Quantitative assessment of the anisotropy of vocal fold tissue using shear rheometry and traction testing

The human vocal folds are layered structures with intrinsically anisotropic elastic properties. Most testing methods assume isotropic behavior. Biaxial testing of vocal folds is strictly difficult because the very soft tissue tends to delaminate under transverse traction loads. In the present study, a linear transversely isotropic model was used to characterize the tissue in-vitro. Shear rheometry was used in conjunction with traction testing to quantify the elasticity of porcine vocal fold tissue. Uniaxial traction testing along with optical measurements were used to obtain the longitudinal modulus. The alternate vocal fold of each animal was subjected to a test-specific sample preparation and concurrently tested using dynamic shear rheometry. The stiffness ratio (i.e., the ratio of the longitudinal modulus and the transverse modulus) varied between ∼5 and ∼7 at low frequencies. The proposed methodology can be applied to other soft tissues.