Sputtered porous Pt for wafer-scale manufacture of low-impedance flexible microelectrodes

Bo Fan,Caleb Kemere,Alexander V Rodriguez,Daniel G Vercosa,Jacob T Robinson

doi:10.1088/1741-2552/ab965c

Bo Fan, Caleb Kemere + Show 3 more

Open Access

https://doi.org/10.1088/1741-2552/ab965c

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Abstract

Objective. Recording electrical activity from individual cells in vivo is a key technology for basic neuroscience and has growing clinical applications. To maximize the number of independent recording channels as well as the longevity, and quality of these recordings, researchers often turn to small and flexible electrodes that minimize tissue damage and can isolate signals from individual neurons. One challenge when creating these small electrodes, however, is to maintain a low interfacial impedance by applying a surface coating that is stable in tissue and does not significantly complicate the fabrication process. Approach. Here we use a high-pressure Pt sputtering process to create low-impedance electrodes at the wafer scale using standard microfabrication equipment. Main results. We find that direct-sputtered Pt provides a reliable and well-controlled porous coating that reduces the electrode impedance by 5–9 fold compared to flat Pt and is compatible with the microfabrication technologies used to create flexible electrodes. These porous Pt electrodes show reduced thermal noise that matches theoretical predictions. In addition, we show that these electrodes can be implanted into rat cortex, record single unit activity, and be removed all without disrupting the integrity of the coating. We also demonstrate that the shape of the electrode (in addition to the surface area) has a significant effect on the electrode impedance when the feature sizes are on the order of tens of microns. Significance. Overall, porous Pt represents a promising method for manufacturing low-impedance electrodes that can be seamlessly integrated into existing processes for producing flexible neural probes.

Highlights

Microelectrodes for recording neural activity have been used for decades, and are commonplace for applications including basic neuroscience research[1] and clinical diagnosis[2]
We compared the impedance of porous and flat electrodes to each other at 1 kHz, because most neural activity is especially strong at this frequency and it is well established for comparing impedance values reported in literature
Here, we demonstrated a new method for direct sputtering of a porous platinum electrode that shows a 5-9 fold impedance reduction compared to flat platinum

Summary

Introduction

Microelectrodes for recording neural activity have been used for decades, and are commonplace for applications including basic neuroscience research[1] and clinical diagnosis[2]. As the size of electrode sites decrease, total impedance of the electrode will naturally increase: at the interface between the electrode and the brain ( known as the electrode-electrolyte interface), a double layer of polarized ions separates the electrode from the brain, resulting in a capacitive interface[9]. This interface is often modeled as a “Randles cell”, with a capacitor and resistor in parallel representing the double layer capacitance and faradaic resistance of the interface, respectively[9,10]. In developing high-channel count small electrodes, the geometric area of the electrode sites is constrained by the implant size and the desired distance between electrodes, the most common solution is to increase the effective surface area of the electrode sites by adding volumetric conductive polymers[13,14], rough or porous materials[6,7,15,16,17], or materials with 3D topography[18,19,20,21]

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