Iranian Journal of Veterinary Medicine 201 IJVM (2016), 10 (3): Mechanical behavior measurement of the sheep small intestine using experimental tests Rassoli, A., Fatouraee, N. * Biological Fluid Dynamics Laboratory, Biomechanics Department, Biomedical Engineering Faculty, Amirk- abir University of Technology, Tehran, Iran Abstract: BACKGROUND: There is no consistent data on the mechani- cal properties of sheep intestine. OBJECTIVES: We performed a series of biaxial strain measurement experiments and extracted the constitutive model to describe the mechanical characteris- tics of the sheep intestinal tissue. METHODS: Eleven specimens were obtained freshly from sacrifced sheep and the planar bi- axial tests were performed on the tissue specimens by applying simultaneous loads along the circumfer-ential and longitudinal directions. Then the measured data were ftted into the aniso- tropic four-parameter Fung-type model and also to the modi- fed Mooney-Rivlin model. RESULTS: The specimens showed some degree of anisotropy; the stiffer direction is not gener-ally predictable. Some of the specimens were stiffer in the circum- ferential direction, and the others in the longitudinal direction. However, the average results state the circumferential di-rec- tion as the stiffer orientation. CONCLUSIONS: It can be con- cluded that sheep intestine be-haves normally as a nonlinear anisotropic tissue which is well-characterized by the modifed Mooney-Rivlin model. Key words: anisotropic constitutive mod- el, fung-type model, nonlinear tissue, modify Mooney-Rivlin model, strain-energy function Correspondence Fatouraee, N. Biological Fluid Dynamics Lab- oratory, Biomechanics Depart- ment, Biomedical Engineering Faculty, Amirkabir University of Technology, Tehran, Iran Tel: +98(21) 64542368 Fax: +98(21) 66468186 Email: Nasser@aut.ac.ir Received: 25 January 2016 Accepted: 7 June 2016 Introduction From the anatomical view, the small intes- tine consists of three adjacent segments; duo- denum, jejunum and ileum. The wall of these segments has a multi-layered structure; the four layers are asserosa, muscolaris, submu- cosa and mucosa. The mechanical p roperties of the small intestine are mainly dominated by the submucosa and the muscular layers, while the other layers contribution is negligible (Egorov et al., 2002). To estimate intestine mechanical proper- ties, researchers performed uniaxial tensile tests on human intestine and proposed that the mechanical properties of intestine in axial and transversal directions are different (Egorov et al., 2002). The infation tests have also been applied to porcine intestine (Slatkin, 1999), human duodenum (Frøkjaer et al., 2006) and pig small intestine (Liao et al., 2010). Al- though the uniaxial tensile test data were pre- sented comprehensively, they are inadequate to determine the mechanical properties, such as nonlinear and anisotropic responses. More- over, the infation tests lack the adaptability of a planar test in addition to the fact that the sample sizes in these tests are dependent on the pressure and size of the nozzle (Chong et al., 2005). Meanwhile biaxial planar test is a proper alternative for approaching a better view on the mechanics of soft tissues. De- spite the fact that the researchers performed planar biaxial tensile tests on porcine duode- num, jejunum, and ileum and used anisotropic four-parameter Fung-type model and isotro- 201-207