Flow-induced oscillations of vocal-fold replicas with tuned extensibility and material properties.

Human vocal folds are highly deformable non-linear oscillators. During phonation, they stretch up to 50% under the complex action of laryngeal muscles. Exploring the fluid/structure/acoustic interactions on a human-scale replica to study the role of the laryngeal muscles remains a challenge. For that purpose, we designed a novel in vitro testbed to control vocal-folds pre-phonatory deformation. The testbed was used to study the vibration and the sound production of vocal-fold replicas made of (i) silicone elastomers commonly used in voice research and (ii) a gelatin-based hydrogel we recently optimized to approximate the mechanics of vocal folds during finite strains under tension, compression and shear loadings. The geometrical and mechanical parameters measured during the experiments emphasized the effect of the vocal-fold material and pre-stretch on the vibration patterns and sounds. In particular, increasing the material stiffness increases glottal flow resistance, subglottal pressure required to sustain oscillations and vibratory fundamental frequency. In addition, although the hydrogel vocal folds only oscillate at low frequencies (close to 60 Hz), the subglottal pressure they require for that purpose is realistic (within the range 0.5-2 kPa), as well as their glottal opening and contact during a vibration cycle. The results also evidence the effect of adhesion forces on vibration and sound production.

Versatile fiber-reinforced hydrogels to mimic the microstructure and mechanics of human vocal-fold upper layers.

Human vocal folds are remarkable soft laryngeal structures that enable phonation due to their unique vibro-mechanical performances. These properties are tied to their specific fibrous architecture, especially in the upper layers, which comprise a gel-like composite called lamina propria. The lamina propria can withstand large and reversible deformations under various multiaxial loadings. Despite their importance, the relationships between the microstructure of vocal folds and their resulting macroscopic properties remain poorly understood. There is a need for versatile models that encompass their structural complexity while mimicking their mechanical features. In this study, we present a candidate model inspired by histological measurements of the upper layers of human vocal folds. Bi-photonic observations were used to quantify the distribution, orientation, width, and volume fraction of collagen and elastin fibers between histological layers. Using established biomaterials, polymer fiber-reinforced hydrogels were developed to replicate the fibrillar network and ground substance of native vocal fold tissue. To achieve this, jet-sprayed poly(ε-caprolactone) fibrillar mats were successfully impregnated with poly(L-lysine) dendrimers/polyethylene glycol hydrogels. The resulting composites exhibited versatile structural, physical and mechanical properties that could be customized through variations in the chemical formulation of their hydrogel matrix, the microstructural architecture of their fibrous networks (i.e., fiber diameter, orientation and volume fraction) and their assembly process. By mimicking the collagen network of the lamina propria with polymer fibers and the elastin/ground substance with the hydrogel composition, we successfully replicated the non-linear, anisotropic, and viscoelastic mechanical behavior of the vocal-fold upper layers, accounting for inter/intra-individual variations. The development of this mimetic model offers promising avenues for a better understanding of the complex mechanisms involved in voice production. STATEMENT OF SIGNIFICANCE: Human vocal folds are outstanding vibrating soft living tissues allowing phonation. Simple physical models that take into account the histological structure of the vocal fold and recapitulate its mechanical features are scarce. As a result, the relations between tissue components, organisation and vibro-mechanical performances still remain an open question. We describe here the development and the characterization of fiber-reinforced hydrogels inspired from the vocal-fold microstructure. These systems are able to reproduce the mechanics of vocal-fold tissues upon realistic cyclic and large strains under various multi-axial loadings, thus providing a mimetic model to further understand the impact of the fibrous network microstructure in phonation.

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 gelatin-based hydrogels during finite strain tension, compression and shear.

Introduction: Among the biopolymers used to make hydrogels, gelatin is very attractive due to its biocompatibility, biodegradability and versatile physico-chemical properties. A proper and complete characterization of the mechanical behavior of these hydrogels is critical to evaluate the relevance of one formulation over another for a targeted application, and to optimise their processing route accordingly. Methods: In this work, we manufactured neat gelatin and gelatin covalently cross-linked with glutaraldehyde at various concentrations, yielding to hydrogels with tunable mechanical properties that we characterized under finite strain, cyclic tension, compression and shear loadings. Results and Discussion: The role of both the chemical formulation and the kinematical path on the mechanical performances of the gels is highlighted. As an opening towards biomedical applications, the properties of the gels are confronted to those of native soft tissues particularly complicated to restore, the human vocal folds. A specific cross-linked hydrogel is selected to mimic vocal-fold fibrous tissues.

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.

Realistic glottal motion and airflow rate during human breathing.

The glottal geometry is a key factor in the aerosol delivery efficiency for treatment of lung diseases. However, while glottal vibrations were extensively studied during human phonation, the realistic glottal motion during breathing is poorly understood. Therefore, most current studies assume an idealized steady glottis in the context of respiratory dynamics, and thus neglect the flow unsteadiness related to this motion. This is particularly important to assess the aerosol transport mechanisms in upper airways. This article presents a clinical study conducted on 20 volunteers, to examine the realistic glottal motion during several breathing tasks. Nasofibroscopy was used to investigate the glottal geometrical variations simultaneously with accurate airflow rate measurements. In total, 144 breathing sequences of 30s were recorded. Regarding the whole database, two cases of glottal time-variations were found: "static" or "dynamic" ones. Typically, the peak value of glottal area during slow breathing narrowed from 217 ± 54 mm(2) (mean ± STD) during inspiration, to 178 ± 35 mm(2) during expiration. Considering flow unsteadiness, it is shown that the harmonic approximation of the airflow rate underevaluates the inertial effects as compared to realistic patterns, especially at the onset of the breathing cycle. These measurements provide input data to conduct realistic numerical simulations of laryngeal airflow and particle deposition.

