Conductive Hydrogel Electrodes for Delivery of Long-Term High Frequency Pulses

Abstract

Nerve block waveforms require the passage of large amounts of electrical energy at the neural interface for extended periods of time. It is desirable that such waveforms be applied chronically, consistent with the treatment of protracted immune conditions, however current metal electrode technologies are limited in their capacity to safely deliver ongoing stable blocking waveforms. Conductive hydrogel (CH) electrode coatings have been shown to improve the performance of conventional bionic devices, which use considerably lower amounts of energy than conventional metal electrodes to replace or augment sensory neuron function. In this study the application of CH materials was explored, using both a commercially available platinum iridium (PtIr) cuff electrode array and a novel low-cost stainless steel (SS) electrode array. The CH was able to significantly increase the electrochemical performance of both array types. The SS electrode coated with the CH was shown to be stable under continuous delivery of 2 mA square pulse waveforms at 40,000 Hz for 42 days. CH coatings have been shown as a beneficial electrode material compatible with long-term delivery of high current, high energy waveforms.

Keywords: conductive hydrogel; high frequency stimulation; nerve block; neural interfaces; peripheral nerve cuff array.

Figure 1

Stereoscopic images of electrode arrays, as received and with CH coatings. (A) Pre-curled commercial cuff array; (B) Opened cuff showing PtIr electrode sites without coating; (C) Opened cuff with CH coating on PtIr electrode sites; (D) Uncoated planar SS array; (E) CH coated planar SS array.

Figure 2

CSC of SS and PtIr electrode arrays before and after CH coating. Error bars are 1 SD, ^*p < 0.05, (n = 8). Note log scale on y-axis required to enable SS data to be visualized.

Figure 3

Frequency dependant response of SS (top) and PtIr (bottom) nerve cuff electrode arrays. Both bare and CH coated arrays are characterized. Error bars are 1 SD, (n = 8).

Figure 4

Charge injection limit for both PtIr and SS nerve cuff arrays, bare, and with CH coatings. Inset figure shows charge injection behavior of bare PtIr and SS at short phase lengths. Performed in saline. Error bars are 1SD (n = 8).

Figure 5

Total voltage drop across commercial PtIr electrode pairs under continuous high frequency stimulation, comparing performance for both bare electrodes and CH coated electrodes. Red arrow indicate an electrode pair that is considered to have failed due to sudden increase in potential transient.

Figure 6

Corrosion of wire and pad connections on PtIr pre-curled cuff arrays, showing (A) delamination of CH coating and (B) presence of discolored precipitate at bonding point on back of electrode sites.

Figure 7

Fluid ingress to commercial cuff connection points, enabling corrosion with continuous electrical stimulation.

Figure 8

Total voltage drop across SS electrode pairs under continuous high frequency stimulation, comparing performance for both bare electrodes and CH coated electrodes. Error bars are 1 SD (n = 4).

Figure 9

CSC of SS electrodes under high frequency stimulation (40 kHz, 2 mA peak-to-peak). Comparison of CH coated SS and bare SS electrodes over 42 days or 150 billion pulses (n = 8).

Figure 10

EIS over 42 day period of high frequency stimulation, showing performance of CH coated SS in comparison to uncoated SS. Passive controls are shown at termination of the study (n = 8).

Figure 11

Impedance magnitude changes in the CH coating, comparing Day 0 to Day 42, unstimulated and stimulated at a high frequency of 40 kHz and 2 mA. Error bars are 1 SD, (n = 8).