Mitigation of Hearing Damage With Liraglutide Treatment in Chinchillas After Repeated Blast Exposures at Mild-TBI.

INTRODUCTION: Although hearing protection devices (HPDs) have been widely used during training and combat, over one million veterans experience service-connected hearing loss. Hearing damage has been reported to be associated with blast-induced mild traumatic brain injury (mTBI) and there is a lack of understanding and treatment. Liraglutide is a glucagon-like peptide-1 receptor agonist and a potential treatment for TBI-induced memory deficits. This study aims to investigate the function of the liraglutide to prevent damage and facilitate hearing restoration in chinchillas exposed to multiple high-intensity, mTBI-level blasts. MATERIALS AND METHODS: Chinchillas were divided into three treatment groups: blast control, pre-blast drug treatment, and post-blast drug treatment. On day 1, the chinchilla ears were protected by HPDs and exposed to three blasts with peak pressure levels of 15-25 psi. The auditory brainstem response (ABR), distortion product otoacoustic emission (DPOAE), and middle latency response (MLR) were recorded pre- and post-blast on day 1 and on days 4, 7, 14, and 28. RESULTS: Substantial acute damage was observed and progressively recovered in chinchillas after the blast exposures. The pre-blast treatment group exhibited the lowest elevation of the ABR threshold and reduction of the wave I amplitude on day 1 after blasts. The liraglutide treatment insignificantly facilitated the recovery of the DPOAE levels and ABR thresholds on days 14 and 28. The pre-blast treatment chinchillas showed reduced MLR amplitudes on days 4 and 7. CONCLUSIONS: This study indicated that the pre-blast liraglutide administration provided damage protection against blasts in addition to the HPDs. Current evidence suggests that the effect of liraglutide is more prominent in the early phase of the experiment.

3D Finite Element Model of Human Ear with 3-Chamber Spiral Cochlea for Blast Wave Transmission from the Ear Canal to Cochlea.

Blast-induced auditory trauma is a common injury in military service members and veterans that leads to hearing loss. While the inner ear response to blast exposure is difficult to characterize experimentally, computational models have advanced to predict blast wave transmission from the ear canal to the cochlea; however, published models have either straight or spiral cochlea with fluid-filled two chambers. In this paper, we report the recently developed 3D finite element (FE) model of the human ear mimicking the anatomical structure of the 3-chambered cochlea. The model consists of the ear canal, middle ear, and two and a half turns of the cochlea with three chambers separated by the Reissner's membrane (RM) and the basilar membrane (BM). The blast overpressure measured from human temporal bone experiments was applied at the ear canal entrance and the Fluent/Mechanical coupled fluid-structure interaction analysis was conducted in ANSYS software. The FE model-derived results include the pressure in the canal near the tympanic membrane (TM) and the intracochlear pressure at scala vestibuli, the TM displacement, and the stapes footplate (SFP) displacement, which were compared with experimentally measured data in human temporal bones. The validated model was used to predict the biomechanical response of the ear to blast overpressure: distributions of the maximum strain and stress within the TM, the BM displacement variation from the base to apex, and the energy flux or total energy entering the cochlea. The comparison of intracochlear pressure and BM displacement with those from the FE model of 2-chambered cochlea indicated that the 3-chamber cochlea model with the RM and scala media chamber improved our understanding of cochlea mechanics. This most comprehensive FE model of the human ear has shown its capability to predict the middle ear and cochlea responses to blast overpressure which will advance our understanding of auditory blast injury.

Real-time measurement of stapes motion and intracochlear pressure during blast exposure.

