| Esophagus is a distensible muscular tube that connects pharynx and stomach. The function of the esophagus is to move food by peristaltic motion, which is the result of the interaction of the tissue forces in the esophageal wall and the hydrodynamic forces in the food bolus. Since the function of esophagus is mainly mechanical, this work is focused on providing quantitative measurement on passive biomechanical properties of esophagus. The study including four parts: 1, Shear properties of the normal intact esophagus. 2, The effect of experimental diabetes on the morphometric and biomechanical properties of the esophagus. 3, Biomechanical properties of the muscle layer and mucosa layer and the effect of diabetes on these layers. 4, Consolidate the stress-strain data in the form of a proposed exponential strain energy function and to obtain material constants. Diabetes was induced by a single injection of streptozotocin (STZ). The form of diabetes mellitus induced by parenteral STZ administration in rats is insulin-dependent (type I). 27 rats were divided into four groups according to the surviving time after STZ treatment: 4 days (n=7), 7 days (n=7), 14 days (n=7),28 days (n=6). Another 8 rats were used as normal controls. The samples were taken from the middle part of esophagus. Two rings were cut from the each end of the sample to measure the geometric parameters of the no-load state and the opening angle at zero-stress state. The remaining part was excised and studied in vitro using a self-developed biomaterial test machine. Using this machine, the esophagus was stepwise elongated and inflated and continuously twist in circumferential-longitudinal direction. In the normal controls and 28 days of diabetes group, after the intact esophagus was tested, the mucosa and muscle layer were separated using microsurgery and tested under the same loading procedure as mentioned above. The esophagus was treated as a membrane when calculating the stress and strain, the longitudinal and circumferential stresses were considered to be evenly distributed along the wall thickness while the radial stressand other transverse shear stresses were ignored. The torque vs twist-angle relation was approximate to be linear at a specified pressure and longitudinal stretch ratio. Thus, the shear modulus can be computed by the torque, twist angle and polar moment of inertial at this state. However, the shear modulus various greatly with the changing inflation pressure and longitudinal stretch ratio. In the process of inflation at a fixed longitudinal stretch ratio, the shear modulus was linear as function of the transmural pressure, circumferential stress and longitudinal stress, and non-linear as function of the circumferential stretch ratio and strain. In the process of elongation at a fixed transmural pressure, the shear modulus was non-linear as function of the longitudinal stretch ratio and strain. Type I diabetes had the following effect on the biomechanical properties of esophagus: In the intact esophagus, the opening angle increased after 14 days(P=0.002), circumferential stiffness increased after 7, 14 and 28 days diabetes (P<0.05). The longitudinal stiffness and shear modulus increased after 28 days diabetes (P<0.05). In the mucosa and muscle layer, the outer and inner perimeter and luminal area increased after 28 days diabetes in both layers, the increase was more prominent in mucosa layer(P<=0.001) than in the muscle layer (P<0.05). The circumferential stiffness of the mucosa increased significantly after 28 days diabetes (P<0.05). When comparing the biomechanical properties between the intact esophagus, muscle and mucosa layer, the mucosa layer was much stiffer than muscle layer in circumferential, longitudinal and shear directions. The mechanical behavior of intact layer is more like the behavior of muscle layer than the mucosa layer, probably because the muscle layer is much thicker than the mucosa layer and because mucosa buckles in the esophagus. The stress-strain data fitted the proposed exponential strain energy...
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