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

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 ×10(1.6), 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.