Biomedical Engineering Reference
In-Depth Information
may provide insight into optimal methods for cryopreservation and
subsequent thawing and rehydration. 105
The rate and temperature at which tissue samples are frozen
will determine, in part, their quality of preservation. The rate at which
the tissue passes through temperatures of
10°C is critical, as
this is the temperature range where large, destructive ice crystals may
form if given a sufficient period of time. A rapid transition through
this region is achieved by the use of small samples and a freezing
temperature that is much less than
0°C to
10°C. The ultimate storage
temperature, for temperatures less than
10°C, has little effect on the size
of the ice crystals. Rather it is the time that it takes to undergo the
transition through this critical temperature range that is the key factor.
One complication is that uncharacterized changes often occur within the
tissue, and cause micro- or macroscale fractures, during transition from
room temperature to very cold temperatures - such as during immersion
in liquid nitrogen at
196°C. Attention must therefore be paid to artifacts
caused by the freezing process in any experiment.
In general, a conventional refrigerator operates at +4°C and its
companion freezer unit at
4°C. Damage to hard and soft tissues
following freezing at
4°C is extensive. Further, such units typically
include a self-defrosting feature, which cycles the temperature to
eliminate the build-up of excess ice, and will rapidly destroy the
microstructure of any tissue contained within. Bone structure and cells
are reasonably well maintained when frozen at
20°C (the temperature of
a typical 'chest' freezer). However the significant thermal mass of bone,
especially for large sections, causes the exterior of the sample to freeze
much more rapidly than the internal region. It is not uncommon to find
small, microscopic fractures within a bone sample that has been frozen at
−20°C. Storage at −85°C, the temperature of a typical lab freezer used to
preserve proteins and genetic material, will also preserve bone samples
for a long period of time.
Mechanical consequences of freezing and thawing on properties of
whole or large machined sections of bones are not profound. In a survey
of seventeen studies, bone structural variables, rather than material
properties, were the most affected. 83 Overall, strength tended to increase
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