Electrochemical and Nanostructural Characterization of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) Films as Coatings for Neural Electrodes

Abstract

Poly(3, (PEDOT), a wellcharacterized conducting polymer, has been applied for coating metal neural electrodes to improve their stimulating or recording performance. The coated electrodes possess advantages in better neuron attachment, lower impedance, and larger capacitance compared to the bare metal substrate due to the biocompatibility and porous surface of the polymer. However, the PEDOT-coated electrodes have frequently reported issues associated with mechanical instability, such as cracking and delamination. Solving this problem is crucial for stimulating electrodes, whereas a massive film is unnecessary for recording purposes. Moreover, the thickness control for the latter has rarely been investigated. In this work, we systematically studied and characterized poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with cyclic voltammetry and atomic force microscopy (AFM) to evaluate the electropolymerization of PEDOT:PSS from the basis and analyze the surface morphology for a range of deposition times. The polymerization potential was obtained, and the deposition charge density was optimized for recording neural electrodes. In addition, high-resolution AFM height and phase images reveal the heterogeneity of the polymer surface. The modified electrode was also tested for its electrochemical performance in a small potential window with both a standard electrochemical cell setup and stainless steel microscrews. The results showed that despite a shift of potential (0.42 V) due to the change of setup, the electrode functions well in the capacitive region without triggering redox reactions.

Double Electrode Experiments Reveal the Processes Occurring at PEDOT-Coated Neural Electrode Arrays

“…The electropolymerization was conducted galvanostatically in a solution of 10 mM EDOT and 0.1 mM NaPSS [0.7% (w/v)] or 10 mM EDOT and 0.1 M NaCl . A constant current of 20 nA was applied for 13 s to achieve an average charge deposition density of 50 mC cm –2 following our previous protocol .…”

“…Therefore, thickness control is a crucial part of a recording neuron electrode, which has been extensively discussed in our previous paper. 41 Despite the benefits of PEDOT as a biocompatible conductive polymer, its application has been primarily limited to single, needle-like electrodes 31 or large, flat slides 30 rather than to densely packed electrode configurations such as tetrodes. This limitation is partly due to the challenges associated with polymer overgrowth, which can lead to crossconnection of detection sites.…”

“…PEDOT: Poly(3,4-ethylene dioxythiophene); PSS: polystyrene sulfonate. 41 activity and its contribution to the signals recorded at the tetrode, the potential of one electrode (WE1) within the tetrode can be suddenly altered to replicate the rapid depolarization changes of active neurons (Figure 1(b)). Meanwhile, the response of a nearby second electrode (WE2) within the same tetrode is monitored (Figure 1(b)).…”

“…Conversely, a massive film is unnecessary for a recording electrode, in particular where little outperformance can be gained over the impedance. Therefore, thickness control is a crucial part of a recording neuron electrode, which has been extensively discussed in our previous paper …”

“…This chain has a positively charged backbone, requiring the incorporation of PSS – ions or other anions from the solution to neutralize the charge. PEDOT: Poly­(3,4-ethylene dioxythiophene); PSS: polystyrene sulfonate …”