(CNN) Cryopreservation is the process by which any living cells, tissues, organs or entire bodies are protected from decay by storing them at extremely low temperatures.
The idea is to preserve them for indefinite periods until the rest of science has caught up and technology is available to revive them, bring them back to life and maybe cure the condition that killed them in the first place. Such technology, however, does not yet exist.
According to the Cryonics Institute, an US organization that offers the service, the fundamental goal is "to give people a second chance at life" and extend human lifespans.
But scientists who work with cryopreservation on smaller scales for research purposes are skeptical.
"It is currently science fiction to suggest that a person could be brought back to life in the future even considering technological advances," Dr. Channa Jayasena, clinical senior lecturer in reproductive endocrinology at Imperial College London, said in a statement. "Cryonics has risks for the patient, poses ethical issues for society, is highly expensive, but has no proven benefit. If this was a drug, it would never get approved."
That said, how does the process work?
The process involves three key steps once someone has been declared legally dead, according to the Cryonics Institute.
First, the body is immediately placed in an ice bath. At the same time, a ventilation mask is used to continue providing oxygen to the body's organs -- particularly the brain -- and the anticoagulant heparin and automated CPR are used to maintain blood flow. Body temperature is monitored to ensure a gradual reduction.
Next, the body is "vitrified," meaning its cells and organs are prepared for the ultra-low temperatures they will soon experience. This involves replacing the body's fluids with injected cryoprotective agents that act as an antifreeze, protecting the body from the damage of becoming frozen.
Now that the body is prepped for the cold, the process of controlled cooling begins. This is the final step before long-term storage and involves slowly cooling the body further.
The body is placed inside a protective insulating bag and then inside a cooling box where liquid nitrogen is fed in at a steady rate. This takes place slowly, over several days, until the body reaches a temperature of minus-200 degrees Celsius.
Those offering the service stress the slow and steady rate of the entire process, to ensure the least risk of damage to a person's body.
Either before or after the second stage, bodies may be transported to the nearest storage facility to complete the process. Once ready for storage, the body is put inside a liquid nitrogen vessel known as a cryostat. The vessels are not powered by electricity, so they are not affected by power outages.
Another, sometimes cheaper, option is neuropreservation, in which only a person's head is preserved on the assumption that information within the brain is the most important for a person to live again and that a new body could be cloned or regenerated.
Companies offer the option of cryopreservation on the belief that science, technology and medicine will someday be able to revive patients and even cure or treat the diseases that killed them in order to give them a new chance at life. The Cryonics Institute believes it is allowing people to "buy time until technology catches up and is able to fully repair and restore the human body."
Major League Baseball player Ted Williams, who died in 2002, underwent neuropreservation. His head is cryogenically frozen, awaiting the day it can be attached to a new body and revived.
The first person to be cryopreserved was Dr. James Bedford, a psychology professor at the University of California, who was cryonically suspended in 1967 at the age of 73 through Arizona-based Alcor. The company's former vice president Jerry Leaf, who died in 1991, is also frozen.
Although only a handful of people have been cryogenically preserved, many more have joined the list to be suspended in liquid nitrogen upon their death. The Cryonics Institute alone has more than 800 members in the United States and more than 100 more worldwide.
The process is expensive. Fees start at $28,000 and go up to $200,000, paid upon death by either the patient or their insurance policy. Companies often also require membership ahead of the procedure and may apply surcharges for people outside the country.
Cryonics UK, a nonprofit organization, says cryopreservation cannot be guaranteed to work and that it's up to people who want to try it.
Barry Fuller, professor in surgical science and low-temperature medicine at University College London, highlighted the benefits the process has had on a smaller level for scientific research.
"Cryopreservation is a remarkable technology which allows us to store living cells, almost indefinitely, at ultra-low temperatures," he said. "It has many useful applications in day-to-day medicine, such as cryopreserving blood cells, sperm and embryos."
But he added that this has not been proven viable for entire bodies, though he hopes it can be proved step by step, starting with organs.
"Cryopreservation has not yet been successfully applied to large structures, such as human kidneys for transplantation, because we have not yet adequately been able to produce suitable equipment to optimise all the steps," he said. "This is why we have to say that at the moment, we have no objective evidence that a whole human body can survive cryopreservation with cells which will function after rearming. ... At the moment, we cannot achieve that."
Clive Coen, professor of neuroscience at King's College London, is even more skeptical. "Despite the claims made by cryonics companies, they've failed to demonstrate that the extraordinary mass of tissue that constitutes the human brain can be protected by the antifreeze that they try to pump through the body after death," he said. "This tissue may indeed be safe once it's in liquid nitrogen, but it will have already sustained incalculable and irreversible damage during the preparatory processes."
Coen questions whether revival will become a reality.
"Advocates of cryonics are unable to cite any study in which a whole mammalian brain (let alone a whole mammalian body) has been resuscitated after storage in liquid nitrogen," he said. "The companies selling the packages focus on safe storage of the tissue and openly admit that there's no current procedure for resuscitation -- but they gloss over all the damage that's caused while they're preparing the tissue for storage."