An integrated position-sensing system for a MEMS-based cochlear prosthesis

Nearly 100,000 people worldwide have received cochlear implants to date, in which a bundle of wire electrodes (16-22) is inserted into the cochlea to electrically stimulate receptors in the auditory nerve, restoring hearing to the profoundly deaf. Replacing the traditional wire electrodes with a MEMS-based higher-density thin-film array should not only allow significantly higher frequency discrimination but could also permit monolithic integration of a real-time position-sensing system, which can not only minimize damage to any surviving hair cells during insertion, but also improve the stimulation efficiency by guiding the implant deep into the scala tympani (modiolus) and hugging the interior wall.; In this dissertation, a polysilicon piezoresistive position-sensing system is integrated onto a thin-film cochlear electrode array based on bulk micromachining technology. The thin-film electrode array is supported on a substrate of boron-diffused silicon and sized for the scala tympani of the guinea pig. At the tip of the array is a wall contact sensor and along its length are eight segmented position sensors made by integrating polysilicon and its encapsulating dielectrics. Each position sensor is arranged in half Wheatstone bridge, the signal from which is time-multiplexed, amplified (gain=10 or 30) and band-limited. The effective gauge factors (EGF) of the sensors are typically about 15, permitting the tip position (implant depth) of an implanted array to be determined within 50mum while providing wall-contact output signals of more than 50mV at the tip. The silicon substrate is engineered in width and thickness to improve array flexibility and facilitate a modiolus-hugging shape while parylene-C is monolithically integrated with the array to improve its robustness. A pre-defined curvature of the array is achieved using in-situ residual stresses or post-release thermal actuation.; As part of an implantable MEMS-based cochlear prosthesis, this position-sensing system uses a custom application-specific integrated circuit (ASIC) that is mounted on the rear of the array. This hybrid circuit chip interfaces with a hermetically-packaged WIMS microcontroller and wireless chip over an 8-lead polymeric cable. The 2.4mmx2.4mm ASIC operates from +/-2.5V, validates commands, generates stimulus currents, selects position sensors, compensates offsets using a 5b digital-to-analog converter, and amplifies/filters sensor outputs. As a result, array position can be reliably monitored with an adjustable refresh rate (10 times/sec) and used to improve the accuracy and depth of array placement.