Investigation on the Regional Loss Factor and Its Anisotropy for Aortic Aneurysms

A Comprehensive Review on the Viscoelastic Parameters Used for Engineering Materials, Including Soft Materials, and the Relationships between Different Damping Parameters

Although the physical properties of a structure, such as stiffness, can be determined using some statical tests, the identification of damping parameters requires a dynamic test. In general, both theoretical prediction and experimental identification of damping are quite difficult. There are many different techniques available for damping identification, and each method gives a different damping parameter. The dynamic indentation method, rheometry, atomic force microscopy, and resonant vibration tests are commonly used to identify the damping of materials, including soft materials. While the viscous damping ratio, loss factor, complex modulus, and viscosity are quite common to describe the damping of materials, there are also other parameters, such as the specific damping capacity, loss angle, half-power bandwidth, and logarithmic decrement, to describe the damping of various materials. Often, one of these parameters is measured, and the measured parameter needs to be converted into another damping parameter for comparison purposes. In this review, the theoretical derivations of different parameters for the description and quantification of damping and their relationships are presented. The expressions for both high damping and low damping are included and evaluated. This study is considered as the first comprehensive review article presenting the theoretical derivations of a large number of damping parameters and the relationships among many damping parameters, with a quantitative evaluation of accurate and approximate formulas. This paper could be a primary resource for damping research and teaching.

What Are the Biomechanical Properties of an Aortic Aneurysm Associated with Quadricuspid Aortic Valve?

Association of quadricuspid aortic valve (QAV) with ascending aortic aneurysms (AsAA) is rare. A 63-year-old female with hypertension was found (on MRI) to have an ascending aortic aneurysm (52 mm in maximum diameter) and dilatation at the level of the sinotubular junction (38 mm in diameter) associated with quadricuspid aortic valve. An ascending aortic wall replacement surgery was performed. In this study, we focus on the behavior of the aorta associated with QAV considering the in vitro biomechanical characteristics and histology. The properties of QAV are closer to bicuspid aortic valve than tricuspid aortic valve, but with higher wall thickness.

3D printed ascending aortic simulators with physiological fidelity for surgical simulation

Introduction

Three-dimensional (3D) printed multimaterial ascending aortic simulators were created to evaluate the ability of polyjet technology to replicate the distensibility of human aortic tissue when perfused at physiological pressures.

Methods

Simulators were developed by computer-aided design and 3D printed with a Connex3 Objet500 printer. Two geometries were compared (straight tube and idealised aortic aneurysm) with two different material variants (TangoPlus pure elastic and TangoPlus with VeroWhite embedded fibres). Under physiological pressure, β Stiffness Index was calculated comparing stiffness between our simulators and human ascending aortas. The simulators’ material properties were verified by tensile testing to measure the stiffness and energy loss of the printed geometries and composition.

Results

The simulators’ geometry had no effect on measured β Stiffness Index (p>0.05); however, β Stiffness Index increased significantly in both geometries with the addition of embedded fibres (p<0.001). The simulators with rigid embedded fibres were significantly stiffer than average patient values (41.8±17.0, p<0.001); however, exhibited values that overlapped with the top quartile range of human tissue data suggesting embedding fibres can help replicate pathological human aortic tissue. Biaxial tensile testing showed that fiber-embedded models had significantly higher stiffness and energy loss as compared with models with only elastic material for both tubular and aneurysmal geometries (stiffness: p<0.001; energy loss: p<0.001). The geometry of the aortic simulator did not statistically affect the tensile tested stiffness or energy loss (stiffness: p=0.221; energy loss: p=0.713).

Conclusion

We developed dynamic ultrasound-compatible aortic simulators capable of reproducing distensibility of real aortas under physiological pressures. Using 3D printed composites, we are able to tune the stiffness of our simulators which allows us to better represent the stiffness variation seen in human tissue. These models are a step towards achieving better simulator fidelity and have the potential to be effective tools for surgical training.

Towards a Clinical Implementation of Measuring the Elastic Modulus of the Aorta from Cardiac Computed Tomography Images

OBJECTIVE

The elasticity of the aortic wall varies depending on age, vessel location, and the presence of aortic diseases. Noninvasive measurement will be a powerful tool to understand the mechanical state of the aorta in a living human body. This study aimed to determine the elastic modulus of the aorta using computed tomography images.

METHODS

We constructed our original formulae based on mechanics of materials. Then, we performed computed tomography scans of a silicon rubber tube by applying four pressure conditions to the lumen. The segment elastic modulus was calculated from the scanned images using our formulae. The actual modulus was measured using a tensile loading test for comparison.

