A micro-mechanical model for the fibrous tissues of vocal folds.

Composed of collagen, elastin and muscular fibrous networks, vocal folds are soft laryngeal multi-layered tissues owning remarkable vibro-mechanical performances. However, the impact of their histological features on their overall mechanical properties still remains elusive. Thereby, this study presents a micro-mechanical hyperelastic model able to describe the 3D fibrous architecture and the surrounding matrices of the vocal-fold sublayers, and to predict their mechanical behavior. For each layer, the model parameters were identified using available histo-mechanical data, including their quasi-static response for key physiological loading paths, i.e., longitudinal tension, transverse compression and longitudinal shear. Regardless of the loading path, it is shown how macroscale nonlinear, anisotropic tissue responses are inherited from the fiber scale. Scenarios of micro-mechanisms are predicted, highlighting the major role of 3D fiber orientation in tension, steric hindrance in compression, and matrix contribution in shear. Finally, combining these predictions to vibrating hyperelastic Timoshenko beam's theory, the impact of the fibrous architecture of the upper layers on vocal-fold vibratory properties is emphasized.

Mechanics of human vocal folds layers during finite strains in tension, compression and shear.

During phonation, human vocal fold tissues are subjected to combined tension, compression and shear loading modes from small to large finite strains. Their mechanical behaviour is however still not well understood. Herein, we complete the existing mechanical database of these soft tissues, by characterising, for the first time, the cyclic and finite strains behaviour of the lamina propria and vocalis layers under these loading modes. To minimise the inter or intra-individual variability, particular attention was paid to subject each tissue sample successively to the three loadings. A non-linear mechanical behaviour is observed for all loading modes: a J-shape strain stiffening in longitudinal tension and transverse compression, albeit far less pronounced in shear, stress accommodation and stress hysteresis whatever the loading mode. In addition, recorded stress levels during longitudinal tension are much higher for the lamina propria than for the vocalis. Conversely, the responses of the lamina propria and the vocalis in transverse compression as well as transverse and longitudinal shears are of the same orders of magnitude. We also highlight the strain rate sensitivity of the tissues, as well as their anisotropic properties.

3D multiscale imaging of human vocal folds using synchrotron X-ray microtomography in phase retrieval mode.

Human vocal folds possess outstanding abilities to endure large, reversible deformations and to vibrate up to more than thousand cycles per second. This unique performance mainly results from their complex specific 3D and multiscale structure, which is very difficult to investigate experimentally and still presents challenges using either confocal microscopy, MRI or X-ray microtomography in absorption mode. To circumvent these difficulties, we used high-resolution synchrotron X-ray microtomography with phase retrieval and report the first ex vivo 3D images of human vocal-fold tissues at multiple scales. Various relevant descriptors of structure were extracted from the images: geometry of vocal folds at rest or in a stretched phonatory-like position, shape and size of their layered fibrous architectures, orientation, shape and size of the muscle fibres as well as the set of collagen and elastin fibre bundles constituting these layers. The developed methodology opens a promising insight into voice biomechanics, which will allow further assessment of the micromechanics of the vocal folds and their vibratory properties. This will then provide valuable guidelines for the design of new mimetic biomaterials for the next generation of artificial larynges.