Characterizing Longitudinal Changes in the Impedance Spectra of In-Vivo Peripheral Nerve Electrodes

Abstract

Characterizing the aging processes of electrodes in vivo is essential in order to elucidate the changes of the electrode–tissue interface and the device. However, commonly used impedance measurements at 1 kHz are insufficient for determining electrode viability, with measurements being prone to false positives. We implanted cohorts of five iridium oxide (IrOx) and six platinum (Pt) Utah arrays into the sciatic nerve of rats, and collected the electrochemical impedance spectroscopy (EIS) up to 12 weeks or until array failure. We developed a method to classify the shapes of the magnitude and phase spectra, and correlated the classifications to circuit models and electrochemical processes at the interface likely responsible. We found categories of EIS characteristic of iridium oxide tip metallization, platinum tip metallization, tip metal degradation, encapsulation degradation, and wire breakage in the lead. We also fitted the impedance spectra as features to a fine-Gaussian support vector machine (SVM) algorithm for both IrOx and Pt tipped arrays, with a prediction accuracy for categories of 95% and 99%, respectively. Together, this suggests that these simple and computationally efficient algorithms are sufficient to explain the majority of variance across a wide range of EIS data describing Utah arrays. These categories were assessed over time, providing insights into the degradation and failure mechanisms for both the electrode–tissue interface and wire bundle. Methods developed in this study will allow for a better understanding of how EIS can characterize the physical changes to electrodes in vivo.

An implant for long-term cervical vagus nerve stimulation in mice

“…Previous reports from rat and large animal models implemented a similar approach to adjust stimulation intensity using respiratory twitching [15,71], and heart rate [40,72,73] to standardize stimulation protocols, but not on a dose-by-dose basis. Notably, electrical impedance, which is commonly used as a measure of electrode integrity and performance [74], did not correlate well with changes in HRT on an individual implant basis as implants aged (Fig 3E). In fact, in cohort 2 animals, impedance values tended to decrease over the first two weeks as HRT increased (Fig 3B and 3D).…”

Non-Enzymatic Electrochemical Detection Of Glutamate Using Templated Polymer-Based Target Receptors

“…This is observed by a rising impedance due to the passivation of the metal surface via the polymer deposition and subsequent Glu binding. The phase plot in Figure 4(c) shows that the peak phase response shifts toward the lower frequency upon binding of polymer and Glu [13]. At the lower frequency domain (f < 100 Hz), the phase differences among the responses is maximized at 25 Hz [14].…”

Classifying Intracortical Brain-Machine Interface Signal Disruptions Based on System Performance and Applicable Compensatory Strategies: A Review

“…One useful diagnostic tool, in vivo impedance spectroscopy, has revealed unique impedance signatures for varying degrees of microelectrode tissue encapsulation (Williams et al, 2007 ; Cody et al, 2018 ). Impedance spectroscopy, in combination with equivalent circuit modeling, can provide insight on abiotic failure modes such as insulation deterioration, wire breakage, and electrode tip degradation (Caldwell et al, 2018 ; Straka et al, 2018 ). Cyclic voltammetry is another method that has proven useful in identifying the formation of current leakage pathways.…”

“…Impedance characterization of devices has historically been reported at 1 kHz because it provides information about the exposed electrode area (Hsu et al, 2009 ) and roughly matches the frequency of an action potential. Importantly, 1 kHz impedance can correlate with recording metrics including array yield and the number of recorded units (Prasad and Sanchez, 2012 ; Black et al, 2018 ), with several studies supporting the notion that considerable information about device integrity exists at higher and lower frequencies (Takmakov et al, 2015 ; Caldwell et al, 2018 ; Straka et al, 2018 ). For the Blackrock Utah array with platinum recording electrodes, 1 kHz impedance <60 kΩ indicates shunting to ground (Barrese et al, 2013 ).…”