Still warming while cooling?
It is happening.
August 21, 2015
We like to think that when we put samples back into the freezer, they immediately begin to cool. That “Phew, they are safe again” feeling, but this is not the case. It turns out that samples continue to warm for several minutes after being returned to a -190°C environment before they slowly cool again. Furthermore, the samples can actually warm more when back in the freezer than when outside the freezer!
At ISCT in Las Vegas this past May, Julian Warhurst presented experimental data which demonstrated in all scenarios that when racks are exposed to ambient conditions then promptly returned back into a ‑190°C LN2 vapour freezer, the samples inside the cryoboxes continued to warm. There are several variables that effect how much and for how long they warm, such as cryobox location in the rack, vial location and population density.
We learned earlier that individual vials can experience warming rates between 55°C and 213°C when outside of an LN2 freezer, but this data showing samples continue to warm when returned to a -190°C environment surprised us.
How does this relate to cryogenic cold chain management? The important learnings from this research are to understand and plan for the warming that is happening to samples after they are returned to the freezer and also the long time it then takes for those samples to equilibrate to the freezer’s temperature.
For example a prudent protocol may account for sample warming in the freezer and estimate a two minute exposure raises sample temperatures by 50°C to -140°C, but does the protocol also account for a repeat exposure several hours later where another two minute exposure may warm the samples above Tg?
Aside from an inclusive SOP, pen, paper and stopwatch, developing intelligent software may be the best way to record, predict and manage sample access to ensure there is no risk of crossing Tg in any scenarios.