Controlled ice nucleation in cryopreservation--a review.

We review here for the first time, the literature on control of ice nucleation in cryopreservation. Water and aqueous solutions have a tendency to undercool before ice nucleation occurs. Control of ice nucleation has been recognised as a critical step in the cryopreservation of embryos and oocytes but is largely ignored for other cell types. We review the processes of ice nucleation and crystal growth in the solution around cells and tissues during cryopreservation with an emphasis on non IVF applications. The extent of undercooling that is encountered during the cooling of various cryocontainers is defined and the methods that have been employed to control the nucleation of ice are examined. The effects of controlled ice nucleation on the structure of the sample and the outcome of cryopreservation of a range of cell types and tissues are presented and the physical events which define the cellular response are discussed. Nucleation of ice is the most significant uncontrolled variable in conventional cryopreservation leading to sample to sample variation in cell recovery, viability and function and should be controlled to allow standardisation of cryopreservation protocols for cells for biobanking, cell based assays or clinical application. This intervention allows a way of increasing viability of cells and reducing variability between samples and should be included as standard operating procedures are developed.