Blast-induced auditory injury is primarily caused by exposure to an overwhelming amount of energy transmitted into the external auditory canal, the middle ear, and then the cochlea. Quantification of this energy requires real-time measurement of stapes footplate (SFP) motion and intracochlear pressure in the scala vestibuli (Psv). To date, SFP and Psv have not been measured simultaneously during blast exposure, but a dual-laser experimental approach for detecting the movement of the SFP was reported by Jiang et al. (2021). In this study, we have incorporated the measurement of Psv with SFP motion and developed a novel approach to quantitatively measure the energy flux entering the cochlea during blast exposure. Five fresh human cadaveric temporal bones (TBs) were used in this study. A mastoidectomy and facial recess approach were performed to identify the SFP, followed by a cochleostomy into the scala vestibuli (SV). The TB was mounted to the "head block", a fixture to simulate a real human skull, with two pressure sensors - one inserted into the SV (Psv) and another in the ear canal near the tympanic membrane (P1). The TB was exposed to the blast overpressure (P0) around 4 psi or 28 kPa. Two laser Doppler vibrometers (LDVs) were used to measure the movements of the SFP and TB (as a reference). The LDVs, P1, and Psv signals were triggered by P0 and recorded simultaneously. The results include peak values for Psv of 100.8 ± 51.6 kPa (mean ± SD) and for SFP displacement of 72.6 ± 56.4 µm, which are consistent with published experimental results and finite element modeling data. Most of the P0 input energy flux into the cochlea occurred within 2 ms and resulted in 10-70 µJ total energy entering the cochlea. Although the middle ear pressure gain was close to that measured under acoustic stimulus conditions, the nonlinear behavior of the middle ear was observed from the elevated cochlear input impedance. For the first time, SFP movement and intracochlear pressure Psv have been successfully measured simultaneously during blast exposure. This study provides a new methodology and experimental data for determining the energy flux entering the cochlea during a blast, which serves as an injury index for quantifying blast-induced auditory damage.

Hearing protection and damage mitigation in Chinchillas exposed to repeated low-intensity blasts.

Repeated exposures to blast overpressure (BOP) introduce hearing complaints in military service members even with the use of hearing protection devices (HPDs). Although epidemiology and animal studies have been performed to investigate the damage formation mechanism of blast-induced hearing damage, there is still a lack of understanding and therapeutic solutions, especially for HPD-protected ears. Recent studies revealed the potential therapeutic function of liraglutide, a glucagon-like peptide-1 receptor agonist, to facilitate post-blast hearing restoration in chinchillas. This study is a continuation and summary of the previous studies performed by Jiang et al. (2021, 2022) to investigate the damage mitigation function of liraglutide treatment in chinchillas with open and protected ears after repeated low-intensity blast exposures within 28 days of observation. Chinchillas were divided into six experimental groups: pre-blast treatment, post-blast treatment, and blast control with ears open or protected by earplug (EP). All animals were exposed to six consecutive blasts at the level of 3-5 psi (21-35 kPa) on Day 1. Hearing function tests including auditory brainstem response (ABR), distortion product otoacoustic emission (DPOAE), and middle latency response (MLR) were performed on Day 1 (pre- and post-blast) and Days 4, 7, 14, and 28 after blast exposure. Results indicated that the damage mitigation function of the liraglutide treatment in the open-ear chinchillas was reflected by the significantly lower ABR threshold shifts in the drug treatment groups than in the blast controls. In EP groups, the higher ABR wave I/V ratio and lower MLR amplitude observed in the drug-treated chinchillas suggested that the post-blast hyperactivities in the auditory system might be potentially ameliorated by the liraglutide treatment. The 28-day-long experiment showed the effect of liraglutide treatment increased with time in both open and EP groups. This study demonstrated that the use of HPDs prevented the blast-induced complications in the middle ear and reduced the damage caused in the central auditory system. The liraglutide treatment showed an effect increasing with time and different outcomes in open and EP chinchillas. This innovation in the animal model of chinchilla provides insights to investigate subtle changes in the higher-level structures of the auditory system.

Mitigation of Hearing Damage After Repeated Blast Exposures in Animal Model of Chinchilla.

