Microelectrode Arrays based on Transfer-free Multilayer Graphene and PEDOT:PSS for Neural Activity Detection

Abstract

Resolving the underlying mechanisms of complex brain functions and associated disorders remains a major challenge in neuroscience, largely due to the difficulty in mapping large-scale neural network dynamics with high temporal and spatial resolution. Multimodal neural platforms that integrate optical and electrical modalities offer a promising solution, with microelectrode arrays (MEAs) being particularly effective for capturing electrophysiological activity across multiple neurons at the cellular level. Graphene has emerged as a highly attractive material for such neural interfaces due to its unique combination of biophysical, electrical, mechanical, biological, and optical properties. However, current graphene-based electrodes face challenges, including the need for transferring pre-grown graphene layers, which causes reliability issues.

In this work, microelectrode arrays based on transfer-free multilayer graphene and PEDOT:PSS are developed, and their feasibility for in vitro neural activity detection is evaluated through a brain slice culture. Firstly, the design and fabrication of graphene-based MEAs (grMEA) on transparent substrates is presented, establishing optimized microfabrication procedures for the integration of transfer-free multilayer graphene technology on in vitro MEAs. Several electrode diameters, ranging from 10 μm to 500 μm, are included to accommodate different experimental requirements and test the limits of the technology. Second, the combination of graphene and PEDOT:PSS conductive polymer is explored to overcome the fundamental limitations associated with graphene electrodes, such as high impedance at small electrode sizes, without compromising the optical transparency. A technique to integrate patterned PEDOT:PSS polymer coating on the electrodes to form graphene/PEDOT:PSS microelectrodes
(grppMEA) is developed.

The electrochemical performance, including charge storage capacity (CSC) and impedance properties, of the grMEA and grppMEA devices are evaluated. Obtained results, comparable with state-of-the-art neural interfaces, show that the PEDOT:PSS coating increases CSC values more than the double and substantially reduces the impedance by 10-20 times (e.g., from 247 kΩ to 13 kΩ for 50 μm diameter electrodes). This demonstrates the potential of the devices for efficient electrical coupling with neural tissue. Additionally, the volumetric capacitance of grppMEA electrodes was found to be 55.7 F/cm3, highlighting the volumetric ionic-electronic interaction coming from the PEDOT:PSS film, a feature absent in metal electrodes.

A custom-made interface that effectively connects the MEA devices to specific recording systems is developed to enable the acquisition of neural signals. The electrophysiological recording capabilities of the grMEA and grppMEA devices were subsequently evaluated through in vitro cerebellar brain slice cultures. Acute recordings of spontaneous activity and spike pattern characteristics of Purkinje cells and other neurons have been successfully obtained. The obtained signal-to-noise ratio values, ∼ 30 dB, demonstrate the great recording potential of the proposed MEAs. Overall, the results presented in this work demonstrate that transfer-free multilayer graphene MEA technology, especially when combined with PEDOT:PSS, overcomes the current limitations and offers the possibility for high-density recordings with single-cell resolution.