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Page 4 of 12 Pejcic et al. Vessel Plus 2019;3:32 I http://dx.doi.org/10.20517/2574-1209.2019.18
[11]
properties of the wall from those of the individual layer behaviour. Khanafer et al. performed uniaxial
tests on the ATAA and highlighted the importance of the elastic modulus in assessing risk of aneurysm
[12]
rupture. Sassani et al. examined the regional uniaxial response (collecting bi-dimensional strain data)
of the intimal, medial, and adventitial layers of the aneurysmal ascending aorta in the circumferential and
longitudinal directions, again separating the specimens into four regions. A decoupled microstructure-
based formulation, resulting in a reduced two-fiber model, was used to describe the uniaxial behaviour
in either direction, concluding that material parameters are highly dependent on differences within the
underlying wall composition. Further studies on these histological effects are presented under the section
titled Factors Affecting Arterial Mechanics.

Biaxial tensile test
While uniaxial tests describe failure properties of vessel walls adequately, as shown by the studies cited
in the previous section, characterising loading response along a singular axis does not reflect in vivo
[13]
conditions, where the blood vessel wall is subject to multi-axial stresses. Guinea et al. performed uniaxial
tests on healthy thoracic aortas, with samples obtained from persons deceased from non-cardiovascular
causes. They acknowledged the limitations of the uniaxial test, stating that it poorly reproduces the
complex loading conditions seen at the physiological level, which are better described by biaxial testing.
In addition, uniaxial testing does not provide a true understanding of the anisotropic properties of the
vessel wall, as the circumferential and axial test strip come from different locations. Biaxial testing is done
with a square sample, which is placed in a loading rig with four arms, generally by means of hooks, at
90 degrees to each other [Figure 1B]. A significant volume of literature has focused on biaxial testing of
[19]
both healthy and diseased tissue [14-18] . Alreshidan et al. used biaxial testing on resected ascending aortic
tissue to compare to and validate the use of in vivo speckle tracking transesophageal echocardiography to
estimate aortic stiffness to better stratify the risk of aortic aneurysm rupture. The advancement of in vivo
imaging techniques gives rise to more reliable data; however, the behaviour of the aorta with these tests is
limited to patient-specific physiological conditions. In addition, in vivo testing presents multivariate data,
as it is not possible to disregard other non-loading effects, such as the effect of surrounding perivascular
adipose tissue. Drawbacks of biaxial testing arise from the attachment method, i.e., the failure of the tissue
usually occurs at the attachment location and rupture is also influenced by the components used to hold
[20]
the tissue . In addition, testing samples of various shapes does not allow for accurate comparison since
the geometry, in addition to the clamping method, can alter the mechanical properties observed, as was
[21]
described by Waldman et al. .
Bulge inflation test
The bulge inflation test, also referred to as the membrane bulge test, gives information on biaxial behaviour,
[22]
and was used most recognizably in the work by Mohan and Melvin on healthy human thoracic aorta
specimens. This test involves using a square specimen, which is secured in the inflation device through
an airtight seal [Figure 1C]. A fluid, generally water, is released at a specified rate, and the expansion of
the specimen is tracked optically with complex digital imaging techniques to obtain the strain field. This
technique has also been applied to analyze the mechanical behaviour of aneurysms [5,23,24] . Romo et al. [23]
utilized the bulge inflation test on the ascending aorta to observe the most likely location of aneurysmal
rupture many stages of deformation before the rupture took place by noting the region that had the
most amount of localized thinning. They created local thickness over pressure maps to predict the site of
aneurysm rupture and they posited that the site of rupture mostly occurs not at the location of maximum
stress but at the location of greatest wall weakening.

Inflation-extension test
An inflation-extension test differs from the aforementioned techniques in that it replicates the in vivo
cylindrical loading scenario of a blood vessel. In general, an intact portion of a blood vessel is extended