Is Liquid Nitrogen The Only Gold Standard For Long-Term Sample Storage?

As we have discussed before, biobankers have two options for ultra-low temperature storage: 1) liquid nitrogen (LN2) (in either the liquid or vapor phase) and 2) mechanical cryopreservation.

For decades now, LN2 storage, which keeps samples at -196°C, has been hailed as the “gold standard” for biosample preservation. Why? Because storing biosamples at a temperature below the glass transition point of water (about -135°C) stops virtually all biological activity and, in theory, minimizes sample degradation over time.

By contrast, mechanical freezers typically operate at (a “balmy”) -80°C . . . leaving some researchers to question whether the higher temperature could exacerbate sample degradation over time.

Even though there are numerous safety and efficiency benefits to using mechanical freezers, it’s clear they won’t become universally accepted for long-term cryopreservation until it can be shown that storage at -80°C doesn’t adversely affect a biosample, even after years in the freezer.

That evidence is starting to accumulate.

For example, a group of researchers from Sweden recently published a study (PDF) evaluating the performance of mechanical freezers for preservation of endocrine tissue samples.To assess possible changes in sample quality, the study evaluated RNA quality and tissue morphology from 153 samples collected over the past 27 years.

RNA is an excellent metric for sample quality because it is the first component of a tissue sample to break down –in other words, any damage to the sample would likely be reflected by diminished RNA quality. In addition, the quality of RNA can be estimated using the RNA integrity number (RIN), a scale of 1-10 (1=lowest; 10=highest).

The main conclusion from the study should come as good news for biobankers who use mechanical freezers: RIN values (and therefore RNA and sample quality) showed no correlation with the time tissues had been frozen. In fact, RIN values were consistently high from samples collected over all 27 years. What’s more, tissue morphology was of a high quality across the board and samples showed no signs of freeze-drying.

Taken altogether, these results indicate that long term storage in mechanical freezers at -80°C does not adversely affect the quality of endocrine tissue samples.

This study –which happens to be the most rigorous and long-term to date –supports others that have evaluated the performance of mechanical freezers. Independent tests like these are important because today’s biobankers can choose from a wide range of storage options and cryopreservation methods, and they need to know which ones are optimal for biosample preservation.

Do the latest results from Sweden agree with what you’ve seen for mechanical cryopreservation? How do you compare LN2 to -80°C storage? Please let us know in the comments. We would love to hear from you!