Circumferential Stretch Ratio Research Articles

Location specific multi-scale characterization and constitutive modeling of pig aorta

The mechanical and structural behavior of the aorta depend on physiological functions and vary from proximal to distal. Understanding the relation between regionally varying mechanical and multi-scale structural response of aorta can be helpful to assess the disease outcomes. Therefore, this study investigated the variation in mechanical and multi-scale structural properties among the major segments of aorta such as ascending aorta (AA), descending aorta (DA) and abdominal aorta (ABA), and established a relation between mechanical and multi-structural parameters. The obtained results showed significant increase in anisotropy and nonlinearity from proximal to distal aorta. The change in periphery length and radii between load and stress free configuration was also found increasing far from the heart. Opening angle was significantly large for ABA than AA and DA (AA/DA vs ABA; p = 0.001). Mean circumferential residual stretch (ratio of mean periphery length at load and stress free configurations) was found decreasing between AA and DA, and then increasing between DA to ABA and its value was significantly more for ABA (AA vs DA; p = 0.041, AA vs ABA; p = 0.001, DA vs ABA; p = 0.001). The waviness of collagen fibers, collagen fiber content, collagen fibril diameter and total protein content were found significantly increasing from proximal to distal. Pearson correlation test showed a significant linear correlation between variation in mechanical and multi-scale structural parameters over the aortic length. Residual stretch was found positively correlated with collagen fiber content (r = 0.82) whereas opening angel was found positively correlated with total protein content (TPC) (r = 0.76).

Predicting complex nonspherical instability shapes of inertial cavitation bubbles in viscoelastic soft matter.

Inertial cavitation in soft matter is an important phenomenon featured in a wide array of biological and engineering processes. Recent advances in experimental, theoretical, and numerical techniques have provided access to a world full of nonlinear physics, yet most of our quantitative understanding to date has been centered on a spherically symmetric description of the cavitation process in water. However, cavitation bubble growth and collapse rarely occur in a perfectly symmetrical fashion, particularly in soft materials. Predicting the onset of dynamically arising, nonspherical instabilities in soft matter has remained a significant, unresolved challenge, in part due to the additional constitutive complexities introduced by the surrounding nonlinear viscoelastic solid. Here, we provide a new theoretical framework capable of accurately predicting the onset of nonspherical instability shapes of a bubble in a soft material by explicitly accounting for all pertinent nonlinear interactions between the cavitation bubble and the solid surroundings. Comparison with high-resolution experimental images from laser-induced cavitation events in a polyacrylamide hydrogel show excellent agreement. Interestingly, and consistent with experimental findings, our model predicts the emergence of various dynamic instability shapes for circumferential bubble stretch ratios greater than 1, in contrast to most quasistatic investigations. Our new theoretical framework not only provides unprecedented insight into the cavitation dynamics in a soft, nonlinear solid, but also provides a quantitative means of interpreting bubble dynamics relevant to a wide array of engineering and medical applications as well as natural phenomena.

A Mathematical Model of the Wall of a Rat Cerebral Resistance Vessel

A mathematical model of the mechanical properties of the rat small cerebral artery wall is presented. It is assumed that the active component of the stress has a circumferential direction and is a function of the concentration of the activator of smooth muscle cell contractions and circumferential and radial stretch ratios; longitudinal stretching is not considered. Calculations showed a significant decrease in both the circumferential stress and circumferential stretch ratios in the vascular wall after activation; the distribution of these values in the transmural direction became more uniform. The active component has a predominant effect on the formation of the total stress. The circumferential stress component was several times higher than the radial and longitudinal ones. The modulus of the ratio of the radial stress component to the circumferential one increased as a result of activation, and a tendency to a decrease in its value with increasing circumferential stretching was observed. Numerical analysis revealed a high sensitivity of both the radius and stress to even a small change in the values of the parameters included in the functional dependence of the stress on the concentration of the smooth muscle contraction activator.

Investigation of the Influence of Smooth Muscle Contractions on the Properties of the Wall of a Small Arterial Vessel

The plane problem of the effect of contractions of smooth muscle cells in the wall of a resistance vessel under the action of transmural pressure on the radius and the distribution of stresses in the vascular wall is considered. It is assumed that in the inactivated state the vessel wall is hyperelastic, and the contraction of smooth muscle cells as a result of activation contributes to the circumferential stress alone. Based on the model and published experimental data, a functional dependence of the active stress on the concentration of the activator of smooth muscle contraction is obtained. Calculations show that the total stress in the wall is determined mainly by the active component. With increasing pressure, contractions of smooth muscle cells lead to decrease in stresses, while the pattern of the distribution of circumferential stresses changes. The circumferential stretch ratios also decrease with activation and their distribution becomes more homogeneous. In both passive and active vessels, the modulus of the ratio of the radial to circumferential stress decreases with increase in tension, this ratio being several times greater in the active vessel than in the passive one.

