Transparent Microelectrode Arrays Fabricated by Ion Beam Assisted Deposition for Neuronal Cell In Vitro Recordings

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.

Electrophysiological Analysis of Brain Organoids: Current Approaches and Advancements

“…Recent advances in bioprinting have been utilized to demonstrate proof-of-concept for patterning and printing MEAs on soft material substrates with mechanical properties similar to brain tissue, assisting with biocompatibility ( Adly et al, 2018 ; Borda et al, 2020 ). To improve electrode properties (i.e., biocompatibility, impedance, structural integrity, transparency), many new materials have been used for design and coating, such as indium tin oxide, gold, titanium nitride, and ruthenium oxide, among others ( Jahnke et al, 2019 ; Koklu et al, 2019 ; Ryynänen et al, 2019 , 2020 ; Atmaramani et al, 2020 ). The continued development of MEAs incorporating these and other materials, as well as improved incorporation methods, may help improve signal-to-noise ratios and fidelity for brain organoid recordings.…”

Functional Characterization of Human Pluripotent Stem Cell-Derived Models of the Brain with Microelectrode Arrays

“…The most wide-spread arrays are the 8 × 8 grids of planar titanium nitride (Multi Channel Systems) or indium tin oxide electrodes (MED64) at the bottom of a cell culture dish. The electrodes of standard MEAs are typically embedded in a thin glass base, which facilitates real time microscopy (e.g., calcium imaging) of the tissues or cultures around the electrodes [ 59 ], even though the electrodes themselves could also be made transparent [ 60 , 61 ]. CMOS technology has also been incorporated into in vitro MEAs, enabling the construction of very high-density arrays for measurements from different parts of the same cell and the analysis of properties such as signal propagation velocity within individual cells [ 62 , 63 ].…”

“…The comparison of burst counts is further complicated by the variety of different burst detection algorithms in the field [ 141 ]. In addition, there are few reports on neural oscillations or LFP analysis in human 2D cultures on MEAs [ 61 ], even though many publications report oscillatory firing behavior [ 119 , 128 , 132 , 142 , 143 ].…”

Editorial on the Special Issue on Microelectrode Arrays and Application to Medical Devices

“…The deposition of ITO and TiN thin films are commonly conducted by the sputter deposition technique. However, Ryynänen et al successfully demonstrated that ion beam-assisted electron deposition (IBAD) can be used as an alternative method for this purpose [ 1 ]. They studied three different approaches for the fabrication of microelectrodes.…”

“…In this editorial note, we briefly review the major findings of the 10 articles published in the Special Issue on microelectrode arrays and application to medical devices. The articles are categorized into three sections, i.e., fabrication techniques [ 1 , 2 , 3 , 4 ], application demonstration [ 5 , 6 , 7 , 8 ], and review of recent advances in this field [ 9 , 10 ].…”