Hamid Motallebzadeh

A non-linear viscoelastic model for the tympanic membrane.

The mechanical behavior of the tympanic membrane displays both non-linearity and viscoelasticity. Previous finite-element models of the tympanic membrane, however, have been either non-linear or viscoelastic but not both. In this study, these two features are combined in a non-linear viscoelastic model. The constitutive equation of this model is a convolution integral composed of a non-linear elastic part, represented by an Ogden hyperelastic model, and an exponential time-dependent part, represented by a Prony series. The model output is compared with the relaxation curves and hysteresis loops observed in previous measurements performed on strips of tympanic membrane. In addition, a frequency-domain analysis is performed based on the obtained material parameters, and the effect of strain rate is explored. The model presented here is suitable for modeling large deformations of the tympanic membrane for frequencies less than approximately 3 rad/s or about 0.6 Hz. These conditions correspond to the pressurization involved in tympanometry.

Visco-hyperelastic law for finite deformations: a frequency analysis

Some biological tissues are repeatedly stimulated under cyclic loading, and this stimulation can be combined with large pressures, thus leading to large deformations. For such applications, visco-hyperelastic models have been proposed in the literature and used in finite-element studies. An extensively used quasi-linear model (QLVH), which assumes linear evolution equations, is compared with a nonlinear model (NLVH), which assumes a multiplicative split of the deformation gradient. The comparison is made here using sets of simulations covering a large frequency range. Lost and stored energies are computed, and the additional parameter of the NLVH model is set to two values found in the literature (NLVH-2 and NLVH-30 models). The predicted behaviour is very similar for all models at small strains, with each time constant (and corresponding viscous modulus) being associated with a damping peak and a stored-energy increase. When the strain amplitude is increased, the ratio of lost to stored energy increases for the QLVH model, but decreases for the NLVH models. The NLVH-30 model also displays a shift of the peak damping towards higher frequencies. Before reaching a steady state, all models display a decay of energy independent of the frequency, and the additional parameter of the NLVH model permits the modelling of complex types of evolution of the damping. In conclusion, this study compares the behaviour of two viscous hyper-elastic laws to allow an informed choice between them.

Wideband reflectance measurements in newborns: Relationship to otoscopic findings

OBJECTIVES Newborn hearing screening includes testing with otoacoustic emissions and the auditory brainstem response. Unfortunately, both tests are affected by the presence of material in the ear canal and middle ear such as vernix, meconium, and amniotic fluid. The objective of this study was to determine to what extent occlusion of the ear canal as seen on otoscopy affects wideband energy reflectance measurements in newborns. A secondary objective was to obtain additional normative wideband reflectance data in newborns. METHODS Newborns from a well-baby nursery were enrolled. Wideband energy reflectance measurements and otoscopy were done immediately after the hearing screening. Occlusion of the ear canal as seen on otoscopy was described on a scale of 0-100%. RESULTS A total of 156 babies were enrolled (mean age = 25 hours). A statistically significant difference in the reflectance at ambient pressure was found between the 0-70% and 80-100% occlusion groups. There was no significant difference in reflectance between the right and the left ears. The median reflectance pattern generally followed that of previous studies but in certain frequency regions the present reflectance values were higher. CONCLUSION A significant increase in reflectance occurs when 70%-80% of the ear-canal diameter is occluded. Taking otoscopy findings into account may improve the interpretation of reflectance measurements. However, further studies are required to better establish the relationship between canal occlusion and reflectance.

Finite-element modelling of the newborn ear canal and middle ear

Available hearing-screening procedures cannot distinguish clearly between conductive and sensorineural hearing loss in newborns, and the results of available diagnostic tests in very young infants are difficult to interpret. Admittance measurements can help to detect conductive losses but do not provide reliable results for newborns, where the ear is anatomically different from the adult ear. Finite-element models of the newborn ear canal and middle ear were developed and their responses were studied for frequencies up to 2000 Hz. Material properties were taken from previous measurements and estimates, and the sensitivities of the models to these different parameters were examined. The simulation results were validated through comparison with previous experimental measurements. Simulations indicate that at frequencies up to 250 Hz the admittance of the canal wall is comparable to that of the middle ear in the newborn. Above 250 Hz, the canal-wall admittance remains almost constant but for the middle ear t...