Impact of Calcium Quantifications on Stent Expansions.

Severely calcified plaque is of great concern when planning and implementing a stenting intervention. In this work, computational models were developed to investigate the influence of calcium characteristics on stenting outcomes. The commonly used clinical measurements of calcium (i.e., the arc angle, maximum thickness, length, and volume) were varied to estimate stenting outcomes in terms of lumen gain, stent underexpansion, strut malapposition, and stress or strain distributions of the stenotic lesion. Results have shown that stenting outcomes were most sensitive to the arc angle of the calcium. A thick calcium with a large arc angle resulted in poor stenting outcomes, such as severe stent underexpansion, D-shaped lumen, increased strut malapposition, and large stresses or strains in the plaque. This was attributed to the circumferential stretch of the tissue. Specifically, the noncalcium component was stretched significantly more than the calcium. The circumferential stretch ratios of calcium and noncalcium component were approximately 1.44 and 2.35, respectively, regardless of calcium characteristics. In addition, the peak stress or strain within the artery and noncalcium component of the plaque occurred at the area adjacent to calcium edges (i.e., the interface between the calcium and the noncalcium component) coincident with the location of peak malapposition. It is worth noting that calcium played a protective role for the artery underneath, which was at the expense of the overstretch and stress concentrations in the other portion of the artery. These detailed mechanistic quantifications could be used to provide a fundamental understanding of the impact of calcium quantifications on stent expansions, as well as to exploit their potential for a better preclinical strategy.

Mechanical behavior and matrisome gene expression in the aneurysm-prone thoracic aorta of newborn lysyl oxidase knockout mice.

Mutations in lysyl oxidase (LOX) are associated with thoracic aortic aneurysm and dissection (TAAD). Mice that do not express Lox (Lox-/- ) die soon after birth and have 60% and 40% reductions in elastin- and collagen-specific cross-links, respectively. LOX inactivation could also change the expression of secreted factors, the structural matrix, and matrix-associated proteins that constitute the aortic matrisome. We hypothesized that absence of Lox will change the mechanical behavior of the aortic wall because of reduced elastin and collagen cross-linking and alter the expression levels of matrisome and smooth muscle cell (SMC) genes in a vascular location-specific manner. Using fluorescence microscopy, pressure myography, and gene set enrichment analysis, we visualized the microarchitecture, quantified the mechanical behavior, and examined matrisome and SMC gene expression from ascending aortas (AAs) and descending aortas (DAs) from newborn Lox+/+ and Lox-/- mice. Even though Lox-/- AAs and DAs have fragmented elastic laminae and disorganized SMCs, the unloaded outer diameter and wall thickness were similar to Lox+/+ AAs and DAs. Lox-/- AAs and DAs have altered opening angles, circumferential stresses, and circumferential stretch ratios; however, only Lox-/- AAs have increased pressurized diameters and tangent moduli. Gene set enrichment analysis showed upregulation of the extracellular matrix (ECM) regulator gene set in Lox-/- AAs and DAs as well as differential expression of secreted factors, collagens, ECM-affiliated proteins, ECM glycoproteins, and SMC cell cycle gene sets that depend on the Lox genotype and vascular location. These results provide insights into the local chemomechanical changes induced by Lox inactivation that may be important for TAAD pathogenesis.NEW & NOTEWORTHY Absence of lysyl oxidase (Lox) causes thoracic aortic aneurysms. The aortic mechanical behavior of Lox-/- mice is consistent with reduced elastin and collagen cross-linking but demonstrates vascular location-specific differences. Lox-/- aortas show upregulation of matrix remodeling genes and location-specific differential expression of other matrix and smooth muscle cell gene sets.

Mechanical behavior of porcine thoracic aorta in physiological and supra-physiological intraluminal pressures.

