Freezing sperm cells, cryopreservation, is important mainly to young men with testicular cancer who after removal of the affected testicle face a course of chemotherapy. Occasionally a man presents for steril­ization, while expressing the wish that his sperm be stored in advance of the procedure. Such a wish cannot be met with the normal health service, though he can have sperm frozen by commercial institutions. Anyone thinking that sperm banks are a twentieth-century human invention is wrong: for several million years the male springtail has been consistently distributing scores of sperm droplets in the form of a chain. When the female insect’s eggs are mature, she goes to the sperm depot and takes one.

Simple freezing of sperm cells leads to shrinkage of the cell, loss of fluid and sometimes cracks, resulting in loss of function. There was a great step forward after the discovery of glycerol, which could counter the above processes. In freezing for cryopreservation liquid nitrogen is used, cooled to -196 degrees Celsius. One sperm sample can be used to fill between five and fifteen ampoules containing 0.3 millilitres. Depending on the situation between one and three batches of sperm will be frozen. After freezing, one ampoule is defrosted in order to assess the mobility of the spermatozoa: the percentage of mobile sperm that continues to move after freezing or defrosting respectively varies from 5 to 50 per cent. With a concentration of mobile sperm of at least one million per millilitre in the initial sample, there is a reasonable chance of mobile sperm after the defrosting of the whole amount. This is important information when icsi is being considered, since only mobile sperm can be used.

The ampoules of frozen sperm are distributed across two vats in order to reduce the chance of loss as far as possible. It is impossible to assign every individual a deep-freeze vat of their own, so that the sperm of several men is stored in a single deep-freeze vat. Occasionally tiny cracks in the ampoules appear during freezing, so that the contents may come into contact with the liquid nitrogen. In this way viruses and microbes can in theory be released and come into contact with the frozen seed of other men. Because of this risk the man is required by law to be tested in advance for infectious diseases such as aids and hepatitis.