The opened-up configuration of the artery wall has long been assumed to be stress-free. This is questionable in a microscopic level. The aortic media is made of concentric layers whose unit is called a lamellar unit, a pair of elastic lamina (EL) and a smooth muscle-rich layer (SML). Recently, we found that the EL was about 2.5 times stiffer than the SML. If the circumferential stress in the in vivo condition is the same between the two layers, residual stress of each layer should be different because the stress-strain relationships differ. Such residual stress is not released fully by radial cutting, but is released in the area close to the cut surface, causing hills and valleys on the surface due to residual stress. To check this hypothesis, we have developed a scanning micro indentation tester, a scaled-up version of the atomic force microscope, and measured the topography and the stiffness distribution of the cut surface. The surface of the section of porcine thoracic aortas shows hill and valley pattern corresponding with their histology. The hills were more than three times stiffer than the valleys, indicating that the hills are the ELs and the valleys the SMLs, and the ELs are compressed and the SMLs stretched in the lamellar unit. A finite element analysis showed that the residual stress in the EL and the SML is much higher than those estimated in the unloaded ring-like segments. Fairly large stress may still reside in the opened-up aortic wall.
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