Nonlinear viscoelastic properties of human vocal fold tissues under large-amplitude oscillatory shear

Abstract

Traditionally, viscoelastic shear properties of human vocal fold tissues have been described by the linear viscoelastic moduli G' and G” at small strain amplitudes. However, once the mechanical behavior becomes nonlinear these moduli are no longer sufficient for viscoelastic characterization. MITlaos, a rheological framework developed for better describing such behavior, was used to characterize the nonlinear viscoelastic properties of the vocal fold lamina propria when subjected to large-amplitude oscillatory shear (LAOS). MITlaos involved Fourier transform processing to convert raw stress–strain signals into a filtered/smoothed total stress signal based on the odd-integer harmonic components, with nonlinear viscoelastic parameters simultaneously derived. Rheometric testing of vocal fold cover specimens was performed with increasing strain amplitudes using a controlled-strain, simple-shear rheometer. Smoothed Lissajous–Bowditch curves generated from MITlaos were plotted in Pipkin space, and the nonlinear analysis was summarized by variations of rheological fingerprints. Pipkin diagrams served as a geometric means for identifying the onset of nonlinear behavior, and any distortions due to noise. Results showed that the human vocal fold cover when subjected to LAOS demonstrated intracycle strain stiffening and intercycle strain softening with increasing strain amplitude, as well as shear thinning with increasing strain-rate amplitude. [Work supported by NIH.]