그래핀-PMMA 진동막을 적용한 고감도 고막 설치형 마이크로폰 = High Sensitivity Trans-tympanic Microphone with Graphene-PMMA Diaphragm|RISS 상세보기

다국어 초록 (Multilingual Abstract)

Fully implantable hearing devices (FIHDs) have been developed to overcome the problems of existing air-conductive hearing aids, and two models have been used commonly so far. The microphone is an important input component that can affect the overall performance of FIHDs. Therefore, vigorous research on implantable microphones has been carried out. In particular, the microphone of FIHDs needs to have an excellent frequency response in the auditory range and must be easy to implant, insensitive to external vibration noise, and miniaturized.
Typically, the FIHD microphone is implanted under the skin near the temporal bone or inside the middle ear. However, these types of implantation make it hard to improve the performance of the FIHDs. In particular, in the case of the microphone of FIHDs implanted under the skin, because of the filtering effect of the skin, severe degradation of the microphone sensitivity is unavoidable in high-frequency regions and the pickup problem of noise occurs during mastication. In the case a piezoelectric microphone implanted in the middle ear, a considerably flat frequency response is shown. However, its implantation requires invasive surgery and the ossicles must be dislocated in part to prevent feedback signals caused by the actuator that is located on the same path. Another type of microphone is based on a capacitive acceleration sensor that collects sounds by using the movement of the tympanic membrane. However, it has a low sensitivity of less than 0.8 kHz due to the acceleration characteristics of the sensor. Thus, new implantable microphone designs that can overcome the problems of existing microphones are needed.
In this paper, we propose a trans-tympanic microphone with a high-performance diaphragm made of graphene–polymethylmethacrylate (PMMA) and with high displacement up to the 7 kHz region in the auditory range. The proposed microphone is placed at the pars tensa of the tympanic membrane. Thus, it has the advantage of using the auditory path (i.e., the pinna, the ear canal). Additionally, the trans-tympanic microphone is easy to implant with the use of ventilation tube technology, which is normally used in otorhinolaryngologic surgery for otitis media. However, the diameter of the microphone diaphragm must be limited to below 4 mm because of the anatomy of the tympanic membrane, making it difficult to obtain excellent sensitivity.
Typically, diaphragm of an electret condenser microphone (ECM) is fabricated with polyphenylenesulfide (PPS) affiliated with based polymer because of the low cost and high efficiency of mass production. The commercial ECM with a diaphragm of 4 mm in diameter has an effective diameter of 2.65 mm and an elastic modulus of 2.0–3.3 GPa and a thickness of 3.6 μm. Therefore, it has a resonant frequency of 7–10 kHz, and its displacement is 30–40 nm. However, the microphone capacitance change between diaphragm and back plate is limited due to this displacement, so the sensitivity of the microphone is –30 to 35 dB (0 dB = 1V/Pa). A hearing aid with an excellent dynamic range should have a microphone with high sensitivity and a small minimum detection level. Thus, diaphragm for the trans-tympanic microphone with large displacement according to the sound pressure in auditory range needs to be designed.
By using the stacked structure of the PMMA with low density and a low elastic modulus and graphene with a high elastic modulus, it is designed to have high displacement up to the 7 kHz region in the auditory range. The graphene–PMMA diaphragm is designed to have a relatively low elastic modulus and low density compared with a commercial ECM diaphragm of the same size, thereby giving it high displacement and a resonant frequency close to 7 kHz. The high displacement of the diaphragm gives it very high sensitivity, thereby giving the trans-tympanic microphone wide dynamic range.
For the design of the graphene–PMMA structure diaphragm, the elastic modulus of the diaphragm according to the thickness ratio of the stacked structure was measured using the indentation system. Then, the density of the stacked structure, which is a combination of different densities, was calculated. Using the measured properties and mathematical analysis of the diaphragm, the resonant frequency and displacement were predicted and the optimal ratio of graphene–PMMA was deducted. We used a laser Doppler vibrometer to evaluate the fabricated diaphragm, which had an effective diameter of 2.65 mm and a thickness of 3.6 μm, and found that it had a high displacement of 6.8dB up to the 7 kHz region compared with that of a commercial ECM diaphragm of the same size.
We also fabricated a trans-tympanic microphone package of 4 mm in diameter and 3.2 mm in height incorporated with the graphene–PMMA diaphragm. We evaluated the quality of the frequency responses and the total harmonic distortion of the microphone and found that it had an excellent sensitivity of –20 dB up to the 7 kHz region, which is, on average, 9dB higher than that of a microphone with a commercial ECM diaphragm. Finally, we conducted experiments using a human cadaver to evaluate the usefulness of the microphone as an input device for FIHDs. The result of the experiment showed a high sensitivity of −18 dB on average in the auditory range. As a result, we demonstrated the high performance of the proposed trans-tympanic microphone. Thus, it is highly expected to be utilized practically as a microphone for FIHDs.

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