Circumferential Stress Strain Research Articles

Characterization of Potting Epoxy Resins Performance Parameters Based on a Viscoelastic Constitutive Model.

To describe the evolution of residual stresses in epoxy resin during the curing process, a more detailed characterization of its viscoelastic properties is necessary. In this study, we have devised a simplified apparatus for assessing the viscoelastic properties of epoxy resin. This apparatus employs a confining cylinder to restrict the circumferential and radial deformations of the material. Following the application of load by the testing machine, the epoxy resin sample gradually reduces the gap between its surface and the inner wall of the confining cylinder, ultimately achieving full contact and establishing a continuous interface. By recording the circumferential stress-strain on the outer surface of the confining cylinder, we can deduce the variations in material bulk and shear moduli with time. This characterization spans eight temperature points surrounding the glass transition temperature, revealing the bulk and shear relaxation moduli of the epoxy resin. Throughout the experiments, the epoxy resin's viscoelastic response demonstrated a pronounced time-temperature dependency. Below the glass transition temperature, the stress relaxation response progressively accelerated with increasing temperature, while beyond the glass transition temperature, the stress relaxation time underwent a substantial reduction. By applying the time-temperature superposition principle, it is possible to construct the relaxation master curves for the bulk and shear moduli of the epoxy resin. By fitting the data, we can obtain expressions for the constitutive model describing the viscoelastic behavior of the epoxy resin. In order to validate the reliability of the test results, a uniaxial tensile relaxation test was conducted on the epoxy resin casting body. The results show good agreement between the obtained uniaxial relaxation modulus curves and those derived from the bulk and shear relaxation modulus equations, confirming the validity of both the device design and the testing methodology.

Advanced Maternal Age Impairs Uterine Artery Adaptations to Pregnancy in Rats.

Advanced maternal age (≥35 years) is associated with pregnancy complications. Aging impairs vascular reactivity and increases vascular stiffness. We hypothesized that uterine artery adaptations to pregnancy are impaired with advanced age. Uterine arteries of nonpregnant and pregnant (gestational day 20) young (4 months) and aged (9 months; ~35 years in humans) Sprague-Dawley rats were isolated. Functional (myogenic tone, n = 6–10/group) and mechanical (circumferential stress-strain, n = 10–24/group) properties were assessed using pressure myography and further assessment of elastin and collagen (histology, n = 4–6/group), and matrix metalloproteinase-2 (MMP-2, zymography, n = 6/group). Aged dams had worse pregnancy outcomes, including smaller litters and fetal weights (both p < 0.0001). Only in arteries of pregnant young dams did higher pressures (>100 mmHg) cause forced vasodilation. Across the whole pressure range (4–160 mmHg), myogenic behavior was enhanced in aged vs. young pregnant dams (p = 0.0010). Circumferential stress and strain increased with pregnancy in young and aged dams (p < 0.0001), but strain remained lower in aged vs. young dams (p < 0.05). Arteries from young nonpregnant rats had greater collagen:elastin ratios than the other groups (p < 0.05). In aged rats only, pregnancy increased MMP-2 active capacity. Altered functional and structural vascular adaptations to pregnancy may impair fetal growth and development with advanced maternal age.

Complete stress–strain constitutive model considering crack model of brittle rock

The microcrack propagation-coalescence mechanism and stress–strain constitutive model of brittle rock is of great significance for prediction of mechanical properties. To establish a more applicable constitutive model of brittle rock, the crack model based on fracture mechanics was introduced. First, based on the crack slip model and maximum circumferential stress theory, a theoretical equation for crack propagation length was established. The microcrack propagation-coalescence mechanism of brittle rock material under loading was studied. On this basis, a macro-coalescence zone model was proposed. Through the relationship between the deformation characteristics of stress–strain curves at different crack growth stages and the behavior of microcracks, a complete stress–strain constitutive model of brittle rock based on the crack model was theoretically deduced. The constitutive model was verified by the results of a coupling hydro-mechanical test of granite carried out after different heat treatment temperatures. The advanced intelligent algorithm, genetic algorithm, was adopted to simulate experimental data. The research results showed that: (1) The derived stress–strain constitutive model of brittle rock is not only high in accuracy of stress–strain simulation, but also can simulate both the axial and circumferential stress–strain curves. Moreover, the dilatancy and crack initiation stresses can be obtained directly. (2) The parameters of the stress–strain constitutive model not only have clear physical meaning, but the mechanisms of rock mechanical properties, such as dilatancy, acoustic emission, and stress fluctuation, can be explained.

