Experimental study and biomechanical characterization for the passive small intestine: Identification of regional differences.

This article reports regional differences in the inflation/extension properties and the no-load and zero-stress geometry, serving as the reference state for defining the multiaxial response, within and across the individual duodenum, jejunum, and ileum of the small intestine of middle-aged rats in the normal condition. The descriptive/predictive capacity of three phenomenological models, regularly appearing in the biomechanics literature, to characterize the anisotropic response of intestinal tissue was investigated in terms of best-fit parameters. Our inflation/extension results showed that the pressure-radius relationship was nonlinear, dissimilar to the near-constant, linear force-pressure relationship at all axial stretches, suggesting an energetically-favorable response in the entire loading range. The geometrical results showed that small intestinal dimensions were greatest in the proximal duodenum and smallest in the proximal jejunum, unlike the opening angle and circumferential residual strains that were less spatially variable; contrasting previous findings on relatively immature animals. The quadratic and exponential model was the most suitable descriptor of the passive pseudo-elastic response for the entire loading range from all regions, while the four- and seven-parameter exponential models afforded good data representations at the physiologic loading range primarily. The stress-strain and material characterization results showed progressive softening in the axial direction within the duodenum and jejunum, unlike the ileum that stiffened axially towards the caecum. Much remains to be learned as regards regional variations in biomechanical properties and the adaptive response during aging, but the reported data provide input for a quantitative analysis of physiological functions, mechanobiology, and clinical interventions, e.g. small intestine-repair device interaction.