Study Of Implant Performance Of Piezoelectric Middle Ear Implants Based On A Human Ear Model Considering Material Nonlinearity

Implantable middle ear hearing devices or middle ear implants(MEI) are currently a new treatment for sensorineural hearing loss. Because it directly stimulate middle ear structures such as the ossicular chain in a mechanical stimulating way, the problems of conventional hearing aids such as acoustic feedback and low gain at high frequency can be solved and the operative wound of inner ear due to cochlear implant can also be avoided. Therefore, MEI is an ideal compensation method to cure moderate to severe sensorineural hearing loss. At present, the existing electromagnetic MEI is limited by its high power consumption and vulnerable to be influenced by the external magnetic field, and the piezoelectric MEI using piezo bimorphs has the limitations of low compensation frequency and insufficient gain. As a result, there is a realistic necessity to study and design an implantable middle ear hearing devices that has a better compensation ability at high frequency, low power consumption and high reliability.In the theoretical design stage of MEI, it is usually an important step to study the implant characteristics such as equivalent sound pressure and power consumption combined with the mechanical behavior of human ear. However, in the reported mechanical models of human ear, material properties of soft tissues were still widely assumed to be linear elasticity. On one hand, this assumption is not consistent with the nonlinear material property of real human ear and therefore can’t accurately reflect the biomechanical characteristics of human ear system. On the other hand, numerical evaluation of MEI based on this assumption is unable to be effectively applied into the structural design and theoretical study of MEI.Based on the above problems, this work was firstly dedicated to construct an accurate and complete mechanical model of human ear. In this model, hyperelastic and viscoelastic material properties of soft tissues were taken into consideration, and the external ear canal, middle ear and inner ear were integrated to realize a whole ear model. On this basis, implant performances including equivalent sound pressure and power consumption were studied for piezoelectric MEIs. Then an incus-body stimulating type actuator using piezoelectric stack as the vibration source was designed. Finally, the constructed ear model and designed piezoelectric actuator were verified by means of acoustic stimulation and mechanical stimulation in human temporal bone experiments, respectively. The main work and contributions of this dissertation are given as follows:1. The method of establishing a constitutive model including the quasi-static hyperelastic and dynamic viscoelastic material properties were studied for the soft tissues in human ear. The third-order Ogden strain energy and firstorder Prony series were separately employed to fit the published experimental measurements, and nonlinear material parameters of each soft tissue were consequently obtained. Furthermore, by measuring the equivalent strain in static analysis, the expression of long-term modulus for dynamic analysis was finally obtained. Therefore, the constitutive connection of hyperelasticity and viscoelasticity was fulfilled and modeling approach of considering material nonlinearity in human ear model was also confirmed.2. A three-dimensional finite element model of human ear was completely and accurately constructed, then this model was used to analyze and study the mechanical characteristics and sound transmission mechanism in human ear. Geometric models of the external ear canal and middle ear were modeled through high-precision Micro-CT imaging data and reverse engineering technology, and the cochlea was simplified as an uncoiled, two-chambered and fluid-filled duct. Moreover, the multi-field coupling modeling method was applied to accomplish the physical connection of ear canal air, middle ear structures and cochlear fluid, and thereby finished the transmission pathway simulation of acoustic signal from the ear canal, via middle ear and finally into cochlea. Furthermore, by means of incorporating hyperelastic and viscoelastic material properties of soft tissues, the constructed model could be used in the analysis under dynamic sound pressure load and static pressure load. Based on this model, dynamic and static behavior of human ear system were simulated and model-derived results showed a reasonable agreement with published experimental measurements. The model was ultimately adopted to analyze the effects of geometry and material nonlinearity on the transmission characteristics of human ear.3. Taking advantage of the coupled model of human ear and the piezoelectric actuator, implant performances of forward and reverse type MEIs were analyzed respectively. The piezoelectric stack was used as the vibration source of piezoelectric MEI, and its electro-mechanical coupling characteristic and modeling approach was firstly studied. As for the forward stimulation type MEI, the peripheral structures of a floating type piezoelectric actuator was designed and its equivalent sound pressure, power consumption and structural optimization were accomplished based on the coupled model. Then, by considering the material nonlinearity of soft tissues, the effects of middle-ear static pressure and viscoelasticity on performances of MEI were analyzed. As for the reverse stimulation type MEI, effect of coupling conditions on the round window stimulation was studied by means of placing the actuator onto the round window membrane. Furthermore, a comparison was made among three kinds of MEI stimulating positions and an incus-body stimulating type actuator was confirmed as the final design solution. Structural design and implant performance analysis of the actuator was subsequently accomplished.4. An experimental study was carried on for the incus-body stimulating type piezoelectric actuator based on the human temporal bone platform. According to the designed structural parameters and technical criteria, the piezoelectric actuator was firstly manufactured and assembled. Under the free boundary condition of the actuator’s end, dynamic output characteristic of the actuator was measured. Experimental results indicated that the actuator’s output displacement satisfied the design requirements with a good linearity and wide dynamic range, which would benefit the processing algorithm development and hearing compensation at high frequency. Furthermore, the experimental platform of temporal bone was established to measure the stapes displacements under the sound stimulation in the ear canal and actuator stimulation, respectively. Experimental results demonstrated that the constructed finite element model could accurately reflect the dynamic behavior of human ear. Besides, through comparisons of stapes displacements under two stimulation forms, the piezoelectric actuator was confirmed having the features of low power consumption and high frequency compensation ability, which can be used in the treatment for sensorineural hearing loss.