Ralph Merkle wants to be frozen when he dies, because he believes his specialism of nanotechnology will one day be able to bring him back to life. Justin Mullins reports.

Ralph Merkle wears a silver bracelet carrying an important message for the unfortunate person who discovers his body should he suddenly drop dead. The bracelet asks the finder to call Alcor, a company based in Scottsdale, Arizona, that deals in "life extension". Alcor will dispatch a team to retrieve the body as soon as possible and prepare it for freezing.

Merkle, a highly intelligent scientist who knows better than most what subtle forces would gradually destroy his frozen body, has nevertheless signed up with Alcor in the hope that one day medicine will be advanced enough to bring him back to life. He revealed the fact to delegates at a recent conference on nanotechnology where, as a researcher at the United States ideas laboratory Xerox Parc, he talked enthusiastically about the quest to design and build molecular machines. It is, he believes, the science of nanotechnology that offers him the best chance of returning from the dead.

But in freezing his body, time will be of the essence. Cells need a constant supply of oxygen and nutrients to stay alive - while the heart is still pumping and the lungs breathing, this supply is guaranteed. Switch off this supply and the cells soon begin to produce toxins that poison the body. This process is reversible for at least some time after death, but nobody agrees on just how long. Victims of cardiac arrest have been resuscitated many minutes after their heart stopped beating, and people who have drowned in cold water have been brought back to life an hour later. The process of cooling clearly extends the grace period in which the victim can be brought back to life.

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This period of reversibility does not last long and when it is over, chemical reactions begin to eat away at the cell structure and the body starts to decay. One of Merkle's greatest fears is dying in a foreign hotel room and his rotting mass being found three days later when he will be beyond even Alcor's help. If Alcor gets to the body in time - ideally the team should be called in advance of death - the company reasons that by carefully controlling the way a body is cooled, it should be possible to extend the grace period indefinitely so that the patient can be revived at some unforeseen time in the future. There is some evidence that it might be right.

A variety of animals appear to survive freezing temperatures with no ill effects. Many fish, for example, live quite happily below arctic ice floes at temperatures close to zero and wood frogs sometimes freeze solid during the long winter only to thaw out with no apparent ill effects in spring.

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Human cells do not take kindly to freezing, however. Ice crystals grow into the molecular equivalent of needles, piercing membranes and cutting the cell to pieces. Any water left tends to be drawn out of the cells so that the molecular scaffolding that helps a cell keep its shape collapses. Chemicals become so concentrated in this fluid that they alone are enough to prevent a cell functioning when it thaws. And if all that were not bad enough, the fatty layers that regulate the flow of molecules across cell walls begin to coagulate, like grease on a cold dinner plate, and the cell starts to leak.

So how do frogs and fishes cope? Part of their trick is to prevent freezing at all. Fish blood contains antifreeze that prevents their freezing solid. Frogs employ two tricks. First their blood contains proteins that prevent ice crystals growing large enough to damage cells. Second, frogs produce glucose, which lowers the freezing point of the fluid inside its cells so that while the water outside its cells freezes solid, the cells remain syrupy. Although wood frogs appear frozen solid, their cells are actually mushy inside.

Alcor goes some way to copy the techniques already used by nature. As soon as possible after death, the patient is injected with heparin, a drug that prevents blood from clotting, and a mechanical thumper is employed to keep the heart pumping. The blood is replaced with a fluid that rapidly cools the body - and particularly the brain - to 5C. The body is then transported to the company's headquarters in Arizona, where the blood is replaced with an antifreeze and the body frozen to the temperature of liquid nitrogen (-197C), at which the company believes further deterioration will be minimal.

Alcor cares for a couple of dozen frozen cadavers, including James Bedford, who in 1967 was the first human to be frozen, and Fred Chamberlain, the company's founder and father of Alcor's chief executive.

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No one is quite sure how the thawing can be done safely. Scientists have had some success in freezing and thawing human tissue such as sperm. Freezing destroys about 30 per cent of sperm cells but the rest are quite capable of fertilisation. And freezing human eggs, which are much larger than sperm cells, has recently become possible. In 1997, scientists announced the first successful birth of babies produced from eggs that had been frozen for two years. Efforts to freeze human organs have been less successful - they tend to crack after long periods of freezing - but researchers have had some promising results with rats' livers.

Alcor concedes that there is no chance of resuscitating its patients using existing technology. As a scientific adviser to the company, Merkle is well aware of these problems, but says that the fact that we are unable to thaw out body parts today does not mean that we will never be able to. In fact, Alcor is not even bothering with whole bodies. Instead it offers patients the option of having only their heads frozen, trusting that one day the rest of the body can be cloned from a single DNA sample, much as Dolly the sheep was grown.

Nanotechnology has raised the possibility that the damage to human tissue from freezing might be repairable. The idea is that molecular machines will be able to enter the human body and repair whatever damage the freezing process has done, much in the way that the body's own molecular machinery - the DNA molecules and ribosomes inside cells - can repair some damage. Of course these machines will also have to patch up whatever it was that killed the patient in the first place. But Merkle and Eric Drexler, head of the Foresight Institute - the US think-tank on nanotechnology - and another Alcor client, see no reason why this will not also be possible.

Even if these machines can repair the damage of freezing and if cloning technology can regenerate organs, limbs and bodies, how does Merkle know that it will really be him - the consciousness "me" - that wakes up on an operating table in the year 3000? He admits that scientists have no idea how to explain the phenomenon of consciousness or how it can be preserved. But he does say that whatever the mechanism may be, the part of it that makes him the way he is relies heavily on his memories. If his memory can be saved, he believes there is a good chance that "he" will be preserved.

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Again he points to scientific evidence to backs him up. Many scientists believe that long-term memories are stored, at least in part, in the pattern of connections that neurones make with each other in the brain. If the freezing process can preserve this pattern then the memories can be saved as well. It is a big if, but it keeps Merkle's hopes alive.

He plans to pay for his cryostorage with an insurance policy payable to Alcor on his death. Current prices are $120,000 for whole body preservation and a mere $50,000 for the head only, and Alcor already has a fund of $2 million to keep its patients suspended. High demand means there are now proposals to form an English association of Alcor, and a number of other organisations, such as the Cryonics Institute, also offer "life extension" plans.

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The big dread is that these organisations will not survive long enough for their patients to be resuscitated. In the 1960s, the Cryonics Society of California froze a number of patients but was unable to look after them. All of the patients were lost. Nobody really knows what lies in store for Alcor, the Cryonics Institute or the others, but Merkle argues that patients have nothing to lose. In future, he says, humans will be repaired with new parts that will keep them running more or less for ever, like London buses. "We will be among the last generation to die," he says. "Imagine that time in the distant future when historians look back at us and realise that we had the opportunity to save ourselves, to live for ever. They will say: 'What fools, they had the chance and missed it.'"