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General Student Poster Award Winner, 1st Place)
Organic Electrochemical Transistors for High-Performance Electrocorticography Recording
Authors: Khoury Fadi , Saleh Sahera , Badawe Heba , Obeid Makram , Khraiche Massoud 
Published: August 2024

Instrumentation used in biopotential recordings is paramount in ensuring correct representation of electrophysiological activity, affecting medical diagnosis and intervention. Additionally, high signal fidelity challenges the engineering of future neural interfaces, requiring high Signal to Noise Ratios (SNR), high spatial and temporal resolution, and low-cost. Recently, transistor-based technologies have garnered increased attention and are being used as bio-electronic interfaces due to their inherent amplification capabilities. Specifically, Organic Electrochemical Transistors (OECTs) stand at the forefront of this technological shift given their low voltage operation, miniature size, biocompatibility, and high transconductance (gm), a measure of amplification. Within an electrolyte environment, OECT channel conductivity is modulated by the charge carrier concentration, changing with the voltage level inside the medium, which, if tied to a biopotential, can be concluded by measuring the drain current.

In this work, we designed and developed an OECT-based neural interface employing an in-plane gate, pioneering its use in-vivo to guarantee operation at peak gm. For device fabrication, we employed inkjet printing, thus avoiding cumbersome multi-step cleanroom microfabrication, reducing costs and facilitating production.

OECTs were fabricated following a three-step fabrication process. First, gold nanoparticle ink was used to pattern the source, drain, and gate on a polyimide backbone. Next, polyimide ink was deposited over the connecting leads for insulation, and finally, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was layered to form the semiconducting channel. The devices’ electrical characteristics were assessed in a phosphate buffer solution (PBS) with either the gold in-plane electrode or an Ag/AgCl probe as the gate. To achieve efficient in-plane gating, the gold electrode was modified with PEDOT:PSS layering to decrease the voltage drop across the gate/electrolyte interface and minimize spatial footprint. To showcase the use and effectiveness of including an in-plane gate, artificial hippocampal neuronal spikes were fed to the OECT at different gate biases. The in-vivo performance of the OECT was evaluated by implanting it onto the cortex of a rat model and assessing its efficacy in recording electrocorticographic (ECoG) activity. Furthermore, low impedance PEDOT:PSS-coated gold passive electrodes were fabricated adjacent to the transistor’s channel for performance comparison. Seizures were induced via intra-amygdala injection of kainic acid (KA), and recordings were acquired from the sensory-motor cortex.

Inkjet-printed OECTs realized in this work had a channel length of 15µm, a width of 130µm and a channel thickness of 250nm. To efficiently gate the device, the voltage drop across the gate/electrolyte interface must be negligible compared to the one experienced at the channel. This can be achieved by increasing the channel-relative gate capacitance, and we show that PEDOT:PSS coating of the electrode efficiently minimizes the drop compared to increasing its size, which is favorable in a spatially limited application like ECoG recordings. In addition, the developed OECTs achieved a high gm of 14mS, a quick response time of 0.5ms, and a compact channel area of 1950µm2, setting our OECT apart from existing high performing devices in literature. In addition, we highlight the importance of adequate material selection for source and drain metals and channel thickness in controlling the operational bandwidth of the device, given that high frequency oscillations, which are typically seen in seizure activity, present a challenge to OECTs that experiencing intrinsic gm drops at high frequencies. Upon comparison of gold and silver nanoparticle inks, results show that the usage of gold increases cut-off frequency for both thick and thin channels. Furthermore, we demonstrate that applying a cathodic potential through an in-plane gate placed in proximity to the channel ensures operation at peak transconductance where amplification is at its highest. At optimal operation, OECT peak amplitudes increased by up to 133% compared to the absence of the gate, accompanied by an increase in SNR. In vivo, OECTs outperform low-impedance passive electrodes and successfully record distinct seizure features, mainly sharp spike-wave discharges, and bursts, achieving a high SNR of 48dB.

Next generation neural interfaces must provide high fidelity and localized recordings. Electrochemical solutions arise as a promising candidate to replace conventional passive electrodes, offering high resolution and SNR. In this work, we designed and built low-cost OECTs using inkjet printing, coupled with an in-plane gate to maximize performance. The developed sensor is employed in vivo and shown to outperform low-impedance passive electrodes by recording distinct seizure phases. This technology shows great promise to develop into a general-purpose active recording component in the diagnosis and monitoring of neurological disorders or other applications in biopotential sensing.