Cochlear amplification and tuning depend on the cellular arrangement within the organ of Corti

Significance While the near-crystalline structure of the organ-of-Corti cytoarchitecture in the mammalian cochlea has been known for some time, its functional consequences on hearing remain to be established. The present computational-modeling studies show that individual outer hair cells (OHCs) can work together to produce high hearing sensitivity and frequency selectivity because of the overlapping asymmetrical Y-shaped structures that they form with the Deiters’ cells (DCs) and phalangeal processes (PhPs). Altering the geometry and material properties of these structures reveals that all three components have a profound effect on basilar-membrane and reticular-lamina amplification and tuning. One implication is that the DCs and PhPs are not just supporting structures, but that they must also be properly restored in emerging therapies to regenerate OHCs. The field of cochlear mechanics has been undergoing a revolution due to recent findings made possible by advancements in measurement techniques. While it has long been assumed that basilar-membrane (BM) motion is the most important determinant of sound transduction by the inner hair cells (IHCs), it turns out that other parts of the sensory epithelium closer to the IHCs, such as the reticular lamina (RL), move with significantly greater amplitude for weaker sounds. It has not been established how these findings are related to the complex cytoarchitecture of the organ of Corti between the BM and RL, which is composed of a lattice of asymmetric Y-shaped elements, each consisting of a basally slanted outer hair cell (OHC), an apically slanted phalangeal process (PhP), and a supporting Deiters’ cell (DC). Here, a computational model of the mouse cochlea supports the hypothesis that the OHC micromotors require this Y-shaped geometry for their contribution to the exquisite sensitivity and frequency selectivity of the mammalian cochlea. By varying only the OHC gain parameter, the model can reproduce measurements of BM and RL gain and tuning for a variety of input sound levels. Malformations such as reversing the orientations of the OHCs and PhPs or removing the PhPs altogether greatly reduce the effectiveness of the OHC motors. These results imply that the DCs and PhPs must be properly accounted for in emerging OHC regeneration therapies.

Finite-element model of the nonlinear distortion and linear reflection sources of the otoacoustic emissions within the mouse cochlea

It has been hypothesized that the otoacoustic emissions (OAEs) are generated at least by two fundamental mechanisms; the nonlinear distortion and linear reflection within the cochlea. The recent studies show that different components of the organ of Corti (OoC) vibrate unsynchronized both in phase and magnitude. The difference is dramatically frequency dependent. We have developed and validated a finite-element model of the mouse cochlea against two sets of measurement at the cochlear base and apex. The nonlinear distortion and linear reflections have been implemented in the model- the former one by the active outer hair cells (OHCs) and the later one by introducing the impedance irregularity in the baseline model components (e.g., stiffness, mass and geometrical parameters). To differentiate the contribution of different components of OoC on the total OAE response, the baseline parameters of the model has been adjusted to vary vibration magnitudes of those components. The preliminary results show that th...

The efficiency of the cytoarchitecture of the organ of Corti for active cochlear amplification

A “Y”-shaped structure of cells formed by the basally tilted force generating outer hair cells (OHC) and apically tilted passive phalangeal process (PhP) connected to a Deiter cell (DC) has been hypothesized to be an essential organ of Corti (OoC) building block for cochlear amplification (Yoon et al., 2011). We developed a COMSOL finite-element model of the mouse cochlea, taking into account the spatial arrangement of Y-shaped elements and the 3-D fluid-structure interaction. The model was validated by comparison with previously reported basilar membrane (BM) displacement for both passive and active cases (Lee et al., 2015). The baseline model can reproduce an increase of 40 dB gain of the BM (re stapes velocity) for low level sounds. However, the absence of the PhP results in an unstable response of the BM for active cases. The model with the roles of the PhP and OHC switched, could not reproduce the active gain. Finally, the model with reversed titling orientation of OHC and PhP cells generated signifi...