Transparent, Low‐Impedance Inkjet‐Printed PEDOT:PSS Microelectrodes for Multimodal 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 charge‐transfer properties. Herein, microelectrodes are demonstrated that take advantage of the transparency and volumetric capacitance of 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.

Polymer Skulls With Integrated Transparent Electrode Arrays for Cortex‐Wide Opto‐Electrophysiological Recordings

“…Further, we demonstrated that the polyethylene terephthalate (PET) film, the transparent window material in the polymer skulls, can be used as a substrate to pattern inkjet‐printed transparent electrodes and interconnects. [ 31 ] Combining these capabilities, we developed the eSee‐Shell, a fully‐integrated transparent polymer skull with ten inkjet‐printed ECoG electrodes ( Figure ).…”

“…Printed PEDOT:PSS interconnects and electrodes were ≈200 nm thick, as previously reported. [ 31 ] The devices were encapsulated in Parylene‐C, and then, an oxygen plasma etch was used to create electrode contact openings in the Parylene‐C. The unique electrode geometry used in eSee‐Shells creates a low‐impedance and transparent interface by using a large volume of encapsulated PEDOT:PSS spread laterally around a small ≈50 µm contact site.…”

“…The unique electrode geometry used in eSee‐Shells creates a low‐impedance and transparent interface by using a large volume of encapsulated PEDOT:PSS spread laterally around a small ≈50 µm contact site. The extra volume of PEDOT:PSS surrounding the contact site acts to reduce the channel impedance, [ 31 ] without diminishing transparency with the use of thicker PEDOT:PSS films. In addition, as this electrode geometry leaves the majority of the PEDOT:PSS electrode encapsulated, and thus, mechanically reinforced by the encapsulation layer, the electrodes are protected against fracture and delamination.…”

“…The mechanical robustness of these electrodes was previously demonstrated through repeated uniaxial bending to a radius of 2 mm. [31] The fully assembled implant, including the titanium headplate, only weighs ≈1.4 g which is well below the weight that mice can tolerate for chronically implanted devices (generally considered to be 15% of the mouse's body weight). [37] A photomicrograph of a completed electrode array is shown in Figure 1b.…”

Fully Desktop Fabricated Flexible Graphene Electrocorticography (ECoG) Arrays

“…Further, most neuroscience laboratories require rapid and flexible design alterations to adapt to various experimental contexts, which is hard to achieve in traditional microfabrication procedures. To simplify the fabrication procedure, inkjet printing conductive materials such as silver nanoparticle inks [9] and conductive polymers like PEDOT:PSS [10] has been used to create flexible and reconfigurable ECoG electrode arrays. Currently, these approaches require expensive printers and still rely on specialized or microfabrication techniques to insulate the electrode [11].…”