Understanding the mechanical behavior of aorta under supra-physiological loadings is an important aspect of modeling tissue behavior in various applications that involve large deformations. Utilizing inflation-extension experiments, the mechanical behavior of porcine descending thoracic aortic segments under physiological and supra-physiological intraluminal pressures was investigated. The pressure was changed in the range of 0-70 kPa and the deformation of the segment was determined in three dimensions using a custom-made motion capture system. An orthotropic Fung-type constitutive model was characterized by implementing a novel computationally efficient framework that ensured material stability for numerical simulations. The nonlinear rising trend of circumferential stretch ratio [Formula: see text] from outer toward inner wall was significantly increased at higher pressures. The increase in [Formula: see text] from physiological pressure (13 kPa) to 70 kPa was 13% at the outer wall and 22% at the inner wall; in this pressure range, the longitudinal stretch ratio [Formula: see text] increased 20%. A significant nonlinearity in the material behavior was observed as in the same pressure range, and the circumferential and longitudinal Cauchy stresses at the inner wall were increased 16 and 18 times, respectively. The overall constitutive model was verified in several loading paths in the [Formula: see text] space to confirm its applicability in multi-axial loading conditions.

Biaxial deformation of collagen and elastin fibers in coronary adventitia

The microstructural deformation-mechanical loading relation of the blood vessel wall is essential for understanding the overall mechanical behavior of vascular tissue in health and disease. We employed simultaneous mechanical loading-imaging to quantify in situ deformation of individual collagen and elastin fibers on unstained fresh porcine coronary adventitia under a combination of vessel inflation and axial extension loading. Specifically, the specimens were imaged under biaxial loads to study microscopic deformation-loading behavior of fibers in conjunction with morphometric measurements at the zero-stress state. Collagen fibers largely orientate in the longitudinal direction, while elastin fibers have major orientation parallel to collagen, but with additional orientation angles in each sublayer of the adventitia. With an increase of biaxial load, collagen fibers were uniformly stretched to the loading direction, while elastin fibers gradually formed a network in sublayers, which strongly depended on the initial arrangement. The waviness of collagen decreased more rapidly at a circumferential stretch ratio of λθ = 1.0 than at λθ = 1.5, while most collagen became straightened at λθ = 1.8. These microscopic deformations imply that the longitudinally stiffer adventitia is a direct result of initial fiber alignment, and the overall mechanical behavior of the tissue is highly dependent on the corresponding microscopic deformation of fibers. The microstructural deformation-loading relation will serve as a foundation for micromechanical models of the vessel wall.

Mechanical characterization of human aortas from pressurization testing and a paradigm shift for circumferential residual stress

Material properties needed for accurate stress analysis of the human aorta are still incompletely known, especially as many reports have ignored the presence of residual stresses in the aortic wall. To contribute new material regarding these issues, we carried out measurements and pressurization testing on ascending, thoracic and abdominal aortic samples from 24 human subjects aged 38–77 years, and evaluated the opening angle describing the circumferential residual stress level present in the aorta. We determined material constants for the aorta by gender, anatomic location and age group, according to a simple phenomenological constitutive model. The unpressurized aortic radius positively correlated with age, and the circumferential and longitudinal stretch ratios under systemic pressure negatively correlated with age, confirming the known enlargement and stiffening of the aorta with aging. The opening angle was measured to range from a minimum of 89° to above 360° for extreme cases. For given aortic dimensions and material properties, analysis of the in vivo circumferential and longitudinal mural stress distributions indicated a profound influence of the opening angle. For instance, in the thoracic aorta of males aged 38–66, opening angles in the range of 0° to 80° (resp. 60°) may equalize the gradient of in vivo circumferential (resp. longitudinal) stress between the inner and outer layers of the aorta, as commonly expected; however, opening angles above 160° (resp. 120°) may cause the gradient of circumferential (resp. longitudinal) stress to reverse and increase compared to the case without residual stress, putting the maximum stresses toward the adventitia instead of the intima. Even though the analysis of the aortic wall excluded possible longitudinal residual stresses as well as material inhomogeneities, such as constitutive differences between the intimal, medial and adventitial layers, the experimental data reported herein are important to aortic modeling at large and the better understanding of aortic pathophysiology in particular.

