Reactive Sputtered Silicon Nitride as an Alternative Passivation Layer for Microelectrode Arrays in Sensitive Bioimpedimetric Cell Monitoring

Abstract

Microelectrode arrays (MEAs) are widely used to study the behavior of cells noninvasively and in real time. While the design of MEAs focuses mainly on the electrode material or its application-dependent modification, the passivation layer, which is crucial to define the electrode area and to insulate the conducting paths, remains largely unnoticed. Because often most cells are in direct contact with the passivation layer rather than the electrode material, biocompatible photoresists such as SU-8 are almost exclusively used. However, SU-8 is not without limitations in terms of optical transmission, optimal cell support, or compatibility within polymer-based microfluidic lab on chip systems. Here, we established a silicon nitride (SiN) passivation by physical vapor deposition (PVD), which was optimized and evaluated for impedance spectroscopy-based monitoring of cells. Surface characteristics, biocompatibility, and electrical insulation capability were investigated and compared to SU8 in detail. To investigate the influence of the SiN passivation on the impedimetric analysis of cells, HEK-293 A and MCF-7 were chosen as adherent cell models and measured on microelectrodes of 50–200 μm in diameter. The results clearly revealed an overall suitability of SiN as alternative passivation. While for the smallest electrode size a cell line dependent comparable or slightly decreased cell signal could be observed in comparison with SU-8, a significant higher cell signal was observed for microelectrodes larger than 50 μm in diameter. Furthermore, a high suitability for the bonding of PEGDA and PDMS microfluidic structures on the SiN passivation layer without any leakage could be demonstrated.

Nanomaterial-based microelectrode arrays for in vitro bidirectional brain–computer interfaces: a review

“…More information about the modification of the conducting layer will be discussed later. However, the insulation layer to protect the conducting wires, which is critical for defining the electrode area and acquiring cell signals, remains the most neglected and awaits innovation 47 .…”

In Vivo Penetrating Microelectrodes for Brain Electrophysiology

“…Since silicon is a semiconductor material, it is covered with various insulating materials [ 101 ]. Some insulating materials include: varnish [ 102 ], glass [ 49 , 103 ], Teflon [ 104 ], silicon dioxide (SiO 2 ) [ 105 , 106 ], silicon nitride (SiN) [ 105 , 107 ], silicon carbide [ 108 , 109 ], silicon dioxide with hafnium dioxide (HfO 2 /SiO 2 ) bilayers [ 110 ], aluminum oxide (Al 2 O 3 ) [ 111 ] and diamond-based coatings [ 101 , 112 ]. However, microelectrodes fabricated on a rigid substrate, such as silicon, cause inflammatory reactions, which further leads to microelectrode failure [ 31 , 75 , 113 ].…”