Alternative Electrode Materials for Prototyping Cell Model-Specific Microelectrode Arrays
|Tila||Julkaistu - 8 marraskuuta 2019|
|Nimi||Tampere University Dissertations|
A microelectrode array, MEA, is a tool used by biologists for measuring the electrical activity of cells in vitro. Instead of only studying random cell clusters and monolayers, an increasing number of biological research questions are aimed at studying welldefined cell networks or single cells. This places special demands on the location, size, and overall performance of the MEA electrodes, which the standard, commercially available layouts cannot usually meet. Therefore, custom-designed MEAs are needed for a wide range of applications from basic cell biology and disease model development to toxicity testing and drug screening. This thesis focuses on the fabrication of microelectrodes made of titanium, atomic layer deposited (ALD) iridium oxide (IrOx), and ion beam-assisted e-beam deposited (IBAD) titanium nitride (TiN). These MEAs are characterized, for example, in terms of their impedance, noise level, and surface morphology, and their biocompatibility and functionality are verified by simple experiments with human stem cell-derived neuronal cells and cardiomyocytes. The aim of these studies is to offer more alternatives for MEA fabrication, enabling researchers and practitioners to choose the electrode material that best fits their application from their available resources. Pure titanium is commonly disregarded as an electrode material because of its oxidation tendency, which destabilizes the electrical performance. However, when prototyping customised MEAs, the time and cost of fabricating the subsequent iterations of the prototype can be more decisive factors than the device’s ultimate electrical performance, which is typically evaluated by the impedance value at 1 kHz. As might be expected, although titanium electrodes underperformed in terms of
impedance (>1700 kΩ), when used in the cell experiments, the field potentials from both neuronal cells and cardiomyocytes were still easily distinguishable from the noise. There are a number of benefits to using titanium as an electrode material. Besides the fact that it is about hundred times cheaper than other commonly-used materials, such as gold or platinum, it usually requires fewer and often simpler process steps than the most common alternatives.
IrOx and TiN are common electrode coatings which, when applied on top of e.g.
a titanium electrode, can lower the impedance and the noise level of the electrode. In this study, two alternative deposition methods, ALD and IBAD, were used for IrOx and TiN in MEA applications. Even if the impedance of these 30 μm electrodes (450 kΩ for ALD IrOx and ~90 kΩ for IBAD TiN) did not quite reach the impedance levels of the industry standards, i.e. sputtered TiN (30-50 kΩ) and Pt black (20-30 kΩ), in cell experiments the IBAD TiN electrodes in particular showed no tangible differences in peak amplitudes and noise levels compared with sputtered TiN electrodes. This makes IBAD TiN an attractive alternative material for those who prefer to use TiN electrodes, but do not have access to a sputter coater, for example. ALD IrOx, on the other hand, relies on the potential of the general properties of ALD and IrOx (yet unverified) to provide exceptional performance in designs requiring excellent step coverage or stimulation capability. Finally, as an application example of a custom-designed MEA, a version capable of measuring cardiomyocytes at the single-cell level was developed. The benefit of such an MEA is to offer a unique noninvasive method to study single cells without destroying them with the time-consuming patch clamp method, and without losing cell-specific information, which often occurs if the cell clusters studied with standard MEAs are too heterogenous. This was achieved with a number of innovations. For example, the electrodes were placed near the perimeter of the cell culturing area and had a larger diameter (80 μm) than the usual 30 μm electrodes. This simplified the plating of the cells to the electrodes and enabled the beating of the cells to be electrically recorded. It is also possible to combine that with image-based analysis of mechanical beating through transparent indium tin oxide (ITO) electrodes.