To The Future By Ferroliquid!

If you freeze cells quick enough, it prevents ice crystals forming and so the cells can be stored frozen. This is used already with small bundles of cells.

The problem for achieving suspended animation, though, is how do you freeze something as big as a human body quickly? If you just dunk it in liquid nitrogen, it will take a while for the cold to permeate, and the inner cells will not freeze quick enough to avoid the dreaded ice crystals. Result: when you dethaw your astronaut/time tourist/terminally ill patient in the future, you just get a lot of mush.

This is where magnetic cooling comes in. When you apply a magnetic field to a magnetisable material, it heats up; when you switch off the field, it cools back down. This is because of entropy and different characteristic curves or some such bollocks; anyway, it?s a well-established phenomena.

So, you've got your volunteer. You give 'em a general anaesthetic (they probably don't want to be awake through the next part). You then insert tubes into some of their major arteries and veins, and pump out all their blood, while simultaneously flooding their body with the ferroliquid - tiny magnetisable particles suspended in a liquid. (Designing your ferroliquid is going to be the tricky part. Some materials are much better at magnetic cooling than others, e.g. gadolinium. Also, it has to be non-toxic, and an appropriate pH, etc).

The magnetic cooling effect is very small, only a matter of a few degrees at most, so the liquid would have to be cooled as it was pumped in. You would probably have to suffuse the body several times to get all the tissue close to zero degrees Celsius, as your liquid can never be below that or you would get areas of freezing, with the dreaded ice crystals. (Obviously at some point in the cooling process you would have stopped the heart.)

This whole process is carried out in a strong magnetic field. When the whole body has reached just above zero degrees C, you just switch off this field, and every magnetic particle in the body instantly(well, pretty damn quickly) cools down a couple of degrees C. The cells, surrounded by the ferrofluid, are cooled down with it, and hey presto, the entire body is frozen solid, with no ice-crystals. You then just transfer said volunteer to the freezer, and wait till its time to revive them(probably using a reverse of this process).

Of course, this does require the use of very high, very uniform magnetic fields. But as luck would have it, that's exactly what MRI scanners are designed to produce, so you could just use one of them.