Gross damage accumulation on frozen rabbit liver due to mechanical stress at cryogenic temperatures.

The second phase of a pilot study dealing with the mechanical response of frozen biological tissues to external compressive load is presented. This stage deals with histological observations of the damage accompanying mechanically induced permanent deformation in frozen rabbit liver. no significant gross histological damage was observed in the liver samples due to either processing the tissue in the frozen state, due to slow cooling of the liver tissues down to -20 degrees C, or due to rapid cooling of the samples down to -196 degrees C. No histological changes were observed in tissue samples that were loaded within the elastic regime, that is, below the yield strength of the material. Therefore, it is concluded that histological changes due to mechanical stresses are associated with plastic (permanent) deformations. Histological observations indicate that linear cracks which appear to have no preferred orientation develop due to mechanical stress beyond the yield strength of the frozen tissue. These cracks accumulate until final failure of the frozen tissue, when the tissue sample collapses to rubble. Based on histological observations and concepts from solid mechanics, an interaction between crack formation and irregularities in the frozen medium is suggested. Significant sources for such irregularities, in an homogeneous tissue such as the liver, are blood vessels and bile ducts. These irregularities may either initiate crack formation or, on the other hand, may also arrest propagating cracks.