Neutral axis location in bending and Young's modulus of different layers of arterial wall.

With few exceptions, experimental results on the blood vessel elasticity have been analyzed with the blood vessel wall treated as a homogeneous material, probably because the experiments have been limited to inflation and stretching. To advance the subject, we must evaluate the force-deformation relationship of different layers of the vessel. A key strategy to do this is to study nonaxisymmetric deformation of the vessel wall so that the different layers of the vessel may deform in different amounts at different places. One of the most effective nonaxisymmetric deformations that can be imposed on the vessel wall is bending. The first important question to ask about bending is, Where is the neutral axis? In this study, a method to determine the neutral axis is presented. We found that the neutral axis of the aorta of the pig lies in the medial layer about one-third of the wall thickness from the endothelium. We measured the strain distribution in the vessel wall by optical methods. Using the load-deflection relationship, we evaluated the Young's modulus of the intima-media layer and that of the adventitia. They differ by an order of magnitude. Our results show that the Young's modulus of the intima-media layer of the pig thoracic aorta is 43.2 +/- 15.8 kPa, whereas the Young's modulus of the adventitial layer is 4.70 +/- 1.72 kPa, in a linear range of the stress-strain relationship including the zero-stress state and the no-load state.