Planar radial extension for constitutive modeling of anisotropic biological soft tissues
Characterizing the mechanical properties of anisotropic biological soft tissues poses unique challenges. One among these challenges is the lack of
a priori information on the underlying fiber orientations that drive its anisotropic behavior. While fiber orientations in very thin planar tissues may be characterized by non-destructive means prior to mechanical testing, this is not possible with thick tissues. A planar radial extension test of circular planar biological soft tissue specimen is proposed for simultaneously ascertaining both the preferred fiber orientations and the mechanical properties. When pulled by a radial load, a circular specimen will deform into an elliptical shape revealing its stiffest direction (minor axis of ellipse) and its anisotropic properties. Numerical investigations were used to assess the stress and strain fields generated and justify the analytical equations to calculate stress and strain. A simple manual force-controlled planar radial extension device was constructed. A porcine aortic specimen was subjected to planar radial extension testing and the test data was fit to an anisotropic finite strain constitutive model to demonstrate the feasibility of this method for assessment of anisotropic biological soft tissues whose material symmetry is not known a priori.