Using Impedance Measurement to Characterize Cell-Electrode Adhesion with Customized 3D-Nanostructured Multielectrode Arrays
Authors: Decker Dominique , Hempelmann Rolf , Rabe Holger , Schäfer Karl-Herbert , Saumer Monika 
Published: February 2020

Multielectrode array (MEA) technology is used as an extracellular recording method to investigate the electrical activity of complex excitable cell networks and tissue slices. In this context, an array of single-contactable, sensing electrodes record the electrical field potentials induced by the ion flux across the cell membranes. Therefore, MEAs are an ideal tool to investigate network activities for prolonged periods. Still, MEA technology suffers from low-signal-to-noise ratios caused by a dissatisfied cell-electrode contact, thus revealing only limited measurements. To improve cell recording, a tight cell-electrode contact is inalienable. This can be achieved by three-dimensional micro- or nanostructures on top of the sensing electrodes. Beside the fact of increasing the electrochemically active surface area, thus leading to a decrease in the overall chip impedance, the nano-sized structures can also trigger an engulfment process of the cells. By minimizing the gap between cell and electrode, a high seal resistance can be achieved which in return results in recording of higher signal amplitudes. In this work, electrochemical concepts are used for fabrication as well as for characterizing the chips and to investigate the cell-electrode interaction.

The developed process allows the fabrication of half-structured MEAs with different three-dimensional nanostructure shapes, geometries and pitch lengths and is mainly based on thermal nanoimprint lithography (NIL), gold electroplating and microstructuring techniques. During thermal NIL a customized, nanostructured master mold is pressed into a polymer resist of a wafer substrate, thus creating a negative copy of the master´s nanostructures. The subsequent electroplating step can be adjusted in a way, that it fills or overgrows the NIL templates, thereby creating different shapes. Photolithography, chemical vapor deposition and etching steps are used to fabricate the overall MEA geometry.

Cyclic voltammetry as well as electrochemical impedance spectroscopy have been applied to characterize the effect of different nanostructure geometries on charge transfer, double-layer capacitance and impedance of the MEA electrodes. Compared to unstructured electrodes, charge transfer and double-layer capacitance of the nanostructured electrodes are increased, whereas the impedance of the nanostructured electrodes are decreased. These results can directly be related to an increase of the electrochemically active surface area and vary with the investigated nanostructure layout.

To examine the effect of the nanostructures on cell-electrode coupling in detail and to quantify cell adhesion, impedance sensing at a fixed frequency of 1 kHz has been performed using the impedance-stable human embryonic kidney cell line HEK 293. Impedance values have been measured with and without cells. Due to the chip layout where non- and nanostructured electrodes have been included on the chip, the relative change of the impedance values could directly be correlated with cell-electrode adhesion. By this, a significant increase or decrease of cell-electrode coupling depending on the nanostructure layout could be demonstrated. For verification, action potential recordings with neural stem cells of the subventricular zone of postnatal mice have been performed, showing a distinct increase of the recorded signal amplitudes of the nanostructured electrodes compared to unstructured electrodes of the same chip. The latter might be correlated to an engulfment process of the cells.

Figure: Left: Impedance measurement to quantify cell-electrode adhesion. Right: SEM picture of attached SVZ cells on top of overgrown pillar structures (Inset: Magnification, scale bar: 2 µm).

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