Predictably, cryonics would be expensive because of the large amounts of resources required for prolonged preservation and the scarcity of expertise in the practice. In 2018, affording the procedure was valued at US$28,000 to $200,000 (16) which is significantly high especially for a clinically unproven theory that depends presently on inexistent technology. Notwithstanding, cryonics might be worth the cost should reanimation be achieved in the future.

There is currently no unified global perspective on cryonics. Countries like France and British Columbia have reservations (17) concerning the practice while some others (Germany, Russia and USA) are less stringent (18). Regardless of the technological advancements, cryonics raise some ethical and legal arguments that must be explored. For example, some believe it to be unethical while others think that employing cryonics is justifiable and humane. Other intriguing questions are the legal status of cryopreserved people and if cryothanasia is a feasible option to increase the chances of revival (1).

Proving the revival of cryopatients is the most daring obstacle because there are currently no proven interventions that can extend human lifespan (19) talk more of resurrection after legal death. Revival would involve repairing damage caused by freezing (in unvitrified tissues), hypoxia, cryoprotectant toxicity, thermal stress, then cure and possibly tissue regeneration if necessary but valid questions to consider in this regard would be: what does it really entail to be alive? would reversing the cause of death guarantee resurrection or a “healthy corpse”? Presently, research has revealed that different cells, tissues and organs require specifically tailored cryopreservation protocols to promote survival of the cryopreserved material. This requirement would pose a serious challenge to cryonics because the whole body is subjected to the same cryopreservation procedure which may not guarantee revival. Also, a revival protocol would have to be designed for every cryonic patient because people die of innumerable causes.

Increasing the survival of complex tissues and organs remains a difficulty in cryopreservation. Considering the complexity of the human body, recovery from cryoinjury in cryonics would be even more challenging because prolonged vitrification at extremely low temperatures predisposes huge organs to rupture (20, 21). Ice crystals also damage intercellular junctions required for organ functioning (22) and without safe/effective cryoprotectants, cell survival is diminished by dehydration, high salt concentrations and cryoprotectant induced toxicity (23).

Although cryobiology research especially those on cryopreservation of more complex tissues and organs forms the core of cryonics, they cannot count as cryonics studies. According to Alcor, “THE SOCIETY FOR CRIOBIOLOGY has discouraged scientist from doing work that could advance cryonics ….” (24). The predictable outcome of this stance is minor research contributions as seen in Table 1, and slow scientific progress which could be improved upon by encouraging specialization of pro-cryonic scientists and research.

Hypothermia considerably extends tolerance to irreversible ischemic injury (28–30) and many clinical reports, especially pediatrics', exist to support this cerebroprotective property of low temperatures (31). Research with animal models have also confirmed the potential benefits of hypothermia following cardiac arrest (32–34) with most of the neuroprotection attributable to the suppression of neuronal injury (35). Techniques to induce quicker hypothermia are being researched and developed such as cold saline aortic flush tested in pigs (36) and the invention of transnasal high flow dry air (37, 38). Furthermore, candidates like vasopressin (39), dihydrocapsaicin (DHC) (40, 41) and mesenchymal stem cell (MSC) (42) have been evaluated for their neuroprotective enhancing properties.

The closest procedure to cryonics and cryopreservation of whole animals is the cryopreservation of tissues and more complex organs including ovarian tissue (43), bones (44), bone marrow (45), skin grafts (46), pancreatic islet grafts (46), hearts (47), and lungs (48) with some of them being clinically applied during organ transplantation. Several studies are presently geared at improving extending cryopreservation time using suitable techniques and cryoprotectants and concomitantly preserving quality of the cryopreserved material. These studies can serve as insights toward the precise conditions necessary for effective long-term preservation and suggest to cryonics whether to review cryopreservation strategies for increased chances of reanimation.

The brain is highly complex in anatomy and function (49) therefore more difficult to recover after cryopreservation. Anoxia, reperfusion injury, oedema, and metabolic alterations could challenge the integrity of the brain after cryonics exposure and averting them is the aim of some research. Some neuroscientists suggest that memory retention is encoded in the physical structures of the brain, particularly neuropil connectivity and long-term retention of synaptic strengths (50) and possibly within individual neurons through DNA modifications (51). Vrselja et al. developed a cytoprotective and pulsatile-perfusion system that restored brain circulation and cellular functions up to 4 h post-mortem in the pig model (52).

Humans have been subjected to deep hypothermic (16, 24°C) cardiac arrest lasting more than 1 h without gross neurological deficits (53) confirming that for as much as the anatomy is untampered, the brains' electrical activity can be lost and fully recovered (9, 54). Also, the brain has been found intrinsically capable of resisting structural alterations for some time after death. 70–90% of Neurons from autopsied geriatric humans obtained approximately 2.6 h post-mortem remained viable after two (2) weeks in vitro (55). Theses research discoveries provide evidence that cryonically preserved subjects might have the chance to recover from ischemia and cryopreservation injury. Neural stem cell therapy (56), digital neural and soma reconstruction (57, 58) using bioabsorbable materials, e.g., polyglycolic acid conduit (59) nanobot cellular repair (60) and neural prosthesis (61) are under investigation as means to improve on restoring brain function. The other less complex organs can be replaced artificially, by tissue engineering (62) or stem cell based regenerative medicine. Regeneration of limbs in salamanders (63) and the use of lab-grown artificial ears (64) are green lights in this direction. Soon, organ replacement by 3D Tissue printing (65) and construction from biodegradable materials, e.g., ligaments with silk collagen scaffold (66) might be put to clinical application.

