Nonlinear viscoelastic characterization of human vocal fold tissues under large-amplitude oscillatory shear (LAOS)

Synthetic, self-oscillating vocal fold models for voice production researcha)

Sound for the human voice is produced by vocal fold flow-induced vibration and involves a complex coupling between flow dynamics, tissue motion, and acoustics. Over the past three decades, synthetic, self-oscillating vocal fold models have played an increasingly important role in the study of these complex physical interactions. In particular, two types of models have been established: "membranous" vocal fold models, such as a water-filled latex tube, and "elastic solid" models, such as ultrasoft silicone formed into a vocal fold-like shape and in some cases with multiple layers of differing stiffness to mimic the human vocal fold tissue structure. In this review, the designs, capabilities, and limitations of these two types of models are presented. Considerations unique to the implementation of elastic solid models, including fabrication processes and materials, are discussed. Applications in which these models have been used to study the underlying mechanical principles that govern phonation are surveyed, and experimental techniques and configurations are reviewed. Finally, recommendations for continued development of these models for even more lifelike response and clinical relevance are summarized.

Linear and nonlinear rheology of liberase-treated breast cancer tumors

Extracellular matrix (ECM) rigidity has been shown to increase the invasive properties of breast cancer cells, promoting transformation and metastasis through mechanotransduction. Reducing ECM stiffness via enzymatic digestion could be a promising approach to slowing breast cancer development by de-differentiation of breast cancer cells to less aggressive phenotypes and enhancing the effectiveness of existing chemotherapeutics via improved drug penetrance throughout the tumor. In this study, we examine the effects of injectable liberase (a blend of collagenase and thermolysin enzymes) treatments on the linear and nonlinear rheology of allograft 4T1 mouse mammary tumors. We perform two sets of in vivo mouse studies, in which either one or multiple treatment injections occur before the tumors are harvested for rheological analysis. The treatment groups in each study consist of a buffer control, free liberase enzyme in buffer, a thermoresponsive copolymer called LiquoGel (LQG) in buffer, and a combined, localized injection of LQG and liberase. All tumor samples exhibit gel-like linear rheological behavior with the elastic modulus significantly larger than the viscous modulus and both independent of frequency. Tumors that receive a single injection of localized liberase have significantly lower tumor volumes and lower tissue moduli at both the center and edge compared to buffer- and free liberase-injected control tumors, while tissue viscoelasticity remains relatively unaffected. Tumors injected multiple times with LQG and liberase also have lower tissue volumes but possess higher tissue moduli and lower viscoelasticities compared to the other treatment groups. We propose that a mechanotransductive mechanism could cause the formation of smaller but stiffer tumors after repeated, localized liberase injections. Large amplitude oscillatory shear (LAOS) experiments are also performed on tissues from the multiple injection study and the results are analyzed using MITlaos. LAOS analysis reveals that all 4T1 tumors from the multiple injection study exhibit nonlinear rheological behavior at high strains and strain rates. Examination of the Lissajous-Bowditch curves, Chebyshev coefficient ratios, elastic moduli, and dynamic viscosities demonstrate that the onset and type of nonlinear behavior is independent of treatment type and elastic modulus, suggesting that multiple liberase injections do not affect the nonlinear viscoelasticity of 4T1 tumors.

Dynamic Ocular Response to Mechanical Loading: The Role of Viscoelasticity in Energy Dissipation by the Cornea

We have used vibrational optical coherence tomography (VOCT) to measure the resonant frequency, elastic modulus, and loss modulus of components of the anterior segment of pig eyes in vitro. Such basic biomechanical properties of the cornea have been shown to be abnormal not only in diseases of the anterior segment but also in posterior segment diseases as well. This information is needed to better understand corneal biomechanics in health and disease and to be able to diagnose the early stages of corneal pathologies. Results of dynamic viscoelastic studies on whole pig eyes and isolated corneas indicate that at low strain rates (30 Hz or less), the viscous loss modulus is as high as 0.6 times the elastic modulus for both whole eyes and corneas. This large viscous loss is similar to that of skin, which has been hypothesized to be dependent upon the physical association of proteoglycans with collagenous fibers. The energy dissipation properties of the cornea provide a mechanism to dissipate energy associated with blunt trauma, thereby preventing delamination and failure. The cornea possesses the ability to store impact energy and transmit excess energy to the posterior segment of the eye through its serial connection to the limbus and sclera. In this manner, the viscoelastic properties of the cornea, in concert with that of the posterior segment of the pig eye, function to prevent mechanical failure of the primary focusing element of the eye. Results of resonant frequency studies suggest that the 100-120 Hz and 150-160 Hz resonant frequency peaks reside in the anterior segment of the cornea since the removal of the anterior segment of the cornea decreases the peak heights at these resonant frequencies. These results suggest that there is more than one collagen fibril network found in the anterior portion of the cornea that provides structural integrity to prevent corneal delamination and that VOCT may be useful clinically to diagnose corneal diseases.

