Ali Hemmasizadeh

Multilayer material properties of aorta determined from nanoindentation tests

In a wide range of biomechanical modeling of aorta from traumatic injury to stent grafts, the arterial wall has been considered as a single homogeneous layer vessel, ignoring the fact that arteries are composed of distinct anatomical layers with different mechanical characteristics. In this study, using a custom-made nanoindentation technique, changes in the mechanical properties of porcine thoracic aorta wall in the radial direction were characterized using a quasi-linear viscoelastic model. Two layers of equal thickness were mechanically distinguishable in descending aorta based on the radial variations in the instantaneous Youngs modulus E and reduced relaxation function G(t). Overall, comparison of E and G(∞) of the outer half (70.27±2.47 kPa and 0.35±0.01) versus the inner half (60.32±1.65 kPa and 0.33±0.01) revealed that the outer half was stiffer and showed less relaxation. The results were used to explain local mechanisms of deformation, force transmission, tear propagation and failure in arteries.

Correlations between transmural mechanical and morphological properties in porcine thoracic descending aorta.

Determination of correlations between transmural mechanical and morphological properties of aorta would provide a quantitative baseline for assessment of preventive and therapeutic strategies for aortic injuries and diseases. A multimodal and multidisciplinary approach was adopted to characterize the transmural morphological properties of descending porcine aorta. Histology and multi-photon microscopy were used for describing the media layer micro-architecture in the circumferential-radial plane, and Fourier Transform infrared imaging spectroscopy was utilized for determining structural protein, and total protein content. The distributions of these quantified properties across the media thickness were characterized and their relationship with the mechanical properties from a previous study was determined. Our findings indicate that there is an increasing trend in the instantaneous Young׳s modulus (E), elastic lamella density (ELD), structural protein (SPR), total protein (TPR), and elastin and collagen circumferential percentage (ECP and CCP) from the inner towards the outer layers. Two regions with equal thickness (inner and outer halves) were determined with significantly different morphological and material properties. The results of this study represent a substantial step toward anatomical characterization of the aortic wall building blocks and establishment of a foundation for quantifying the role of microstructural components on the functionality of aorta.

Investigation of inhomogeneous and anisotropic material behavior of porcine thoracic aorta using nano-indentation tests.

This study investigates the inhomogeneity and anisotropy of porcine descending thoracic aorta in three dimensions using a custom-made nano-indentation technique and a quasi-linear viscoelastic modeling approach. The indentation tests were conducted in axial, circumferential, and radial orientations with about 100 μm spatial resolution. The ratio of the elastic moduli obtained in different orientations was used to quantify the tissue local anisotropy. The distal sections were generally stiffer than the proximal ones in both axial and circumferential indentations. Four distinct layers were identified across the thickness with significantly different mechanical properties. The stiffness of the medial quadrant was significantly lower than all other quadrants in axial indentation. The anisotropic behavior of the tissue was more pronounced in the lateral quadrant of the distal sections. The results of this study can be used to better understand the mechanisms of aorta deformation and improve the spatial accuracy of computational models of aorta.

Material properties of different layers of aorta

The arterial wall has been considered as a single homogeneous layer vessel in a wide range of biomechanical modeling of aorta from traumatic injury to stent grafts, ignoring the fact that arteries are composed of distinct anatomical layers with different mechanical characteristics. In this study, changes in the mechanical properties of porcine thoracic aorta wall in the radial direction were characterized using a custom-made nanoindentation technique and a quasi-linear viscoelastic model. Two layers of equal thickness were mechanically distinguishable based on the radial variations in the instantaneous Youngs modulus E and reduced relaxation function G(t). The results showed that the outer half layer was stiffer and showed less relaxation than the inner half one. These layers may correspond to media and adventitia in the specimens.

Mechanical characterization of Polyacrylamide for Prostate Tissue-Mimicking Phantoms

Tissue-mimicking phantoms are essential for understanding the mechanics of needle-tissue interactions. Recent developments in active needles, especially for prostate brachytherapy, involve self actuation of the needle using actuators placed on the body of the needle, where actuation forces is generated by resistively heating shape memory alloy wires with electric current [1]. Therefore, the phantom materials should be able to mimic both the mechanical and thermal properties of the real tissue. Polyacrylamide (PA) gels, apart from meeting these requirements, can simulate the tissue damage from excessive heating by adding proteins to the tissue [2]. As an initial step, this work will synthesize PA gels that mimic the prostate tissue and then study the mechanical properties of these gels. Mechanical properties of the PA gels were determined by unconstrained compression and indentation tests.

Characterization of Multilayer Material Properties of Descending Aorta

In a wide range of biomechanical modeling of aorta from traumatic injury to stent grafts, the arterial wall has been considered as a single homogeneous layer vessel, ignoring the fact that arteries are composed of distinct anatomical layers with different mechanical characteristics. In this study, using a custom-made nanoindentation technique, changes in the mechanical properties of porcine thoracic aorta wall in the radial direction were characterized using a quasi-linear viscoelastic model. Two layers of equal thickness were mechanically distinguishable based on the radial variations in the instantaneous Young’s modulus E and reduced relaxation function G(t). The overall results showed that the outer half was stiffer and showed less relaxation than the inner half. These layers may correspond to media and adventitia in the specimens.Copyright

Material Properties of Aorta From Nanoindentaion Tests

Aorta is assumed as a homogenous material in current finite element models, but in fact aorta is composed of three major layers: intima, media, and adventitia with different microstructural organizations that result in different material properties. Understanding the material properties of these layers is essential for studying the local mechanisms of deformation, force transmission, and failure in aorta. The material properties of aorta wall layers were determined from nano-indentation tests. The results show that the value of shear modulus generally increases from the inner wall toward the outer wall. Aorta material time dependency was independent from the radial orientation. This study explains why aorta rupture initiates near the inner wall and propagates toward the outer wall.Copyright