Impedance Characterization and Modeling of Subcellular to Micro-sized Electrodes with Varying Materials and PEDOT:PSS Coating for Bioelectrical Interfaces

Abstract

Electrode-to-cell/tissue interfaces with high biocompatibility, low impedance, and long-term chemical and mechanical stability are of paramount importance in numerous biological and biomedical applications. For meticulous monitoring of biological parameters, there is a rapidly growing interest in sensing at subcellular levels with radically improved spatiotemporal resolution, which necessitates ultra-miniaturized electrodes with significant reduction in electrode contact sizes. Such aggressive electrode downsizing inevitably impacts the electrochemical interfaces, with the consequences still poorly understood. This paper reports the first systematic analysis of the interfacial electrochemical impedance spectroscopy of electrodes comprising a variety of biocompatible electrode materials consisting of gold (Au), platinum (Pt), indium tin oxide, and titanium nitride (TiN) coated with/without an organic polymer, poly­(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), with electrode diameters (D) ranging from millimeter to subcellular (<10 μm) dimensions. PEDOT:PSS-coated electrodes have greater Faradaic charge-transfer capability and capacitive coupling compared to their uncoated counterparts. At D = 10–200 μm, the PEDOT:PSS coating reduces the electrode interfacial impedance at 1 kHz by up to ×101.6, while at D > 200 μm, the effect is lessened due to the dominance of solution, or bulk electrolyte, and routing resistance. The low interfacial impedance of PEDOT:PSS-coated electrodes makes them promising candidates for next-generation bioelectrical interfaces with subcellular spatial resolution.

A CMOS Multi-Electrode Array for Four-Electrode Bioimpedance Measurements

“…To generalize, we have not included cells, but instead modelled the effect of decreasing the conductivity of the solution. The surface impedance's at 10 kHz (2 • 10 −5 Ω • m 2 ) and 100 kHz (4 • 10 −6 Ω • m 2 ) for circular electrodes with radius 2.5 µm were extracted from Wang et al [21]. It should be noted that the polarization impedance will in general vary with the conductivity of the solution [22].…”

“…The electrode impedance for miniaturized electrodes can be large (several MΩ) [21], thus requiring a high gain from the TIA to amplify the current. In Fig.…”

In Vivo Penetrating Microelectrodes for Brain Electrophysiology

“…Small size microelectrodes (<15 µm of thickness for Michigan-type and <15 µm of diameter for wire- and needle-types) are most preferable for high-quality, long-term recording of neuronal activity [ 30 , 31 ] since their high spatial resolution allows them to record signals with a higher amplitude compared to larger microelectrodes [ 175 ] and they significantly reduce inflammatory processes at the site of the implantation. However, the microelectrode size (registration sites) reduction leads to an increase in impedance and negatively affects the signal-to-noise ratio (SNR) [ 176 , 177 ]. Low electrical impedance is an important characteristic that should be considered for neuronal activity recording because high impedance values, above 2–5 MΩ [ 86 , 178 ], can cause attenuation of the recorded signal [ 176 , 178 ].…”

A Multimodal and Multifunctional CMOS Cellular Interfacing Array for Digital Physiology and Pathology Featuring an Ultra Dense Pixel Array and Reconfigurable Sampling Rate

“…However, the 2-point impedance measurements will become problematic when electrodes are scaled down because the electrode-electrolyte interfacial impedance starts to dominate sample impedance, especially at lower frequencies [66][67][68]. With 4-point impedance, the double layer capacitance (Cdl) and charge transfer resistance (Rct) from the electrode-electrolyte impedance presented in Fig.…”

“…Eventually, a Ti/Pt layer is sputter deposited over all the electrodes in the pixel array followed by a lift-off without the need for ultrasonication. Pt was chosen instead of Au due to Pt's lower electrode-electrolyte impedance while possessing similar inertness and stability [66][67][68]. Our in-house process possesses some key advantages over previously reported methods of eliminating Al and using EBPVD [14,16].…”