shows the relationship between the reaction rate and temperature, as depicted in Figure 2. We will use the enzyme lactate dehydrogenase to illustrate the relationship between K and T [4]. With its activation energy defined as 54, 810 J/mol, we can explore the enzyme’s reaction rate at a 10°C difference. With T1 and T2 at 40°C and 30°C respectively, we get a reaction rate ratio of 2.004. This tells us that a 10°C difference is enough to cut the reaction rate of the enzyme exactly in half.  

The relationship between reaction rates and temperature, as expressed by the Arrhenius equation, lends weight to the viability of cryopreservation. If a 10°C difference is enough to cut a reaction rate in half, imagine how much the reaction rate would be reduced within cryonically preserved individuals at extremely low temperatures. Furthermore, the biochemical processes that are occurring in the body at these levels are paused—not in the sense of being physically stopped, but rather the time needed for the processes to go to completion is relatively infinite. 

Figure 1. The Arrhenius equation	Figure 2. Manipulation of the Arrhenius equation to compare reaction rates at two different temperatures

Human Cryopreservation

By understanding that at these extremely low temperatures, biochemical reaction rates are suppressed, the practice of cryogenically preserving a whole individual became a reality [14]. For this process to begin, the individual must be induced in the death state. Once an individual enters the initial stages of death, the human body initiates its decomposition phase. The body’s cell walls begin to break down and in turn, release digestive enzymes that process the tissues in the body [11]. Because the body begins to break down at such a rapid pace, it is imperative that the patient, once induced in the death stage, be worked on immediately.

 The process of chilling the human body to extremely low temperatures is a delicate and slow process and is very important in the initial steps of the cryopreservation procedure. Once the patient arrives in the death state, the circulation and respiration of the cryonic subject is restored and they are ready to be cooled [4]. First, the subject’s blood is replaced with 10% cryoprotectants to prevent ice formation. A small percentage of cryoprotectants are added initially to avoid an elevated osmotic shrinking response. Once the intracellular and extracellular cryoprotectant volume reaches equilibrium, the cells are ready for cooling which is done at a very slow pace (1°C/min) [5].

The cryoprotectant used typically consists of nutritional salts, buffers, osmogens, and apoptosis inhibitors, ingredients necessary in the maintenance of isotonic concentrations of the cell [5]. In doing so, cells within the human body can avoid swelling and shrinking. Additionally, another key formulation of cryoprotectant mixture is non-penetrating cryoprotectants which are typically large molecular polymers. These play a large part in the inhibition of ice growth and prevention of injury due to being subjected to the extreme cold [5]. 

To understand the prospects of human cryopreservation, it is helpful to redirect ourselves back to the definition of death. In 1988, the scientific community reviewed and redefined the definition of death from being in cardio-respiratory arrest to brain death [8]. In cryonically preserved patients, the extremely cold temperatures are thought to preserve the neural structures, which store long-term memory and the identity of the person. In this way, utilizing extremely low temperatures to preserve neural structures and prevent them from being compromised is a prospect worth noting. Individuals who are cryonically preserved should not be viewed as being dead or alive, but rather be viewed as being temporarily suspended in time [8]. The normal cycles of biological processes such as growth and decay are paused, providing an opportunity for resuscitation and reanimation in the future [10]. To give a new perspective, cryopreservation can be viewed similarly to frozen embryos: just as embryos preserved in extremely cold temperatures gain life once implanted in a uterus, the cryopreserved patient may reenter the living state through the process of human reanimation. 

Prospects for Immortality

The process of human cryopreservation aims to allow individuals to escape imminent death by first being induced into a transient death state [8]. Essentially, individuals are given the opportunity to bypass human mortality. Dr. James Hiram Bedford, a former psychology professor at UC Berkeley had his life threatened by renal cancer. He decided to undergo the cryopreservation process and became the first human to be cryonically preserved in 1967 [13]. By agreeing to enter this process, he hoped that, in the future, technology would be advanced enough to revive him and cure his illness. Ever since interests in cryopreservation have increased substantially, and as of 2014, about 250 corpses have been cryogenically preserved in the US [13].

Shifting Views on Aging

Aging is a degradative process that entails a whole array of pathologies. If we were to view aging as a disease itself that can be treated, cryopreservation opens a wide range of possibilities. Specifically, the process of cryopreservation allows an individual to avoid the effects of aging pathologies by having the opportunity to be treated once technology has advanced enough. This provides hope to bypass the mechanically built-in lifespans of humans, and essentially, provides prospects for immortality.

On a larger scale, as we age, the probability of dying increases significantly [7]. To put it simply, as we age, there are more health factors in place to compete for our lives and the chance of survival through older ages decreases. In such cases, aging can be correlated with functional decline. Similarly, on the molecular scale, aging can be seen as a direct consequence of telomere shortening [6]. Telomeres are nucleoprotein structures that exist at the ends of chromosomes and are essential to the integrity of our DNA. During the process of DNA replication, telomeres protect the ends of chromosomes and prevent loss of genetic information [16]. However, as we age, and as our body continues to undergo DNA replication, the telomeres shorten leading to the joining of ends of various chromosomes, pathological cell division, genomic instability and apoptosis. 

In short, the health consequences that come with aging are inevitable but human cryopreservation can be seen to offset these inevitable aging phenomena. The process allows an individual who is suffering from a presently incurable disease to be temporarily frozen in time. In this way, they may be revived when society is advanced enough to deal with the disease successfully. In essence, the human cryopreservation process can be seen to bypass inevitable health consequences, providing rejuvenating possibilities for any individual.

Technological Limitations 

Although successfully preserving an individual through extreme temperatures is certainly an exciting prospect, little evidence exists to indicate that successful preservation and remanimation is possible [15]. At present, there are many challenges that need to be overcome to even support the viability of such an extensive process. According to Professor Armitage, the director of tissue banking at the University of Bristol, preserving the whole human body is an entirely new challenge [15]. Society is not even at the stage of cryopreserving organs. Organs, alone, are very complex, containing different types of cells and blood vessels that all need to be preserved. Similarly, Barry Fuller, another professor at the University of College London, has stated that before exploring the prospects of human cryopreservation, society must be able to demonstrate that human organs can be cryopreserved for transplantation [15]. Hence, as of current, there is close to zero evidence that a whole human body can survive cryopreservation. 

In the previous section, we discussed the arrhenius equation which derived the relationship between temperature and metabolic rates. However, the equation itself does not explore the consequences of raising the temperature of the human body during reanimation. While thawing, the frozen tissues and cells can experience physical disruptions which can damage them [3]. To a greater extent, an individual’s epigenetic markers can even be affected, causing epigenetic reprogramming, which can change the expression of certain genes. However, the biggest hurdle is the successful preservation of the brain. The human brain is arguably one of the most important organs in the body, and cryopreservation must be successful in preserving the integrity of the neural structures. Prospects of successfully cryopreserving whole human brains are slim due to minimal research. Moreover, experiments with frozen whole animals’ brains have not been reported since the 1970s [3]. Obviously, research on this matter is severely limited.

Discussion 

Despite the overwhelming uncertainties surrounding human cryopreservation and society’s current limits, the prospects of being able to defy death or possibly avoiding it in the future are becoming a topic of increasing interest. When an individual is brought to the brink of death, the uncertainties around the cryopreservation procedure, specifically its unproven track record of success, seem inconsequential in the long run. If society were to overlook the field of preservation based purely on unsubstantiated results and the unlikelihood of success, advancements would never occur. All in all, the increase in technological advancements and research within cryogenics is making the prospects of reviving a frozen individual in the future ever so likely. 

References:

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