A linear, physiologically based, three-dimensional finite element model of the cochlea is developed. The model integrates the electrical, acoustic, and mechanical elements of the cochlea. In particular, the model includes interactions between structures in the organ of Corti (OoC), piezoelectric relations for outer hair cell (OHC) motility, hair bundle (HB) conductance that changes with HB deflection, current flow in the cross section and along the different scalae, and the feed-forward effect. The parameters in the model are based on guinea-pig data as far as possible. The model is vetted using a variety of experimental data on basilar membrane motion and data on voltages and currents in the OoC. Model predictions compare well, qualitatively and quantitatively, with experimental data on basilar membrane frequency response, impulse response, frequency glides, and scala tympani voltage. The close match of the model predictions with experimental data demonstrates the validity of the model for simulating cochlear response to acoustic input and for testing hypotheses of cochlear function. Analysis of the model and its results indicates that OHC somatic motility is capable of powering active amplification in the cochlea. At the same time, the model supports a possible synergistic role for HB motility in cochlear amplification.

A new class of micromechanically motivated chain network models for soft biological tissues is presented. On the microlevel, it is based on the statistics of long chain molecules. A wormlike chain model is applied to capture the behavior of the collagen microfibrils. On the macrolevel, the network of collagen chains is represented by a transversely isotropic eight chain unit cell introducing one characteristic material axis. Biomechanically induced remodeling is captured by allowing for a continuous reorientation of the predominant unit cell axis driven by a biomechanical stimulus. To this end, we adopt the gradual alignment of the unit cell axis with the direction of maximum principal strain. The evolution of the unit cell axis' orientation is governed by a first-order rate equation. For the temporal discretization of the remodeling rate equation, we suggest an exponential update scheme of Euler-Rodrigues type. For the spatial discretization, a finite element strategy is applied which introduces the current individual cell orientation as an internal variable on the integration point level. Selected model problems are analyzed to illustrate the basic features of the new model. Finally, ARTICLE IN PRESS www.elsevier.com/locate/jmps 0022-5096/$ -see front matter r (E.M. Arruda), grosh@engin.umich.edu (K. Grosh).the presented approach is applied to the biomechanically relevant boundary value problem of an in vitro engineered functional tendon construct. r

This composite article is intended to give the experts in the field of cochlear mechanics an opportunity to voice their personal opinion on the one mechanism they believe dominates cochlear amplification in mammals. A collection of these ideas are presented here for the auditory community and others interested in the cochlear amplifier. Each expert has given their own personal view on the topic and at the end of their commentary they have suggested several experiments that would be required for the decisive mechanism underlying the cochlear amplifier. These experiments are presently lacking but if successfully performed would have an enormous impact on our understanding of the cochlear amplifier.

Most mathematical models of the mammalian cochlea neglect structural longitudinal coupling. However, recent experimental data suggest that viscoelastic longitudinal coupling, in the basilar membrane (BM) and the tectorial membrane (TM), is non-negligible. In this paper, mathematical models for BM and TM longitudinal coupling are presented to determine the influence of such a coupling on the tuning of the BM. The longitudinal coupling models are added to a macroscopic linear model of the guinea pig cochlea that includes the micromechanics of the organ of Corti and outer hair cell (OHC) somatic motility. The predictions of the BM response to acoustic stimulus show that the characteristic frequency is controlled by a TM radial resonance and that TM longitudinal coupling has a more significant effect than BM longitudinal coupling. TM viscoelasticity controls the sharpness of the BM frequency response and the duration of the impulse response. The results with realistic TM longitudinal coupling are more consistent with experiments. The model predicts that OHC somatic electromotility is able to supply power to the BM at frequencies well above the cutoff of the OHC basolateral membrane. Moreover, TM longitudinal coupling is predicted to stabilize the cochlea and enable a higher BM sensitivity to acoustic stimulation.

Surgical tendon repair is limited by the availability of viable tissue for transplantation. Because of its relatively avascular nature, tendon is a prime candidate for engineered tissue replacement. To address this problem, cells isolated from rat Achilles tendon were grown to confluence in culture and allowed to self-assemble into a cylinder between two anchor points. The resulting scaffold-free tissue was composed of aligned, small-diameter collagen fibrils, a large number of cells, and an excess of noncollagenous extracellular matrix; all characteristics of embryonic tendon. The stress-strain response of the constructs also resembles the nonlinear behavior of immature tendons, and the ultimate tensile strength is approximately equal to that of embryonic chick tendon, roughly 2 MPa. These physical and mechanical properties indicate that these constructs are the first viable tendons engineered in vitro, without the aid of artificial scaffolding.

A constitutive model is developed to characterize a general class of polymer and polymer-like materials that displays hyperelastic orthotropic mechanical behavior. The strain energy function is derived from the entropy change associated with the deformation of constituent macromolecules and the strain energy change associated with the deformation of a representative orthotropic unit cell. The ability of this model to predict nonlinear, orthotropic elastic behavior is examined by comparing the theory to experimental results in the literature. Simulations of more complicated boundary value problems are performed using the finite element method.

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Otoacoustic emissions, sounds generated by the inner ear, are widely used for diagnosing hearing disorders and studying cochlear mechanics. However, it remains unclear how emissions travel from their generation sites to the cochlear base. The prevailing view is that emissions reach the cochlear base via a backward-traveling wave, a slow-propagating transverse wave, along the cochlear partition. A different view is that emissions propagate to the cochlear base via the cochlear fluids as a compressional wave, a fast longitudinal wave. These theories were experimentally tested in this study by measuring basilar membrane (BM) vibrations at the cubic distortion product (DP) frequency from two longitudinal locations with a laser interferometer. Generation sites of DPs were varied by changing frequencies of primary tones while keeping the frequency ratio constant. Here, we show that BM vibration amplitude and phase at the DP frequency are very similar to responses evoked by external tones. Importantly, the BM vibration phase at a basal location leads that at a more apical location, indicating a traveling wave that propagates in the forward direction. These data are in conflict with the backwardtraveling-wave theory but are consistent with the idea that the emission comes out of the cochlea predominantly through compressional waves in the cochlear fluids. basilar membrane vibration ͉ cochlear traveling wave ͉ laser interferometer ͉ otoacoustic emission K emp (1, 2) discovered that the cochlea can generate sounds that are transmitted to the external ear canal through the middle ear. Such otoacoustic emissions (OAEs) are considered a by-product of normal cochlear performance (3), relying on a putative cochlear amplifier (4-6). Because OAEs can provide information on the health status of the cochlea and can be noninvasively measured, they are increasingly used for diagnosing auditory disorders and as a research tool for studying cochlear physiology (3). However, the applications of OAEs have been limited because the generation and transmission mechanisms are not clear.There are two different theories about OAE propagation from the generation site to the cochlear base. Propagation may either be dominated by a traveling wave moving in the backward direction (1, 7-9) or occur through a compressional wave in the cochlear fluids (10-16). Backward-traveling waves are slow and vibrate in the transverse direction, whereas compressional waves are longitudinal and travel at the speed of sound in water.Backward-traveling waves were first postulated in Kemp's original reports (1, 2). Support for this theory comes from indirect measurements, primarily of the delay between the stimulus and the appearance of the emission in the ear canal (17). Although the interpretation of such measurements is not unambiguous, it is commonly believed that the backward delay is the same as the forward delay. Consequently, the emission delay would be twice the forward delay (7, 9, 18). However, a direct proof of the theory requires backward waves to be me...