An advanced cochlear implant hearing prosthesis for profound to total deafness

© 1988 Dr. Hugh Joseph McDermottThis thesis describes the design and application of an advanced cochlear implant for a hearing prosthesis. When used with a suitable external speech processor, the device is intended to convey more auditory information to deaf implantees than existing prostheses. The implant comprises an electrode array, a capsule containing a receiver-stimulator integrated circuit, and a coil to which radio frequency (RF) signals are transmitted from the speech processor. The electrode array includes 20 platinum bands mounted on a flexible carrier. It is surgically inserted into the scala tympani of the cochlea (inner ear) to bring the electrodes into proximity with surviving auditory neurons. Each electrode is connected individually to the receiver-stimulator. It is the task of the receiver-stimulator to demodulate the RF signals picked up by the receiving coil and to deliver stimulating current pulses to the electrodes under control of the received data. The entire device is implanted beneath the skin, and derives an internal power supply by rectification of the RF signal transmitted from the speech processor. The receiver-stimulator chip, which contains approximately 8000 transistors in digital and analogue circuits, was custom designed. On receipt of each data sequence, it can generate three pulsatile stimuli controlled independently in onset time, duration and current level. Each stimulus can be delivered to any two electrodes selected from the array and configured as a bipolar pair. Bipolar stimulation is used to reduce the spread of neural excitation distributions so as to minimize undesirable interactions between stimuli. The stimuli can be presented sequentially or effectively simultaneously. In the simultaneous stimulation mode, a novel current waveform is generated which was developed to ensure that chronic stimulation would be biologically safe. The implant also contains a telemetry system which enables a selected electrode voltage waveform to be conveyed to an external receiver for display and analysis. The receiver-stimulator chip was fabricated externally using a low-power high-reliability process. Subsequently, the complete advanced cochlear implant was constructed and tested intensively. The device has been implanted in two patients so far. In initial audiological evaluations, both implantees are achieving good results