Microstructure and mechanics of healthy and aneurysmatic abdominal aortas: experimental analysis and modelling

Abstract

Soft biological tissues such as aortic walls can be viewed as fibrous composites assembled by a ground matrix and embedded families of collagen fibres. Changes in the structural components of aortic walls such as the ground matrix and the embedded families of collagen fibres have been shown to play a significant role in the pathogenesis of aortic degeneration. Hence, there is a need to develop a deeper understanding of the microstructure and the related mechanics of aortic walls. In this study, tissue samples from 17 human abdominal aortas (AA) and from 11 abdominal aortic aneurysms (AAA) are systematically analysed and compared with respect to their structural and mechanical differences. The collagen microstructure is examined by analysing data from second-harmonic generation imaging after optical clearing. Samples from the intact AA wall, their individual layers and the AAA wall are mechanically investigated using biaxial stretching tests. A bivariate von Mises distribution was used to represent the continuous fibre dispersion throughout the entire thickness, and to provide two independent dispersion parameters to be used in a recently proposed material model. Remarkable differences were found between healthy and diseased tissues. The out-of-plane dispersion was significantly higher in AAA when compared with AA tissues, and with the exception of one AAA sample, the characteristic wall structure, as visible in healthy AAs with three distinct layers, could not be identified in AAA samples. The collagen fibres in the abluminal layer of AAAs lost their waviness and exhibited rather straight and thick struts of collagen. A novel set of three structural and three material parameters is provided. With the structural parameters fixed, the material model was fitted to the mechanical experimental data, giving a very satisfying fit although there are only three material parameters involved. The results highlight the need to incorporate the structural differences into finite-element simulations as otherwise simulations of AAA tissues might not be good predictors for the actual in vivo stress state.

Mechanical strength of aneurysmatic and dissected human thoracic aortas at different shear loading modes

“…Even though the layers could be easily peeled off in several cases, the borderlines between the intima/media and media/adventitia, especially for aneurysmatic samples, were not always clearly visible due to the process of aneurysm formation. For more discussion on that issue see Niestrawska et al (2016). All tests were conducted with an extension rate of 1.0 mm/min, although it is not established that this rate corresponds to a physiological value.…”

Numerical analyses of the interrelation between extracellular smooth muscle orientation and intracellular filament overlap in the human abdominal aorta

“…To capture the layer-specific passive mechanical response of the aorta a micro-structural constitutive model is used. The model was recently proposed by [28] and mechanical and structural parameters can be found in [48]. Briefly, the aortic wall is treated as a thick-walled composite material, i.e.…”

“…Because of the fact that there are no suitable experimental data available on the contractile behavior of human aortic tissue these active length-tension and tension-time data are chosen and slightly modified, see Appendix A. Material and structural parameters for the proposed passive constitutive model are taken from [48], and are summarized in Table 2. Micro-structural investigations (second-harmonic generation imaging) and biaxial mechanical tensile tests of healthy abdominal aortas are performed, and the experimental data are fitted to the non-symmetric collagen fiber dispersion model provided in this work.…”

“…The medial strip is modeled with a referential length of 15.0 mm (circumferential direction), a referential width 3.0 mm (axial direction) and a referential thickness of 0.8467 mm. [48] Due to symmetry, only one-eighth of the medial strip is modeled. We use 250 eight-node hexahedral elements, and apply the mixed 1∕ 0 element formulation throughout the computation to avoid the well-known locking phenomena.…”

“…The inner radius of the ring is set to i = 6.8375 mm, [29,30] the medial wall thickness is med = 0.8467 mm, while the adventitial wall thickness is adv = 0.5884 mm in the reference configuration. [48] Due to symmetry only one half of the aortic ring is simulated and discretized by 640 eight-node hexahedral elements (320 elements per layer), using the mixed 1∕ 0 element formulation throughout the simulation. All nodes at the media/adventitia interface are linked together.…”