High-intensity sound or blast-induced hearing impairment is a common injury for Service members. Epidemiology studies revealed that the blast-induced hearing loss is associated with the traumatic brain injury (TBI), but the mechanisms of the formation and prevention of auditory injuries require further investigation. Liraglutide, a glucagon-like peptide-1 receptor (GLP-1R) agonist, has been reported as a potential treatment strategy for TBI-caused memory deficits; however, there is no study on therapeutics of GLP-1R for blast-induced hearing damage. This paper reports our current study on progressive hearing damage after repeated exposures to low-level blasts in the animal model of chinchilla and the mitigation of hearing damage using liraglutide. Chinchillas were divided into three groups (N = 7 each): blast control, pre-blast treatment, and post-blast treatment. All animals were exposed to six consecutive blasts at the level of 3-5 psi (21-35 kPa) on Day 1. The auditory brainstem response (ABR) was measured on Day 1 (pre- and post-blast) and Days 4, 7, and 14 after blast exposure. Upon the completion of the experiment on Day 14, the brain tissues of animals were harvested for immunofluorescence studies. Significant damage was revealed in blast-exposed chinchillas by increased ABR thresholds, decreased ABR wave I amplitudes, and cell apoptosis in the inferior colliculus in the blast control chinchillas. Treatment with liraglutide appeared to reduce the severity of blast-induced hearing injuries as observed from the drug-treated chinchillas comparing to the blast controls. This study bridges the gap between TBI and hearing impairment and suggests a possible intervention for blast-induced hearing loss for Service members.

A comprehensive finite element model for studying Cochlear-Vestibular interaction.

We present a 3-D finite element (FE) model of the chinchilla's inner ear consisting of the entire cochlea structure and the vestibular system. The reaction of the basilar membrane to the head rotation and the reaction of ampulla to the stapes movement were investigated. These results demonstrate the existence of hearing-vestibular system interaction. They provide an explanation to the clinical finding on the coexistence between hearing loss and equilibration dysfunction. It is a preliminary, yet critical step toward the development of a comprehensive FE model of an entire ear for mechano-acoustic analysis.

Three-Dimensional Finite Element Modeling of Blast Wave Transmission From the External Ear to a Spiral Cochlea.

Blast-induced injuries affect the health of veterans, in which the auditory system is often damaged, and blast-induced auditory damage to the cochlea is difficult to quantify. A recent study modeled blast overpressure (BOP) transmission throughout the ear utilizing a straight, two-chambered cochlea, but the spiral cochlea's response to blast exposure has yet to be investigated. In this study, we utilized a human ear finite element (FE) model with a spiraled, two-chambered cochlea to simulate the response of the anatomical structural cochlea to BOP exposure. The FE model included an ear canal, middle ear, and two and half turns of two-chambered cochlea and simulated a BOP from the ear canal entrance to the spiral cochlea in a transient analysis utilizing fluid-structure interfaces. The model's middle ear was validated with experimental pressure measurements from the outer and middle ear of human temporal bones. The results showed high stapes footplate (SFP) displacements up to 28.5 µm resulting in high intracochlear pressures and basilar membrane (BM) displacements up to 43.2 µm from a BOP input of 30.7 kPa. The cochlea's spiral shape caused asymmetric pressure distributions as high as 4 kPa across the cochlea's width and higher BM transverse motion than that observed in a similar straight cochlea model. The developed spiral cochlea model provides an advancement from the straight cochlea model to increase the understanding of cochlear mechanics during blast and progresses toward a model able to predict potential hearing loss after blast.

A 3D Printed Human Ear Model for Standardized Testing of Hearing Protection Devices to Blast Exposure.

Hypothesis: A 3D printed human temporal bone (TB) that is anatomically accurate would cost-effectively reproduce the responses observed in blast testing of human cadaveric TBs with and without passive hearing protection devices (HPDs). Background: HPDs have become critical personal protection equipment against auditory damage for service members. Acoustic test fixtures and human TBs have been used to test and develop HPDs; however, the lack of a cost-effective, standardized model impedes the improvement of HPDs. Methods: In this study, the 3D printed TB model was printed with flexible and rigid polymers and consisted of the ear canal, tympanic membrane (TM), ossicular chain, middle ear suspensory ligaments/muscle tendons, and middle ear cavity. The TM movement under acoustic stimulation was measured with laser Doppler vibrometry. The TB model was then exposed to blasts with or without HPDs and pressures at the ear canal entrance (P0) and near the TM in the ear canal (P1) were recorded. All results were compared with that measured in human TBs. Results: Results indicated that in the 3D printed TB, the attenuated peak pressures at P1 induced by HPDs ranged from 0.92 to 1.06 psi (170-171 dB) with blast peak pressures of 5.62-6.54 psi (186-187 dB) at P0, and measured results were within the mean and SD of published data. Vibrometry measurements also followed a similar trend as the published results. Conclusions: The 3D printed TB model accurately evaluated passive HPDs' protective function during blast and the potential for use as a model for acoustic transmission was investigated.