Interdependency between mechanical parameters and afferent nerve discharge in remodeled diabetic Goto-Kakizaki rat intestine.

BackgroundGastrointestinal disorders are very common in diabetic patients, but the pathogenesis is still not well understood. Peripheral afferent nerves may be involved due to the complex regulation of gastrointestinal function by the enteric nervous system.ObjectiveWe aimed to characterize the stimulus–response function of afferent fibers innervating the jejunum in the Goto-Kakizaki (GK) type 2 diabetic rat model. A key question is whether changes in afferent firing arise from remodeled tissue or from adaptive afferent processes.DesignSeven 32-week-old male GK rats and seven age-matched normal Wistar rats were studied. Firing from mesenteric afferent nerves was recorded in excised jejunal segments of seven GK rats and seven normal Wistar rats during ramp test, stress relaxation test, and creep test. The circumferential stress–strain, spike rate increase ratio (SRIR), and single unit firing rates were calculated for evaluation of interdependency of the mechanical stimulations and the afferent nerve discharge.ResultsElevated sensitivity to mechanical stimuli was found for diabetic nerve bundles and single unit activity (P<0.05). The stress relaxed less in the diabetic intestinal segment (P<0.05). Linear association between SRIR and the thickness of circumferential muscle layer was found at high stress levels as well as for SRIR and the glucose level.ConclusionAltered viscoelastic properties and elevated mechanosensitivity were found in the GK rat intestine. The altered nerve signaling is related to muscle layer remodeling and glucose levels and may contribute to gastrointestinal symptoms experienced by diabetic patients.

Morphometric and biomechanical remodeling of the small intestine during aging in rats

The present study aimed to study the morphometric and biomechanical remodeling of the small intestine during aging in rats. Twenty-four male Wistar rats, aged from 6 to 22 months, were used in the study. The body weight and the wet weight per length of duodenal and ileal segments were measured at the termination of the experiments. Morphometry data was obtained by measuring the wall thickness and cross-sectional area. The mechanical test was done as a step-wise distension experiment. The intestinal diameter and length were obtained from digitized images of the segments at pre-selected pressure levels and at the no-load and zero-stress states. Circumferential and longitudinal stresses (force per area) and strains (deformation) were computed from the length, diameter and pressure data and from the zero-stress state geometry. The duodenal and ileal dimensions increased slightly from 6 to 22 months, e.g. the wall thickness and the wall cross-sectional area increased about 4% and 25% for duodenum and 5% and 8% for ileum. The opening angle gradually decreased from 154 to 117 degrees for duodenum and from 144 to 87 degrees for ileum during aging. The circumferential stress–strain curves significantly shifted to the left after 22 months (p<0.05) whereas the longitudinal stress–strain curves significantly shifted to the left after 18 months (p<0.01) both for duodenum and ileum. The intestinal wall became stiffer circumferentially and longitudinally during the aging. Furthermore, the intestinal wall was stiffer longitudinally than circumferentially. In conclusion, pronounced morphometric and biomechanical remodeling occurred in the rat intestine during aging.

Mechanics and composition of middle cerebral arteries from simulated microgravity rats with and without 1-h/d -Gx gravitation.

