Low-Impedance, High Surface Area Pt-Ir Electrodeposited on Cochlear Implant Electrodes

The Advanced Bionics HiFocus 1j Electrode is a commercially available cochlear implant electrode array consisting of 16 rectangular platinum stimulating contacts. Each contact measures approximately 0.4 mm wide by 0.5 mm long (0.2 mm²). On each array, Pt-Ir was electrodeposited onto 12 electrode contacts using a method described previously⁸ and 4 were left uncoated as a control.

Optical images were taken using a digital microscope (Keyence VHX-1000). Scanning electron microscopy (SEM) and EDS scans were performed using a JEOL JSM-6480 (accelerating voltage: 10 kV, working distance: 9 to 12 mm). Electrodes were grounded to the SEM stage, thus no additional sample preparation was required.

A custom-built Advanced Bionics current pulse generator based on the HiRes 90K implant electronics was used to control current pulses. Voltage transients were captured using a Tektronix DPO 3014 oscilloscope, which measured the voltage across the working electrode (HiFocus 1j contacts) and a large graphite rod counter electrode (Princeton Applied Research, G0091) while trains of biphasic, cathodic-first, charge-balanced pulses were injected through each electrode contact. The electrolyte used was prepared using Type 2 purified water (EMD Millipore Elix Advantage, resistivity >5 MΩ-cm, TOC < 30 ppb) and consisted of a physiologically relevant 0.01 M phosphate-buffered saline solution (Sigma-Aldrich, P5368) composed of 2.7 mM KCl, 138 mM NaCl, 1.5 mM KH₂PO₄, and 8.5 mM Na₂HPO₄. All experiments were performed at room temperature (∼22°C) in open air (dissolved O₂ was present in solution). (Cyclic voltammetry and electrochemical impedance spectroscopy was also performed. Details of which can be found in the supplemental materials.) Four pulse designs within a clinically relevant parameter space¹⁰ were tested (Table II). Waveforms were recorded once the waveform shape reached a steady state (generally after 10–20 s).

The access resistance (R_a) (1) and polarization impedance (|Z_p|) (2) were calculated based on the access voltage (V_a) and polarization potential (E_p) taken from voltage transients measured in response to biphasic current pulses¹¹ (also see supplemental material), with i_c being the amplitude of the cathodic current pulse.

SEM (Figures 1a and 1c) and optical images (Figures 1b and 1d) of bare Pt (Figures 1a and 1b) and Pt-Ir coated Pt (Figures 1c and 1d) show the Pt-Ir coating has an anisotropic, rough appearance with microscopic nodules characteristic of electrodeposited Pt-Ir.⁸ The Pt-Ir coating masked the striations visible on the surface of the bare electrode and did not extend beyond the edge of the base Pt electrode, indicating a well-controlled plating process.

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Electrode impedance was directly compared by measuring voltage transients during clinically-relevant biphasic pulses. The R_a 1 of the coated and uncoated electrodes were not statistically different (Table II). The R_a is proportional to the V_a which is the near instantaneous drop in voltage during the leading edge of the pulse train and are nearly identical when comparing coated and uncoated electrodes (Figure 2). This is to be expected because access voltage/resistance is generally determined by the geometric area of the electrode¹¹ and the electrolyte resistivity, which the coating did not change either.

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In contrast, |Z_p|—generally determined by the electrochemical surface area (ESA) and the electrode material¹¹—was decreased by 91% for the long, low amplitude pulses and 93% for short, high amplitude pulses (Table II). The polarization impedance is proportional to the E_p, defined as the drop in potential between the onset (after the access potential drop) and the end of the cathodic phase of the pulse. For uncoated arrays, the drop appears as a negatively sloped line, whereas the polarization is small enough for the coated arrays that the potential appears to remain constant during the pulse (Figure 2). E_p is associated with the electrochemical processes occurring at the electrode interface such as double-layer charging and redox reactions. Since these processes are dependent on electrode material and the ESA of the electrode, E_p for the Pt-Ir coated samples is greatly reduced due to the presence of Ir and the coating's 3-dimensional structure. Many other electrode materials/coatings listed in Table I show low polarization under slow rates of charge transfer (e.g. during cyclic voltammetry or under biphasic stimulation with long pulse widths), but their impedance advantage does not hold up under biphasic pulsing at short pulse widths like those used in the present study. This may be due to the porous nature of other electrode materials, which does not allow sufficient ion flux in and out of pores at fast timescales.