A Study on the Liquid Crystal Polymer-Based Intracochlear Electrode Array

Abstract

Owing to the chemical stability, low water absorption rate and low water vapor transmission rate, liquid crystal polymer (LCP) is considered a good material for neural interfaces where long-term reliability in an aqueous body environment is required. Several studies of LCP-based neural interfaces using micro-fabrication processes such as photolithography, thermal bonding and laser machining have been reported. These studies employed pre-opened cover LCP film with high melting temperature (high-temp LCP) as an encapsulation. The micro-sized metal patterns were deposited also on the high-temp LCP. An LCP layer with low melting temperature (low-temp LCP) was used as a bonding layer between high-temp LCP films. Subsequently, the bonded films were cut by laser machine with automated alignment setup. However, the resolution of the micro-sized lead wires was only 50-µm-wide with 100-µm-wide pitch, which did not fully utilize the advantages of micro-fabrication process. Moreover, the alignment error of laser machining process was not defined. First, in the present study, we report advances in fabrication methods for LCP-based neural interface. Previous studies on LCP-based electrode arrays utilized photoresist for attachment of an LCP film to a silicon wafer. The flatness of the aforementioned LCP coated wafer worsened due to the bubbles caused by solvent in photoresist, resulted in low resolution of photolithography process. In contrast, we employed a novel LCP film preparation process using silicone-elastomer-coated silicon wafer. The substrate prepared by suggested method enabled 10-µm-wide lead wires with 20-µm-wide pitch. Laser machining and lamination alignment errors were defined for optimization of electrode designs. Secondly, we developed a highly flexible intracochlear electrode array using the micro-fabrication process stated above. The first LCP-based cochlear electrode array was reported in 2006, however, it was too stiff to be applied in human clinical use. The thickness and number of layers might affect its stiffness. In this study, we used two LCP layers for 16-channel electrode array. The thickness of single LCP film was 25 µm, which was half of single LCP layer of previously reported LCP-based intracochlear electrode array. The thickness of the finalized LCP-based electrode array was one-fifth of that of previous LCP-based intracochlear electrode array. For higher flexibility, the LCP substrate remained such a minimal extent as only to maintain lead wires. To protect the cochlear tissues from sharp edges of the LCP-film-based electrode array, we encapsulated the LCP-based electrode array using medical grade silicone elastomer. For this, a novel micro-molding process employed self-alignment scheme was demonstrated. The diameters of the finalized electrode arrays were 0.5 mm (tip) and 0.8 mm (base). And the length was 28 mm. Lastly, to assess the feasibility of the developed intracochlear electrode array in human clinical use, we investigated electrochemical and mechanical properties. The charge storage capacity and impedance at 1 kHz were 38.0 mC/cm2 and 391 Ω, respectively. The insertion force into a transparent plastic cochlear model with displacement of 8 mm from a round window was 8.2 mN, and the maximum extraction force was 110.4 mN. In the human temporal bone insertion trial, the insertion depths were measured as 405˚. Two cases of human temporal bone insertion showed no observable trauma while three cases showed a rupture of the basilar membrane.