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.