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Electrochemical Impedance Spectroscopy for Assessment of MEA Insulator Durability

Antti Karttu1*, Tomi Ryynänen1, Timo Salpavaara1 and Jukka Lekkala1
  • 1 Tampere University of Technology, BioMediTech Institute and Faculty of Biomedical Sciences and Engineering, Finland

Motivation Study and development related to electrical properties of microelectrode arrays (MEAs) have been largely focused on optimizing impedance of the electrode surface. However, the benefits gained from low impedance electrodes for measurement and stimulation of cell activity can be compromised if the insulation layer surrounding the electrodes is unable to withstand the cell culture environment. Degradation of insulator materials has been reported after extended exposure to electrolyte solutions [1][2][3]. Estimating insulator performance and detecting failures with actual MEA structures is difficult due to opened electrodes [4]. A method was needed to study the electrical properties and possible physical degradation of insulator materials during extended exposure to cell culture media over several weeks. Electrochemical impedance spectroscopy (EIS) is capable of easily repeatable non-contact measurement of thin films and has been used extensively in material science to study corrosion [5][6][7]. Here it is applied for estimating both electrical integrity and film thickness of the insulation layer while immersed in cell culture medium. Materials and Methods Simplified test electrode array was designed and fabricated on glass slide with six scaled-up electrodes of 1.5 to 2 mm diameter. Aluminum conductor tracks and elec-trodes were evaporated on the glass and fully covered with 500 nm of silicon nitride by plasma-enhanced chemical vapor deposition (PECVD). Contact pads for the EIS measurement were opened with reactive ion etching (RIE). A culture chamber was formed out of polydimethylsiloxane (PDMS) and bonded onto the silicon nitride surface. Samples were incubated at 37 ˚C and high humidity to simulate the conditions during a stem cell culture. Three solutions were used to test the durability of the insulator: phosphate-buffered saline (PBS) without Mg2+ and Ca2+ ions (Lonza, Belgium), BrainPhys Neuronal Medium (STEMCELL Technologies, USA) and N3 neuronal supplement [8]. EIS measurements were performed periodically to investigate the thickness changes and possible failures of the coating. Impedance measurements were performed with a potentiostat using a two-electrode setup, where a platinum wire immersed in the medium acted as the counter electrode, while the electrode underneath the insulation layer on the sample slide was connected to the system as the working electrode. Frequency range used for the impedance analysis was from 1 Hz to 1 MHz. The insulation layer was modelled as a simple parallel plate capacitor, for which the capacitance value was calculated from the imaginary part of complex impedance. As capacitance is inversely proportional to the distance between capacitor plates, it can be used to estimate the proportional change in the layer thickness. In addition, localized failures in the insulator would be detected as deviations from capacitive behavior in graphs of impedance magnitude and phase angle as a function of frequency. Profilometry was used to confirm the calculated thickness changes after the experiment and the insulator surfaces were imaged with a microscope to observe damages. Results Each sample in the test showed varying degree of corrosion depending on the immersion media. Changes in coating thickness were successfully calculated from the impedance spectra, with the results being close to the profilometer measurements after incubation. Waveforms of impedance magnitude and phase angle showed clearly the effects of the failed insulation layer, which could be seen as leakage resistance through the previously capacitive coating. Microscopic images showed extensive damage to the electrodes in regions where the insulator had corroded completely. Two failure modes were identified, including constant decrease of insulator thickness and localized crevices through the coating, both of which caused impedance of the insulation layer to decrease considerably. Thickness change was smallest although noticeable during the test with PBS, while significant damage to the insulator was observed with BrainPhys and N3. Discussion and Conclusions The tested measurement method allows continuous assessment of insulator material integrity in contact with physiological media. EIS measurements were found to be an effective method for detecting thickness changes and local failures of the insulator when exposed to conditions resembling cell culture studies. The measurement method can be applied to durability testing of insulators currently used in MEA fabrication and to study new possible alternatives. Results obtained with silicon nitride raise questions on its long-term stability in contact with physiological media and should be considered, especially when reusing silicon nitride insulated MEAs for several subsequent experiments.

Acknowledgements

This work was supported by the Academy of Finland and Business Finland (formerly the Finnish Funding Agency for Technology and Innovation (Tekes)).

References

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Keywords: Electrochemical impedance spectroscopy, EIS, Insulator, silicon nitride, corrosion

Conference: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays, Reutlingen, Germany, 4 Jul - 6 Jul, 2018.

Presentation Type: Poster Presentation

Topic: Microelectrode Array Technology

Citation: Karttu A, Ryynänen T, Salpavaara T and Lekkala J (2019). Electrochemical Impedance Spectroscopy for Assessment of MEA Insulator Durability. Conference Abstract: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays. doi: 10.3389/conf.fncel.2018.38.00025

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Received: 17 Mar 2018; Published Online: 17 Jan 2019.

* Correspondence: Mr. Antti Karttu, Tampere University of Technology, BioMediTech Institute and Faculty of Biomedical Sciences and Engineering, Tampere, Finland, antti.karttu@tut.fi