BackgroundTo elucidate further from the biomechanical aspect whether microgravity-induced cerebral vascular mal-adaptation might be a contributing factor to postflight orthostatic intolerance and the underlying mechanism accounting for the potential effectiveness of intermittent artificial gravity (IAG) in preventing this adverse effect.Methodology/Principal FindingsMiddle cerebral arteries (MCAs) were isolated from 28-day SUS (tail-suspended, head-down tilt rats to simulate microgravity effect), S+D (SUS plus 1-h/d −Gx gravitation by normal standing to simulate IAG), and CON (control) rats. Vascular myogenic reactivity and circumferential stress-strain and axial force-pressure relationships and overall stiffness were examined using pressure arteriography and calculated. Acellular matrix components were quantified by electron microscopy. The results demonstrate that myogenic reactivity is susceptible to previous pressure-induced, serial constrictions. During the first-run of pressure increments, active MCAs from SUS rats can strongly stiffen their wall and maintain the vessels at very low strains, which can be prevented by the simulated IAG countermeasure. The strains are 0.03 and 0.14 respectively for SUS and S+D, while circumferential stress being kept at 0.5 (106 dyn/cm2). During the second-run pressure steps, both the myogenic reactivity and active stiffness of the three groups declined. The distensibility of passive MCAs from S+D is significantly higher than CON and SUS, which may help to attenuate the vasodilatation impairment at low levels of pressure. Collagen and elastin percentages were increased and decreased, respectively, in MCAs from SUS and S+D as compared with CON; however, elastin was higher in S+D than SUS rats.ConclusionsSusceptibility to previous myogenic constrictions seems to be a self-limiting protective mechanism in cerebral small resistance arteries to prevent undue cerebral vasoconstriction during orthostasis at 1-G environment. Alleviating of active stiffening and increasing of distensibility of cerebral resistance arteries may underlie the countermeasure effectiveness of IAG.

Biomechanical characterization of ascending aortic aneurysm with concomitant bicuspid aortic valve and bovine aortic arch

Studies have shown that patients harboring bicuspid aortic valve (BAV) or bovine aortic arch (BAA) are more likely than the general population to develop ascending aortic aneurysm (AsAA). A thorough quantification of the AsAA tissue properties for these patient groups may offer insights into the underlying mechanisms of AsAA development. Thus, the objective of this study was to investigate and compare the mechanical and microstructural properties of aortic tissues from AsAA patients with and without concomitant BAV or BAA. AsAA (n=20), BAV (n=20) and BAA (n=15) human tissues were obtained from patients who underwent elective AsAA surgery. Planar biaxial and uniaxial failure tests were used to characterize the mechanical and failure properties of the tissues, respectively. Histological analysis was performed to detect medial degenerative characteristics of aortic aneurysm. Individual layer thickness and composition were quantified for each patient group. The circumferential stress–strain response of the BAV samples was stiffer than both AsAA (p=0.473) and BAA (p=0.152) tissues at a low load. The BAV samples were nearly isotropic, while AsAA and BAA samples were anisotropic. The areal strain of BAV samples was significantly less than that of AsAA (p=0.041) and BAA (p=0.004) samples at a low load. The BAA samples were similar to the AsAA samples in both mechanical and failure properties. On the microstructural level, all samples displayed moderate medial degeneration, characterized by elastin fragmentation, cell loss, mucoid accumulation and fibrosis. The ultimate tensile strength of BAV and BAA sampleswere also found to decrease with age. Overall, the BAV samples were stiffer than both AsAA and BAA samples, and the BAA samples were similar to the AsAA samples. The BAV samples were thinnest, with less elastin than AsAA and BAA samples, which may be attributed to the loss of extensibility of these tissues at a low load. No apparent difference in failure mechanics among the tissue groups suggests that each of the patient groups may have a similar risk of rupture.