In Vitro Evaluation of Biomaterials for Vocal Fold Injection: A Systematic Review

Vocal fold injection is a preferred treatment in glottic insufficiency because it is relatively quick and cost-saving. However, researchers have yet to discover the ideal biomaterial with properties suitable for human vocal fold application. The current systematic review employing PRISMA guidelines summarizes and discusses the available evidence related to outcome measures used to characterize novel biomaterials in the development phase. The literature search of related articles published within January 2010 to March 2021 was conducted using Scopus, Web of Science (WoS), Google Scholar and PubMed databases. The search identified 6240 potentially relevant records, which were screened and appraised to include 15 relevant articles based on the inclusion and exclusion criteria. The current study highlights that the characterization methods were inconsistent throughout the different studies. While rheologic outcome measures (viscosity, elasticity and shear) were most widely utilized, there appear to be no target or reference values. Outcome measures such as cellular response and biodegradation should be prioritized as they could mitigate the clinical drawbacks of currently available biomaterials. The review suggests future studies to prioritize characterization of the viscoelasticity (to improve voice outcomes), inflammatory response (to reduce side effects) and biodegradation (to improve longevity) profiles of newly developed biomaterials.

Dynamic Biomechanical Analysis of Vocal Folds Using Pipette Aspiration Technique

The voice producing process is a complex interplay between glottal pressure, vocal folds, their elasticity and tension. The material properties of vocal folds are still insufficiently studied, because the determination of material properties in soft tissues is often difficult and connected to extensive experimental setups. To shed light on this less researched area, in this work, a dynamic pipette aspiration technique is utilized to measure the elasticity in a frequency range of 100–1000 Hz. The complex elasticity could be assessed with the phase shift between exciting pressure and tissue movement. The dynamic pipette aspiration setup has been miniaturized with regard to a future in-vivo application. The techniques were applied on 3 different porcine larynges 4 h and 1 d postmortem, in order to investigate the deterioration of the tissue over time and analyze correlation in elasticity values between vocal fold pairs. It was found that vocal fold pairs do have different absolute elasticity values but similar trends. This leads to the assumption that those trends are more important for phonation than having same absolute values.

Probing gels and emulsions using large-amplitude oscillatory shear and frictional studies with soft substrate skin surrogates

Water swellable crosslinked polymers are widely used in oil-in-water emulsions for the healthcare and cosmetic industries due to their thickening properties. In this study, we investigate the rheological and lubrication behavior of a microgel-forming polymer, a lightly-crosslinked hydrophobically modified polyacrylic acid (HMPAA), in an aqueous medium and in an emulsion. Hydrogenated phosphatidylcholine, a class of phospholipids, is used as a surfactant in the emulsions composed of different oil content. Rheological behavior is probed both in the linear and non-linear regimes using small strain amplitude and large amplitude oscillatory shear (LAOS) experiments, respectively. We observe all systems to exhibit gel-like behavior with the elastic modulus (G') dominating and being frequency independent. Lissajous-Bowditch plots and nonlinear parameters obtained under large deformation show that the emulsions can resist greater deformations with smaller increase in the viscous dissipation when compared to a HMPAA gel. For tribology experiments, friction curves in a range of entrainment speeds are examined using substrates to mimic the skin surface (PDMS and Bioskin®). The role of polymer hydrophobicity on the different substrates are also explored by comparing the behavior of HMPAA to that of its hydrophilic analog, a polyacrylic acid highly crosslinked. We find the friction coefficient to be dependent on the hydrophobicity of the substrate and the polymer as well as the substrate roughness. These results taken together provide insights in the formulation of skincare products with efficient lubrication properties for different skin characteristics.