The Layered Structure of Coronary Adventitia under Mechanical Load

The mechanical loading-deformation relation of elastin and collagen fibril bundles is fundamental to understanding the microstructural properties of tissue. Here, we use multiphoton microscopy to obtain quantitative data of elastin and collagen fiber bundles under in situ loading of coronary adventitia. Simultaneous loading-imaging experiments on unstained fresh coronary adventitia allowed morphometric measurements of collagen and elastin fibril bundles and their individual deformation. Fiber data were analyzed at five different distension loading points (circumferential stretch ratio λθ = 1.0, 1.2, 1.4, 1.6, and 1.8) at a physiological axial stretch ratio of λaxial = 1.3. Four fiber geometrical parameters were used to quantify the fibers: orientation angle, waviness, width, and area fraction. The results show that elastin and collagen fibers in inner adventitia form concentric densely packed fiber sheets, and the fiber orientation angle, width, and area fraction vary transmurally. The extent of fiber deformation depends on the initial orientation angle at no-distension state (λθ = 1.0 and λaxial = 1.3). At higher distension loading, the orientation angle and waviness of fibers decrease linearly, but the width of collagen fiber is relatively constant at λθ = 1.0–1.4 and then decrease linearly for λθ ≥ 1.4. A decrease of the relative dispersion (SD/mean) of collagen fiber waviness suggests a heterogeneous mechanical response to loads. This study provides fundamental microstructural data for coronary artery biomechanics and we consider it seminal for structural models.

Experimental characterization of the distribution of collagen fiber recruitment

The passive biomechanical behavior of blood vessels is generally modeled by a parallel arrangement of elastin and collagen, with collagen recruitment depending on vessel strain. We experimentally determined the collagen recruitment distribution using confocal microscopy. Digital images from sections of rabbit carotid artery under increasing circumferential stretch ratio were acquired. The straightness of the fibers was measured to compute the fraction of recruited fibers for each stretch ratio. The experimental distribution obtained was then used in the model proposed by Zulliger et al. This model is based on a strain energy function (SEF), which provides a constituent-based description of the wall mechanics. Using this model, a fit of the pressure–radius curve was then performed by using the experimental collagen recruitment distribution with the collagen elastic constant as a free fit parameter. The fit was good, but the value of the collagen elastic constant obtained was below values reported in the literature. A second fit of the pressure–radius curve was also performed, whereby the collagen distribution was left free to adapt while the collagen elastic constant was set to a physiological value. The differences between the experimental collagen engagement distribution and the distribution obtained when the collagen elastic constant was fixed were analyzed. Good qualitative agreement was found between the experimental distribution of collagen recruitment and the model. Some experimental limitations and modeling approximations remain. Nevertheless, experimental proof of progressive collagen recruitment was established, which validates the basic hypothesis of the model.

T0201-3-6 円周方向引張負荷に伴うブタ胸大動脈壁内微視的変形挙動の観察([T0201-3]細胞の構造と流れにおけるマイクロ・ナノスケール解析(3))

The artery walls consist of structural components that have various mechanical properties: elastin, collagen and smooth muscle cells. Such variety may cause heterogeneous deformation of the components at a microscopic level when arterial tissue is subjected to macroscopic uniform stretch. To examine this hypothesis, we sliced 15-μm-thick specimens from porcine thoracic aorta with cryotome, and stretched them under a transmitted light microscope to observe their microscopic deformation caused by macroscopic stretch. Microscopic strain was found to be highly heterogeneous: stretch ratio in the elastic lamina (EL) and smooth muscle layer (SML) was 1.1-2.3 and 1.1-2.1, respectively, at circumferential stretch ratio of 1.5. Strain difference between an EL and a SML adjacent to each other could be as high as 6 times. These results suggest that the microscopic deformation in the artery wall is highly heterogeneous. Mechanical environment of the artery wall might not be so homogeneous at a cellular level.

Mechanical behavior of human aortas: Experiments, material constants and 3-D finite element modeling including residual stress

Segments of fresh human ascending, thoracic descending and abdominal aortas from eight male sexagenarians were pressurized under closed-end and free extension conditions. The median unpressurized inner radii for the ascending, thoracic and abdominal locations were 14.21, 9.67 and 7.16 mm, respectively. The median thickness was similar in the ascending and thoracic regions, at about 1.6 mm, while it was 1.2 mm in the abdominal region. The opening angle was not statistically different between regions, with a median of −38°. Under 13.3 kPa pressure, the median circumferential stretch ratio was about 1.26 in all three aortic locations; the median longitudinal stretch ratio was similar in the ascending and thoracic regions, at about 1.13, while it was 1.05 in the abdominal region. Material constants for a three-dimensional hyperelastic anisotropic constitutive model were determined. Experimental, analytical and finite element results showed excellent agreement, validating the novel experimental approach and the numerical methods used. When residual stress was not taken into account, stresses were highest on the inside of the aorta, with a gradient across the wall of about 200 and 50 kPa in the circumferential and longitudinal directions, respectively. When residual stress was included as described by negative opening angles, stresses were highest on the outside of the aorta, with a gradient across the wall in excess of 400 kPa for the circumferential direction, and on the order of 150 kPa for the longitudinal direction. The mechanical consequences of negative opening angles had not been appreciated so far, and deserve further investigation.

