What Is Cryopreservation?

Cryopreservation is the preservation of cells, tissues, and other biological material by cooling to temperatures low enough to halt metabolic and chemical activity, then holding it there until it is needed. The practice, sometimes called cryobanking when organized around long-term inventories, is an applied branch of cryogenics: it depends on liquid nitrogen handling, controlled-rate cooling hardware, and low-temperature thermometry. Below roughly minus 130 degrees Celsius, molecular diffusion in aqueous systems effectively stops and biological material can in principle be held for decades without measurable degradation.

The engineering problem is not reaching low temperature but surviving the journey to it. Water makes up most of a cell's volume, and freezing water expands, concentrates solutes, and forms crystals with sharp growth fronts. The interval between about minus 5 and minus 60 degrees Celsius, sometimes called the zone of injury, is where most damage occurs, so protocol design concentrates almost entirely on how a sample transits that range in both directions.

Mechanisms of Freezing Injury

Two competing modes of damage govern outcomes. Cooling too slowly gives extracellular ice time to form, which draws water out of the cell osmotically and raises intracellular solute concentration to levels that denature proteins and destabilize membranes, an effect known as solution effects injury. Cooling too quickly traps water inside the cell, where it nucleates intracellular ice that ruptures organelles and membranes. The result is the classic inverted-U survival curve, with an optimal cooling rate specific to each cell type and set by its surface-to-volume ratio and water permeability. A review of cryopreservation principles and cell-specific considerations sets out why that optimum differs so widely, from about one degree per minute for many mammalian cells to hundreds of degrees per minute for others. Cryoprotective agents such as dimethyl sulfoxide, glycerol, and ethylene glycol shift the curve by colligatively depressing the freezing point and reducing the fraction of water available to crystallize, at the cost of osmotic stress during addition and removal and of chemical toxicity at high concentrations.

Slow Freezing and Vitrification

Two protocol families dominate. Controlled-rate slow freezing uses a programmable freezer to cool at a fixed rate through the nucleation point, often with an induced seeding step to control where ice first forms. Vitrification instead loads the sample with high concentrations of cryoprotectant and cools fast enough that the solution passes into an amorphous glassy state without any crystallization at all. The principles of cryopreservation by vitrification described by Fahy and Wowk explain how the approach removes mechanical ice injury and the need to identify an optimal cooling rate, while raising the difficulty of managing cryoprotectant toxicity and osmotic loading. Vitrification succeeds routinely for oocytes and embryos, where sample volumes are small enough to cool and warm quickly.

Scaling to Tissues and Organs

Cell suspensions preserve reliably; whole organs largely do not. As sample mass grows, heat and mass transfer become limiting, since the interior of a large tissue cools far more slowly than its surface, and cryoprotectant must diffuse through the vasculature rather than across a single membrane. Warming introduces its own failure mode, devitrification, in which a vitrified sample crystallizes on the way back up unless rewarmed fast and uniformly. Work reviewed on directional freezing and vitrification of large tissues and organs covers approaches including controlled advance of a freezing interface, nanoparticle-mediated volumetric rewarming, and isochoric confinement. Long-term storage itself is comparatively simple: samples are held in liquid nitrogen vapor or liquid phase in dewars, with vapor-phase storage preferred where cross-contamination between vials is a concern.

Applications

Cryopreservation has applications in a range of fields, including:

• Assisted reproduction, including oocyte, sperm, and embryo banking
• Cord blood and hematopoietic stem cell banking for transplantation
• Cell and gene therapy manufacturing and distribution
• Blood product and tissue banking for surgery
• Agricultural livestock breeding and germplasm conservation
• Seed and plant germplasm repositories for biodiversity preservation
• Microbial culture collections and research cell line archives