Perspective: A Guide to Successful ml to L Scale Vitrification and Rewarming

Cryopreservation by vitrification to achieve an "ice free" glassy state is an effective technique for preserving biomaterials including cells, tissues, and potentially even whole organs. The major challenges in cooling to and rewarming from a vitrified state remain ice crystallization and cracking/fracture. Ice crystallization can be inhibited by the use of cryoprotective agents (CPAs), though the inhibition further depends upon the rates achieved during cooling and rewarming. The minimal rate required to prevent any ice crystallization or recrystallization/devitrification in a given CPA is called the critical cooling rate (CCR) or critical warming rate (CWR), respectively. On the other hand, physical cracking is mainly related to thermomechanical stresses, which can be avoided by maintaining temperature differences below a critical threshold. In this simplified analysis, we calculate ΔT as the largest temperature difference occurring in a system during cooling or rewarming in the brittle/glassy phase. This ΔT is then used in a simple "thermal shock equation" to estimate thermal stress within the material to decide if the material is above the yield strength and to evaluate the potential for fracture failure. In this review we aimed to understand the limits of success and failure at different length scales for cryopreservation by vitrification, due to both ice crystallization and cracking. Here we use thermal modeling to help us understand the magnitude and trajectory of these challenges as we scale the biomaterial volume for a given CPA from the milliliter to liter scale. First, we solved the governing heat transfer equations in a cylindrical geometry for three common vitrification cocktails (i. e., VS55, DP6, and M22) to estimate the cooling and warming rates during convective cooling and warming and nanowarming (volumetric heating). Second, we estimated the temperature difference (ΔT) an d compared it to a tolerable threshold ( ΔTmax) based on a simplified "thermal shock" equation for the same cooling and rewarming conditions . We found, not surprisingly, that M22 achieves vitrification more easily during convective cooling and rewarming for all volumes compared to VS55 or DP6 due to its considerably lower CCR and CWR. Further, convective rewarming (boundary rewarming) leads to larger temperature differences and smaller rates compared to nanowarming (volumetric rewarming) for all CPAs with increasing failure at larger volumes. We conclude that as more and larger systems are vitrified and rewarmed with standard CPA cocktails, this work can serve as a practical guide to successful implementation based on the characteristic length (volume/surface area) of the system and the specific conditions of cooling and warming.