Abstract
Microelectrode array (MEA) is a tool used for recording bioelectric signals from electrically active cells in vitro. In this paper, ion beam assisted electron beam deposition (IBAD) has been used for depositing indium tin oxide (ITO) and titanium nitride (TiN) thin films which are applied as transparent track and electrode materials in MEAs. In the first version, both tracks and electrodes were made of ITO to guarantee full transparency and thus optimal imaging capability. In the second version, very thin (20 nm) ITO electrodes were coated with a thin (40 nm) TiN layer to decrease the impedance of Ø30 µm electrodes to one third (1200 kΩ → 320 kΩ) while maintaining (partial) transparency. The third version was also composed of transparent ITO tracks, but the measurement properties were optimized by using thick (200 nm) opaque TiN electrodes. In addition to the impedance, the optical transmission and electric noise levels of all three versions were characterized and the functionality of the MEAs was successfully demonstrated using human pluripotent stem cell-derived neuronal cells. To understand more thoroughly the factors contributing to the impedance, MEAs with higher IBAD ITO thickness as well as commercial sputter-deposited and highly conductive ITO were fabricated for comparison. Even if the sheet-resistance of our IBAD ITO thin films is very high compared to the sputtered one, the impedances of the MEAs of each ITO grade were found to be practically equal (e.g., 300–370 kΩ for Ø30 µm electrodes with 40 nm TiN coating). This implies that the increased resistance of the tracks, either caused by lower thickness or lower conductivity, has hardly any contribution to the impedance of the MEA electrodes. The impedance is almost completely defined by the double-layer interface between the electrode top layer and the medium including cells.
Highlights
Microelectrode or multielectrode arrays (MEAs) are a common measurement platform in various in vitro studies where, for example, neuronal cells or cardiomyocytes are applied for drug screening, toxicity testing, cell model development or for increasing understanding of cell behavior [1,2,3,4]
Partial transparency was still maintained unlike with the thicker 200 nm ion beam assisted electron beam deposition (IBAD) titanium nitride (TiN) layer, whose transmission was not measured for its obvious opaqueness
In version 2, the thin TiN layer resulted in a drop of the impedance to one quarter or even more when compared to the bare indium tin oxide (ITO) electrodes, ranging between 300 kΩ to 370 kΩ
Summary
Microelectrode or multielectrode arrays (MEAs) are a common measurement platform in various in vitro studies where, for example, neuronal cells or cardiomyocytes are applied for drug screening, toxicity testing, cell model development or for increasing understanding of cell behavior [1,2,3,4]. The field potential [5] or impedimetric [6] measurements typically performed with MEA are usually complemented with fluorescence imaging or microscopic inspection during or after the MEA recordings. Opaque low-impedance platinum black [12] or titanium nitride (TiN) [13] electrodes are usually used with ITO tracks Both Ryynänen et al [14] and Mierzejewski et al [15] have proposed the use of a very thin TiN layer on the electrodes, which would benefit from TiN’s columnar structure and capability of decreasing impedance by increasing the effective surface area, while still maintaining the transparency, at least to some extent. Graphene [16,17], diamond [18] and conducting polymers [19,20] have been demonstrated as potential candidates for transparent electrodes, but they have their own challenges especially related to the ease of fabrication and stability
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