Cylindrical cochlear electrode array for use in humans

Cochlear electrode arrays: past, present and future

Cochlear implants are very successful devices: more than 60000 people use them throughout the world. Key to the success of these prostheses is the development of electrode arrays that place contacts close to the target neurons, survive for decades in the tissues of the inner ear, and that provide reliable and repeatable excitation to the cells of the auditory nerve. This article describes the early electrode arrays and their development into the arrays that are used presently in clinical cochlear prostheses. While integrated circuit techniques were proposed and tested in the laboratory two decades ago, the present clinical devices still are hand built and made of wire-based technologies. Current approaches that seek to automate the construction of cochlear electrode arrays are described and discussed.

A Model for Cochlear Implant Electrode Insertion and Force Evaluation: Results with a New Electrode Design and Insertion Technique

OBJECTIVES AND HYPOTHESIS

This study has the specific aim of evaluating the insertion characteristics of a new cochlear implant electrode. Techniques for evaluation of fluoroscopic real time mechanical insertion dynamics, histologic electrode position and trauma results, hydraulic force, and mechanical insertion forces are presented. In addition, this study should serve to present a novel model for cochlear implant electrode insertion evaluations.

STUDY DESIGN

Prospective analysis using a series of analytical techniques.

METHODS

All studies are conducted in fixed cadaveric temporal bones. Real time fluoroscopic insertion evaluations, histologic evaluations for trauma and electrode position in embedded bones, hydraulic measures, and mechanical intracochlear force measurements are conducted with a current and new electrode.

RESULTS

The Contour Advance electrode provides a more reliable and less traumatic insertion when deployed with the Advance Off Stylet technique. This is largely because of a reduction in intracochlear outer wall force generation. Fluoroscopic and histologic analysis reveal a smooth insertion without reliance on cochlear outer wall contact. No hydraulic forces were detected when measured from the superior semicircular canal ampulla.

CONCLUSION

The model used for this study provides valuable information to cochlear implant surgeons and design engineers. The Contour Advance electrode, inserted with the Advance Off Stylet technique, represents an improvement over the Contour electrode inserted with the standard insertion technique.

Electrochemical platinum coatings for improving performance of implantable microelectrode arrays

The formation and properties of electrochemical platinum films grown on platinum contacts contained in implantable flexible microelectrodes were investigated. The resulting platinum deposits were obtained by applying cyclic voltammetry to baths containing concentrations around 70 mM of chloroplatinic acid. A pre-activation step was necessary before the platinum-electroplating step in order to achieve good adhesive properties. The benefits of this process were ascribed to higher corrosion resistance, lower impedance and improved adhesion to the sputtered platinum. These improvements can make the application of this electrochemical technique highly useful for increasing the lifetime of implantable microelectrode arrays, such as cuff structures (IEEE Trans. Biomed. Eng. 40 (1993) 640). These medical devices, obtained by semiconductor technology could be used for selective stimulation of nerve fascicles, although, poor long-term performance has been achieved with them. The dissolution rate for platinum thin-film microelectrodes under fixed corrosion test conditions was 38.8 ng/C. Lower rates were observed for electroplated microelectrodes, obtaining a dissolution rate of 7.8 ng/C under analogous experimental ageing conditions. The corrosion behaviour of the electroplated platinum during stimulation experimental conditions was estimated by electrochemical impedance spectroscopy.

Strategies to improve electrode positioning and safety in cochlear implants

An injection-molded internal supporting rib has been produced to control the flexibility of silicone rubber encapsulated electrodes designed to electrically stimulate the auditory nerve in human subjects with severe to profound hearing loss. The rib molding dies, and molds for silicone rubber encapsulation of the electrode, were designed and machined using AutoCad and MasterCam software packages in a PC environment. After molding, the prototype plastic ribs were iteratively modified based on observations of the performance of the rib/silicone composite insert in a clear plastic model of the human scala tympani cavity. The rib-based electrodes were reliably inserted farther into these models, required less insertion force and were positioned closer to the target auditory neural elements than currently available cochlear implant electrodes. With further design improvements the injection-molded rib may also function to accurately support metal stimulating contacts and wire leads during assembly to significantly increase the manufacturing efficiency of these devices. This method to reliably control the mechanical properties of miniature implantable devices with multiple electrical leads may be valuable in other areas of biomedical device design.

Silicon ribbon cables for chronically implantable microelectrode arrays

This paper describes the design, fabrication, and testing of miniature ultraflexible ribbon cables for use with micromachined silicon microprobes capable of chronic recording and/or stimulation in the central nervous system (CNS). These interconnects are of critical importance in reliably linking these microelectrodes to the external world through a percutaneous connector. The silicon cables allow the realization of multilead, multistrand shielded local interconnects that are extremely flexible and yet strong enough to withstand normal handling and surgical manipulation. Cables 5 microns thick, 1-5 cm long, and from 60 to 250 microns wide have been fabricated with up to eight leads. The series lead resistance is typically 4 k omega/cm for polysilicon and 500 omega/cm for tantalum, with shunt capacitance values of 5-10 pF/cm and an interlead capacitance below 10 fF/cm. Soak tests in buffered saline performed under electrical and mechanical stress have been underway for over three years and show subpicoampere leakage levels. Silicon microprobes with built-in ribbon cables have remained functional for up to one year in the guinea pig CNS, recording driven single-unit activity and maintaining impedance levels in the 1-7 M omega range.