Preferential sub-surface gold layer dissolution during electrical stimulation stability testing of neural electrodes

Electrode stability is critical for ensuring long-term neural stimulation and recording. This work aims to identify and characterize abiotic failure mechanisms resulting in electrode degradation during stimulation. We studied the stimulation stability of flexible cuff electrodes designed for vagus nerve stimulation, comprised of thin-film Ti/Ir/IrOx electrodes with Ti/Pt/Au traces, under accelerated conditions. Approach: Microfabricated polyimide-based flexible electrodes were stimulation stability (stim-stab) tested using 109 cathodic-leading biphasic current pulses at 12 (n=3) or 16 mA (n=2) (current density of 4.98 A/cm2 or 6.64 A/cm2, respectively). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and voltage transients (VT) were utilized before and after the stimulation to characterize electrochemical changes. Optical microscopy was used to screen for damage, backscattered scanning electron microscopy to evaluate electrode degradation, and energy-dispersive X-ray spectroscopy to examine the composition of the metal stack. Additionally, focused ion beam with scanning electron microscopy captured cross-sectional images of the metal stack, and X-ray photoelectron spectroscopy was used to measure and image changes in electrode surface composition compared to controls. Main Results: Optical microscopy identified damaged regions on some electrodes, corroborated by backscattered scanning electron microscopy showing damaged areas through morphological and compositional differences. Energy dispersive X-ray analysis determined that visibly damaged regions lost Ir and gold from the electrode stack, while Pt and Ti from electrical traces remained. Focused ion beam cross-sectional analysis revealed regions where highly selective degradation of the gold layer occurred, implicating this process in the failure mechanism. Significance: Gold is frequently employed for neural electrodes due to its bio-inert character, conductivity, and compatibility with microfabrication. Our findings show it can selectively dissolve during neural stimulation relative to the adjacent Ti and Pt layers, beneath a surface that appears intact.