A Polymer Cochlear Electrode Array: Atraumatic Deep Insertion, Tripolar Stimulation, and Long-Term Reliability

Abstract

Biocompatible polymers have gained widespread interest in implantable biomedical applications due to their flexibility and compatibility with micro-fabrication processes. Liquid crystal polymer (LCP) is an inert, highly water-resistant, and thermoplastic polymer suitable for the encapsulation of electronic components and as a base material for fabricating neural interfaces. Feasibility of monolithic integration of neural interface and electronics packaging and its extremely low water absorption rate (< 0.04 %) enable LCP-based implantable devices that have salient benefits in terms of performance and reliability. In this dissertation, new design of LCP-based neural interfaces, especially cochlear electrode arrays, are proposed and evaluated within their purposes on fabrication process, functionality, and reliability. The following issues of LCP-based cochlear electrode arrays are studied: atraumatic deep insertion, tripolar stimulation, and long-term reliability. Flexible LCP-based cochlear electrode array has been studied, but, there is no electrode design for an atraumatic insertion in terms of structural approach. An atraumatic cochlear electrode array has become indispensable to high-performance cochlear implants such as electric acoustic stimulation (EAS), wherein the preservation of residual hearing is significant. A new design of cochlear electrode array based on LCP for an atraumatic implantation using precise batch fabrication and thermal lamination process unlike conventional wire-based cochlear electrode array is proposed. Multi-layered structure with variable layers of LCP films depending on the parts of the array to achieve a sufficient degree of basal rigidity and a flexible tip is devised and a peripheral blind via contributes to reduction of the width of the array. The diameters of the finalized electrode arrays are 0.3 mm (tip) and 0.75 mm (base). In vitro force measurements in a customized experimental setup reveal that the insertion force with a displacement of 8 mm from a round window and the maximum extraction force are 2.4 mN and 34.0 mN, respectively. Five human temporal bone insertion trials show that the electrode arrays can be inserted from 360˚ to 630˚ without trauma at the basal turn. Electrically evoked auditory brainstem responses are successfully recorded acutely in a guinea pig model, which confirms the efficacy of the array. Hearing preservation and tissue reaction are investigated during and after 4 weeks implantation of LCP electrode array. For high-density and pitch-recognizable cochlear implant, channel interaction should be concerned. There have been efforts to increase distinct stimulation channel using advanced focused current stimulation methods including tripolar stimulation. In this dissertation, structural considerations on electrode sites are approached for locally focused stimulation. A 3-dimensional arrangement of electrode site in multi-layered structure is practicable to be fabricated because differently patterned LCP layers can be merged into one substrate by thermal compression bonding. 3-dimensional electrode site structures for locally tripolar stimulation are simulated about electrical field distribution using finite element method. LCP electrode array of center stimulation channel with side wall auxiliary channel for tripolar stimulation is fabricated from the result of simulation. Compared with conventional monopolar and tripolar stimulation, locally tripolar stimulation on the proposed electrode site structure is more focused through in vitro measurements, which shows that spreading of electrical stimulation in electrolyte. Device reliability is one of the most significant issues in polymer-based neural prostheses. Two technical strategies are suggested in this dissertation. One strategy adopts mechanical interlocking structure at metal-polymer interface, which is started by J. H. Kim. This study deepens his works and analyzes the impact of the strategy in terms of device reliability. The polymer-metal interface is vulnerable to water penetration that causes detrimental device failure. A goal is to suggest a feasible fabrication method using mechanical interlocking to improve polymer-metal adhesion in polymer-based neural electrodes and evaluate its impact on device reliability quantitatively through in vitro measurements. After the metal patterns with undercut profile cross-sections are fabricated using a dual photolithography process and electroplating, the LCP interlocks with the metal during the lamination process. In a 180° peel test, the average maximum adhesion force of the samples with and without mechanical interlocking was 19.24 N and 14.27 N, respectively. In vitro accelerated soak tests that consist of interdigitated electrode patterns and a customized system for measuring the leakage current show that samples with and without interlock fail to function after 224 days and 185 days, respectively, in a 75°C saline. Scanning electron microscopy images reveal that the interlocked LCP-metal interfaces remained intact after water leakage. The other strategy is to use dielectric materials in LCP-based neural implants. Dielectric materials such as silicon dioxide and silicon nitride have been used in neural implants to prevent water and ion penetration. In addition to these features, dielectric materials can maintain the metal patterning during lamination bonding process which causes migration of metal patterning on LCP substrate. With consideration on the role of dielectric materials in the LCP-based device and their effects on device reliability, preliminary tests, including peel test to evaluate adhesion strength between LCP and dielectric materials compared with that of LCP-LCP interface and thermo-compression bonding process of LCP and dielectric materials with metal patterning to observe metal migration, are performed. LCP-dielectric materials interface is more adhesive than weakly bonded LCP-LCP interface and there is no metal migration after lamination process (295 °C, 1 MPa). The results confirm the possibility of the strategy. Finally, a review of long-term reliability in LCP-based neural prosthetic devices including recently developed enabling technologies, demonstrated prototype devices and their performance capabilities as well as theoretical fundamentals is presented in the dissertation. Verification foretells the development of cochlear electrode array for an atraumatic deep insertion, advanced stimulation, and long-term clinical implant.