Biomedical Engineering Reference
In-Depth Information
and engineered tissues are required to be preserved. Preservation is important for
modern medicine/healthcare and for many other areas:
1. The banking of a large quantity of living cells/tissues for genetic typing and
matching between the recipients and donors to meet the increased clinical
needs and sometimes the urgent needs (e.g., in a war or in events such as
terrorist attack, or natural disasters);
2. Facilitating the transport of cells/tissues between different medical centers;
3. Allowing sufficient time for the screening of transmissible diseases (e.g.,
HIV) in donated cells/tissues before transplantation;
4. Engineered tissues needing to be successfully preserved before their practi-
cal use in any applications and commercialization;
5. The preservation of sperm and oocytes/eggs of endangered or transgenic
species.
A fact that permits the long-term preservation of the living cells and tissues is
that biological metabolism in living cells diminishes and eventually stops at low
temperatures. Thus, for short-term storage (less than couple of days), cells and
organs can be stored at 4
C. This is a clinically employed organ preservation tech-
nique and there are specific preservation solutions that minimize cellular swelling
and membrane pump activity. Cell survival in cold storage depends on the cell type.
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7.5.1 Long-Term Storage of Cells
For long-term storage (months to years), cells must be stored frozen at as low a
temperature as possible; the colder the temperature, the longer the storage. Storage
in liquid phase nitrogen (
C) allows the lowest possible storage temperature
to be maintained with absolute consistency, but requires the use of large volumes
(depth) of liquid nitrogen. There could be the possibility of cross-contamination by
virus pathogens via the liquid nitrogen medium. For these reasons low temperature
storage is commonly in the vapor phase nitrogen. For vapor phase nitrogen storage,
cells present in small containers (called ampules) are positioned above a shallow
reservoir of liquid nitrogen. Since a vertical temperature gradient exists in the vapor
phase, the liquid level should be properly monitored.
While preserving, cooling the cells to required temperature plays a significant
role in the survival. When cells are cooled too slowly (Figure 7.12), the extracel-
lular environment freezes first, and extracelluar ice forms. Extracellular ice creates
a chemical potential difference across the membrane of cells, creating an osmosis
of water that dehydrates and shrinks the cell. The slower the cells are cooled, the
longer the cells are dehydrated, causing irreparable damage called a solution injury.
On the other hand, when cells are cooled too quickly, the cell retains water within
the cell. Inside the cell water freezes and expands, as the density of ice is less than
that of water. The abrasive ice crystals physically destroy the cell itself; this is called
an intracellular ice injury. If the cooling rate is optimized properly, maximum sur-
vival is achievable where the total cell damage from both mechanisms is minimized.
However, this is only a narrow range of a cooling rate, which permits a high viabil-
ity of cryopreserved cells. Specific cell properties such as membrane permeability
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