EmbryoMail News - Comments on definitions of vitrification, devitrification, and recrystallization

[Mazur, Peter]

   In EmbryoMail V. 169, issue 3, Mitch Scheiwe correctly stated that it is erroneous to use the term ‘devitrification’ in place of the word ‘thawing’ when referring to the warming of a cell system that has vitrified upon cooling. But he confuses the formation of ice crystals during warming with their growth during warming. The latter process is recrystallization. I think the time is over due to make the definitions of the events during cooling and warming precise and based on precise thermodynamic quantities.

   First, the vitrification of water or aqueous solutions is the conversion of a super viscous supercooled liquid into a glass, a conversion that occurs when the sample is cooled below the glass transition temperature, Tg. For pure water, Tg is about –137°C. For solutions, it is higher, with the value depending on the solutes present and their concentration. The conversion to a glass is not accompanied by exothermic heat of fusion but it is accompanied by changes in the heat capacity of the sample [Wowk, Cryobiol. 60 (2010)11; Moore and Molinero, Nature 479 (2011) 506]. When the vitrified sample is warmed, it goes through a three-step process: First. when it is warmed above Tg, it should in theory be converted from a glass back into a highly viscous extensively supercooled normal liquid. This is devitrification; i,e., the reverse of vitrification. Second, since the liquid at that point is supercooled 50°C or more below the homogeneous ice nucleation temperature, it will be converted to ice crystals with great rapidity. We already have an excellent term for this conversion; namely, “freezing”. The only difference is that here it is occurring during warming and not during cooling.  Nevertheless, it will manifest all the usual thermodynamic characteristics of the process, especially, the release of a large amount of latent heat of fusion. Unfortunately, the term “devitrification” has, in my view, been erroneously applied to the second step instead of the first step for many decades
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        If the results of this freezing during warming are the formation of numerous small crystals, these crystals become subject to a third step; namely, the phenomenon referred to as recrystallization. Small ice crystals have higher surface free energies than large crystals, and so, given appropriate time or low warming rates, they will become subsumed by the large crystals. This process of recrystallization has a very large temperature coefficient (the rate quadruples every five degrees). It is this high temperature coefficient or activation energy that makes the warming rate so critical. That is, the cells have to be warmed rapidly enough to prevent the ice crystals from growing to a damaging size.
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      The final step in the warming process is the conversion of ice back into liquid water under equilibrium conditions beginning above the eutectic point and ending at the melting point. This is melting or thawing, It absorbs the full latent heat of fusion, and is relatively innocuous.

Peter Mazur    04/11/12