 The differences in the mechanical properties of the aneurysmal blood vessels and the healthy blood vessels stem from the compositional differences of the vessels.Compared to normal aortas, aneurysmal aortas have a much higher volume fraction of collagen and ground substance (54.8% vs. 95.6%) and a much lower volume fraction of elastin (22.7% vs. 2.4%) and smooth muscles (22.6% vs. 2.2%), which contribute to higher initial stiffness.It was also found that the ultimate tensile strength, or the strength to withstand rupture, of aneurysmal vessel wall is 50% lower than that of normal aortas.