A very critical stage in preparing patients for cryostasis is the replacement of body fluids (blood and water) with CPAs which are additives used to minimize ice induced injury to cryopreserved materials and support their post-thaw recovery. Different classes of CPAs exist, each functioning by one or more mechanisms including the induction of thermal hysteresis, ice nucleation and recrystallization inhibition and ice shaping. The most commonly used CPAs in cryobiology are dimethylsulfoxide (DMSO), glycerol, and different polyols like propylene glycol. Vitrification is the preferred cooling method in cryonics as the body is preserved in a stable glassy state (8, 67) but achieving this requires high CPA concentrations which can be damaging and toxic. As the future of cryonics is reanimation, a significant amount of research is currently focused on developing safer and effective CPA options including antifreeze proteins (AFPs) (68), trehalose (69), DP6 (70), nanotechnology engineered cryoprotectants (71), neutral amino acids (6). A major milestone was recorded in 2007 when major cryonics organizations claimed to have successfully vitrified the brain without ice formation (9), still post-thaw recovery not being promised drove the award-winning research of G. Fahy and R. McIntyre in 2015 (72). They infused rabbit brain with glutaraldehyde before vitrification allowing for recovery of an almost perfect brain post-thaw (73).

Some researchers have suggested that cryothanasia (subjecting a terminally ill patient to cryopreservation prior to legal death) might increase the chances of reanimation (74) but this concept is also hindered by lack of proof and ethical restraints. Experimenting on the concept of cryothanasia might hold the required breakthrough in cryonics but suitable live models have to be used and proven in studies prior to application in humans. In marine and aquatic science, several animals are naturally capable of adapting and surviving hypothermia for prolonged periods during overwintering. This act shares some similarities with cold preserving bodies before death and simulating the natural mechanisms that occur in overwintering animal models may make significant contributions to cryonics.

For instance, ranid frogs are highly sensitive to anoxia and during hypothermia, they maintain normal oxygen uptake and circulation at low oxygen partial pressures by cutaneous gas exchange and diverse physiological changes while energy is supplied by reserves in adipose tissues, liver and muscles (75). This example shows the significance of oxygenation, and justifies the artificial restoration of respiration and blood circulation performed in cryonics. It also reveals that it may be more beneficial to continue the supply of oxygen and energy to the cryopreserved subject throughout the entire cryopreservation process thereby reducing damage induced by hypoxia.

Similarly, Ultsch in 2006 studied different species of turtles undergoing overwintering where it was discovered that most of the marine species respond by migrating to avoid the negative effects of extensive exposure to cold (76). The Terrapene turtle adapts by freeze tolerance while some non-marine aquatic turtles are anoxia-tolerant (76). Mimicking the freeze tolerance and anoxia tolerance mechanisms could be advantageous to improving cryopreservation results.

Furthermore, a strain of Gibel Carp (Carassius gibelio) has been discovered to possess genetic resistance to overwintering starvation with less apoptosis (77), pointing to fact that certain genetic manipulations might confer resistance to tissue damage on cryopreserved subjects.

Cardiopulmonary resuscitation (CPR) combined with external defibrillation has restored life to many declared clinically dead from cardiac arrest (78) however, studies have suggested that delayed defibrillation is associated with lower survival rates (79, 80). This poses a serious challenge to cryonics and prolonging survival after delayed time to CPR is therefore critical in reanimating cryonics patients. In a related study using cats, brain activity was restored in a significant population after an hour of global cerebral ischemia and administration of norepinephrine/dopamine, heparin, insulin, and acidosis buffers (81). These agents are potentially applicable in increasing the chances of survival.

Reperfusion injury is a term used to describe damage to tissues incurred from inflammatory responses and oxidation by toxic free radicals when blood circulation returns following ischemia lasting more than approximately 20 min (82, 83). In order to prevent reperfusion injury, different interventions have been employed. For example, microcirculation remodeling (84), administration of alpha-tocopherol (vitamin E) (85) and cilostazol (86) have been found beneficial against reperfusion injury in the spinal cord of rats. Pretreatment of cryonics subjects with vitamin E could be of added advantage as it reduces blood clotting and lacks the risk of gastric bleeding associated with aspirin (9) and administration of iptakalim has been found to improve cerebral microcirculation in mice (87). At Alcor, drugs including excitotoxicity inhibitors, nitric oxide synthase inhibitors and Poly ADP-ribose polymerase inhibitors are administered against reperfusion injury with further resistance conferred on the brain by anesthesia (10).