Central and peripheral auditory abnormalities in chinchilla animal model of blast-injury.

Exposure to blast overpressure or high-intensity sound can cause injuries to the auditory system, which leads to hearing loss or tinnitus. In this study, we examined the involvement of peripheral auditory system (PAS), and central auditory system (CAS) changes after exposure to blast overpressure (15-25 psi) on Day 1 and additionally during 7 days of post blast time period in chinchillas. Auditory brainstem response (ABR), distortion product otoacoustic emission (DPOAE), and cochlear hair cell changes were measured or identified in post-blast period within 7 days to detect injuries in the PAS. In the CAS, changes in NMDAR1 (excitatory receptor) and GABAA (inhibitory receptor) as well as changes in serotonin (5-HT2A) and acetylcholine (AChR) receptors were examined in different brain regions: auditory cortex (AC), geniculate body (GB), inferior colliculus (IC) and amygdala by immunofluorescence staining. We observed the PAS abnormalities of increased ABR threshold and decreased DPOAE response in animals after blast exposure with hearing protection devices (e.g., earplug). Blast exposure also caused a reduction in both NMDAR1 and GABAA receptor levels in acute condition (post-blast or Day 1) in AC and IC, while serotonin and acetylcholine receptor levels displayed a biphasic response at Day 1 and Day 7 post-exposure. Results demonstrate that the earplug can protect the tympanic membrane and middle ear against structural damage, but the hearing level, cochlear outer hair cell, and the central auditory system (levels of excitatory and inhibitory neurotransmitter receptors) were only partially protected at the tested blast overpressure level. The findings in this study indicate that blast exposure can cause both peripheral and central auditory dysfunctions, and the central auditory response is independent of peripheral auditory damage. The CAS dysfunction is likely mediated by direct transmission of shockwaves in all the regions of central nervous system (CNS), including nerves and surrounding tissues along the auditory pathways. Hence, targeting central auditory neurotransmitter abnormalities may have a therapeutic benefit to attenuate blast-induced hearing loss and tinnitus.

Dual-laser measurement of human stapes footplate motion under blast exposure.

Hearing damage is one of the most frequently observed injuries in Service members and Veterans even though hearing protection devices (HPDs, e.g. earplugs) have been implemented to prevent blast-induced hearing loss. However, the formation and prevention mechanism of the blast-induced hearing damage remains unclear due to the difficulty for conducting biomechanical measurements in ears during blast exposure. Recently, an approach reported by Jiang et al. (2019) used two laser Doppler vibrometers (LDVs) to measure the motion of the tympanic membrane (TM) in human temporal bones during blast exposure. Using the dual laser setup, we further developed the technology to detect the movement of the stapes footplate (SFP) in ears with and without HPDs while under blast exposure. Eight fresh human cadaveric temporal bones (TBs) were involved in this study. The TB was mounted in a "head block" after performing a facial recess surgery to access the SFP, and a pressure sensor was inserted near the TM in the ear canal to measure the pressure reaching the TM (P1). The TB was exposed to a blast overpressure measuring around 7 psi or 48 kPa at the entrance of the ear canal (P0). Two LDVs were used to measure the vibrations of the SFP and TB (as a reference). The exact motion of the SFP was determined by subtracting the TB motion from the SFP data. Results included a measured peak-to-peak SFP displacement of 68.7 ± 31.6 µm (mean ± SD) from all eight TBs without HPDs. In five of the TBs, the insertion of a foam earplug reduced the SFP displacement from 48.3 ± 6.3 µm to 21.8 ± 10.4 µm. The time-frequency analysis of the SFP velocity signals indicated that most of the energy spectrum was concentrated at frequencies below 4 kHz within the first 2 ms after blast and the energy was reduced after the insertion of HPDs. This study describes a new methodology to quantitatively characterize the response of the middle ear and the energy entering the cochlea during blast exposure. The experimental data are critical for determining the injury of the peripheral auditory system and elucidating the damage formation and prevention mechanism in an ear exposed to blast.

Prevention of Blast-induced Auditory Injury Using 3D Printed Helmet and Hearing Protection Device - A Preliminary Study on Biomechanical Modeling and Animal.

