Biomechanical testing and material characterization for the rat large intestine: regional dependence of material parameters.

The function of the large bowel is to absorb water from the remaining indigestible food matter and subsequently pass useless waste material from the body, but there has been only a small amount of data in the literature on its biomechanical characteristics that would facilitate our understanding of its transport function. Our study aims to fill this gap by affording comprehensive inflation/extension data of intestinal segments from distinct areas, spanning a physiologically relevant deformation range (100-130% axial stretches and 0-15 mmHg lumen pressures). These data were characterized by the Fung-type exponential model in the thick-walled setting, showing reasonable agreement, i.e. root-mean-square error ~30%. Based on optimized material parameters, i.e. a(1)<a(2) and the anisotropy degree (a(1) + a(4))/(a(2) + a(4))<1, with a(1) and a(2) representing circumferential and axial stiffness, and a(4) the stiffness interaction, the tissue was stiffer axially. The transverse colon was the stiffest of all regions both circumferentially (a(1) = 5.304 ± 0.952 versus proximal colon: 4.043 ± 0.643, distal colon: 1.505 ± 0.222 and rectum: 2.339 ± 0.285) and axially (a(2) = 16.639 ± 0.792 versus proximal colon: 16.580 ± 1.042, distal colon: 13.209 ± 1.185 and rectum: 12.553 ± 0.689). Our biomechanical testing and material characterization results for the large intestine of healthy young animals are expected to aid in comprehending the adaptation/remodeling that occurs with ageing, pathological conditions and surgical procedures, as well as for the development of suitable biomaterials for replacement.