Jan Vesely

Correlations between age, prestrain, diameter and atherosclerosis in the male abdominal aorta

The longitudinal prestrain of arteries facilitates their physiological function. Remodeling, adaptation and aging result in an age-dependent magnitude of the pretension. Although the phenomenon is known, detailed statistics, especially for human arteries, are lacking. This study was designed to propose the regression model capable of estimating the prestrain of the human abdominal aorta. The length of the abdominal aorta before, l, and after excision from the body, L, the diameter, heart weight, thickness of left ventricle and degree of atherosclerosis were collected in autopsies of 156 male cadavers of known age. Longitudinal prestrain was quantified by means of the stretch ratio λ=l/L. Statistical analysis revealed significant dependence between age, prestrain, diameter and atherosclerosis, which were best fitted to the power law equation. Longitudinal prestretch reduced with age significantly; λmean=1.30±0.07 for age<30 (n=29), whereas λmean=1.06±0.03 for age>59 (n=31) with p-value<0.0001. Raw data gave linear correlation coefficients as follows: λ-age (R=-0.842); l-age (R=0.023); L-age (R=0.476); (l-L)-age (R=-0.811). It was concluded that longitudinal prestrain decreases nonlinearly with age and both age and diameter are suitable predictors of the prestrain. Data suggests that unloaded length elongates with age in contrast to the elastic retraction.

Age-related distribution of longitudinal pre-strain in abdominal aorta with emphasis on forensic application.

It is a well-known fact that the length of an artery in situ and the length of an excised artery differs. Retraction of blood vessels is usually observed. This pre-tension plays crucial role in arterial biomechanics. It augments an artery wall load-bearing capacity. This paper presents the longitudinal pre-strain of the human aorta as an index of human age. The length of abdominal aortas was measured during autopsies before and after segment resection. The longitudinal pre-strain was calculated in 130 donors; 100 male and 30 female bodies. The pre-strain was defined as the ratio between in situ length and the length after the excision. The mean pre-strain was found to be 1.18±0.10 for male and 1.14±0.10 for female sample (mean±standard deviation). The age in the male group was 41.6±15.9 years; and 47.7±17.7 years in the female group. Statistical analysis revealed the correlation coefficient between age and pre-strain r=-0.821 and r=-0.839 in male and female group, respectively. The analysis also confirmed close correlation between aortic circumference and age; and between circumference and pre-strain. Linear and power law regression equations were employed and prediction intervals were computed. The power law estimates the age more accurately than linear one model. Nevertheless, especially for small values of the pre-strain (aged individuals) the linear model can be advantageous.

Effect of Polyvinyl Alcohol Concentration on the Mechanical Properties of Collagen/Polyvinyl Alcohol Blends

The effects of the polyvinyl alcohol (PVA) concentration on mechanical properties of hydrogels based on blends of native or denatured collagen / PVA were examined. Blends of PVA with collagen were obtained by mixing the solutions in different ratios, using glycerol as a plasticizer. The solutions were cast on polystyrene plates and the solvent was allowed to evaporate at room temperature. Uniaxial tensile tests were performed in order to obtain the initial modulus of elasticity (up to deformation 0.1), the ultimate tensile stress and the deformation at failure of the material in the water-saturated hydrogel form. It was found that the material was elastic and the addition of PVA helped to enhance both the ultimate tensile stress and modulus of elasticity of the films. Samples prepared from denaturated collagen showed the higher ultimate tensile stress and the deformation at failure in comparison with those prepared from native collagen. The results suggest that we could expect successful application of the collagen/PVA biomaterial for tissue engineering.

Constitutive Modelling and Histology of Vena saphena

The inflation-extension test was performed in order to obtain the mechanical response (stress-strain curves) of the human vein - vena saphena magna (usually used for coronary artery bypass graft surgery). Tubular samples of the vein were inflated four times up to the pressure approx. 4 kPa (vein pressure) and then four times up to approx. 16 kPa (systolic pressure). The experiments were recorded by the CCD camera. The longitudinal and circumferential deformations of the tube were evaluated using the edge detection method. The experimental data were fitted by anisotropic, nonlinear, constitutive model in order to obtain model parameters, especially the parameter which can be explained as collagen fibres orientation approximation. This parameter was then compared with the findings from histology. The histology analyses based on label-free imaging were performed additionally to the mechanical testing. Collagen (most important load-bearing component of the vein wall) was visualized using second harmonic generation imaging (SHG, excitation at 860 nm by a tunable IR pulse laser, detection at 430±10 nm). This method enabled us to observe collagen through the vein wall. It was found that the collagen fibres are helically aligned within the vein at an angle 37±6° measured from circumferential axis. The results of collagen orientation angle show a good agreement of findings obtained from histology and from constitutive model.