Arterial stiffness identification of the human carotid artery using the stress–strain relationship in vivo

Arterial stiffness is well accepted as a reliable indicator of arterial disease. Increase in carotid arterial stiffness has been associated with carotid arterial disease, e.g., atherosclerotic plaque, thrombosis, stenosis, etc. Several methods for carotid arterial stiffness assessment have been proposed. In this study, in vivo noninvasive assessment using applanation tonometry and an ultrasound-based motion estimation technique was applied in seven healthy volunteers (age 28 ± 3.6 years old) to determine pressure and wall displacement in the left common carotid artery (CCA), respectively. The carotid pressure was obtained using a calibration method by assuming that the mean and diastolic blood pressures remained constant throughout the arterial tree. The regional carotid arterial wall displacement was estimated using a 1D cross-correlation technique on the ultrasound radio frequency (RF) signals acquired at a frame rate of 505–1010 Hz. Young’s moduli were estimated under two different assumptions: (i) a linear elastic two-parallel spring model and (ii) a two-dimensional, nonlinear, hyperelastic model. The circumferential stress ( σ θ ) and strain ( ɛ θ ) relationship was then established in humans in vivo. A slope change in the circumferential stress–strain curve was observed and defined as the transition point. The Young’s moduli of the elastic lamellae ( E 1), elastin–collagen fibers ( E 2) and collagen fibers ( E 3) and the incremental Young’s moduli before ( E 0 ⩽ ε θ < ε θ T ) and after the transition point ( E ε θ T ⩽ ε θ ) were determined from the first and second approach, respectively, to describe the contribution of the complex mechanical interaction of the different arterial wall constituents. The average moduli E 1, E 2 and E 3 from seven healthy volunteers were found to be equal to 0.15 ± 0.04, 0.89 ± 0.27 and 0.75 ± 0.29 MPa, respectively. The average moduli E 0 ⩽ ε θ < ε θ T Int and E ε θ T ⩽ ε θ Int of the intact wall (both the tunica adventitia and tunica media layers) were found to be equal to 0.16 ± 0.04 MPa and 0.90 ± 0.25 MPa, respectively. The average moduli E 0 ⩽ ε θ < ε θ T Me and E ε θ T ⩽ ε θ Ad of the tunica adventitia were found to be equal to 0.18 ± 0.05 MPa and 0.84 ± 0.22 MPa, respectively. The average moduli E 0 ⩽ ε θ < ε θ T Me and E ε θ T ⩽ ε θ Me of the tunica media were found to be equal to 0.19 ± 0.05 MPa and 0.90 ± 0.25 MPa, respectively. The stiffness of the carotid artery increased with strain during the systolic phase. In conclusion, the feasibility of measuring the regional stress–strain relationship and stiffness of the normal human carotid artery was demonstrated noninvasively in vivo.

Hydrogel–Electrospun Mesh Composites for Coronary Artery Bypass Grafts

The aim of the present study was to investigate the potential of hydrogel-electrospun mesh hybrid scaffolds as coronary artery bypass grafts. The circumferential mechanical properties of blood vessels modulate a broad range of phenomena, including vessel stress and mass transport, which, in turn, have a critical impact on cardiovascular function. Thus, coronary artery bypass grafts should mimic key features of the nonlinear stress-strain behavior characteristic of coronary arteries. In native arteries, this J-shaped circumferential stress-strain curve arises primarily from initial load transfer to low stiffness elastic fibers followed by progressive recruitment and tensing of higher stiffness arterial collagen fibers. This nonlinear mechanical response is difficult to achieve with a single-component scaffold while simultaneously meeting the suture retention strength and tensile strength requirements of an implantable graft. For instance, although electrospun scaffolds have a number of advantages for arterial tissue engineering, including relatively high tensile strengths, tubular mesh constructs formed by conventional electrospinning methods do not generally display biphasic stress-strain curves. In the present work, we demonstrate that a multicomponent scaffold comprised of polyurethane electrospun mesh layers (intended to mimic the role of arterial collagen fibers) bonded together by a fibrin hydrogel matrix (designed to mimic the role of arterial elastic fibers) results in a composite construct which retains the high tensile strength and suture retention strength of electrospun mesh but which displays a J-shaped mechanical response similar to that of native coronary artery. Moreover, we show that these hybrid constructs support cell infiltration and extracellular matrix accumulation following 12-day exposure to continuous cyclic distension.

