Ultraminiature Piezoelectric Implantable Acoustic Transducers for Biomedical Applications

Abstract

Miniature piezoelectric acoustic transducers have been developed for numerous applications. Compared to other transduction mechanisms like capacitive or piezoresistive, piezoelectric transducers do not need direct current (DC) bias voltage and can work directly exposed to fluid. Hence, they are good candidates for biomedical applications that often require the transducer to work in water based fluid. Among all piezoelectric materials, aluminum nitride (AlN) is a great choice for implantable sensors because of the high electrical resistance, low dielectric loss, and biocompatibility for in vivo study. This thesis presents the design, modeling, fabrication, and testing of the AlN acoustic transducers, miniaturized to be implantable for biomedical applications like hearing or cardiovascular devices.

To design and model the transducer in air and in water, a 3D finite element analysis (FEA) model was built to study the transducer in a viscous fluid environment. An array of AlN bimorph cantilevers were designed to create a multi-resonance transducer to increase the sensitivity in a broad band frequency range. A two-wafer process using microelectricalmechanical systems (MEMS) techniques was used to fabricate the xylophone transducer with flexible cable. Benchtop testing confirmed the transducer functionality and verified the FEA model experimentally.

The transducer was then implanted inside a living cochlea of a guinea pig and tested in vivo. The piezoelectric voltage output from the transducer was measured in response to 80-95 dB sound pressure level (SPL) sinusoidal excitation spanning 1-14 kHz. The phases showed clear acoustic delay. The measured voltage responses were linear and above the noise level. These results demonstrated that the transducer can work as a sensor for a fully implantable cochlear implant.

The second generation device, an ultraminiature diaphragm transducer, was designed to be smaller, and yet with an even lower noise floor. The transducer was designed and optimized using a 2D axial-symmetric FEA model for a better figure of merit (FOM), which considered both minimal detectable pressure (MDP) and the diaphragm area. The low-frequency sensitivity was increased significantly, because of the encapsulated back cavity. Because of this merit, cardiovascular applications, which focus on low frequency signals, were also investigated. The diaphragm transducers were fabricated using MEMS techniques. Benchtop tests for both actuating and sensing confirmed the transducer functionality, and verified the design and model experimentally. The transducer had a better FOM than other existing piezoelectric diaphragm transducers, and it had a much lower MDP than the other intracochlear acoustic sensors.