Theoretical And Experimental Research On Hearing Compensation Of A Novel Middle Ear Implant's Piezoelectric Vibrator

Hearing impairment is one of the most common diseases in our society. With the development of audiology and otology micro-surgery, most of the conductive hearing loss can benefit from operation. Whereas, there still lack of effective treatment to sensorineural hearing loss. The majority of these hearing-impaired individuals can only turn to traditional hearing aids. However, traditional hearing aids have several inherent disadvantages, such as sound distortion, limited amplification, noise and ringing, discomfort and cosmetic appearance. To overcome these shortcomings of traditional hearing aids, middle ear implants (MEI), which compensate hearing loss by direct mechanical stimulation to the ossicular chain, become a dynamic area of research. Until now, two types of vibrator have been developed for middle ear implant: piezoelectric vibrator and electromagnetic vibrator. In contrast, the piezoelectric vibrator has demonstrated many advantages including ease of fabrication, wider bandwidth, lower power consumption and more compatibility with external magnetic environment. But, most of developed piezoelectric vibrators using bimorphic principle, which makes them have small output gain and can't compensate effectively for severe sensorineural hearing loss.Accordingly, to invent a new piezoelectric MEI, this work was carried out to study the possibility of a novel piezoelectric vibrator. A human ear mechanical model, which can reflect the sound transmission process, was established. The coupling characteristics of the vibrator and the human auditory system were investigated and used to help the design of two types of piezoelectric vibrators, which can be implanted by a simple surgical operation. And finally, the capability of the incus driving type piezo-vibrator's hearing loss compensation was verified, based on a human temporal bone experiment. The main work and innovative contributions of this dissertation are as follows:1. A three dimensional model of the human cochlea at macroscopic level was established and used to calculate cochlear equivalent input impedance. First, a simplified three dimensional human cochlear model was developed and used to calculate basilar membrane velocity. And the model derived results were compared with reported experimental data to confirm the model's validity. Then, the intracochlear pressure consists of both the travelling wave and the compressive wave was analyzed. Finally, the cochlear input impedance was calculated, and a single mass-spring- damper equivalent model of the cochlea was derived.2. A high quality three dimensional human middle ear finite element model was constructed and used to analyze its sound transmission properties. A high quality three-dimensional finite element model of the human middle ear was constructed, based on microcomputer tomography and reverse engineering technology. To model the boundary condition of the ossicular suspension veritably, the solid models of ligaments and tendons were also established by reverse engineering technology. The elastic modulus values of the ligaments and tendons in the present FE model were calibrated by mode analysis and dynamic responses comparisions, so that the individual dynamic characteristic of the middle ear was retained. The validity of this model was confirmed by comparing the stapes displacement, stapes velocity transfer function, umbo displacement and ossicular lever ratio obtained by this model with published experimental measurements on human temporal bones. The reason of the lever ratio's sudden change at 2000 Hz was also analyzed by this FE model.3. The effects of the vibrator's main design parameters on human ear sound transmission were investigated. Based on the previous established human middle ear mechanical model, the influence of the vibrator's stimulation sites on its hearing loss compensation effect was investigated. A middle ear finite element model with a floating mass block clamped was constructed, given that the floating mass type vibrator likely to worsen patients'residual hearing. Then the mechanical model was used to study effects of the vibrator's main design parameters on human ear sound transmission. The results show that, floating vibrator produces mass loading effect prominently at high frequencies, the force needed to drive the incus to the equivalent of 100 dB SPL is about 89μN, and placing the clamp point of the floating vibrator close to the incudostapedial joint enhances the driving effect. Besides, incus long process is an ideal attachment point for vibrator as only a small excitation force is required to compensate a same level of hearing loss, and its compensation effect is less sensitive to surgical error.4. Based on corresponding piezoelectric vibrator-human middle ear coupling mechanical model, two types of piezoelectric vibrator which can efficiently stimulate the ossicular chain were proposed and designed. Aiming at the problem that existing middle ear implants can not compensate high frequency hearing loss properly, the author put forward two types of piezoelectric vibrator utilizing piezo-stack's inverse piezoelectric effect. Accordingly, two sets of the corresponding piezoelectric vibrator-human middle ear coupling mechanical models were construced. The final constructed coupling models were used to aid the design process of these proposed two types of piezoelectric vibrators: the floating mass type and the incus driving type. In addition, the hearing loss compensation performance and power consumption of these two types of vibrators were analyzed, respectively. The results show that: these two types of piezoelectric vibrators can compensate hearing loss efficiently by lower power consumption, comparing with electromagnetic vibrator. And they both perform well at high frequency which is a valuable aspect of their performance, given that the most commonly encountered pattern of sensorineural hearing loss affects the high frequencies more than the low frequencies. 5. The performance of the designed incus driving type piezoelectric vibrator was evaluated using human temporal bone experiment. A human temporal bone experimental plateform, which used for middle ear implants' performance test, was self-designed and built. And the hearing loss compensation capability of the designed incus driving type piezoelectric vibrator was tested on this plateform. Besides, to classify the intelligibility of middle ear implants' hearing loss compensation, the specification of hearing aids characteristics on harmonic distortion was brought in middle ear implants' test. The results show that, the incus driving type piezoelectric vibrator can compensate hearing loss efficiently under lower power consumption and lower voltage. Besides, it demonstrates high performance at high frequencies. On one hand, it has high output gain at high frequencies. On the other hand, it has small harmonic distortion at high frequencies.