Sputtered iridium oxide films for neural stimulation electrodes

Abstract

Sputtered iridium oxide films (SIROFs) deposited by DC reactive sputtering from an iridium metal target have been characterized in vitro for their potential as neural recording and stimulation electrodes. SIROFs were deposited over gold metallization on flexible multielectrode arrays fabricated on thin (15 microm) polyimide substrates. SIROF thickness and electrode areas of 200-1300 nm and 1960-125,600 microm(2), respectively, were investigated. The charge-injection capacities of the SIROFs were evaluated in an inorganic interstitial fluid model in response to charge-balanced, cathodal-first current pulses. Charge injection capacities were measured as a function of cathodal pulse width (0.2-1 ms) and potential bias in the interpulse period (0.0 to 0.7 V vs. Ag|AgCl). Depending on the pulse parameters and electrode area, charge-injection capacities ranged from 1-9 mC/cm(2), comparable with activated iridium oxide films (AIROFs) pulsed under similar conditions. Other parameters relevant to the use of SIROF on nerve electrodes, including the thickness dependence of impedance (0.05-10(5) Hz) and the current necessary to maintain a bias in the interpulse region were also determined.

Figure 1.

SEM image of a seven-electrode cluster identifying the five electrode sizes (50–400 μm diameter) evaluated in the present study. The light areas in the image are SIROF. Two unused SIROF sites and the approximately rectangular unused SIROF return site surrounding the charge-injection electrodes are also indicated.

Figure 2.

Representative voltage transient of a SIROF electrode in response to a biphasic, asymmetric current pulse with a 240 μA cathodal-first leading phase and a 60 μA anodal phase (t_c = 0.4 ms, t_a = 1.6 ms, t_d = 20 μs).

Figure 3.

Representative voltage transient of a SIROF electrode in response to a monophasic, 256 μA cathodal current pulse (pulse width = 0.4 ms) using a positive potential bias of 0.6 V (Ag|AgCl).

Figure 4.

SEM images of the surface morphology of 240, 500, and 770 nm thick SIROF.

Figure 5.

Cyclic voltammograms of SIROF at 50 mV/s sweep rate in PBS as a function of thickness. The corresponding CSC_c’s are 78 mC/cm² (240 nm), 133 mC/cm² (500 nm), and 194 mC/cm² (770 nm).

Figure 6.

Changes in the CV response of a 1300 nm thick SIROF after 393 cycles at 50 mV/s between −0.9 V and 1.05 V versus Ag|AgCl.

Figure 7.

Changes in the CV response of a 1300 nm thick SIROF after 393 cycles at 50 mV/s between −0.6 V and 0.8 V versus Ag|AgCl.

Figure 8.

Impedance magnitude as a function of frequency for 200–1300 nm thick SIROF. For clarity, only every fourth data point is indicated with a symbol.

Figure 9.

Charge-injection capacity of a 300 nm SIROF in response to biased, biphasic asymmetric current pulses showing the dependence on bias level (E_ipp in Figure 3) and waveform asymmetry (t_a:t_c). Filled and open symbols represent cathodally and anodally limited charge-injection limits, respectively.

Figure 10.

Charge-injection capacity of a 300 nm SIROF as a function of pulse width. Similar sized AIROF and PtIr microelectrode data from Cogan et al are included for comparison.

Figure 11.

SIROF charge-injection capacity as a function of electrode area. Each data point is the average of five thickness levels at each area (mean ± SD).

Figure 12.

Current necessary to sustain a non-equilibrium interpulse bias in model-ISF for three SIROF thickness levels.