Science Questions

How can you freeze sperm, then have them still be alive when you thaw them? Shouldn’t the water in the cells expand and damage them? Chris, Poland

You can freeze all sorts of things, sperm, eggs, embryos… They aren’t just frozen in water, they are mixed with a chemical such as dimethyl sulphoxide, which alters the freezing properties of the water in the cells and acts an antifreeze. This stops the water making large crystals and damaging the cells.

June 2007