Differences in the mechanical properties of intestinal tissue based on preservation freezing duration and temperature

In this study, tissue samples were stress tested to determine if freezing duration and temperature alter their mechanical properties. Tissue samples taken from the small intestine of pigs were assigned to 5 groups: fresh tissue, -28.9 °C for 7 days, -62.2 °C for 7 days, -28.9 °C for 30 days, and -62.2 °C for 30 days. Tissue was stored in PBS for the assigned temperature and duration until testing occurred with the exception of fresh tissue which was tested at sample collection. Before testing, samples were thawed in a room temperature bath, and the thickness was measured. Samples were then mounted in a biaxial test system using four anchoring rakes. Each sample was pulled to a strain of 0.2 with the corresponding forces recorded. This cycle of relaxation to 0.2 strain was repeated 5 times per sample. The thickness and force values were used to find the first Piola-Kirchhoff stress experienced at 0.2 strain and the strain energy. The average stress values in the circumferential direction were: fresh tissue: 22.3 ± 9.85 kPa; -28.9 °C for 7 days: 37.8 ± 14.1 kPa; -62.2 °C for 7 days: 46.5 ± 19.0 kPa; -28.9 °C for 30 days: 46.4 ± 22.7 kPa; -62.2 °C for 30 days: 40.1 ± 19.5 kPa. The stress and strain energy values of frozen tissue were statistically higher than the fresh tissue, although no statistical difference was found by varying duration or temperature. Based on this result, we determined that freezing tissue at any of the tested temperatures or durations increases the stiffness of the thawed tissue. This possibly occurs due to the directional formation of ice, which increases ion concentrations and glycosaminoglycan (GAG) interactions near collagen fibrils.