In vivo characterization of the aortic wall stress–strain relationship

Arterial stiffness has been shown to be a good indicator of arterial wall disease. However, a single parameter is insufficient to describe the complex stress–strain relationship of a multi-component, non-linear tissue such as the aorta. We therefore propose a new approach to measure the stress–strain relationship locally in vivo noninvasively, and present a clinically relevant parameter describing the mechanical interaction between aortic wall constituents. The slope change of the circumferential stress–strain curve was hypothesized to be related to the contribution of elastin and collagen, and was defined as the transition strain ( ε θ T ). A two-parallel spring model was employed and three Young’s moduli were accordingly evaluated , i.e., corresponding to the: elastic lamellae ( E 1), elastin–collagen fibers ( E 2) and collagen fibers ( E 3). Our study was performed on normal and Angiotensin II (AngII)-treated mouse abdominal aortas using the aortic pressure after catheterization and the local aortic wall diameters change from a cross-correlation technique on the radio frequency (RF) ultrasound signal at 30 MHz and frame rate of 8 kHz. Using our technique, the transition strain and three Young’s moduli in both normal and pathological aortas were mapped in 2D. The slope change of the circumferential stress–strain curve was first observed in vivo under physiologic conditions. The transition strain was found at a lower strain level in the AngII-treated case, i.e., 0.029 ± 0.006 for the normal and 0.012 ± 0.004 for the AngII-treated aortas. E 1, E 2 and E 3 were equal to 69.7 ± 18.6, 214.5 ± 65.8 and 144.8 ± 55.2 kPa for the normal aortas, and 222.1 ± 114.8, 775.0 ± 586.4 and 552.9 ± 519.1 kPa for the AngII-treated aortas, respectively. This is because of the alteration of structures and content of the wall constituents, the degradation of elastic lamella and collagen formation due to AngII treatment. While such values illustrate the alteration of structure and content of the wall constituents related to AngII treatment, limitations regarding physical assumptions (isotropic, linear elastic) should be kept in mind. The transition strain, however, was shown to be a pressure independent parameter that can be clinically relevant and noninvasively measured using ultrasound-based motion estimation techniques. In conclusion, our novel methodology can assess the stress–strain relationship of the aortic wall locally in vivo and quantify important parameters for the detection and characterization of vascular disease.

Stretch and Shear Interactions Affect Intercellular Junction Protein Expression and Turnover in Endothelial Cells

Complex hemodynamics play a role in the localization and development of atherosclerosis. Endothelial cells (ECs) lining blood vessel walls are directly influenced by various hemodynamic forces: simultaneous wall shear stress (WSS), normal stress, and circumferential stress/strain (CS) due to pulsatile flow, pressure, and diameter changes. ECs sense and transduce these forces into biomolecular responses that may affect intercellular junctions. In this study, a hemodynamic simulator was used to investigate the combined effects of WSS and CS on EC junctions with emphasis on the stress phase angle (SPA), the temporal phase difference between WSS and CS. Regions of the circulation with highly negative SPA, such as the coronary arteries and carotid bifurcation, are more susceptible to the development of atherosclerosis. At 5 h, expression of the tight junction protein zonula occludens-1 was significantly higher for the atheroprotective SPA = 0° compared to the atherogenic SPA = -180° while the apoptosis rate was significantly higher for SPA = -180° than SPA = 0°. This decrease in tight junction protein and increase in apoptosis and associated leaky junctions suggest a decreased junctional stability and a higher paracellular permeability for atherogenic macromolecules for the atherogenic SPA = -180° compared to SPA = 0°.

