Mass Transfer in Cryopreservation at Multiple Scales: Cells, Tissues, and Organs.

The ability to survive cryopreservation almost universally requires the use of exogenous molecules known as cryoprotective agents (CPAs), which are typically used at concentrations exceeding 1 mol/L. In the case of slow freezing, these molecules help mitigate damage associated with formation of extracellular ice. For vitrification methods, CPAs are used at even higher concentrations to completely suppress ice formation, instead promoting the formation of an amorphous glassy state throughout the sample. In either case, effective CPA delivery is critical for survival. However, CPA exposure can itself be damaging, either because of osmotically driven volume changes or inherent CPA toxicity. Therefore, a central challenge in cryopreservation is developing CPA equilibration methods that deliver sufficient CPA for protection without excessive osmotic damage or chemical toxicity. At the heart of this problem is the transport of water and CPA. Thus, mass transfer modeling can provide a foundation for rational design CPA equilibration methods. In this chapter, we review the key physical phenomena affecting transport of water and CPA at the level of cells, tissues, and organs, summarize mass transfer modeling strategies, and highlight opportunities for using these models to design improved CPA equilibration methods.