Transparent, Low-Impedance Inkjet-Printed PEDOT:PSS Microelectrodes for Multi-modal Neuroscience

Abstract

Transparent microelectrodes that facilitate simultaneous optical and electrophysiological interfacing are desirable tools for neuroscience. Electrodes made from transparent conductors such as graphene and indium tin oxide (ITO) show promise but are often limited by poor interfacial charge-transfer properties. Here, microelectrodes are demonstrated that take advantage of the transparency and volumetric capacitance of the mixed ion-electron conductor Poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Ring-shaped microelectrodes are fabricated by inkjet-printing PEDOT:PSS, encapsulating with Parylene-C, and then exposing a contact site that is much smaller than the microelectrode outer diameter. This unique structure allows the encapsulated portion of the microelectrode volume surrounding the contact site to participate in signal transduction, which reduces impedance and enhances charge storage capacity. While using the same 100 μm diameter contact site, increasing the outer diameter of the encapsulated electrode from 300 to 550 μm reduces the impedance from 294±21 to 98±2 kΩ, respectively, at 1 Hz. Similarly, the charge storage capacity is enhanced from 6 to 21 mC cm^-2. The PEDOT:PSS microelectrodes provide a low-haze, high-transmittance optical interface, demonstrating their suitability for optical neuroscience applications. They remain functional after a million 1 V stimulation cycles, up to 600 μA of stimulation current, and more than 1000 mechanical bending cycles.

Keywords: PEDOT:PSS; impedance modeling; printing; transparent microelectrodes; volumetric capacitance.

Figure 1.

a) Fabrication diagram. b) Schematic of the side- and top-views of the microelectrode interface. Note that PEDOT:PSS and Parylene thicknesses are not to scale.The diameter of the contact opening at the top surface is roughly 100 μm (Figure S2), and the geometric surface area (GSA) of the microelectrode is indicated by the red highlights. c) Photograph of a single PEDOT:PSS microelectrode. d) Distributions of microelectrode impedance magnitudes at 1 Hz for microelectrodes of varying outer diameters but constant contact opening diameter. Middle of boxes is the median, edges of boxes show 25% and 75% interquartile ranges, and whiskers show max and min values after removing outliers (n=14, 14, and 13 microelectrodes for 300 μm, 400 μm and 550 μm outer diameters, respectively).

Figure 2.

a) Impedance spectra for microelectrodes of varying outer diameters. Closed circles represent the average measured value, while the shaded regions show the standard deviation (n=7 microelectrodes for each outer diameter), and solid lines show the spectra generated by the average fit values extracted from fitting the impedance model shown in b). b) Impedance model for ring-shaped PEDOT:PSS microelectrodes. c) Distributions of extracted C_v values from model fitting showing 25% to 75% interquartile range (box) and min/max values after removing outliers (whiskers).

Figure 3.

a) Total transmittance and b) haze spectra of material stacks present in devices for visible-NIR light.

Figure 4.

a) Representative cyclic voltammograms from microelectrodes with varying outer diameters. b) Distributions of extracted charge storage capacity (CSC) for microelectrodes of varying outer diameters (n=7, 7, 6 microelectrodes for 300 μm, 400 μm and 550 μm outer diameters, respectively).

Figure 5.

a) Current waveforms of increasing amplitude applied to a 550 μm outer diameter microelectrode. b) Maximum measured cathodic current for each intended stimulation current amplitude, for three ring-shaped microelectrodes of each outer diameter. c) Voltage needed to drive 300 μA for each sample, and d) Impedance magnitude at 1 kHz for each sample after 1000 pulses at each stimulation amplitude.

Figure 6.

a) Photograph of a microelectrode array taped to a glass rod for bending stress testing. Impedance spectra measured after b) tensile, and c) compressive bending cycles. Changes in impedance magnitude at 10 Hz for d) tensile and e) compressive bending. Each series represents one individual sample. Separate samples are used for tensile and compressive bending tests.

Figure 7.

Responses of fully encapsulated microelectrodes with 550 μm outer diameters to a) tensile, and b) compressive bending stress. Each series represents the response of a single microelectrode, and different sets of microelectrodes are used for each bend direction.