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

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.

Impedance scaling for gold and platinum microelectrodes

“…Fabrication: All the electrodes are fabricated using a similar method as previously reported 12 . We first sputtered a release layer of 70 nm Al on a Si wafer(University Wafer).…”

“…The important trade-offs in performance as a function of electrode size, makes it important to understand how the impedance scales with the electrode area, particularly for ultra-small electrodes. For planar electrodes it is typically assumed that impedance is inversely proportional to the electrode area 12,14 and thus electrode materials are typically characterized by a specific impedance (impedance times area) that is assumed to be a constant for that material. For volumetric conductors, like poly(3,4‐ethylenedioxythiophene)(PEDOT) the impedance scales inversely with the volume of the electrode coating 15 .…”

“…Recently we noted deviations from the linear relationship between impedance and electrode area when investigating planar and porous Pt electrodes which became pronounced for small feature sizes 12 . Here, we systematically study the impedance of Pt and Au electrodes as a function of size from diameters ranging from 3 - 125 µm to see if the impedance deviates from the expected linear relationship and what mechanism may explain this deviation.…”

“…This high impedance can decrease the signal-to-noise ratio(SNR) in two ways. First, it leads to higher thermal noise, which increases as impedance increases 11,12 . Second, it creates a voltage divider, which consists of the electrode and the recording system in series [11][12][13] .…”

“…First, it leads to higher thermal noise, which increases as impedance increases 11,12 . Second, it creates a voltage divider, which consists of the electrode and the recording system in series [11][12][13] . As a result, higher impedance above 2-5 MΩ 11,13 can cause greater signal attenuation.…”