Most commercially available cochlear implants and hearing aids use microphones as sensors for capturing the external sound field. These microphones are in general located in an external element, which is also responsible for processing the sound signal. However, the presence of the external element is the cause of several problems such as discomfort, impossibility of being used during physical activities and sleeping, and social stigma. These limitations have driven studies with the goal of developing totally implantable hearing devices, and the design of an implantable sensor has been one of the main challenges to be overcome. Different designs of implantable sensors can be found in the literature and in some commercial implantable hearing aids, including different transduction mechanisms (capacitive, piezoelectric, electromagnetic, etc), configurations microphones, accelerometers, force sensor, etc) and locations (subcutaneous or middle ear). In this work, a detailed technical review of such designs is presented and a general classification is proposed. The technical characteristics of each sensors are presented and discussed in view of the main requirements for an implantable sensor for hearing devices, including sensitivity, internal noise, frequency bandwidth and energy consumption. The feasibility of implantation of each sensor is also evaluated and compared.

The presence of external elements is a major limitation of current hearing aids and cochlear implants, as they lead to discomfort and inconvenience. Totally implantable hearing devices have been proposed as a solution to mitigate these constraints, which has led to challenges in designing implantable sensors. This work presents a feasibility analysis of a MEMS piezoelectric accelerometer coupled to the ossicular chain as an alternative sensor. The main requirements of the sensor include small size, low internal noise, low power consumption, and large bandwidth. Different designs of MEMS piezoelectric accelerometers were modeled using Finite Element (FE) method, as well as optimized for high net charge sensitivity. The best design, a 2 × 2 mm²annular configuration with a 500 nm thick Aluminum Nitride (AlN) layer was selected for fabrication. The prototype was characterized, and its charge sensitivity and spectral acceleration noise were found to be with good agreement to the FE model predictions. Weak coupling between a middle ear FE model and the prototype was considered, resulting in equivalent input noise (EIN) lower than 60 dB sound pressure level between 600 Hz and 10 kHz. These results are an encouraging proof of concept for the development of MEMS piezoelectric accelerometers as implantable sensors for hearing devices.

Totally implantable hearing devices have been proposed as solution to mitigate limitations associated to external elements in traditional hearing devices. However, requirements for an implantable sensor are difficult to be achieved concurrently, and therefore no sensor developed fulfills them entirely. In this work, we investigate how various geometries of MEMS piezoelectric accelerometers, optimized via different optimization techniques, behave in terms of the equivalent input noise (EIN) in sound pressure level (SPL) when coupled to the middle ear ossicular chain. An alternative optimization goal based on the minimization of the EIN is proposed in contrast to the traditional charge sensitivity maximization, and this alteration rendered near 20 dB improvement in EIN in low frequencies. Prototypes are fabricated through a multi-user batch process featuring a 500 nm Aluminum Nitride piezoelectric layer, and experimentally characterized for validation of the finite element analysis and analytic modeling of spectral noise. Despite limitations imposed by the current batch fabrication process, we are able to design a highly competitive implantable sensor in the form of a 2 ×2mm2 footprint cantilever accelerometer, which should be able to detect 60 dB SPL between 250 Hz and 8 kHz. These promising results establish piezoelectric sensors as a viable alternative for totally implantable hearing devices and put an implantable sensor which fulfills all requirements concurrently within our grasp.