Dimensions and Circumferential Stress-Strain Relation in the Porcine Esophagus in Vitro Determined by Combined Impedance Planimetry and High-Frequency Ultrasound

The mechanical properties of the esophagus are important for its function because the esophagus is subjected to changes in wall stress and strains caused by the passage of boli and the action of peristalsis. Electrodes for impedance planimetry and an ultrasound transducer were placed on the probe inside a fluid-filled bag and used to study the circumferential stress and strain relation of the porcine esophagus in vitro. Impedance planimetry was used to determine the luminal cross-sectional area (CSA) and high-frequency ultrasound was used to determine the esophageal wall thickness during bag distension. Circumferential stress and strain were computed from steady-state values of pressure, CSA, and wall thickness. The incremental elastic modulus was obtained from the slope of the stress-strain curve and was plotted as a function of strain. The steady state pressure-CSA relation was nonlinear. At the lowest and highest luminal pressure load of 1 and 5 kPa, the steady state CSA was 159+/-20 and 338+/-25 mm(2), respectively. In the same pressure range, the wall thickness decreased from 1.93+/-0.08 to 1.44+/-0.08 mm. The slope of the stress-strain curve was 2.58+/-0.35 kPa. The circumferential stress and the incremental elastic modulus as function of the strain were exponential, that is, the tissue was soft at physiologic pressures and stiffer in the supraphysiologic pressure range. These biomechanical properties of the esophageal wall seem to prevent overstretch of the esophageal wall when luminal loading becomes supraphysiologic.

Manual pressure distension of the human saphenous vein changes its biomechanical properties—implication for coronary artery bypass grafting

Patency rates of saphenous vein grafts following coronary artery bypass grafting (CABG) depend on multiple factors. Information regarding the impact of biomechanical properties of vein grafts on patency rates is not available. The objective of the present study was to evaluate whether uncontrolled manual pressure distension during routine preparation of the saphenous vein in CABG-induced changes in the biomechanical properties of the vein. The morphometric and stress–strain properties were studied in isolated segments of the saphenous vein from 12 patients undergoing elective CABG. Six segments were manually distended without pressure control and six were not distended. The mechanical test was performed as a ramp inflation using syringe pump. The vein dimensions were obtained from digitised images at different pressures as well as at the no-load and zero-stress states. The circumferences, the wall and lumen area, the wall thickness, and the outer diameter as function of the applied pressure were largest in the segments with uncontrolled manual distension compared to those without distension ( P < 0.05 ) . The opening angle and the absolute value of the residual strains were lower ( P < 0.01 ) and the circumferential stress–strain curve shifted to the left, indicating the wall became stiffer with uncontrolled manual distension compared to those without distension ( P < 0.05 ) . In conclusion, manual pressure distension changed the morphometric and biomechanical properties of the saphenous vein. The perspective is that studies on biomechanical properties on the saphenous vein may guide surgeons how to handle graft material without causing major changes of the biomechanical properties during harvesting and preparation.

A Computational Study of Flow in a Compliant Carotid Bifurcation–Stress Phase Angle Correlation with Shear Stress

The present study presents a three-dimensional, unsteady supercomputer simulation of the coupled fluid-solid interaction problem associated with flow through a compliant model of the bifurcation of the common carotid artery into the internal and external carotid arteries. The fluid wall shear stress (WSS) and solid circumferential stress/strain (CS) are computed and analyzed for the first time using the complex ratio of CS to WSS (CS/WSS). This analysis reveals a large negative phase angle between CS and WSS (stress phase angle--SPA) on the outer wall of the carotid sinus where atherosclerotic plaques are localized. This finding is consistent with other measurements and computations of the SPA in coronary arteries and the aortic bifurcation that show large negative SPA correlating with sites of plaque location and in vitro studies of endothelial cells showing that large negative SPA induces pro-atherogenic gene expression and metabolite release profiles.

Morphometric and biomechanical remodelling following reopening of the obstructed bile duct