Ventricular-fold dynamics in human phonation.

PURPOSE: In this study, the authors aimed (a) to provide a classification of the ventricular-fold dynamics during voicing, (b) to study the aerodynamic impact of these motions on vocal-fold vibrations, and (c) to assess whether ventricular-fold oscillations could be sustained by aerodynamic coupling with the vocal folds. METHOD: A 72-sample database of vocal gestures accompanying different acoustical events comprised high-speed cinematographic, audio, and electroglottographic recordings of 5 subjects. Combining the physiological correlates with a theoretical model of phonation, the vocal-ventricular aerodynamic interactions were investigated. RESULTS: A ventricular-fold motion is found during (de)crescendos, shout, throat singing, yodel, growls, and glides with transitions between registers. Three main types of dynamics are identified: slow nonoscillatory motion and fast oscillatory motion with aperiodical or periodical vibrations. These patterns accompany a change in voice quality, pitch, and/or intensity. Alterations of glottal-oscillatory amplitude, frequency, and contact were predicted. It is shown that a ventricular oscillation can be initiated and sustained by aerodynamic coupling with the vocal folds. CONCLUSIONS: Vocal-ventricular aerodynamic interactions can alter, enhance, or suppress vocal-fold vibrations or leave them unchanged, depending on the ventricular-fold dynamics involved. Depending on its variation in time, a similar level of ventricular-fold adduction impacts the glottal vibratory magnitude and contact much differently.

Flow of a blood analogue fluid in a compliant abdominal aortic aneurysm model: experimental modelling.

The aim of this work is to develop a unique in vitro set-up in order to analyse the influence of the shear thinning fluid-properties on the flow dynamics within the bulge of an abdominal aortic aneurysm (AAA). From an experimental point of view, the goals are to elaborate an analogue shear thinning fluid mimicking the macroscopic blood behaviour, to characterise its rheology at low shear rates and to propose an experimental device able to manage such an analogue fluid without altering its feature while reproducing physiological flow rate and pressure, through compliant AAA. Once these experimental prerequisites achieved, the results obtained in the present work show that the flow dynamics is highly dependent on the fluid rheology. The main results point out that the propagation of the vortex ring, generated in the AAA bulge, is slower for shear thinning fluids inducing a smaller travelled distance by the vortex ring so that it never impacts the anterior wall in the distal region, in opposition to Newtonian fluids. Moreover, scalar shear rate values are globally lower for shear thinning fluids inducing higher maximum stress values than those for the Newtonian fluids. Consequently, this work highlights that a Newtonian fluid model is finally inadequate to obtain a reliable prediction of the flow dynamics within AAA.

Vocal fold and ventricular fold vibration in period-doubling phonation: physiological description and aerodynamic modeling.

Occurrences of period-doubling are found in human phonation, in particular for pathological and some singing phonations such as Sardinian A Tenore Bassu vocal performance. The combined vibration of the vocal folds and the ventricular folds has been observed during the production of such low pitch bass-type sound. The present study aims to characterize the physiological correlates of this acoustical production and to provide a better understanding of the physical interaction between ventricular fold vibration and vocal fold self-sustained oscillation. The vibratory properties of the vocal folds and the ventricular folds during phonation produced by a professional singer are analyzed by means of acoustical and electroglottographic signals and by synchronized glottal images obtained by high-speed cinematography. The periodic variation in glottal cycle duration and the effect of ventricular fold closing on glottal closing time are demonstrated. Using the detected glottal and ventricular areas, the aerodynamic behavior of the laryngeal system is simulated using a simplified physical modeling previously validated in vitro using a larynx replica. An estimate of the ventricular aperture extracted from the in vivo data allows a theoretical prediction of the glottal aperture. The in vivo measurements of the glottal aperture are then compared to the simulated estimations.

Influence of a constriction in the near field of the vocal folds: physical modeling and experimental validation.

The involvement of the ventricular folds is often observed in human phonation and, in particular, in pathological and or some throat-singing phonation. This study aims to explore and model the possible aerodynamic interaction between the ventricular and vocal folds using suitable in vitro setups allowing steady and unsteady flow conditions. The two experimental setups consist of a rigid and a self-oscillating vocal-fold replica, coupled to a downstream rigid ventricular-fold replica in both cases. A theoretical flow modeling is proposed to quantify the aerodynamic impact of the ventricular folds on the pressure distribution and thereby on the vocal-fold vibrations. The mechanical behavior of the vocal folds is simulated by a distributed model accounting for this impact. The influence of the ventricular constriction is measured in both flow conditions and compared to the model outcome. This study objectively evaluates the additional pressure drop implied by the presence of a ventricular constriction in the larynx. It is demonstrated that such constriction can either facilitate or impede the glottal vibrations depending on the laryngeal geometrical configuration. The relevance of using static or dynamic vocal-fold replicas is discussed.