Surrounding tissues affect the passive mechanics of the vessel wall: theory and experiment

The stress and strain in the vessel wall are important determinants of vascular physiology and pathophysiology. Vessels are constrained radially by the surrounding tissue. The hypothesis in this work is that the surrounding tissue takes up a considerable portion of the intravascular pressure and significantly reduces the wall strain and stress. Ten swine of either sex were used to test this hypothesis. An impedance catheter was inserted into the carotid or femoral artery, and after mechanical preconditioning pressure-cross-sectional area relations were obtained with the surrounding tissue intact and dissected away (untethered), respectively. The radial constraint of the surrounding tissue was quantified as an effective perivascular pressure on the outer surface of the vessel, which was estimated as 50% or more of the intravascular pressure. For carotid arteries at pressure of 100 mmHg, the circumferential wall stretch ratio in the intact state was approximately 20% lower than in the untethered state and the average circumferential stress was reduced by approximately 70%. For femoral arteries, the reductions were approximately 15% and 70%, respectively. These experimental data support the proposed hypothesis and suggest that in vitro and in vivo measurements of the mechanical properties of vessels must be interpreted with consideration of the constraint of the surrounding tissue.

Transmural strain distribution in the blood vessel wall

The transmural distributions of stress and strain at the in vivo state have important implications for the physiology and pathology of the vessel wall. The uniform transmural strain hypothesis was proposed by Takamyzawa and Hayashi (Takamizawa K and Hayashi K. J Biomech 20: 7-17, 1987; Biorheology 25: 555-565, 1988) as describing the state of arteries in vivo. From this hypothesis, they derived the residual stress and strain at the no-load condition and the opening angle at the zero-stress state. However, the experimental evidence cited by Takamyzawa and Hayashi (J Biomech 20: 7-17, 1987; and Biorheology 25: 555-565, 1988) to support this hypothesis was limited to arteries whose opening angles (theta) are <180 degrees. It is well known, however, that theta > 180 degrees do exist in the cardiovascular system. Our hypothesis is that the transmural strain distribution cannot be uniform when theta; is >180 degrees. We present both theoretical and experimental evidence for this hypothesis. Theoretically, we show that the circumferential stretch ratio cannot physically be uniform across the vessel wall when theta; exceeds 180 degrees and the deviation from uniformity will increase with an increase in theta; beyond 180 degrees. Experimentally, we present data on the transmural strain distribution in segments of the porcine aorta and coronary arterial tree. Our data validate the theoretical prediction that the outer strain will exceed the inner strain when theta > 180 degrees. This is the converse of the gradient observed when the residual strain is not taken into account. Although the strain distribution may not be uniform when theta exceeds 180 degrees, the uniformity of stress distribution is still possible because of the composite nature of the blood vessel wall, i.e., the intima-medial layer is stiffer than the adventitial layer. Hence, the larger strain at the adventitia can result in a smaller stress because the adventitia is softer at physiological loading.

Pipette aspiration technique for the measurement of nonlinear and anisotropic mechanical properties of blood vessel walls under biaxial stretch

A pipette aspiration technique was proposed for the measurement of nonlinear mechanical properties of arteries under biaxial stretching. A cross-shaped specimen of porcine thoracic aorta whose principal axes corresponded with the axial and circumferential directions of the aortic walls was excised. The intraluminal surface of the specimen was aspirated with a circular cross-sectioned glass pipette while the specimen was stretching in the axial and circumferential directions in 10% increments. The elastic modulus agreed with the incremental elastic modulus obtained through a conventional pressure-diameter test of the same specimen to within an error of 30% at a circumferential stretch ratio below 1.3 and an axial stretch ratio of 1.0, 1.1 or 1.2, which represent lower range of physiological stretch ratios for the porcine aorta. A rectangular cross-sectioned pipette was utilized to measure anisotropic properties of the specimen under biaxial stretching. When aspirated with such a pipette, the specimens’ elastic properties along the length of the rectangular pipette cross section can be neglected. The elastic modulus was found to increase rapidly when the specimen was stretched in the direction of the pipette's width. Thus, pipette aspiration should have many advantages such as well measurement of the local nonlinear and anisotropic mechanical properties of blood vessel walls.