Obstruction in the common bile duct results in duct dilatation, increased wall thickness and stiffening of the wall. Whether these changes are reversible after release of the obstruction has not yet been studied in detail. The aim of this study was to create a porcine model for release of bile duct obstruction and to describe biomechanical and morphometric parameters in a time-dependent design after the reopening. The bile duct of pigs was completely obstructed for one week and then reopened. Eighteen animals were allocated into four groups: unobstructed (group 1, n = 5), one week of obstruction (group 2, n = 5) and one week of obstruction followed by release of the obstruction after one week (group 3, n = 4), and four weeks (group 4, n = 4), respectively. The bile ducts were examined in vitro on the day of termination. One week of obstruction induced pronounced dilatation and wall thickening. Both the diameter and wall thickness decreased after release of the obstruction (p < 0.05) but without reaching the unobstructed level. The biomechanical properties in terms of the circumferential stress–strain relation differed between groups (p < 0.01). Groups 3 and 4 were transposed to the left of the other groups, indicating that they had the stiffest wall. The wall was stiffer in the longitudinal direction when compared to the circumferential direction in all groups (p < 0.05). The collagen amount decreased after releasing the obstruction (p < 0.01). The collagen amount correlated to the wall thickness (r > 0.9 and p < 0.001) and wall thickness-to-radius ratio (r > 0.7 and p < 0.05) in each of the two groups with release of the obstruction.

Structural and mechanical remodelling of the common bile duct after obstruction.

Biliary obstruction in man, most often caused by cholelithiasis, induces remodelling of the bile ducts. Obstruction-induced structural remodelling of the common bile duct (CBD) has been previously described. The mechanical changes that accompany the structural remodelling, however, have not been studied in detail. The aim of this study is to quantify the structural and mechanical changes in the CBD at different time intervals after acute obstruction. The CBD was ligated in the pig, near the duodenum, and studied after 3 h, 12 h, 2 days, 8 days and 32 days (n=5 in each group). One additional animal in each group was sham-operated. At each scheduled time, the CBD was mechanically tested in vitro with a computer-controlled volume infusion system to study the pressure-volume relationship of the CBD segment. A video camera provided simultaneous measurements of the outer dimensions of the CBD at the various pressures. The diameter and wall thickness of the CBD increased about three-fold in the 32-day group compared to the sham group (P < 0.001). The circumferential stress-strain relationship differed between groups (P < 0.001); it was shifted to the right, indicating softening, in the 3-h, 12-h, and 2-day groups and to the left, indicating stiffening, in the 8-day and 32-day group, compared to the sham group. The longitudinal stress-strain curves were all shifted to the left of the circumferential stress-strain curves (P < 0.05). The collagen area increased during obstruction (P < 0.001) but no correlation between the size of the collagen area and the biomechanical parameters was found. A practical implication of the present study serves as a warning to surgeons. A reduction in the wall stiffness in the first several days of obstruction along with an increased duct diameter and a decreased wall thickness suggest that operative procedures such as suturing, anastomosis and procedures related to ERCP must be performed with special care to avoid damage to the CBD.

Effect of re-feeding after starvation on biomechanical properties in rat small intestine

Luminal nutrients are essential for maintaining the structural and functional integrity of the gut. Starvation induces pronounced structural and biomechanical remodelling in the rat small intestine. The present work was done to study the recovery process after resumption of food intake. Twenty-five Wistar rats were allocated to five groups. Four groups fasted for 7 days but had free access to water. One of these groups served as fasted controls and was killed at the end of the fast. The other three groups were re-fed for 2, 4 and 7 days before they were euthanised. The fifth group had free access to food during the whole study (fed controls). The intestinal no-load state, zero-stress state and the stress–strain relationship during distension were studied. The intestinal segments were cut transversely into a series of short ring-shaped segments to obtain the no-load state. Each ring was cut in the radial direction to obtain the zero-stress state. The rats regained the lost body weight (22%) by the 7th day of re-feeding. The lost duodenal mass (40%) and jejunal mass (25%) were regained by the 2nd day whereas the lost mass from ileum (18%) was regained by the 4th day. The fasting-induced morphometric changes were normalised by re-feeding on the 2nd day in the duodenum and jejunum, and on the 4th day in the ileum. The longitudinal stress–strain curves shifted to the right after fasting and shifted back within two days following re-feeding ( P<0.05). The circumferential stress–strain curves in the fasted or re-fed rats changed in a similar though less pronounced way. Normal values were reached within 4–7 days for the circumferential direction. In conclusion, fasting-induced biomechanical and structural remodelling were normalised by re-feeding in a time- and location-dependent way.