It’s not all heads in freezers you know!
Cryopreservation is a tricky business. Not just the process, which can be quite involved, but the amount of baggage that comes with it. A random survey of people stopped in the street* and asked what their idea of ‘cryopreservation’ was muttered: ‘Cryopreservation? That’s frozen heads isn’t it?’ The answer to which is: NO!
And don’t you dare mention Walt Disney!
‘Cryonics’ as practised by ‘cryonicists’ (and yes they really do call themselves that I checked) is err, a somewhat controversial subject. Some might say: ‘a support for the feeble minded who can’t accept the inevitability of death’. I couldn’t possibly comment.
Oh all right then, I will.
Cryopreservation is actually carried out for two different ends. The first objective is to maintain the structure of biological tissue as close as possible to the functional biological state for subsequent analysis. The second objective is to permit freezing, and eventual recovery to a functional state, of the biological material. The two different approaches to cryopreservation do not necessarily go hand in hand.
For ultrastructural and associated analytical studies the biological material must be frozen as quickly as possible e.g. by ‘slamming’ the material against a liquid helium-cooled copper block. I’ve done it; it’s really cool (sorry)! The objective is to freeze the tissues so fast that ice crystals cannot form. The cell water becomes vitrified – glasslike.
However, even with the very fast freezing rates that are produced by slamming you only get ‘vitrification’ in a very superficial (< 10 micron) layer. This is just down to physics. You simply can’t get the heat out of biological material any faster. So you get ice-crystals forming deeper into the tissue and they disrupt the ultrastructure, bust all the membranes and organelles, and that’s what eventually kills the cells.
You don’t come back easily from that state.
Cryopreservation is a different matter when you are considering freezing for subsequent recovery. That’s all about ‘controlling’ the water. Seeds do this very well, by removing most of the water, which is why they ‘last’ so well and can be easily frozen. Some animals, like arctic fish, as is now well known, contain antifreeze agents that are forms of cryoprotectants. Natural cryoprotectants includes sugars, polyols and glycoproteins. Cryoprotectants can be both penetrating and non-penetrating and share the common property of protecting biological material from ice crystal damage, generally by increasing the solute concentration in the cell – making less water available to produce ice crystals. The cryoprotectant therefore needs to be able to get pretty damn close to – if not into the cell – and it needs to do less damage than the ice crystals themselves of course!
However, even in optimum conditions – with isolated cells suspended perfectly in the best of cryoprotectants (like dimethyl suphoxide)- after freezing and recovery cell viability rates, at best, are something like 90%. Now this is actually a terrific figure; very, very impressive – well done amsbio and their CELLBANKER solutions. This is perfect for freezing sperm for example. If you have 4-5 million cells to start off with, you are laughing; as long as the cells aren’t yours, of course. That is ‘yours’ as in, the bit of your body you are thinking with now.
And remember 90% is in perfect conditions, cells in solutions, not where you need to perfuse a cryoprotectant through a dead body – sorry a ‘patient’ as some cryonicists now like to call corpses. Whole body perfusions can be tricky at the best of times – like when the patient is actually alive with a beating heart.
I for one would not want to loose approximately 10 billion neurones from my brain. Call me old-fashioned, but I don’t really fancy coming back as something rather akin, at best, to a zombie.
I will leave the last word to the very talented Helen Arney:
* my friend David.