INTRODUCTION: Repeated blast exposures result in structural damage to the peripheral auditory system (PAS) and the central auditory system (CAS). However, it is difficult to differentiate injuries between two distinct pathways: the mechanical damage in the PAS caused by blast pressure waves transmitted through the ear and the damage in the CAS caused by blast wave impacts on the head or traumatic brain injury. This article reports a preliminary study using a 3D printed chinchilla "helmet" as a head protection device associated with the hearing protection devices (e.g., earplugs) to isolate the CAS damage from the PAS injuries under repeated blast exposures. MATERIALS AND METHODS: A finite element (FE) model of the chinchilla helmet was created based on micro-computed tomography images of a chinchilla skull and inputted into ANSYS for FE analysis on the helmet's protection against blast over pressure. The helmet was then 3D printed and used for animal experiments. Chinchillas were divided into four cases (ears open, with earplug only, with both earplug and helmet, and with helmet only) and exposed to three blasts at blast over pressure of 15 to 20 psi. Hearing function tests (e.g., auditory brainstem response) were performed before and after blast on Day 1 and Days 4 and 7 after blasts. RESULTS: The FE model simulation showed a significant reduction in intracranial stress with the helmet, and the animal results indicated that both earplug and helmet reduced the severity of blast-induced auditory injuries by approximately 20 dB but with different mechanisms. CONCLUSIONS: The biomechanical modeling and animal experiments demonstrated that this four-case study in chinchillas with helmet and hearing protection devices provides a novel methodology to investigate the blast-induced damage in the PAS and CAS.

3D Finite Element Modeling of Blast Wave Transmission from the External Ear to Cochlea.

As an organ that is sensitive to pressure changes, the ear is often damaged when a person is subjected to blast exposures resulting in hearing loss due to tissue damage in the middle ear and cochlea. While observation of middle ear damage is non-invasive, examining the damage to the cochlea is difficult to quantify. Previous works have modeled the cochlear response often when subjected to an acoustic pressure input, but the inner ear mechanics have rarely been studied when the ear is exposed to a blast wave. In this study we aim to develop a finite element (FE) model of the entire ear, particularly the cochlea, for predicting the blast wave transmission from the ear canal to cochlea. We utilized a FE model of the ear, which includes the ear canal, middle ear, and uncoiled two-chambered cochlea, to simulate the cochlear response to blast overpressure (BOP) at the entrance of the ear canal with ANSYS Mechanical and Fluent in a fluid-structure interface coupled analysis in the time domain. This model was developed based on previous middle and inner ear models, and the cochlea was remeshed to improve BOP simulation performance. The FE model was validated using experimentally measured blast pressure transduction from the ear canal to the middle ear and cochlea in human cadaveric temporal bones. Results from the FE model showed significant displacements of the tympanic membrane, middle ear ossicles, and basilar membrane (BM). The stapes footplate displacement was observed to be as high as 60 µm, far exceeding the displacement during normal acoustic stimulation, when the 30 kPa (4.35 psi, 183 dB (SPL), Sound Pressure Level) of BOP was applied at the ear canal entrance. The large stapes movement caused pressures in the cochlea to exceed the physiological pressure level [< 10 Pa, 120 dB (SPL)] at a peak of 49.9 kPa, and the BM displacement was on the order of microns with a maximum displacement of 26.4 µm. The FE model of the entire human ear developed in this study provides a computational tool for prediction of blast wave transmission from the ear canal to cochlea and the future applications for assisting the prevention, diagnosis, and treatment of blast-induced hearing loss.

Investigating the Geometry and Mechanical Properties of Human Round Window Membranes Using Micro-Fringe Projection.

