DOI: 10.5061/dryad.8931zcrvw
title: In vivo and in vitro electrochemical impedance spectroscopy analysis of acute and chronic intracranial electrodes

description: Invasive intracranial electrodes are used in both clinical and research
 applications for recording and stimulation of brain tissue, providing
 essential data in acute and chronic contexts. The impedance
 characteristics of the electrode–tissue interface (ETI) evolve over time
 and can change dramatically relative to pre-implantation baseline.
 Understanding how ETI properties contribute to the recording and
 stimulation characteristics of an electrode can provide valuable insights
 for users who often do not have access to complex impedance
 characterizations of their devices. In contrast to the typical method of
 characterizing electrical impedance at a single frequency, we demonstrate
 a method for using electrochemical impedance spectroscopy (EIS) to
 investigate complex characteristics of the ETI of several commonly used
 acute and chronic electrodes. We also describe precise modeling strategies
 for verifying the accuracy of our instrumentation and understanding
 device–solution interactions, both in vivo and in vitro. Included with
 this publication is a dataset containing both in vitro and in vivo device
 characterizations, as well as some examples of modeling and error
 structure analysis results. These data can be used for more detailed
 interpretation of neural recordings performed on common electrode types,
 providing a more complete picture of their properties than is often
 available to users.
description: <strong>Electrochemical impedance
 spectroscopy</strong>- This technique involves measurement of an
 electrochemical interface by applying a sinusoidal voltage perturbation
 across a range of frequencies. For potentiostatic modulation, a single
 perturbation amplitude is maintained across the frequency spectrum,
 typically lower than amplitudes used in biological stimulation. The
 frequency range chosen for these measurements was 10 Hz-100 kHz, with an
 amplitude of 0.01 V. Data were sampled at 39 frequency intervals across
 this spectrum, or 9.5/dec. Using the complex impedance values obtained by
 our instrument, a fitted model was created to approximate the components
 of the biotic-abiotic interface. <strong>Data
 collection</strong>- Data were primarily collected using a Palmsens4
 Potentiostat instrument in Electrochemical Impedance Spectroscopy mode,
 both <em>in vitro</em> and <em>in vivo</em> in
 non-human primates (NHP). NHP subjects included two adult male rhesus
 macaque monkeys chronically implanted with both penetrating deep brain
 stimulation (DBS) electrodes and surface-contact electrocorticography
 (ECoG) electrodes placed epidurally. Measurements <em>in
 vivo</em> were performed only in 2-electrode mode, using a titanium
 screw or rod mounted to each animal’s skullcap as a return electrode. For
 <em>in vitro </em>measurements, a 2-electrode system was
 employed for direct comparison to typical <em>in vivo
 </em>data collection methods. A 3-electrode system including an
 Ag/AgCl reference electrode was also used <em>in vitro</em> to
 verify the reliability of the 2-electrode system. The Palmsens device was
 operated using battery power to isolate it from electrical line
 noise. Sample fluids used <em>in vitro</em>
 included phosphate-buffered saline (PBS) at 1x concentration and
 artificial cerebrospinal fluid (aCSF). Sample fluid temperature was
 regulated to approximate biological conditions, specifically NHP internal
 body temperature of approximately 37° C. Temperature regulation was
 performed using a Gamry electrochemical flow-cell connected to a thermally
 regulated bath, with measured temperature verified to within plus or minus
 0.1°C. Working electrodes included a selection of
 commercially available intracranial electrodes, designed for both human
 and NHP applications. Electrodes used for <em>in vivo</em>
 measurements were chronically implanted, and connection to the Palmsens
 device was achieved via an external connector. The counter electrode used
 <em>in vivo</em> consisted of titanium screw for Animal 1 and
 a titanium rod for Animal 2, with the same approximate surface area.
 Counter electrodes used <em>in vitro</em> included a titanium
 screw similar to that used <em>in vivo</em> as well as
 platinum wire (7.5 cm length/0.5 mm diameter) and platinum foil (625
 mm<sup>2</sup>). When a 3-electrode measurement system was
 applied, the “reference” consisted of a silver-silver chloride (Ag/AgCl)
 electrode, as is typically employed in non-biological EIS
 applications. For comparison to standard methods of
 electrode impedance verification, measurements were also performed using a
 Bak Electrode Impedance Tester calibrated with a 1 kOhm resistor. In this
 setup, impedance was computed as a function of two contacts: a working
 electrode and a counter electrode. <strong>Setup
 and solution preparation</strong>- Gray and white matter have
 measured conductivities of 0.33 and 0.142 S/m. The required salt
 concentration to mimic gray and white matter solutions was found by
 measuring the conductivity of dissolved salt at intervals of 0.5 g/L at
 37°C +/- 0.1°C until the measured conductivity was greater than 0.33 S/m.
 Laboratory-grade salt (Thermo Fischer) was added between intervals, mixed
 using a vortex at 300 rpm until clear.  A line fit to these conductivity
 values was used to find salt concentrations that approximated the
 conductivity of gray and white matter. The aCSF was made using 12.6 mM
 NaCl, 3mM KCl, 26 mM NaHCO<sub>3</sub>, 1.4 mM
 NaH<sub>2</sub>PO<sub>4</sub>, 10mM Glucose, 2 mM
 CaCl<sub>2</sub>, and 2 mM MgSO<sub>4</sub>,
 osmolarity of 306 mOsm/L, and pH of 7.36.
