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In the past studies, the resistance of magnetically controlled capsules running through the small intestine has been modeled assuming that the small intestine was a circular tube with a constant diameter. Peristalsis is an important character of the human gastrointestinal system, and it would result in some changes in the diameter of the intestine, meaning that the existing resistance models would no longer be applicable. In this paper, based on the assumption that intestinal peristalsis is actually a sinusoidal wave, a resistance model of the capsule running in the peristaltic intestine is established, and then it is validated experimentally. The model provides a realistic foundation for the optimization and control of the magnetically controlled endoscopy.

The small intestine is known for exhibiting complex nonlinear and time-dependent properties, requiring a suitable visco-hyperelastic model to characterize it. In this paper, the development of the constitutive model is divided into two steps. In the first step, we use a generalized anisotropic structure-based model, which has previously been used to model a wide range of tissues, to describe the hyperelastic behavior of the intestine. In the second step, the Prony series is used to describe the viscoelastic behavior of the small intestine; thus, a model characterizing the nonlinear visco-hyperelasticity of the small intestine during large deformation is developed. To verify the accuracy of the model, we use rabbit small intestine for tension tests and relaxation tests performed at 37 °C. The coefficients of determination of the hyperelastic and viscoelastic parts of the constitutive model are greater than 0.99, which is superior to the representative models currently used. The development of the model in this paper is fundamental to the analysis of the interaction between the capsule endoscopy and the small intestine.