(571h) Electrochemical Impedance Characterization of Microelectrodes in Phosphate-Buffered Saline: Optimization of Neurostimulation Devices

Stimulation using microelectrode-implants is considered for the treatment of neurological disorders. When implanted into the nerve tissues, the electrodes send electrical signals activating neurons. Microelectrodes are considered to be breakthroughs for neural stimulation because they are less invasive, biocompatible, provide a precise target of neurons, and have a high spatial resolution of functional responses during brain stimulation. However, electrode degradation, tissue damage, and high impedance may limit the charging current delivered in a stimulation cycle. Hence, a study of the electrochemical reactions at the electrode-electrolyte interface is necessary to establish low impedance, safe charge-injection limits, and improve the overall performance of neurostimulation devices.

Impedance spectroscopy can be used to study electrode-tissue properties. Cogan et al. (1) studied the impedance behavior of iridium microelectrodes in phosphate-buffered saline and found that the system transitioned from a charge-transfer behavior at the high-frequency capacitive loop to diffusion control at low frequencies. Wei (2) investigated the impedance of neural stimulation electrodes in vitro and suggested that faradaic resistance and double-layer capacitance decreased monotonically with increasing frequency. However, precise regression analysis of EIS data is required to determine the process model and error structure, assess data consistency with the Kramers-Kronig relation, and provide reliable data interpretation. Waston and Orazem’s (3) measurement model program has been utilized to regress impedance data using a series of Voigt circuit elements that satisfy Kramers-Kronig relations. The measurement model can generate stochastic error structures of experimental data, error models, and characteristic frequency above which ohmic impedance influences the electrode behavior.

We conducted an in vitro examination for microelectrodes in phosphate-buffered saline (PBS) solution to extract parameters for measurement model analysis. The electrochemical instrumentation used were the Gamry Reference 600+, Autolab Potentiostat/Galvanostat (PGSTAT128N), and Palmsens4 Potentiostat. The regression was weighted by the stochastic error structures of experimental data, and the characteristic frequency or transition point from high-frequency to low-frequency was identified. The high-frequency capacitive loop influenced by ohmic impedance was eliminated, and the resulting fit was found to be consistent with the Kramers-Kronig relations. The process model accounted for ohmic resistance, diffusion and reduction of oxygen, and the constant-phase-element behavior associated with the electrode. The capacitance derived from regression analysis agreed with that obtained from Brug's formula. Finally, the study showed that impedance response was controlled by diffusion at low frequencies, and the high-frequency regions exhibited a constant-phase element behavior.

References:

1. S. F. Cogan, U. M. Twardoch, L. S. Robblee, T. L. Rose, G. S. Jones, Y. P. Liu, “Fundamental Studies of Neural Stimulating Electrodes,” Final Report NO1-NS-4–2310, National Institute of Health, National Institute of Neurological Disorders and Stroke Bethesda, Maryland 20892, (1998).
2. X. F. Wei, “Analysis and Design of Electrodes for Deep Brain Stimulation (Doctoral dissertation),” Department of Biomedical Engineering Duke University (2009).
3. 3. W. Watson, M. E Orazem, “A Python-based Measurement Model Toolbox for Impedance Spectroscopy,” Electrochem Soc Interface (2020). https://ecsarxiv.org/kze9x/