Viscoelastic substrate decouples cellular traction force from other related phenotypes

Survival and maintenance of normal physiological functions depends on continuous interaction of cells with its microenvironment. Cells sense the mechanical properties of underlying substrate by applying force and modulate their behaviour in response to the resistance offered by the substrate. Most of the studies addressing cell-substrate mechanical interactions have been carried out using elastic substrates. Since tissues within our body are viscoelastic in nature, here we explore the effect of substrate's viscoelasticity on various properties of mesenchymal stem cells. Here, we used two sets of polyacrylamide substrates having similar storage modulus (G' = 1.1-1.6 kPa) but different loss modulus (G" = 45 Pa and 300 Pa). We report that human mesenchymal stem cells spread more but apply less force on the viscoelastic substrate (substrate with higher loss modulus). We further investigated the effect of substrate viscoelasticity on the expression of other contractility-associated proteins such as focal adhesion (FA) proteins (Vinculin, Paxillin, Talin), cytoskeletal proteins (actin, mysion, intermediate filaments, and microtubules) and mechano-sensor protein Yes-Associated Protein (YAP). Our results show that substrate viscoelasticity decouples cellular traction from other known traction related phenotypes.

Large amplitude oscillatory rheology of silica and cellulose nanocrystals filled natural rubber compounds

Nanoparticles reinforce rubbers and enhance Payne effect for the compounds experiencing large amplitude oscillatory shear deformation. Herein the effects of silica and cellulose nanocrystals on the Payne effect of natural rubber compounds are investigated by stress decomposition methods for clarifying the elastic and viscous nonlinearities varying with filler content and composition. The Payne effect is in general characterized by intercycle strain softening and shear thinning behaviors and intracycle hardening and thinning behaviors at high strain (strain rate) amplitudes while the filler influences the behaviors markedly at intermediate strain (rate) amplitudes. Especially, the addition of cellulose nanocrystals in the silica filled compounds improves the elastic nonlinearity and greatly weakens the viscous nonlinearity, providing a perspective on understanding the Payne effect for manufacturing high-performance rubber materials.

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.

Nonlinear rheological characteristics of single species bacterial biofilms

Bacterial biofilms in natural and artificial environments perform a wide array of beneficial or detrimental functions and exhibit resistance to physical as well as chemical perturbations. In dynamic environments, where periodic or aperiodic flows over surfaces are involved, biofilms can be subjected to large shear forces. The ability to withstand these forces, which is often attributed to the resilience of the extracellular matrix. This attribute of the extracellular matrix is referred to as viscoelasticity and is a result of self-assembly and cross-linking of multiple polymeric components that are secreted by the microbes. We aim to understand the viscoelastic characteristic of biofilms subjected to large shear forces by performing Large Amplitude Oscillatory Shear (LAOS) experiments on four species of bacterial biofilms: Bacillus subtilis, Comamonas denitrificans, Pseudomonas fluorescens and Pseudomonas aeruginosa. We find that nonlinear viscoelastic measures such as intracycle strain stiffening and intracycle shear thickening for each of the tested species, exhibit subtle or distinct differences in the plot of strain amplitude versus frequency (Pipkin diagram). The biofilms also exhibit variability in the onset of nonlinear behaviour and energy dissipation characteristics, which could be a result of heterogeneity of the extracellular matrix constituents of the different biofilms. The results provide insight into the nonlinear rheological behaviour of biofilms as they are subjected to large strains or strain rates; a situation that is commonly encountered in nature, but rarely investigated.

Ovine Vocal Fold Tissue Fatigue Response to Accumulated, Large-Amplitude Vibration Exposure at Phonatory Frequencies

Purpose The contribution of tissue mechanical response to vocal fatigue is poorly understood. This study investigated the fatigue response of vocal fold tissues to large-amplitude vibration exposure at phonatory frequencies, using an ex vivo ovine model. Method Twelve sheep vocal fold mucosal specimens were subjected to sinusoidal, simple-shear deformation for prolonged cycles, under a large but physiological shear strain (46%) in a frequency range of 100-230 Hz. The duration of shear varied from a critical vibration exposure limit of 1,040 s to 4 times the limit (4,160 s). Tissue viscoelastic response was quantified by the elastic shear modulus (G'), viscous shear modulus (G″), and damping ratio (G″/G'). Results Distinct response patterns were observed at different frequencies. G' and G″ generally decreased with vibration exposure at 100 Hz, whereas they generally increased with vibration exposure at 200 and 230 Hz. Statistically significant differences were found for G″ increasing with vibration exposure at 200 Hz and damping ratio decreasing with vibration exposure at 200 Hz. Significant increases with frequency were also found for all viscoelastic functions. Results suggested that the contribution of tissue viscoelastic response to vocal fatigue could be highly frequency dependent. In particular, increases in G″ with vibration exposure could lead to high phonation threshold pressures and difficulty sustaining phonation at higher frequencies following prolonged vocalization. Conclusion These preliminary findings may help us better understand vocal fatigue and recovery and should be corroborated by studies with human vocal fold tissues.