A cell bank is a facility that stores cells of specific genome for the purpose of future use in a product or medicinal needs. They often contain expansive amounts of base cell material that can be utilized for various projects. Cell banks can be used to generate detailed characterizations of cell lines and can also help mitigate cross-contamination of a cell line. Utilizing cell banks also reduces the cost of cell culture processes, providing a cost-efficient alternative to keeping cells in culture constantly. Cell banks are commonly used within fields including stem cell research and pharmaceuticals, with cryopreservation being the traditional method of keeping cellular material intact. Cell banks also effectively reduce the frequency of a cell sample diversifying from natural cell divisions over time.
Storage
Before putting the donated cell lines into storage, they are first proliferated and multiplied into a large number of identical cells before being stored in a number of cryovials. Along with the cells, cryoprotection agents are also added to the vials to protect the cells from rupturing from ice crystals during the freezing process. 10% DMSO solution is a common cryoprotection agent. These cryovials are then placed into a tray, labeled with the cell line's genetic data, and placed into cryogenic freezers. The freezers contain nitrogen in either liquid or vapor form, and the cells are frozen at a rate of -1 to -3 degrees Celsius per minute until a temperature of -196 degrees Celsius is reached. At a temperature of -196 degrees Celsius, metabolic processes within the cells are significantly slowed to stop all cell growth, thus preserving the cell line, which is especially useful when the cell line has a limited number of cell divisions. Cells can be stored for an extended amount of time in this state, reducing the rate of degradation of cellular material.
Freezing
The general freezing process for mammalian cells involves suspending a small density of the cells of interest in a solution of cryopreservation agents in a cryovial and freezing the cells to a temperature of -196 degrees Celsius. A slow freezing rate is important to maintaining the health of the cell culture. Freezing the cells at a rate of -1 to -3 degrees Celsius per minute is generally acceptable in maintaining cell culture health. Freezing too quickly risks damaging the cells. At a freezing rate of -5 degrees Celsius per minute, significant decreases of the thawed cell culture is observed. Even more pronounced decreases in cell culture health is observed at faster freezing rates, to the point that the cell culture cannot maintain a cell density. The use of cryopreservation agents is also key to the freezing process. A common cryoprotection agent used is 10% solution of DMSO, which acts to protect the cells from the rupturing caused by ice crystals during freezing and during thawing. DMSO has been observed to be toxic to cells, and requires dilution after the cells are thawed.
Thawing
Rapid thaws are recommend in bringing the cells out of cryopreservation and starting up their normal metabolic processes. Minimizing the exposure of the cryovial and its contents to room, or ambient temperatures is important. Rapid thaws are important to prevent the contents of the vial from melting and refreezing rapidly, which could cause ice crystals to form rupture the cells in the vial. Thaws can be performed in a few minutes within a water bath at a temperature around 37 °C. Experimentation has shown that a slower thaw in a controlled environment such as an incubator also can be used to safely thaw cryofrozen cells.Thawing in an incubator avoids the risk of contamination involved in thawing in a water bath, however takes a significantly longer amount of time and resources. Post thaw, the cells need to be transferred from the cryovial into another vessel and resuspended in media. By diluting the concentration of the cryoprotection agent present, negative effects such as toxicity from the cryprotection agents on metabolically active cells can be mitigated.
