Study of Size Reduction, Reliability and Lead Safety of an Intra-Cochlear Lead-Zirconate-Titanate Micro-Actuator

Abstract

To rehabilitate hearing loss, researchers have developed hearing aids with combined electric and acoustic stimulations to attain better speech recognition in the last decade. Such hearing aids incorporate an electrode array implanted inside cochlear (i.e., the electric stimulation) and a traditional acoustics-based hearing aid in the ear canal (i.e., the acoustic stimulation). Since the electric stimulation component is in vitro and the acoustic stimulation component is in vitro, these devices not only pose a challenging task for surgeon to implant but also inherit disadvantages of traditional hearing aids (e.g., occlusion). To realize combined electric and acoustic stimulations that are totally implantable inside the inner ear, the author’s research team has been developing a novel intra-cochlear lead-zirconate-titanate (PZT) micro-actuator to generate acoustic wave directly in the inner ear. Current design of the intra-cochlear micro-actuator is a square PZT diaphragm anchored at all four edges forming a tiny speaker. The purpose of this dissertation is to address three major challenges encountered while developing next-generation micro-actuators. They are size reduction, reliability and lead safety. For size reduction, a partially released diaphragm structure is proposed in the dissertation. As the previous fully anchored diaphragm scales down in size, its deflection is substantially reduced, and the micro-actuator becomes ineffective. To increase actuation strength, two opposite diaphragm edges are released to enhance the flexibility of the diaphragm. A PZT intra-cochlear micro-actuator probe with three partially released diaphragms at the tip of a cantilever is also fabricated by through etching two slots on each diaphragm. To enable the open slots, new fabrication procedures (e.g., treatment of cat ears, annealing of bottom electrodes to remove electrode non-uniformity, etching of PZT and double-side etching to form open slots) are developed and discussed in detail in the dissertation. After the intra-cochlear PZT micro-actuator probe is fabricated, its frequency response function is measured experimentally and predicted via a finite element analysis (FEA). Both measurements and FEA predictions indicate that the sensitivity is dominated by the diaphragm deflection, while the first natural frequency is dominated by the cantilever structure. Furthermore, parametric studies indicate that precise control of the thickness of unetched silicon layer and elimination of residual silicon are critical to achieve designed micro-actuator performance, For reliability, three series of tests are performed to investigate possible failure mechanisms. They are in-air tests, soaking tests, and in-fluid driving tests. The first two tests specifically focus on structural failure and electrical failure, respectively, while the third test investigates their combined effects. Test results indicate that the electrical failure usually occurs prior to the structural failure. Moreover, the electrical failure can be detected by monitoring parallel resistance extracted from electrical impedance measurements. When a sudden and significant drop in the parallel impedance occurs, it implies that the surrounding fluid has infiltrated the encapsulation layer (parylene), resulting in electrical failure. As a result, the parallel resistance can be utilized as an indicator to monitor integrity of the intra-cochlear micro-actuator, especially when it is placed inside the inner ear. On the contrary, the structural failure manifests itself in sudden reduction of frequency response functions. As a result, the structural failure is most likely caused by delamination of the top electrodes from the PZT diaphragm. Finally, methods to improve the encapsulation layer and reliability are proposed. For lead leaching, the possibility of replacing PZT with a lead-free piezoelectric material, especially the biocompatible polymer PVDF, is first explored. An FEA is conducted to assess the feasibility of using PVDF as an alternative material for intra-cochlear micro-actuators. The FEA results indicate that PVDF cannot effectively drive the intra-cochlear micro-actuators in fluid with large enough response due to its small piezoelectric constants. Next, the amount of lead that could possibly be leached from a PZT micro-actuator encapsulated by parylene is investigated. A series of long-term tests are performed by driving PZT micro-actuators in artificial perilymph to complete failure. Then samples from the artificial perilymph are collected, and the leached lead is measured via inductive coupled plasma mass spectrometry (ICP-MS). The test results confirm a concentration of 51ng/mL in the worst-case scenario, while the lead advisory level in blood published by the Center of Disease Control is not to exceed 100 ng/mL.