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Pejcic et al. Vessel Plus 2019;3:32 I http://dx.doi.org/10.20517/2574-1209.2019.18 Page 3 of 12

Figure 1. Schematics of the mechanical tests described for aortic mechanics where A: shows a uniaxial test; B: biaxial testing; C: the
bulge-inflation test; D: depicts inflation-extension testing; E: opening angle testing: Upon cutting an intact circumferential segment of an
artery in an unloaded state (described by radius, r), an expansion of the segment is observed over time, leading to an equilibrium zero-
stress state (described by radius, R, and opening angle, theta). The residual strains can be obtained from comparison between the two
states; F: nano-indentation test: an indenter of known material properties is pressed into the tissue with a known loading pattern, after
which the area of the indentation is observed, and the hardness of the material can be calculated

loading or strain rate, or the strain measure used to calculate elasticity parameters, make lateral
[4,8]
comparisons between papers difficult at best .

A focus of uniaxial testing on the aneurysmal aorta has been considering the region- and layer-dependent
[9]
variations of the wall behaviour. Iliopoulos et al. first showed that the ascending thoracic aortic aneurysm
(ATAA) exhibits heterogeneity between the anterior, posterior, right and left lateral regions. Note that
the following studies presented in this section divided specimens into these same four regions. The data
from the circumferential direction suggested this direction to be stiffer than the longitudinal direction at
physiological and high stresses. While no differences in the peak elastic modulus were observed between
the four regions in the circumferential direction, the longitudinal direction results indicated that the
anterior region was significantly less stiff than the other regions. The results showed that no correlation
[9]
existed between failure stress and the diameter of the whole ATAA . The layer-specific differences of the
[10]
tunica intima, media, and adventitia from uniaxial data of the ATAA were documented by Sokolis et al. . In
general, the circumferential stiffness was recorded as higher than the longitudinal direction in the adventitia
and media, but not the intima, though all layers had a higher failure stress in the circumferential direction.
This is true for all four regions (previously introduced) of the media, but only for the anterior and posterior
regions of the adventitia. This study also brought up the important concern that the adhesive connections
between the three layers needs to be considered when drawing conclusions regarding the overall mechanical