 For <em>in </em>vitro recordings, electrodes were
 suspended in solution with all contacts fully immersed in electrolyte and
 not touching glass side walls. EIS was measured using the Bak Electrode
 Impedance Tester and the PalmSens device. The PalmSens device delivered 10
 Hz-10kHz, 0.1 nA-100 mAmps, 3 or 10 cycles each. The electrodes included
 in this solution comparison were AdTech Depth, AdTech Strip, AdTech Grid,
 Dixi Depth, Neuropace Depth, Neuropace Strip, and AdTech Behnke-Fried. To
 facilitate experimental determination of the stochastic error structure,
 trials were repeated 3x per electrode. For <em>in
 vivo</em> recordings, the exposed distal end of the working
 electrode was accessed, and a titanium screw mounted in the NHP skullcap
 served as a combined counter/reference electrode. A Palmsens MUX8/R2
 Multiplexer facilitated switching between channels of the working
 electrode, with three scans performed per channel to facilitate error
 regression.
description: # Electrochemical Impedance Spectroscopy (EIS) recordings of a series of
 implantable neural electrodes This dataset contains both in-vitro and
 in-vivo EIS recordings of implantable electrodes designed for recording
 and stimulation of human and animal brain tissue. In-vitro data were
 recorded in up to four different electrolyte solutions and two different
 electrode configurations. Solutions included phosphate-buffered saline
 (PBS), artificial cerebrospinal fluid (ACSF) and salt solutions matching
 gray and white matter conductivities, in both two and three-electrode
 configurations. The counter electrode used in in-vivo measurements
 consisted of a titanium screw or titanium rod embedded in a chronically
 affixed skull cap, with its tip contacting brain tissue.
 Counter-electrodes used in vitro included a titanium screw similar to that
 used in vivo as well as platinum wire (7.5 cm length/0.5 mm diameter) and
 platinum foil (625 mm^2 surface area). When a 3-electrode measurement
 system was applied, the “reference” consisted of a silver-silver chloride
 (Ag/AgCl) electrode, as is typically employed in non-biological EIS
 applications. In vivo data were recorded from 2 chronically implanted
 adult male rhesus macaque monkeys, denoted as Animal 1 and Animal 2. In
 most cases, EIS measurements were performed three times to facilitate
 stochastic error structure modeling using the software and methods
 described here:
 [https://ecsarxiv.org/kze9x/](https://ecsarxiv.org/kze9x/). This process
 can be used to assess signal to noise ratio (SNR) and perform other forms
 of modeling on the data. An example of the output from this process is
 included in the folder "Fitted model example", performed on a
 single channel of the segmented Heraeus lead from an in vitro recording.
 Folder names within the dataset correspond to electrode models (name or
 model number), with the exception of "Bak Vs PalmSens", which
 contains data comparing the single-frequency Bak Electrode Impedance
 Tester to the Palmsens4 Potentiostat instrument, and "Fitted Model
 Example", which contains example modeling using the Measurement Model
 software
 ([https://doi.org/10.1149/osf.io/kze9x](https://doi.org/10.1149/osf.io/kze9x)). File names in this dataset contain a description of the electrode model measured, the temperature at which measurements were conducted (for in vitro data), whether the measurement was conducted using a 2 or 3 electrode configuration, and the material/type of counter electrode used (or combined counter/reference for 2 electrode measurements). For example, the file "MDT_c1_37C_platWire_PBS.csv" contains data from the MDT SenSight electrode, contact one, recorded at 37 °C using a platinum wire counter-electrode in phosphate buffered saline solution. All 3-electrode measurements used a silver/silver chloride reference. When not otherwise noted, the fluid used in in-vitro measurements was PBS. Note that temperature measurements were accurate to within a tenth of a degree. Data included here are encoded in both .csv and .pssession formats. The .csv files include columns for frequency, negative phase angle, DC current, complex impedance, real component of impedance, negative imaginary component of impedance, and charge. This file type should be readable by many types of software. The included .pssession files are a proprietary format used by our PalmSens4 Potentiostat instrument's corresponding software, PSTrace ([https://www.palmsens.com/software/ps-trace/](https://www.palmsens.com/software/ps-trace/)). These files may be useful to others who use PalmSens instruments and software. A Python script used to convert from the .pssession file type to .csv format is also included, and is described below. The python file, "pssession_parser.py" converts *.pssession files to *.csv files. This script works for python version 3.8+. The script uses "Pandas" 1.1.1 for handling data. In order to run this script, simply find the if \\_*name*\\\_ == "**main**" statement, change the datapath to either the *pssession file you are looking for or a directory of *pssession files you want to convert. Then run the program to output the contents of the PalmSens .pssession file(s) as a .csv Glossary: \* GM = Gray Matter \* WM = White Matter \* CSF = Cerebrospinal Fluid \* PBS = Phosphate Buffered Saline \* TR1/TR2/TR3=Trial one, Trial two, Trial three (as part of 3 repeated measurements) \* BF = Behnke Fried (Adtech BF) NP=NeuroPace \* 2-Electrode = Setups with 1 working electrode + 1 counter/reference electrode \* 3-Electrode = Setups with 1 working electrode, 1 counter electrode, 1 reference electrode