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Development of aortic aneurysms includes significant morphological changes within the tissue: collagen content increases, elastin content reduces and smooth muscle cells degenerate. We seek to quantify the impact of these changes on the passive mechanical response of aneurysms in the supra-physiological loading range via mechanical testing and constitutive modeling. We perform uniaxial extension tests on circumferentially and axially oriented strips from five thoracic (65.6 years ± 13.4, mean ± SD) and eight abdominal (63.9 years ± 11.4) aortic fusiform aneurysms to investigate both continuous and discontinuous softening during supra-physiological loading. We determine the significance of the differences between the fitted model parameters: diseased thoracic versus abdominal tissues, and healthy (Weisbecker et al., J. Mech. Behav. Biomed. Mater. 12, 93-106, 2012) versus diseased tissues. We also test correlations among these parameters and age, Body Mass Index (BMI) and preoperative aneurysm diameter, and investigate histological cuts. Tissue response is anisotropic for all tests and the anisotropic pseudo-elastic damage model fits the data well for both primary loading and discontinuous softening which we interpret as damage. We found statistically relevant differences between model parameters fitted to diseased thoracic versus abdominal tissues, as well as between those fitted to healthy versus diseased tissues. Only BMI correlated with fitted model parameters in abdominal aortic aneurysmal tissues. publisher: Elsevier articletitle: Human thoracic and abdominal aortic aneurysmal tissues: Damage experiments, statistical analysis and constitutive modeling journaltitle: Journal of the Mechanical Behavior of Biomedical Materials articlelink: http://dx.doi.org/10.1016/j.jmbbm.2014.10.003 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved. ispartof: Journal of the Mechanical Behavior of Biomedical Materials vol:41 pages:92-107 ispartof: location:Netherlands status: published

Experiments on cross-linked actin networks in form of a circular cylinder conducted with a rheometer and parallel-plate geometry resemble a torsional problem of a cylinder undergoing large deformation. A commonly used approximation for the analysis of such experiments is simple shear which is inappropriate for the global analysis of the more complex 3D torsion deformation. We compare the solutions of the torsion of a cylinder with simple shear on the basis of three (phenomenological) rubber models and two network models for cross-linked actin. We start with rubber elasticity and show that the approximation for materials with linear shear elasticity may be reasonable. In the case of cross-linked actin networks, however, the strong strain-stiffening behavior causes higher deviations of simple shear from the more realistic torsional mode. Furthermore, we show that the frequently used eight-chain model cannot account for the correct normal stress behavior of cross-linked actin networks. A recently proposed affine network model reproduces the correct sign for the normal stress for both versions of the boundary conditions. The two solutions, however, differ significantly so that an approximation of the deformation mode in a parallel-plate rheometer by simple shear should be used with caution.

Many treatments for cardiovascular diseases include an endovascular insertion of stents or stent grafts into arteries, a procedure which may cause high tissue stresses and even damage in the arterial wall. In order to study such problems by using finite element methods, both appropriate constitutive models and experimental data on human tissue samples are required. Layer-specific experimental data for human tissue tested up to the supra-physiological loading range are rare in the literature. In this study, intact and layer-separated experimental data from uniaxial extension tests are presented for human thoracic and abdominal aortas with non-atherosclerotic intimal thickening undergoing supra-physiological loading. A novel pseudo-elastic damage model, proposed to describe discontinuous softening in aortic arterial tissues, is fit to the obtained experimental data. Fitting of the model with and without consideration of damage accumulation in the non-collagenous matrix material reveals that tissue damage is primarily related to the collagen fiber fabric. By employing the fit model, the effect of aortic tissue pre-conditioning on the material parameters from the resulting data fits is evaluated. Histological examination of the collagen fibers under different applied stretches is used to gain more insights into the structural changes of the tissue under supra-physiological loading.

Catheter based surgical procedures for the treatment of cardiovascular disease such as the placement of stent-grafts can generate high tissue stresses and may cause damage in the arterial wall. To facilitate studies of such problems, we conducted layer-specific uniaxial extension tests under supra-physiological loading on the three tissue layers (intima, media, adventitia) of the human aorta with non-atherosclerotic intimal thickening. We present a pseudo-elastic damage model describing discontinuous softening and a fit to experimental data. Furthermore, the mechanical properties of the media, treated with either elastase or collagenase, are investigated.