HYPOTHESIS: The geometry and the mechanical property of the round window membrane (RWM) have a fundamental impact on the function of cochlea. BACKGROUND: Understanding the mechanical behavior of RWM is important for cochlear surgery and design for the cochlear implant. Although the anatomy of RWM has been widely studied and described in the literature, argument remains regarding the true shape of RWM. The mechanical properties of RWM are also scarcely reported due to the difficulty of the measurement of the small size RWM. METHODS: In this paper, micro-fringe projection was used to reconstruct the 3-dimensional geometries of 14 RWMs. Mechanical properties of the RWMs were subsequently measured using finite element (FE) model and an inverse method. The three-dimensional surface topographies and the curvatures of the two major directions reconstructed from the micro-fringe projection both demonstrated wide variations among samples. RESULTS: The diameters of the RWMs vary from 1.65 to 2.2 mm and the curvatures vary from -0.97 to 3.76 mm-1. The nonlinear elasticity parameters in the Ogden model for each sample was measured and the average effective Young's modulus is approximately 1.98 MPa. CONCLUSION: The geometries and mechanical properties of the human RWM measured in the work could potentially be applied to surgery design and on modeling analysis for the cochlea.

Mechanical Properties of Baboon Tympanic Membrane from Young to Adult.

Mechanical properties of the tympanic membrane (TM) play an important role in sound transmission through the middle ear. While numerous studies have investigated the mechanical properties of the adult human TM, the effects of age on the TM's properties remain unclear because of the limited published data on the TM of young children. To address this deprivation, we used baboons in this study as an animal model for investigating the effect of age on the mechanical properties of the TM. Temporal bones were harvested from baboons (Papio anubis) of four different age groups: less than 1 year, 1-3 years, 3-5 years, and older than 5 years of age or adult. The TM specimens were harvested from baboon temporal bones and cut into rectangle strips along the inferior-superior direction, mainly capturing the influence of the circumferential direction fibers on the TM's mechanical properties. The elasticity, ultimate tensile strength, and relaxation behavior of the baboon TM were measured in each of the four age groups with a mechanical analyzer. The average effective Young's modulus of adult baboon TM was approximately 3.1 MPa, about two times higher than that of a human TM. The Young's moduli of the TM samples demonstrated a 26 % decrease from newborn to adult (from 4.2 to 3.1 MPa). The average ultimate tensile strength of the TMs for all the age groups was ~ 2.5 MPa. There was no significant change in the ultimate tensile strength and relaxation behavior among age groups. The preliminary results reported in this study provide a first step towards understanding the effect of age on the TM mechanical properties from young to adult.

Hearing Damage Induced by Blast Overpressure at Mild TBI Level in a Chinchilla Model.

INTRODUCTION: The peripheral auditory system and various structures within the central auditory system are vulnerable to blast injuries, and even blast overpressure is at relatively mild traumatic brain injury (TBI) level. However, the extent of hearing loss in relation to blast number and time course of post-blast is not well understood. This study reports the progressive hearing damage measured in chinchillas after multiple blast exposures at mild TBI levels (103-138 kPa or 15-20 psi). MATERIALS AND METHODS: Sixteen animals (two controls) were exposed to two blasts and three blasts, respectively, in two groups with both ears plugged with foam earplugs to prevent the eardrum from rupturing. Auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) were measured in pre- and post-blasts. Immunohistochemical study of chinchilla brains were performed at the end of experiment. RESULTS: Results show that the ABR threshold and DPOAE level shifts in 2-blast animals were recovered after 7 days. In 3-blast animals, the ABR and DPOAE shifts remained at 26 and 23 dB, respectively after 14 days. Variation of auditory cortex damage between 2-blast and 3-blast was also observed in immunofluorescence images. CONCLUSIONS: This study demonstrates that the number of blasts causing mild TBI critically affects hearing damage.

The effect of blast overpressure on the mechanical properties of the human tympanic membrane.

The rupture of the tympanic membrane (TM) is one of the major indicators for blast injuries due to the vulnerability of TM under exposure to blast overpressure. The mechanical properties of the human TM exhibit a significant change after it is exposed to such a high intensity blast. To date, the published data were obtained from measurement on TM strips cut from a TM following an exposure to blast overpressure. The dissection of a TM for preparation of strip samples can induce secondary damage to the TM and thus potentially lead to data not representative of the blast damage. In this paper, we conduct mechanical testing on the full TM in a human temporal bone. A bulging experiment on the entire TM is carried out on each sample prepared from a temporal bone following the exposure to blast three times at a pressure level slightly below the TM rupture threshold. Using a micro-fringe projection method, the volume displacement is obtained as a function of pressure, and their relationship is modeled in the finite element analysis to determine the mechanical properties of the post-blast human TMs, the results of which are compared with the control TMs without an exposure to the blast. It is found that Young's modulus of human TM decreases by approximately 20% after exposure to multiple blast waves. The results can be used in the human ear simulation models to assist the understanding of the effect of blast overpressure on hearing loss.

