Cryogenic biological sample storage
Liquid nitrogen is used in hospitals, clinics and cryo-banks for cell and tissue sample storage. It is also used in animal husbandry for bull semen storage.
Many hospitals and clinics use liquid nitrogen cyrogenic sample storage on site or send samples to a cryo-bank for off site storage. The operating temperature of the cryo bank is permanently regulated at -196°C due to the physical properties of liquid nitrogen. Only if the storage chamber were to run out of liquid nitrogen would it gradually warm up. However, this would be a totally undesirable outcome so a reliable liquid nitrogen supply is essential to maintain sample integrity.
Long-term sample storage relies on cryogenic temperatures being maintained at all times. If the samples warm up they will be permanently damaged. Considering that the samples may be precious embryos or semen for fertility clinics or biopsy tissue samples from oncology wards, it is essential that the samples are preserved with the utmost caution.
Another common deep freeze arrangement is to use electrically-powered mechanical freezers, which operate in the range of -40 to -86°C. In this case it is common to use either liquid nitrogen or carbon dioxide cylinders as a backup in the event of power failure to ensure sample integrity is never compromised.
If liquid nitrogen is used at the hospital site for other applications, it may be economical to install a bulk liquid nitrogen storage tank outside the hospital. In this case, small transportable liquid nitrogen dewars can be filled at the liquid nitrogen storage tank and then rolled back inside to the sample store. Alternatively, liquid nitrogen from the bulk tank can be piped to the sample store using insulated stainless steel piping. If there is no requirement for a bulk liquid nitrogen storage tank on site, smaller portable liquid nitrogen tanks of 200 or 300 litres capacity can be used which can be transported inside the hospital or clinic and kept near the sample store. These tanks are vacuum-insulated to minimise liquid nitrogen boil off.