I'm not sure about what's the problem with slow thawing though: AFAIK most tissues are pretty stable at fridge temperature for a few hours (from 4hours for a he

I'm not sure about what's the problem with slow thawing though: AFAIK most tissues are pretty stable at fridge temperature for a few hours (from 4hours for a heart, up to 36hours for a kidney), but freezing is another matter: the slower you freeze things, the bigger the ice crystal grow, and IIRC you need to freeze things very quickly if you don't want damages from the ice crystal (you want to “vitrify” the water, instead of crystalizing it) and I don't think even liquide nitrogen is enough to freeze big organs quickly enough.