Date

2018

Author

Uluşan, Hasan

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Cochlear implants are one of the most successful neural prosthesis where users could go from being profoundly deaf to enjoying high degree of speech perception. However, subsequent aesthetic concerns, damage risks and high power dissipation associated with bulky external units of cochlear implants have redirected recent studies to fully implantable cochlear implants (FICI). Although, implantable sensors occupies the largest portion of the previous researches on FICIs, design of the low powered signal conditioning interface circuit is a bottle-neck to accomplish a FICI system. In this thesis, a novel fully integrated interface circuit is designed to process signals from an implantable multi-frequency piezoelectric (PZT) cantilever set for stimulation of the auditory neurons. The 1st generation FICI interface is focused on power dissipation of the front-end signal conditioning circuit. Power of the front-end circuit is reduced through a novel logarithmic amplifier design that combines amplification and compression stages. The conditioned signal controls the biphasic rectangular current pulses for neural stimulation. Single channel performance of the circuit has been tested with a thin film pulsed-laser deposition (PLD) PZT sensor. The interface generates biphasic current in the range of 110-430 µA for acoustic input of 60-100 dB sound pressure level (SPL). Power dissipation of the front-end signal conditioning and the overall system for 8-channel operation is projected from measurement as 51.2 and 691.2 µW, respectively. After validating the 1st generation interface performance, power dissipation and input range of the sensor front was improved at the 2nd generation FICI interface through novel current mode circuits. The conditioned signal is converted into biphasic neural stimulation current with a 7-bit user-programmed DAC to enable patient fitting (calibration). The proposed circuit introduces an optimized stimulation current waveform to reduce the electrode voltage and hence supply voltage of the stimulator (most power hungry part) by about 20%. The designed system has been tested with up to 60 dB input dynamic range (40-100 dB SPL) while the minimum threshold and maximum comfort levels of the system are 0 and 1 mA, respectively. The 8-channel interface has been validated to be fully functional with the front-end and the overall circuit power dissipation of 19.7 and 471.7 μW, when excited by a mimicked speech signal. The proposed 2nd generation interface electronics is the first sub-500 μW FICI interface that provides more than 30 years of operational lifetime, and reduces the healthcare cost and risks associated with surgical battery replacements. The implantable device performance of the 2nd generation FICI interface electronics has been validated through in-vivo tests on a guinea pig. After validating healthy and partially-deafened hearing performance of the guinea pig, the electrical stimulation performance of controlled current stimulator and the FICI interface electronics were tested and compared. The neural stimulation capability of acoustically excited FICI system with intra-cochlear electrodes has been validated with 55 dB hearing threshold and 45 dB input dynamic range. The proposed system is the first FICI interface electronics with ultra-low power dissipation and wide dynamic range that is also validated with in-vivo tests.

Subject Keywords

Cochlear implants., Implants, Artificial., Electric stimulation., Electricity in medicine.

URI

http://etd.lib.metu.edu.tr/upload/12622887/index.pdf
https://hdl.handle.net/11511/27855

Collections

Graduate School of Natural and Applied Sciences, Thesis