Biomedical Engineering Reference
In-Depth Information
Intra-cellular compartment
Cell volume,
V
Inactive Volume,
Water
CPA
V
Ice Volume,
V
ice
Aqueous solution:
-
NaCl
Water,
V
water
- lt,
V
NaCl
-
CPA,
V
CPA
NaCl
Cell membrane
Extra-cellular compartment
Volume,
ext
V
Fig. 1
Schematic representation of the cell model
differently sized cells belonging to the same lineage are assumed to be constituted
by the same inactive cell fraction, which is then considered a characteristic feature
of the cellular lineage.
This cell structure is separated from the exterior environment by an oil
semi-permeable membrane that rules the movement of any molecule between the
intra- and the extra-cellular compartments composing the system. Cell membrane
is characterised by a low resistance to water transport, whilst the permeability to a
specific solute depends on its size, electric charge and the features of hydrogen
bonding [
29
]. Traditionally, in cryopreservation the sodium chloride is assumed to
be not permeant through the membrane. Thus, the content of NaCl does not change
in both the intra- and the extra-cellular compartments. On the contrary, a permeant
CPA like glycerol or DMSO may be transferred from the external medium to the
interior of the cells and vice versa, so that its content in the two compartments may
change. The water and solutes permeabilities to membrane are considered char-
acteristic features of a cellular lineage just like the inactive cell fraction.
Apart from osmosis, during the cooling stage of cryopreservation liquid water
may turn into ice. This phase change takes place inside both the intra- and the
extra-cellular compartment, separately. Thus, the water content in any compart-
ment does not change due to ice formation but simply splits between the liquid and
solid fractions therein.
For simplicity, a negligible difference between ice and liquid water densities is
typically assumed. In addition, the densities of water, salt, CPA and inactive
materials are considered constant with respect to temperature variations, so that a
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