RESULTS

The segment moduli of elasticity from the images were 0.525 [0.524, 0.527], 0.524 [0.520, 0.524], 0.520 [0.515, 0.523], and 0.522 [0.516, 0.532] (unit: MPa, median [25%, 75% quantiles]) for the four pressure conditions, respectively. The corresponding measurements in the tensile test were 0.548 [0.539, 0.566], 0.535 [0.528, 0.553], 0.526 [0.513, 0.543], and 0.523 [0.508, 0.530], respectively. These results indicated errors of 4.2%, 2.1%, 1.1%, and 0.2%, respectively.

CONCLUSION

Our formulae provided good estimations of the segment elastic moduli of a silicon rubber tube under physiological pressure conditions using the computed tomography images.

SIGNIFICANCE

In addition to the elasticity, the formulae provide the strain energy as well. These properties can be better predictors of aortic diseases. The formulae consist of clinical parameters commonly used in medical settings (pressure, diameter, and wall thickness).

Determination of the Material Parameters in the Holzapfel-Gasser-Ogden Constitutive Model for Simulation of Age-Dependent Material Nonlinear Behavior for Aortic Wall Tissue under Uniaxial Tension

In this study, computational simulations and experiments were performed to investigate the mechanical behavior of the aorta wall because of the increasing occurrences of aorta-related diseases. The study focused on the deformation and strength of porcine and healthy human abdominal aortic tissues under uniaxial tensile loading. The experiments for the mechanical behavior of the arterial tissue were conducted using a uniaxial tensile test apparatus to validate the simulation results. In addition, the strength and stretching of the tissues in the abdominal aorta of a healthy human as a function of age were investigated based on the uniaxial tensile tests. Moreover, computational simulations using the ABAQUS finite element analysis program were conducted on the experimental scenarios based on age, and the Holzapfel–Gasser–Ogden (HGO) model was applied during the simulation. The material parameters and formulae to be used in the HGO model were proposed to identify the failure stress and stretch correlation with age.

Local variations in material and structural properties characterize murine thoracic aortic aneurysm mechanics

We recently developed an approach to characterize local nonlinear, anisotropic mechanical properties of murine arteries by combining biaxial extension-distension testing, panoramic digital image correlation, and an inverse method based on the principle of virtual power. This experimental-computational approach was illustrated for the normal murine abdominal aorta assuming uniform wall thickness. Here, however, we extend our prior approach by adding an optical coherence tomography (OCT) imaging system that permits local reconstructions of wall thickness. This multimodality approach is then used to characterize spatial variations of material and structural properties in ascending thoracic aortic aneurysms (aTAA) from two genetically modified mouse models (fibrillin-1 and fibulin-4 deficient) and to compare them with those from angiotensin II-infused apolipoprotein E-deficient and wild-type control ascending aortas. Local values of stored elastic energy and biaxial material stiffness, computed from spatial distributions of the best fit material parameters, varied significantly with circumferential position (inner vs. outer curvature, ventral vs. dorsal sides) across genotypes and treatments. Importantly, these data reveal an inverse relationship between material stiffness and wall thickness that underlies a general linear relationship between stiffness and wall stress across aTAAs. OCT images also revealed sites of advanced medial degeneration, which were captured by the inverse material characterization. Quantification of histological data further provided high-resolution local correlations among multiple mechanical metrics and wall microstructure. This is the first time that such structural defects and local properties have been characterized mechanically, which can better inform computational models of aortopathy that seek to predict where dissection or rupture may initiate.

Inhibition of miR-155 attenuates abdominal aortic aneurysm in mice by regulating macrophage-mediated inflammation

The aim of the present study was to identify abdominal aortic aneurysms (AAA)-associated miR-155 contributing to AAA pathology by regulating macrophage-mediated inflammation. Angiotensin II (AngII)-infused apolipoprotein E-deficient (ApoE-/-) mice and THP-1 cells model of miR-155 overexpression and deficiency were used in the experiments. The expression of miR-155 was detected by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Cytokines were evaluated using enzyme-linked immunoabsorbent assay (ELISA). Western blotting was used to measure the levels of MMP-2, MMP-9, iNOS, and monocyte chemoattractant protein (MCP)-1 proteins. Immunostaining and transwell were used to determine CD68, elastic collagen, proliferation, and migration of vascular smooth muscle cells (VSMCs). The results showed that miR-155 and cytokines were up-regulated in AAA patients or ApoE-/- mice. Overexpression of miR-155 enhanced MMP-2, MMP-9, iNOS, and MCP-1 levels, and stimulated the proliferation and migration of VSMCs. Meanwhile, inhibition of miR-155 had the opposite effect. In addition, histology demonstrated accumulation of CD68 and elastic collagen-positive areas significantly decreased in miR-155 antagomir injection group. In conclusion, the results of the present study suggest that inhibiting miR-155 is crucial to prevent the development of AAA by regulating macrophage inflammation.