Engineering Low Noise Organic Electrochemical Transistors for Electrophysiology Applications
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 transistor, bioelectronics, noise analysis, electrophysiology
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