Hello Vinayak

In cell culture, cryopreservation of mammalian cells is done by slow rate of freezing and using cryoprotectant like DMSO or glycerol in order to protect the cells from damage due to ice crystals.

Slow rate of freezing at an optimum cooling rate of -1 degree celsius per minute is needed, and this depends on the heat capacity of alcohol. Isopropanol possesses this property of providing a cooling rate of -1 degree celsius per minute. So, this alcohol is used in cryocoolers.

Isopropanol has to be replaced after every five cycles of cooling and warming because after several such cycles there is enough vapour condensation taking place that the isopropanol in the cryocooler gets replaced by water which in turn affects the heat capacity of isopropanol thereby affecting the cooling rate which has to be maintained at -1 degree celsius per minute.

I hope this information is helpful.

Best Wishes.