Traditionally, upright, mechanical compressor-based sample freezers have been the only solution available for the long-term storage of biological samples at -80 °C. Nitrogen vapor freezers have emerged as an alternative. They are different to conventional cryogenic liquid nitrogen freezers as there is no liquid nitrogen stored inside of the freezer.
All researchers have their chosen method of storage, whether the samples are reproductive cells, stem cells, or other biological samples. This article will compare each method to see which is most effective
What is the Difference Between Nitrogen Vapor and Mechanical Freezers?
Mechanical freezers are usually powered by a compressor or an engine and are designed in an upright position to facilitate easy access. This kind of freezer may utilise a refrigerant like propane (R290) or ethane (R170). Mechanical freezers are normally kept at approximately -80 °C but they can run at lower temperatures for brief periods of time.
In regards to design, nitrogen vapor freezers are similar to liquid nitrogen freezers where a circular profile is used to enable access to the sample racks located inside of the freezer on a turntable. Nitrogen vapor freezers vary with the inclusion of a heat exchange system that utilises the vaporisation energy of LN2 as a coolant.
Utilising this technique of storing samples in nitrogen vapor instead of liquid enables a higher degree of flexibility in temperature, where samples can be stored between -20 and -150 °C. As long as LN₂ is supplied, the fixed temperature can be maintained almost indefinitely.
A lid that can be opened at the top of the freezer allows easy access to samples, and the correct sample rack can be accessed on a turntable inside. Researchers who need to frequently and regularly access samples often prefer mechanical freezers.
The Drawbacks of Mechanical Freezers
While mechanical freezers provide advantages for the retrieval of samples, there is a range of limitations involved. Firstly, mechanical freezers are restricted in the temperatures they can attain.
Even at -80 °C temperatures, the operation of compressor-based freezers can put a strain on the unit components and has substantial energy requirements. This has a significant impact on the environment, even before considering the need for a HVAC system to eliminate the additional heat produced by the freezer.
Hot spots are a further limitation of mechanical freezers. This means that there is a significant fluctuation in temperature inside the freezer. There can be a variation of up to 30 °C in certain cases. Nitrogen vapor freezers, for example, the MVE Vario, can uphold a temperature variation of less than 5 °C.
The strongest disadvantage of mechanical freezers and the one which is most concerning to researchers is that redundancy is not built into the system. If the freezer stops operating or the power is cut, samples can become non-viable quickly. This can also occur with no warning.
A freezer at the Harvard Brain Tissue Resource Center failed in 2012, destroying one-third of the biggest collection of donated autism brain samples found internationally.
Although the external thermostat reported a temperature of -80 °C, the temperature within was similar to that of a household refrigerator. The malfunction was only found when researchers manually inspected the freezer.
The neuropathologist and Associate Professor of Neurology at Johns Hopkins University, Professor Carlos Pardo, explained that the malfunction set research in autism back by a decade.
Are Nitrogen Vapour Freezers More Reliable?
As less mechanical elements are included in nitrogen vapor freezers, the rate of failure is much lower. A small source of power is required for the control panel, but this can operate using a battery back-up which maintains the viability of the samples inside. The freezer will continue to function and keep samples frozen as long as there is a consistent supply of liquid nitrogen.
Tanks located outside of the laboratory can supply the nitrogen vapor freezers, and these can be refilled with frequent deliveries of liquid nitrogen where necessary.
The temperature of the interior can be altered simply by opening the door of a compressor-based freezer. This can be harmful to highly fragile samples. Samples can be unviable in a matter of hours if the power supply fails.
Contemporary nitrogen vapor freezers can maintain significantly low temperatures for a long period of time. For example, when functioning at -80 °C, the MVE Variō™’s superior insulation means that it would take four days for the temperature to reach -60 °C, even when liquid nitrogen is not supplied.
If a malfunction does arise, it is normally cost-effective, efficient, and simple to repair in a nitrogen vapor freezer because there are no costly motors or compressors to replace.
The Hot Topic
The supporters of each storage technique are primarily divided when it comes to temperature. As outlined before, compressor freezers usually run at approximately -80 °C. It is possible to operate these at lower temperatures, but this can result in failure due to an increase in stress on the motor.
Nitrogen vapor ULT freezers, for example, the MVE Variō™ freezers, can easily keep samples at temperatures as cold as -150 °C. This is significantly colder than the point where water transitions to a glassy state (-135 °C), below which essentially all biological functions cease. Many view this as the only method of ensuring that samples do not deteriorate over time.
A number of storage banks will reap the advantages of both methods for their samples. As the cell membranes can be harmed by freezing samples too quickly, a compressor freezer can be utilized in some cases to lower the sample temperature gradually prior to being held in LN₂ storage for a longer period of time.
This is not usually a possibility when using liquid nitrogen freezers, but a nitrogen vapor ULT freezer can alter the temperature. This allows samples to be slowly lowered to the target temperature where they can either be easily transferred to a conventional LN2 freezer or stored for a long period of time.
The Verdict
Each of the sample storage techniques has supporters and critics, and each technique has benefits and weaknesses according to a range of factors such as temperature, reliability, and accessibility.
What can be stated for certain is that for the storage of important samples over medium to long periods of time, nitrogen vapor freezers provide more flexibility compared to LN2 or mechanical freezers, while simultaneously providing the reliability of a conventional liquid nitrogen unit.
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