Search

We examine the factors controlling the neuromorphic response of organic electrochemical transistors (OECTs). In these devices, the gate voltage is used to supply the pre-synaptic input, while the drain current is taken as the post-synaptic output. We analyze the behavior of devices made from polymer blends of a mixed conductor (PEDOT:PSS) and an ion conductor (PSSNa). The experimental results highlight that the post-synaptic response timescale depends on the size of ion in the electrolyte. Modeling shows that the neuromorphic response is controlled by the transient response of the ionic circuit of the OECT. These insights on device response time pave the way for a more rational design of OECT-based neuromorphic devices.

Organic electrochemical transistors (OECTs) have been widely used as transducers in electrophysiology and other biosensing applications. Their identifying characteristic is a transconductance that increases with channel thickness, and this provides a facile mechanism to achieve high signal amplification. However, little is known about their noise behavior. Here, we investigate noise and extract metrics for the signal-to-noise ratio and limit of detection in OECTs with different channel thicknesses. These metrics are shown to improve as the channel thickness increases, demonstrating that OECTs can be easily optimized to show not only high amplification, but also low noise., This work was funded by the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 732032 (BrainCom) the King Abdullah University of Science and Technology (KAUST) Office of sponsored Research (OSR) under Award No. OSR-2016-CRG5-3003. Authors also acknowledge funding from the 2DTecBio (FIS2017-85787-R) funded by the Spanish Ministry of Science, Innovation and Universities, the State Research Agency (AEI), and the European Regional Development Fund.

Understanding how the nervous system functions has always been one of the most difficult challenges the medical community has faced. Despite the progress made so far, the ability of scientists to treat brain disorders has been severely limited by the complexity of the nervous system and the quality of the information derived from recording devices. A device that holds great potential for recording high quality electrophysiology signals is the organic electrochemical transistor (OECT). OECTs have been fabricated onto flexible substrates from biocompatible materials and have been shown to provide higher signal-to-noise ratio (SNR) than electrodes. Their unique properties can pave the way for enhanced performance neural interfaces whilst minimising the invasiveness of the recording method. In this study, a thorough analysis of the noise characteristics of OECTs was performed with the greater aim of utilising these findings to further improve the performance of OECTs in various in vitro and in vivo environments. From an engineering perspective, different parameters such as the device geometry and the thickness of the channel film were tuned to minimise the intrinsic noise of an OECT. OECTs with different overlap between the conducting polymer and the source-drain gold contacts were fabricated. It is shown that the contact overlap reduced the frequency response of OECTs without affecting the noise level, suggesting that a good Ohmic contact is achieved. The effect of the thickness of the polymer film was also studied, indicating that noise can be further reduced as a function of increased channel thickness. This effect was observed up to a specific polymer thickness, above which the noise level reached a plateau. These findings allow to design OECTs that exhibit record low noise and lead to the establishment of new design rules. To further understand the origins of noise in OECTs, we applied these design rules and tested the performance of our devices under different bias conditions. It has been shown that the voltage applied between the three terminals of an OECT can modify the signal amplification, rendering xiv an optimisation on the biasing conditions imperative for high quality recordings. By employing different bias conditions, we managed to combine maximum signal amplification with minimum noise, leading to an increased SNR. Our results highlight that the optimum operation point is achieved at zero gate voltage. Finally, an analysis of the performance of our low noise OECTs in an in vitro and an in vivo application was performed. The in vitro application consisted of a simulated neuron configuration that was employed to test the capability of OECTs to record low amplitude, high frequency signals. It is shown that OECTs have the potential to record a variety of neurological signals, including action potentials. The application of OECTs in electrocorticography (ECoG) recordings was also explored. By using devices that consisted of both transistors and electrodes, we collected and compared the quality of epidural recordings. OECTs were found to outperform electrodes and the quality of the recordings was significantly high, even with minimal filtering. Our findings demonstrate that OECTs are an ideal candidate for chronical neurological recordings due to their ability to provide consistent high signal quality in a minimally invasive set up.

Organic electrochemical transistors (OECTs) from poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) are used as amplifying transducers for bioelectronics. Although the impact on performance of device geometry parameters such as channel area and thickness has been widely explored, the overlap between the semiconductor film and the source and drain contacts has not been considered. Here we vary this overlap and explore its impact on transconductance and noise. We show that increasing contact overlap does not alter the magnitude of the steady-state transconductance but it does decreases the cut-off frequency. Noise was found to be independent of contact overlap and to vary according to the charge noise model. The results show that high-quality contacts can be established in PEDOT:PSS OECTs with minimal overlap.

Transistor-based ion sensors have evolved significantly, but the best-performing ones rely on a liquid electrolyte as an internal ion reservoir between the ion-selective membrane and the channel. This liquid reservoir makes sensor miniaturization difficult and leads to devices that are bulky and have limited mechanical flexibility, which is holding back the development of high-performance wearable/implantable ion sensors. This work demonstrates microfabricated ion-selective organic electrochemical transistors (OECTs) with a transconductance of 4 mS, in which a thin polyelectrolyte film with mobile sodium ions replaces the liquid reservoir. These devices are capable of selective detection of various ions with a fast response time (≈1 s), a super-Nernstian sensitivity (85 mV dec-1 ), and a high current sensitivity (224 µA dec-1 ), comparing favorably to other ion sensors based on traditional and emerging materials. Furthermore, the ion-selective OECTs are stable with highly reproducible sensitivity even after 5 months. These characteristics pave the way for new applications in implantable and wearable electronics.

Funder: King Abdullah University of Science and Technology; Id: http://dx.doi.org/10.13039/501100004052, Organic electrochemical transistors (OECTs) are widely used as amplifying transducers of biological signals due to their high transconductance and biocompatibility. For implantable applications that penetrate into tissue, OECTs need to be integrated onto narrow probes. The scarcity of real estate necessitates the use of small local gate electrodes and narrow interconnects. This work shows that both of these factors lead to a decrease in the maximum transconductance and an increase in gate voltage required to attain this maximum. This work further shows that coating the gate electrode with a thick conducting polymer improves performance. These findings help guide the development of efficient OECTs on implantable probes.