Principles Underlying Cryopreservation and Freeze-Drying of Cells.

Cryopreservation, vitrification, and freeze-drying can be used to preserve biomolecules, cells, or tissues for extended periods until they are needed. This chapter explores the underlying principles how to preserve biological systems in the frozen state at cryogenic temperatures or in the dried state at room temperature. Freezing and drying are different stress vectors that require different types of protective agents. Freezing injury results from solution effects and physical damage if intracellular ice is formed, while drying removes non-freezable water bound to biomolecules resulting in conformational and phase changes. This chapter examines the mechanisms by which cryoprotective and lyoprotective agents work, including reducing ice crystal growth, preferential exclusion, water replacement, and formation of a highly viscous glassy state, i.e., vitrification. It is discussed how protective agents can be introduced into cells or tissues. We also highlight how insights in cell-specific membrane phase and permeability properties can help design more effective (ice-free) cryopreservation and freeze-drying protocols. In addition, a brief overview of emerging trends in cryopreservation and freeze-drying technologies is presented.