ANALYSIS OF STRESS AND PRESSURE IN THE HUMAN ALVEOLAR WALL BEFORE BURSTING

A strain energy function for the human alveolar wall in vivo is developed based on its length-tension properties. Using large deformation theory, this function is employed to determine the relationship between circumferential stress and stretch ratio as well as between alveolar pressure and stretch ratio. We find that both the circumferential stress and the pressure in the alveolar wall rise very rapidly if the wall is stretched even slightly more than that which occurs at the pressures attained during mechanical in-exsufflation (MI-E). MI-E involves producing a high inspiratory air pressure and then quickly switching to a large negative air pressure in order to remove mucus from the airways.

A new method to denude the endothelium without damage to media: structural, functional, and biomechanical validation.

The intimial thickening that occurs in human and animal atherogenesis can be induced by mechanical injury to the endothelium. The objective of the present study was to develop a new method to induce arterial endothelial injury without damage to the media for future investigations of mechanisms of intimal thickening and atherogenesis. A specifically designed catheter was inserted into the common femoral artery of Wistar rats (n = 9) through an arteriotomic mouth. After application of Tyrode solution containing 0.14 M KCl on the surface of the vessel, the vessel contracted onto the catheter. The catheter was then moved back and forth to scrape away the endothelium. The left common femoral artery of the same rat was subjected to the standard balloon injury model. The two models were evaluated structurally, functionally, and biomechanically. Structurally, we verified that both techniques remove the endothelium, but the balloon method damages the media. Functionally, we examined the contractile response of the artery to [K+] and norepinephrine 2 days after the denudation. We found that the right femoral artery underwent contraction in response to [K+], whereas the left artery did not. Furthermore, neither artery responded to norepinephrine. Biomechanically, we measured the pressure-diameter relationship and the zero-stress state of the vessel and computed the stress-strain relation. The circumferential stretch ratios at 120 mmHg were 1.38 +/- 0.08 for the control, 1.41 +/- 0.08 (P > 0.05) for the new method, and 1.56 +/- 0.09 for the balloon injury (P < 0.05). The opening angles at the zero-stress state were 113 +/- 21 degrees for the control, 102 +/- 18 degrees for the new method (P > 0.05), and 8 +/- 13 degrees for the balloon injury (P < 0.001). In conclusion, the new method removes the endothelium while maintaining the structure, contractile function, and biomechanical properties of the vessel.

Compliance of a Biocomposite Vascular Tissue in Longitudinal and Circumferential Directions as a Basis for Creating Artificial Substitutes

Experimental studies on the mechanical behavior of 11 human common carotid arteries at different values of internal pressure and axial force were performed on a device allowing us to measure the internal pressure, axial force, and circumferential and longitudinal deformations of the vessel. The persons age ranged from 20 to 28 years. Two types of experiments were carried out. In the first series, cylindrical samples were gradually loaded by an internal pressure from 0 to 200 mmHg at different longitudinal stretch ratios. The second series included axial extension of the same samples at different circumferential stretch ratios. The undulation level of elastin membranes in longitudinal and circumferential directions for samples fixated at different values of internal pressure and longitudinal stretch ratio were determined from histological data.

Local Strain Measurement of Arterial Wall Based on Longitudinal Observation.

The purpose of this study was to establish a method for measuring the circumferential difference in the deformation of artery walls during pressurization. Thoracic aortas of Japanese white rabbits were stretched to their in vivo length and pressurized to 80 mmHg in a physiological saline at room temperature. Four 5-mm long small needles (φ0.4 mm) were stuck perpendicularly into the aortic wall at equal intervals on a circumference. Displacement of the needles during pressurization from 0 to 200 mmHg was observed from longitudinal direction with a CCD camera and laterally with two cameras placed in an opposite direction across the aorta. The images of the cameras were used to identify the points at penetration of each needle and to obtain circumferential length between the needles. Circumferential stretch ratio was significantly higher in the ventral side than in the dorsal, indicating that mechanical heterogeneity exists even in healthy arteries. The present method may be useful for further understanding of vascular mechanics.