Multilayer material properties of aorta determined from nanoindentation tests

Abstract

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 Young's 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.

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

“…Fat and excessive tissues were removed and six transverse sections (rings), 8–10 mm in length, were excised from the specimens (Figure 1-a). The sections were divided into three groups according to their location along the aortic tree ( z -direction) including Upper-DTA (above the 1 st intercostal artery), Mid-DTA (between the 1 st and the 2 nd intercostal arteries), and Lower-DTA (between the 3 rd and 4 th intercostal arteries) (Hemmasizadeh et al, 2012). In order to investigate the effect of circumferential location (θ-direction), four quadrants, i.e., anterior (ANT), posterior (POST), medial (MED), and lateral (LAT) quadrants, were identified as indicated in Figure 1-b.…”

“…Using a custom-made nano-indentation test setup that is described in Hemmasizadeh et al, 2012, indentation tests were performed for material characterization. Briefly, samples were moved upward toward the conical indenter (62.5° semi-vertical angle) with a ramp and hold displacement with 60 µm indentation depth, 25 ms ramp time and 20 s hold time (Figure 2).…”

“…The formulations used were: $$Pfalse(tfalse)={\int }_0^{t}Gfalse(t-\tau false)\frac{\partial P^{e}false(hfalse)}{\partial h}\frac{\partial h}{\partial \tau }d\tau$$ $$P^e=\frac{1}{\alpha }\frac{2E_0\text{italiccot}false(\beta false)}{\pi false(1-{\nu }^{2}false)}{h}^2$$ $$Gfalse(tfalse)=G_{\infty }+{\sum }_{i=1}^4{G}_i\text{italicexp}false(-{\beta }_{i}tfalse)$$ in which G ( t ) is the reduced relaxation function describing the time dependency of the response and P e ( h ) is the instantaneous elastic force function. E 0 is the instantaneous elastic (Young’s) modulus, and ν is the Poisson's ratio which was assumed to be 0.49. α is the correction factor introduced in Hemmasizadeh et al (2012) that accounts for the non-ideal geometry of the indenter, imperfections in contact area, and the nonlinearity of system compliance among other factors. For the indentation depth used in this study ( h = 60 µm), α ≈ 3.…”

“…In previous studies conducted in our laboratory, the inhomogeneous biomechanical properties of porcine Descending Thoracic Aorta (DTA) in the radial direction and in the medial quadrant were characterized using a nano-indentation technique and a viscoelastic modeling approach (Hemmasizadeh et al, 2015, 2012). Two distinct layers of equal thickness were determined with significantly different material and morphological properties, with an increasing trend in the instantaneous elastic modulus (stiffness), elastic lamella density, total protein, and elastin circumferential percentage from the inner to the outer layers.…”

“…In this study, the method of nano-indentation tests introduced in Hemmasizadeh et al (2012) was expanded to three orientations to characterize the inhomogeneity and anisotropy of porcine DTA in three dimensions, i.e., in the longitudinal, circumferential and radial directions. The spatial resolution is also significantly improved compared to our previously reported results.…”