Surface Motion Changes of Tympanic Membrane Damaged by Blast Waves.

Eardrum or tympanic membrane (TM) is a multilayer soft tissue membrane located at the end of the ear canal to receive sound pressure and transport the sound into the middle ear and cochlea. Recent studies reported that the TM microstructure and mechanical properties varied after the ear was exposed to blast overpressure. However, the impact of such biomechanical changes of the TM on its movement for sound transmission has not been investigated. This paper reports the full-field surface motion of the human TM using the scanning laser Doppler vibrometry in human temporal bones under normal and postblast conditions. An increase of the TM displacement after blast exposure was observed in the posterior region of the TM in four temporal bone samples at the frequencies between 3 and 4 kHz. A finite element model of human TM with multilayer microstructure and orthogonal fiber network was created to simulate the TM damaged by blast waves. The consistency between the experimental data and the model-derived TM surface motion suggests that the tissue injuries were resulted from a combination of mechanical property change and regional discontinuity of collagen fibers. This study provides the evidences of surface motion changes of the TM damaged by blast waves and possible fiber damage locations.

Characterization of Protection Mechanisms to Blast Overpressure for Personal Hearing Protection Devices - Biomechanical Measurement and Computational Modeling.

Hearing damage induced by blast exposure is a common injury in military personnel involved in most operation activities. Personal hearing protection devices such as earplugs come as a standard issue for Service members; however, it is not clear how to accurately evaluate the protection mechanisms of different hearing protection devices for blast overpressures (BOP). This paper reports a recent study on characterization of earplugs' protective function to BOP using human cadaver ears and 3D finite element (FE) model of the human ear. The cadaver ear mounted with pressure sensors near the eardrum (P1) and inside the middle ear (P2) and with an earplug inserted was exposed to BOP in the blast test chamber. P1, P2, and BOP at the ear canal entrance (P0) were simultaneously recorded. The measured P0 waveform was then applied at the ear canal entrance in the FE model and the P1 and P2 pressures were derived from the model. Both experiments and FE modeling resulted in the P1 reduction which represents the effective protection function of the earplug. Different earplugs showed variations in pressure waveforms transmitted to the eardrum, which determine the protection level of earplugs.

Mapping the Young's modulus distribution of the human tympanic membrane by microindentation.

The human tympanic membrane (TM, or eardrum) is composed primarily of layers of collagen fibers oriented in the radial and circumferential directions, as well as epidermal and mucosal layers at the lateral and medial surfaces. The mechanical properties of the TM depend on the microstructures of the collagen fibers, which vary with location, resulting in a spatial variation of Young's modulus. In this study, the Young's modulus of the human TM is measured using microindentation. A 10 µm diameter spherical nanoindenter tip is used to indent the TM at different locations in the lateral and medial surfaces. Through a viscoelastic contact analysis, the steady state out-of-plane (through thickness) Young's modulus at a constant strain rate for the TM is determined from the uniaxial relaxation modulus. The measured spatial distribution of Young's modulus is reported for the entire TM pars tensa on both lateral and medial surfaces. The Young's modulus, for the four TM quadrants, is analyzed statistically using a normal quantile-quantile (Q-Q) plot. The obtained S-shaped curve indicates a bi-modal Gaussian distribution in the Q-Q plot. The spatial distribution of the Young's modulus is modeled by a bivariate Gaussian function in the polar coordinates over the entire TM on both the lateral and medial surfaces. It is shown that the anterior-superior quadrant has the smallest value of Young's modulus. Differences are observed in the spatial distribution of the Young's modulus for both the lateral and medial surfaces. For the medial surface, Young's modulus varies mainly along the radial direction following a small-large-small trend, emanating from the umbo. For the lateral surface, the modulus at the anterior-superior quadrant shows the smallest modulus; the modulus decreases gradually along the radial directions. The quantitative results presented in this paper will help improve future simulation models of the middle ear by using spatial dependence of Young's modulus over the entire TM.