Bringing transparent microelectrodes to market: Evaluation for production and real-world applications

Abstract

Event Abstract Back to Event Bringing transparent microelectrodes to market: Evaluation for production and real-world applications Michael Mierzejewski1, Pranoti Kshirsagar1, Udo Kraushaar1, Gerhard Heusel1, Ramona Samba2 and Peter D. Jones1* 1 Natural and Medical Sciences Institute, Germany 2 NMI Technologie Transfer GmbH, Germany Introduction Microelectrode arrays (MEAs) are used for a number of applications in electrophysiology, with the ability to record field action potentials (fAP) from cardiomyocytes and neurons, as well as selectively induce a potential on an electrode in the array. This technology makes high throughput monitoring of field potentials possible, but the materials used in commercially-available MEAs can inhibit visual recording or stimulation. While transparent conductors such as indium tin oxide (ITO) are used for conducting traces, the need for microelectrodes to have stable, low electrochemical impedance has necessitated the use of opaque materials such as titanium nitride (TiN), gold (Au), platinum (Pt), and platinum iridium (Pt/Ir). Any bright-field microscopy will lead to a shadow being cast onto the cells above the electrode, resulting in a loss of potentially useful information. To remedy this issue, several groups have been working on transparent microelectrode materials such as Au-mesh coated with conducting polymer [1] and carbon nanomaterials [2-5]. To the best of our knowledge, no MEA with transparent microelectrodes is currently commercially available. The purpose of this project is to evaluate multiple transparent materials to complement those opaque materials currently on the market. Specifically, we are investigating the material properties, suitability for scalable production, stability against sterilization and during cell culture, and evaluating their application in opto- and electrophysiology in cardiac and neural applications. This technology has the potential to advance the current trend of combining electrophysiology with optophysiology. Materials and Methods We are evaluating various transparent materials against state-of-the-art titanium nitride (TiN) microelectrodes. Standard (opaque) and transparent titanium nitride microelectrodes have been fabricated in an 8x8 MEA configuration with ITO traces, SiNx as the insulator, electrode size of 30 µm, and pitch of 200 µm. Carbon nanomaterials and conducting polymers are also being considered as potential materials for transparent electrodes. Impedance spectroscopy was performed with a Bio-logic VMP3 potentiostat at an amplitude of 10 mV from 1 Hz to 100 kHz. Noise measurements were performed using 150 mM phosphate buffered saline (PBS) as the medium, utilizing a MEA2100-system from Multi Channel Systems (MCS). Cardiomyocytes were isolated from embryonic chicken ventricles after 13 days in incubation, and 80 k cells were subsequently plated on each MEA after coating with nitrocellulose. Recordings were taken of these samples in a 3 % Fetal Calf Serum (FCS) medium after 6 days in vitro. Optical transmission measurements were taken with a bright-field Zeiss microscope, performing a line scan across the electrodes. Results and Discussion Characterization of the transparent electrodes showed an increase in transparency, noise, and impedance in comparison to the standard electrodes. An increase in impedance of ~150 kΩ (Figure 1b) led to an increase in peak-to-peak noise of 10 µV when compared against standard opaque electrodes. This increase of impedance and noise is accompanied by a 40 % absolute increase in transmission of light in the visible spectrum (400–700 nm) as shown in Figure 1d. The electrodes were then compared against each other in a real world application. Measurement of fAP from the cardiomyocytes showed the effect of the increased noise and transmission observed during the characterization. This increase in noise affected the clarity of the cardiomyocyte recordings, but cellular activity such as the repolarization event in Figure 1c was still visible. While the recordings were impacted by the characteristics of the transparent material, the increased transmission allowed for cells on top of the electrode to be visible under bright-field spectroscopy. Conclusion and Outlook Preliminary results show that increased transparency can be attained in exchange for increased impedance, and subsequent increased noise. The increase in transparency does allow for the ability to view cells which would normally be obstructed by the electrode. This will provide new capabilities for specific applications where observation or optical stimulation of the cells is important to the application. With continued work on this project we aim to further evaluate transparent TiN as well as materials including carbon nanomaterials and conducting polymers. Along with testing the impedance and noise of all the electrodes, we plan to put the MEAs through real world scenarios to see how well they can be used in research applications. This will include investigating the limits that the MEAs can withstand regarding standard cleaning and sterilization procedures, as well as how well the MEAs withstand extended or repeated cell cultures. Figure 1 Figure 2 Acknowledgements We thank the NMI TT GmbH for supporting MEA fabrication, Sandra Buckenmaier for cardiomyocyte cell culturing and Simon Dickreuter for optical transmittance measurements.