Biaxial mechanical properties of human vocal fold cover under vocal fold elongation

Abstract

Mechanical properties of the human vocal fold cover layer were experimentally investigated in uniaxial and biaxial tensile tests. The results showed a coupling effect between the stress conditions along the anterior-posterior and transverse directions, with vocal fold elongation increasing vocal fold stiffness along both directions, thus allowing more efficient control of the fundamental frequency of voice through vocal fold elongation. This study also shows that vocal folds were nearly isotropic at resting conditions, thus a tendency to vibrate with incomplete glottal closure, but became increasingly anisotropic with increasing vocal fold elongation. IntroductionThe mechanical conditions within the human vocal folds play an important role in determining the fundamental frequency of vocal fold vibration and the resulting voice quality. As the vocal folds are postured for the production of different voice types, the mechanical conditions are also expected to vary (Hirano, 1974). A better understanding of the mechanical conditions of the vocal folds across different voice types would facilitate the development of biomaterials for vocal fold injection or tissue engineered vocal fold replacement with similar material properties. Computationally, a more quantitative characterization of the vocal fold mechanical properties would provide data for developing material constitutive models, linear or nonlinear, for use in computational models of phonation.There have been many previous reports experimentally characterizing vocal fold mechanical properties (e.g., Hirano and Kakita, 1985;Alipour-Haghighi and Titze, 1991;Haji et al., 1992;Chan and Titze, 1999;Alipour and Vigmostad, 2012;Kelleher et al., 2013;Kazemirad et al., 2014). While these studies have provided valuable contribution to our understanding of the vocal fold mechanical properties, most of these studies were performed in a uniaxial tensile test, different from the physiological conditions in which vocal folds are subject to tensions or constraints along multiple directions. Although elongating the vocal folds along the anterior-posterior (AP) direction increases vocal fold stiffness and tension along this direction, which is considered the primary means of regulating the fundamental frequency (F0) of voice (Titze, 1994;Zhang, 2016b), little is known about how vocal fold elongation affects the mechanical conditions in the transverse plane (the plane perpendicular to the AP direction), which may also contribute to F0 control. Previous studies have shown a cross-axis coupling effect such that AP elongation may also stiffen the vocal folds in the transverse plane (e.g., Yin and Zhang, 2013). With this extra vocal fold stiffening in the transverse plane, vocal fold elongation would be more effective in regulating the overall vocal fold stiffness and F0 of vocal fold vibration, compared to F0 control through varying the stiffness and tension along the AP direction alone. This cross-axis coupling effect of vocal fold elongation cannot be investigated in a ...

Structural constitutive modeling of the anisotropic mechanical properties of human vocal fold lamina propria

“…3 for both structural conditions. Similar to Zhang et al (2017), Fig. 3 shows that AP elongation not only increases the AP stiffness, but also significantly increases the transverse stiffness, particularly when elongation leads to recruitment of collagen fibers (for stretches above 1.1).…”

“…Compared with uniaxial tensile tests or indentation tests, biaxial tensile testing allows a better quantification of the anisotropic mechanical properties at different vocal fold longitudinal elongations and vibration amplitudes in the transverse plane, thus better approximating the realistic mechanical conditions that occur in human phonation. Indeed, the results from our recent biaxial tensile testing (Zhang et al, 2017) showed a strong mechanical coupling between the AP and transverse directions that was not observed in uniaxial tensile tests.…”

“…where the subscripts c and e denote collagen and elastin fibers, respectively. The inclusion of the fiber-matrix interaction term is based on the observation in biaxial tensile testing (Zhang et al, 2017) that AP elongation significantly increases the transverse stiffness. Our initial modeling effort showed that this cross-axis coupling is stronger than that would be predicted by fiber dispersion alone and thus an explicit fibermatrix interaction term is required for the model to successfully curve fit the biaxial testing data.…”

“…Our initial modeling effort showed that this cross-axis coupling is stronger than that would be predicted by fiber dispersion alone and thus an explicit fibermatrix interaction term is required for the model to successfully curve fit the biaxial testing data. Since such coupling is stronger at strain ranges with collagen recruitment (Zhang et al, 2017), only collagen-matrix interaction is considered in Eq. (1) for simplicity,…”

“…The lamina propria layer of a male larynx was dissected and used in the tensile tests. The same experimental setup and procedure as described in Zhang et al (2017) was used, with a five-test protocol. In the first test, the specimen was vibrated sinusoidally along the AP direction while the transverse direction was held constant at zero strain.…”