2025 AIChE Annual Meeting

(536f) Understanding the Electrode-Tissue Interface in Neural Stimulation through Impedance Spectroscopy

Authors

Bernard Biney - Presenter, University of Florida
QiWei Dong, Purdue University
Kevin Otto, University of Florida
Mark E. Orazem, University of Florida
Electrochemical impedance spectroscopy (EIS) is a highly sensitive, non-destructive technique that enables real-time monitoring of electrode performance.1 This makes it particularly well-suited for investigating the complex interactions between neural stimulation electrodes and surrounding biological tissue. A critical challenge in neural interface design lies in balancing miniaturization with functional efficiency. Ultramicroelectrodes, despite their high spatial resolution and minimal invasiveness, exhibit increased impedance, which can limit charge transfer and reduce stimulation efficiency. As neural stimulation relies on efficient charge injection, understanding the impedance characteristics of the electrode-tissue interface is central to optimizing therapeutic performance and device longevity. Analysis of impedance spectra can help identify resistive losses and differentiate between capacitive and faradaic charge transfer.2 Additionally, changes in impedance magnitude and phase can reflect tissue response, inflammation, glial encapsulation, among others.

In this paper, we present a combined experimental and modeling investigation of the impedance behavior of sputtered iridium oxide film (SIROF) ultramicroelectrodes with diameters of 5, 10, 15, and 20 µm (19.63, 78.54, 176.71, and 314.16 µm2, respectively). In-vitro impedance measurements were conducted in phosphate buffered saline using a GAMRY Reference 600+ potentiostat in a three-electrode configuration to establish baseline electrode characteristics. Chronic in-vivo measurements were subsequently conducted in the somatosensory cortex of rats using an Autolab PGSTAT12 potentiostat in a two-electrode setup. Surgical procedures performed at the University of Florida were conducted under the approval and guidance of the UF Institutional Animal Care and Use Committee (IACUC). Surgical procedures performed at Purdue University were conducted under the approval and guidance of the Purdue IACUC.

An equivalent circuit model was developed to account for the parasitic capacitance, ohmic resistance, and the constant-phase-element (CPE) behavior of the electrode in both in-vitro and in-vivo measurements. The model also accounted for the redox behavior of the sputtered iridium oxide film (SIROF) coating the electrode sites. The iridium reaction was modeled by a parallel combination of a resistance and capacitance, capturing the kinetics and storage capacity associated with the iridium oxidation-reduction process. The model fit both in vitro and in vivo spectra across the frequency range of 0.1 Hz to 100 kHz (in vitro) and 10 Hz to 100 kHz (in vivo), allowing for quantitative comparison of interfacial properties under controlled and physiological conditions.

Acknowledgement:

This work was supported in part by NIH UO1 Grant Number: 1U01NS126052-01, Engineering the Neuronal Response to Electrical Microstimulation.

References:

  1. Wang, S.; Zhang, J.; Gharbi, O.; Vivier, V.; Gao, M.; Orazem, M. E. Electrochemical Impedance Spectroscopy. Nature Reviews Methods Primers. 2021. 1 (1), 41
  2. Vivier, V.; Orazem, M. E. Impedance Analysis of Electrochemical Systems. Chem Rev 2022, 122 (12), 11131–11168.