Jakub Kronek

Site-Specific Mechanical Properties of Aortic Bifurcation

This study describes stiffness of arterial wall near aortic bifurcation. Uni-axial tensile tests with specimens cut out from human abdominal aorta, aortic bifurcation and common iliac arteries were carried out. Material anisotropy was verified as well as site-specific stiffness. The stiffness was described by initial secant modulus of elasticity (EINIT). This parameter was gained from stress/strain data at 12% of deformation. The tissue samples in various regions and directions were concluded as described as follows: region of aorto-iliac bifurcation is significantly stiffer in longitudinal direction than in the circumferential direction in all of three examined parts of arterial tree. The comparison between tissue samples cut and loaded in longitudinal direction indicates that arterial tree stiffens in caudal direction. No significant change in material properties was found in the apex of bifurcation in comparison with adjacent tissue. Stiffening of arterial tree during ageing was verified.

Inflation-Extension Test of Silicon Rubber-Nitinol Composite Tube

Composite tube fabricated from elastomer matrix reinforced with NiTi wires was tested within simultaneous inflation and extension. Four different longitudinal weights were applied (m=41g; 188g; 488g; and 785g) to induce initial prestrain of the tube. The inflation was induced with repeating pressurization up to 200kPa; four times with each weight. Displacements and loading pressure were recorded into PC.

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.

A distribution of collagen fiber orientations in aortic histological section

Distributions of collagen fibrils and smooth muscle cells nuclei (SMC) orientations were investigated in histological sections obtained from medial layer of human thoracic aorta. The sections were stained with van Gieson. Digital image of the sections was converted to binary pixel map with target component represented in white (logical unity). Selected image was indicated to elucidate the sensitivity to threshold conditions and three different binary conversions were performed. Consecutively images were processed by in house developed software BinaryDirections which uses an algorithm of the rotation line segment to determine significant directions in digital images. The algorithm operates in the way that in each target pixel a line segment is rotated step by step to explore neighborhood of the pixel. Exploring the neighborhood the number of unity pixels in each rotating step is determined. The distribution of orientations in the entire image is gained after normalization either as averaged density distribution from all pixels or as an histogram of the most abundant directions in the image. It was found that both collagen fibrils and SMC nuclei analyses give significant peak in distributions. Its value ranges between 45° - 65° (defined as declination from longitudinal axis of an artery in a tubular configuration) depending on the method. It implies that preferred direction in aortic medial layer was oriented circumferentially rather than longitudinally. This conclusion was almost independent of the threshold setup. Results suggest that the orientations of SMC nuclei and collagen fibrils are mutually correlated and determination of collagen fibril orientations, which may be stained in insignificant manner, could be supported with SMC nuclei orientations to obtain more realistic models.

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.