The anisotropic hyperelastic biomechanical response of the vocal ligament and implications for frequency regulation: a case study.

One of the primary mechanisms to vary one's vocal frequency is through vocal fold length changes. As stress and deformation are linked to each other, it is hypothesized that the anisotropy in the biomechanical properties of the vocal fold tissue would affect the phonation characteristics. A biomechanical model of vibrational frequency rise during vocal fold elongation is developed which combines an advanced biomechanical characterization protocol of the vocal fold tissue with continuum beam models. Biomechanical response of the tissue is related to a microstructurally informed, anisotropic, nonlinear hyperelastic constitutive model. A microstructural characteristic (the dispersion of collagen) was represented through a statistical orientation function acquired from a second harmonic generation image of the vocal ligament. Continuum models of vibration were constructed based upon Euler-Bernoulli and Timoshenko beam theories, and applied to the study of the vibration of a vocal ligament specimen. From the natural frequency predictions in dependence of elongation, two competing processes in frequency control emerged, i.e., the applied tension raises the frequency while simultaneously shear deformation lowers the frequency. Shear becomes much more substantial at higher modes of vibration and for highly anisotropic tissues. The analysis was developed as a case study based on a human vocal ligament specimen.

Empirical measurements of biomechanical anisotropy of the human vocal fold lamina propria.

The vocal folds are known to be mechanically anisotropic due to the microstructural arrangement of fibrous proteins such as collagen and elastin in the lamina propria. Even though this has been known for many years, the biomechanical anisotropic properties have rarely been experimentally studied. We propose that an indentation procedure can be used with uniaxial tension in order to obtain an estimate of the biomechanical anisotropy within a single specimen. Experiments were performed on the lamina propria of three male and three female human vocal folds dissected from excised larynges. Two experiments were conducted: each specimen was subjected to cyclic uniaxial tensile loading in the longitudinal (i.e., anterior-posterior) direction, and then to cyclic indentation loading in the transverse (i.e., medial-lateral) direction. The indentation experiment was modeled as contact on a transversely isotropic half-space using the Barnett-Lothe tensors. The longitudinal elastic modulus E(L) was computed from the tensile test, and the transverse elastic modulus E(T) and longitudinal shear modulus G(L) were obtained by inverse analysis of the indentation force-displacement response. It was discovered that the average of E(L) /E(T) was 14 for the vocal ligament and 39 for the vocal fold cover specimens. Also, the average of E(L)/G(L), a parameter important for models of phonation, was 28 for the vocal ligament and 54 for the vocal fold cover specimens. These measurements of anisotropy could contribute to more accurate models of fundamental frequency regulation and provide potentially better insights into the mechanics of vocal fold vibration.

A new generation of sodium chloride porogen for tissue engineering.

Porogen leaching is a widely used and simple technique for the creation of porous scaffolds in tissue engineering. Sodium chloride (NaCl) is the most commonly used porogen, but the current grinding and sieving methods generate salt particles with huge size variations and cannot generate porogens in the submicron size range. We have developed a facile method based on the principles of crystallization to precisely control salt crystal sizes down to a few microns within a narrow size distribution. The resulting NaCl crystal size could be controlled through the solution concentration, crystallization temperature, and crystallization time. A reduction in solution temperature, longer crystallization times, and an increase in salt concentration resulted in an increase in NaCl crystal sizes due to the lowered solubility of the salt solution. The nucleation and crystallization technique provides superior control over the resulting NaCl size distribution (13.78 ± 1.18 µm), whereas the traditional grinding and sieving methods produced NaCl porogens 13.89 ± 12.49 µm in size. The resulting NaCl porogens were used to fabricate scaffolds with increased interconnectivity, porous microchanneled scaffolds, and multiphasic vascular grafts. This new generation of salt porogen provides great freedom in designing versatile scaffolds for various tissue-engineering applications.