Experimental and Numerical Characterization of the Mechanical Masseter Muscle Response During Biting.

Predictive simulations of the mastication system would significantly improve our understanding of temporomandibular joint (TMJ) disorders and the planning of cranio-maxillofacial surgery procedures. Respective computational models must be validated by experimental data from in vivo characterization of the mastication system's mechanical response. The present pilot-study demonstrates the feasibility of a combined experimental and numerical procedure to validate a computer model of the masseter muscle. An experimental setup is proposed that provides a simultaneous bite force measurement and ultrasound-based visualization of muscle deformation. The direct comparison of the experimentally observed and numerically predicted muscle response demonstrates the predictive capabilities of such anatomically accurate biting models. Differences between molar and incisor biting are investigated; muscle deformation is recorded for three different bite forces in order to capture the effect of increasing muscle fiber recruitment. The three-dimensional (3D) muscle deformation at each bite position and force-level is approximatively reconstructed from ultrasound measurements in five distinct cross-sectional areas (four horizontal and one vertical cross section). The experimental work is accompanied by numerical simulations to validate the predictive capabilities of a constitutive muscle model previously formulated. An anatomy-based, fully 3D model of the masseter muscle is created from magnetic resonance images (MRI) of the same subject. The direct comparison of experimental and numerical results revealed good agreement for maximum bite forces and masseter deformations in both biting positions. The present work therefore presents a feasible in vivo measurement system to validate numerically predicted masseter muscle contractions during mastication.

On the rheological and sensory properties of a novel natural salivary substitute.

The prevalence of xerostomia, the sensation of dry mouth, is estimated at 20 % in the general population and up to 50 % in older adults. Saliva plays different roles during bolus formation: lubrication, mixing, coating, hydration, dissolution, and comminution of food particles. This study proposes and tests artificial saliva formulations mimicking human saliva rheological and sensory perceptions. Shear and extensional rheology were assessed to select the type of formulation closest to saliva rheological characteristics. After evaluating three alternative sources, an extract simulating saliva rheology was produced from flax seeds. Friction coefficient and rheological properties, such as flow curves, relaxation times, and Trouton ratios, were compared favorably with human saliva. The sensory evaluation demonstrated that flaxseed extracts induce perceived mouth hydration, slipperiness, and adhesion exceeding that of human saliva. The flaxseed extract proposed in this can i) be used to study in vitro food oral processing and ii) pave the way to novel natural salivary substitutes to alleviate the symptoms of xerostomia.

Wicking in a powder.

We investigate the wicking in granular media by considering layers of grains at the surface of a liquid and discuss the critical contact angle below which spontaneous impregnation takes place. This angle is found to be on the order of 55° for monodisperse layers, significantly smaller than 90°, the threshold value for penetrating assemblies of tubes. Owing to geometry, impregnating grains is more demanding than impregnating tubes. We also consider the additional effects of polydispersity and pressure on this wetting transition and discuss the corresponding shift observed for the critical contact angle.

Coupled tomography and distinct-element-method approach to exploring the granular media microstructure in a jamming hourglass.

We describe an approach for exploring microscopic properties of granular media that couples x-ray microtomography and distinct-element-method (DEM) simulations through image analysis. We illustrate it via the study of the intriguing phenomenon of instant arching in an hourglass (in our case a cylinder filled with a polydisperse mixture of glass beads that has a small circular shutter in the bottom). X-ray tomography provides three-dimensional snapshots of the microscopic conditions of the system both prior to opening the shutter, and thereafter, once jamming is completed. The process time in between is bridged using DEM simulation, which settles to positions in remarkably good agreement with the x-ray images. Specifically designed image analysis procedures accurately extract the geometrical information, i.e., the positions and sizes of the beads, from the raw x-ray tomographs, and compress the data representation from initially 5 gigabytes to a few tens of kilobytes per tomograph. The scope of the approach is explored through a sensitivity analysis to input data perturbations in both bead sizes and positions. We establish that accuracy of size--much more than position--estimates is critical, thus explaining the difficulty in considering a mixture of beads of different sizes. We further point to limits in the replication ability of granular flows away from equilibrium; i.e., the difficulty of numerically reproducing chaotic motion.

An in vitro experiment to simulate how easy tablets are to swallow.

The compliance of patients to solid oral dosage forms is strongly conditioned by the perceived ease of swallowing, especially in geriatric and pediatric populations. This study proposes a method, based on an in vitro model of the human oropharyngeal cavity, to study quantitatively the oral phase of human swallowing in presence of single or multiple tablets. The dynamics of swallowing was investigated varying the size and shape of model tablets and adjusting the force applied to the mechanical setup to simulate tongue pressure variations among individuals. The evolution of the velocity of the bolus, the oral transit time, and the relative position of the solid oral dosage form within the liquid bolus were measured quantitatively from high speed camera recordings. Whenever the solid dosage forms were big enough to interact with the walls of the in vitro oral cavity, a strong effect of the volume of the medication in respect of its swallowing velocity was observed, with elongated tablets flowing faster than spherical tablets. Conversely, the geometrical properties of the solid oral dosage forms did not significantly affect the bolus dynamics when the cross section of the tablet was lower than 40% of that of the bolus. The oral phase of swallowing multiple tablets was also considered in the study by comparing different sizes while maintaining a constant total mass. The predictive power of different theories was also evaluated against the experimental results, providing a mechanistic interpretation of the dynamics of the in vitro oral phase of swallowing. These findings and this approach could pave the way for a better design of solid oral medications to address the special needs of children or patients with swallowing disorders and could help designing more successful sensory evaluations and clinical studies.

