MEMS thin film piezoelectric acoustic transducer for cochlear implant applications

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2018
İlik, Bedirhan
In this thesis, a multi-frequency thin film piezoelectric acoustic sensor concept to be placed on the eardrum has been proposed for the development of next generation and fully implantable cochlear implants (FICIs). The design consists of several thin film piezoelectric cantilever beams, each of which resonates at a specific frequency within the daily acoustic band. The device will exploit the functional parts of the natural hearing mechanism and mimic the function of the hair cells in the cochlea, where the signal generated by the piezoelectric transducers will be processed by interface electronics to stimulate the auditory neurons. The limited volume (<0.1 cm3) in the middle ear, the mass tolerance (<25 mg) and the size of the eardrum (9 mm × 10 mm) and the requirement for covering the audible frequency band (250-5000 Hz) with enough number of channels are the main limitations/challenges for obtaining an adequate voltage output for neural stimulation. In this direction, design, modeling, fabrication and characterization of a multi-frequency thin film piezoelectric acoustic sensor have been accomplished to overcome the main bottlenecks of CIs. Pulsed Laser Deposited (PLD) Lead Zirconate Titanate (PZT) is preferred among other thin film piezoelectric alternatives due to their superior ferroelectric and piezoelectric properties for acoustic sensing. To demonstrate the feasibility of the proposed fabrication scheme, a single cantilever thin film PLD-PZT transducer prototype is fabricated. The realized device is assembled onto a flexible parylene carrier and placed on a parylene membrane, mimicking the operation of the eardrum. The mechanical, electrical and acoustical properties are characterized by a shaker table and an acoustic setup. Acceleration characteristic of the sensor attached to the membrane is obtained by using a Laser Doppler Vibrometer (LDV) as the output voltage was measured by an oscilloscope. A maximum voltage output of 114 mV is obtained, when the single channel device was excited at 110 dB Sound Pressure Level (SPL) at 1325 Hz. Experimental results show that the voltage output of the device exceeds the minimum required sensing voltage (100 µV) for the neural stimulation circuitry. Fabricated single-channel prototype is modeled using finite element modeling (FEM) which are within 92% agreement with the experimental results. Based on this model, a multi-channel thin film piezoelectric acoustic sensor is designed. The total volume, area and mass of the transducer are 5×5×0.2 mm3, 5×5 mm2, and 12.2 mg, respectively. The multi-channel prototype is fabricated and characterized. The electromechanical properties are measured by a shaker table and LCR meter. The test results show that, fabricated device, consists of several piezoelectric cantilever beams, each of which resonates at a specific frequency within the daily acoustic band (500 Hz – 2600 Hz). Consequently, the device provides mechanical filtering and shows a clear frequency selectivity mimicking the operation of the cochlea. Experimental results show that the voltage output of the device exceeds the minimum required sensing voltage for the neural stimulation circuitry and decreases the required power for readout circuitry. Expected to satisfy all the requirements (volume, mass, area, and stimulation signal at hearing band) of FICI applications for the first time in the literature, the fabricated device has a groundbreaking nature and it can be referred to as the next generation FICIs since it revolutionizes the operational principle of conventional CIs.

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Citation Formats
B. İlik, “MEMS thin film piezoelectric acoustic transducer for cochlear implant applications,” M.S. - Master of Science, Middle East Technical University, 2018.