The Influence of the Opening Angle on the Stress Distribution through the Saphenous Vein Wall

In the present study, the inflation test of human vena saphena magna was conducted to obtain data suitable for multi-axial constitutive modeling at overloading conditions (pressures up to approximately 18 kPa). Subsequently the data were fitted with a hyperelastic, nonlinear and anisotropic constitutive model based on the theory of the closed thick-walled tube. It was observed that initial highly deformable behavior in the pressure–circumferential stretch response is followed by progressive large strain stiffening, which is in contrast to the pressure–axial stretch response. The effect of possible residual stress was evaluated in a simulation of the intramural stress distribution with the opening angle prescribed to 0°, 10°, 20°, 30°, 40°, and 50°. The result suggests that the optimal opening angle making the stress distribution through the wall thickness uniform is about 20°.

EFFECT OF STERILIZATION ON MECHANICAL PROPERTIES OF BIOLOGICAL COMPOSITE

In this study, composite tubes were manufactured from biological collagenous matrix and reinforcing polyester mesh. The effect of sterilization on mechanical properties of this structure was evaluated using inflation-extension tests. Samples were exposed to two types of sterilization (ethylene oxide and irradiation). The control (non-sterilized) samples were also tested. It was found that the process of sterilization (especially irradiation) dramatically affects the final mechanical properties of the material. These findings should be taken into account when such collagenous material is assumed to be used in tissue engineering.

Collagen structures in pericardium and aortic heart valves and their significance for tissue engineering

Biological prostheses of human heart valves are prepared from autologous heart valves and from xenogeneic heart valves or pericardium. Xenogenous and allogenous biological prostheses are associated with adverse immune reactions, thrombosis and degeneration, and thus they have a high rate of reoperation. An optimum autologous tissue for heart valve grafts therefore needs to be found. Human pericardium was assessed as a potential source, and was compared with native human heart valves and porcine and bovine pericardium and heart valves. The tissues were evaluated for their mechanical properties, their collagen content and structure, and their histological structure. We observed differences in mechanical properties, collagen fibre orientation, the shape of the collagen bundles, and the collagen content among the pericardium samples and heart valves of different origin.

LONGITUDINAL PRESTRETCH IN HUMAN INFRARENAL AORTA

Elastic arteries in situ undergo significant longitudinal prestretch. Longitudinal prestretch is advantageous from a biomechanical viewpoint; it helps to pressure pulse wave transmission. Direct measurement of the prestretch in living individuals is problematic. In such a situation statistics of post mortal measurements can be helpful. Systematic measurement of the axial prestretch was added to regular autopsy protocol in our laboratory. The results of two-year-study are summarized herein.

In vitro Coronary Stent Implantation: Vessel Wall-Stent Interaction

This study was designed to assess the biomechanical interactions between the coronary artery wall and intracoronary stents following implantation in vitro. Balloon expandable stent was deployed in vitro into the sample of the left anterior descending coronary artery of a 67 years old female with multiple atherosclerotic lesions. The stent was selected to match approximately the internal diameter of the healthy segment of the target artery. The stent implantation procedure was recorded with CCD camera. Digital images were subsequently processed with the edge detector based on Canny algorithm. Obtained coordinates of the surface contours were used in the deformation analysis. For the sake of simplicity the deformation was considered as the ratio between the distances of deformed and reference contour points at the same longitudinal position. We found that the stent expansion induced significant over-stretching of the external coronary artery. We have concluded that optical tracking of the external surface of the artery during the stent deployment provides sufficiently accurate deformation analysis potentially useful in the assessment of biomechanical interactions during intracoronary stenting.

Mullins effect in human aorta described with limiting extensibility evolution

Cyclic uni-axial tensile tests with samples of human aorta were performed with an aim to obtain data describing the Mullins effect of arterial tissue. According to presumed anisotropy, reported widely, both samples oriented longitudinally and circumferentially were tested. Each of tested samples underwent cyclic tension up to a particular value of a stretch four times, consecutively maximum limit of reached stretch was increased and subsequent four cycles were performed. Significant stress softening of aortic tissue and residual strains were confirmed. An idealization was made in such a way that reloading and unloading curves are coincident. It was hypothesized that the stress softening observed within reloading of previously loaded tissue may be described by an evolution of material parameters. These parameters should be related to an alternation of internal structure. We proposed a model based on changes in limiting fiber extensibility of fibrillar component of the aortic wall, primarily represented by a collagen. The arterial wall was assumed to be hyperelastic transversely isotropic material with different response under primary loading and unloading. A stored energy function was additively split into isotropic and anisotropic part. Preferred direction in continuum, defined in referential configuration, was assumed to be unchanged with cyclic loading. Every straining level in the cyclic test had its own value of fiber extensibility related to strain maximum previously reached. The isotropic matrix response was modeled using Neo-Hooke term with shear modulus values different under primary loading and reloading, however all reloading values were held the same. The predictions of the model described above were in good agreement with observations.