Elastic and Rupture Properties of Porcine Aortic Tissue Measured Using Inflation Testing

Abstract

A new inflation test device was developed to study the mechanical properties of aortic tissue. The device was used to measure failure (rupture) strength and to determine the nonlinear, anisotropic elastic properties of porcine thoracic aorta. The tester was designed to stretch initially flat, circular tissue specimens to rupture under uniform biaxial loading. Water was chosen as the pressurizing fluid. Mechanical stretch and radius of curvature during inflation were measured optically in two orthogonal directions, and the Cauchy stress components were calculated from the deformation and the applied pressure. All porcine samples that ruptured successfully did so via a tear in the circumferential direction. Thus, the failure strength was taken to be the stress in the axial direction immediately prior to rupture. The mean failure strength was 1.75 MPa and mean axial stretch at failure was 1.52. These values agree well with published data for other arterial tissues. The nonlinearly elastic deformation behavior was modeled using a hyperelastic constitutive law of the type proposed by Holzapfel et al. [Holzapfel GA, Gasser TC, Ogden RW. J Elasticity 2000;61:1–48]. The results showed that the dominant directions of anisotropy in the porcine aortas were approximately 45° to the axial and circumferential directions, and that the isotropic contribution to the constitutive model was insignificant.

Appendix—Approximate Stress Caused by Flattening the Specimen Prior to Testing

A rough estimate of that stress caused by flattening out an originally cylindrical specimen can be determined using a mechanics of materials approach (Craig 2000). The bending moment, M, required to flatten a cylindrical specimen of initial radius R is approximately:

$$M=\frac{EI}{R}$$

(9)

where E is the (linear) modulus of elasticity and I is the moment of inertia of the specimen

The maximum bending stress would then be approximately:

$$T=\frac{M({h/2})}{I}=\frac{Eh}{2R}$$

(10)

where h is the specimen thickness. Using the data from Fig. 8, the approximate elastic modulus for the initial portion of the stress–stretch curve is 3 kPa. For an aorta with initial diameter 1.5 cm and thickness 1.5 mm, the stress resulting from Eq. 10 is then about 0.1 kPa.