Vertical ordering of rods under vertical vibration.

Granular media composed of elongated particles rearrange and order vertically upon vertical vibration. We perform pseudo-two-dimensional discrete element model simulations and show that this phenomenon also takes place with no help from vertical walls. We quantitatively analyze the sizes of voids forming during vibrations and consider a void-filling mechanism to explain the observed vertical ordering. Void filling can explain why short rods are less prone to align vertically than long ones. We cannot, however, explain, invoking just void filling, the existence of an optimum acceleration to promote vertical ordering and its dependence on particle length. We finally introduce an interpretation of the phenomenon, by considering the energetic barriers that particles have to overcome to exit a horizontal or a vertical lattice. By comparing these energetic thresholds with the peak mean particle fluctuant kinetic energy, we identify three different regimes. In the intermediate regime a vertical lattice is stable, while a horizontal is not. This interpretation succeeds in reconciling both dependencies on vibration acceleration and on particle length.

Gray free-energy multiphase lattice Boltzmann model with effective transport and wetting properties.

The paper shows that it is possible to combine the free-energy lattice Boltzmann approach to multiphase modeling of fluids involving both liquid and vapor with the partial bounce back lattice Boltzmann approach to modeling effective media. Effective media models are designed to mimic the properties of porous materials with porosity much finer than the scale of the simulation lattice. In the partial bounce-back approach, an effective media parameter or bounce-back fraction controls fluid transport. In the combined model, a wetting potential is additionally introduced that controls the wetting properties of the fluid with respect to interfaces between free space (white nodes), effective media (gray nodes), and solids (black nodes). The use of the wetting potential combined with the bounce-back parameter gives the model the ability to simulate transport and sorption of a wide range of fluid in material systems. Results for phase separation, permeability, contact angle, and wicking in gray media are shown. Sorption is explored in small sections of model multiscale porous systems to demonstrate two-step desorption, sorption hysteresis, and the ink-bottle effect.

In vivo observations and in vitro experiments on the oral phase of swallowing of Newtonian and shear-thinning liquids.

In this study, an in vitro device that mimics the oral phase of swallowing is calibrated using in vivo measurements. The oral flow behavior of different Newtonian and non-Newtonian solutions is then investigated in vitro, revealing that shear-thinning thickeners used in the treatment of dysphagia behave very similar to low-viscosity Newtonian liquids during active swallowing, but provide better control of the bolus before the swallow is initiated. A theoretical model is used to interpret the experimental results and enables the identification of two dynamical regimes for the flow of the bolus: first, an inertial regime of constant acceleration dependent on the applied force and system inertia, possibly followed by a viscous regime in which the viscosity governs the constant velocity of the bolus. This mechanistic understanding provides a plausible explanation for similarities and differences in swallowing performance of shear-thinning and Newtonian liquids. Finally, the physiological implications of the model and experimental results are discussed. In vitro and theoretical results suggest that individuals with poor tongue strength are more sensitive to overly thickened boluses. The model also suggests that while the effects of system inertia are significant, the density of the bolus itself plays a negligible role in its dynamics. This is confirmed by experiments on a high density contrast agent used for videofluoroscopy, revealing that rheologically matched contrast agents and thickener solutions flow very similarly. In vitro experiments and theoretical insights can help designing novel thickener formulations before clinical evaluations.

A model experiment to understand the oral phase of swallowing of Newtonian liquids.

A model experiment to understand the oral phase of swallowing is presented and used to explain some of the mechanisms controlling the swallowing of Newtonian liquids. The extent to which the flow is slowed down by increasing the viscosity of the liquid or the volume is quantitatively studied. The effect of the force used to swallow and of the gap between the palate and the roller used to represent the contracted tongue are also quantified. The residual mass of liquid left after the model swallow rises strongly when increasing the gap and is independent of bolus volume and applied force. An excessively high viscosity results in higher residues, besides succeeding in slowing down the bolus flow. A realistic theory is developed and used to interpret the experimental observations, highlighting the existence of an initial transient regime, at constant acceleration, that can be followed by a steady viscous regime, at constant velocity. The effect of the liquid viscosity on the total oral transit time is lower when the constant acceleration regime dominates bolus flow. Our theory suggests also that tongue inertia is the cause of the higher pressure observed at the back of the tongue in previous studies. The approach presented in this study paves the way toward a mechanical model of human swallowing that would facilitate the design of novel, physically sound, dysphagia treatments and their preliminary screening before in vivo evaluations and clinical trials.

About the influence of friction and polydispersity on the jamming behavior of bead assemblies.

We study the jamming of bead assemblies placed in a cylindrical container whose bottom is pierced with a circular hole. Their jamming behavior is quantified here by the median jamming diameter, that is the diameter of the hole for which the jamming probability is 0.5. Median jamming diameters of monodisperse assemblies are obtained numerically using the Distinct Element Method and experimentally with steel beads. We obtain good agreement between numerical and experimental results. The influence of friction is then investigated. In particular, the formation of concentric bead rings is observed for low frictions. We identify this phenomenon as a boundary effect and study its influence on jamming. Relying on measures obtained from simulation runs, the median jamming diameter of bidisperse bead assemblies is finally found to depend only on the volume-average diameter of their constituting beads. We formulate this as a tentative law and validate it using bidisperse assemblies of steel beads.