Functional Biomembranes in Organical Electrochemical Transistor—Analysis, Modelling and Working Regimes

In bioelectronics, the rise of electrolyte-gated transistors, based on conducting polymers (CPs), enabled the development of efficient platforms interfacing with biological tissues¹. Recently, thanks to the development of inexpensive, tuneable and energy efficient devices, organic neuromorphic engineering emerged as promising approach to overcome conventional silicon technology limitations. In particular, PEDOT:PSS-based organic electrochemical transistors (OECTs), have come to play an important role for their natural ionic-to-electronic signal transduction and biocompatibility. In addition, the intrinsic bulk capacitance changes, resulting from the doping/de-doping processes occurring upon the application of electrical signals, endows such devices with increased sensitivity and possibility to operate at low voltages².<br/>To date, the operation mechanism of numerous OECTs has been widely studied: top and planar gating configurations were investigated, along with polarizable and non-polarizable gate materials³. On the other hand, their combination with biological systems makes it difficult to obtain a proper model able to incorporate complex phenomena like diffusion and surface reactions.<br/>Importantly, supported lipid bilayers (SLBs), widely employed to study membrane functions and transmembrane proteins’ activities⁴, are often coupled with OECTs and have been studied and modelled through passive electrical models⁵. This modelling is crucial in biosensing applications, however there is a lack of studies of dynamical modelling of OECTs coupled to such membranes.<br/>In this regard, the present work aims to study and model the physics and the operation regimes of OECTs coupled with SLBs, investigating how the presence and the position of such biomembranes affect the behaviour of transistors fabricated on different inorganic conductors. In fact, PEDOT:PSS is patterned on both gold and indium tin oxide (ITO) substrates. The two obtained architectures are characterized and differences in terms of response time and ion diffusion inside the bulk of the CP channel are highlighted. An SLB formed by POPC and a brain cell-like SLB composition are presented and characterized through impedance measurements and pulsed transistor operations, particularly relevant in neuromorphic operation regime of the above-mentioned devices. As a result, the two examined SLBs compositions lead to different modulations of the ionic circuit of the devices, in terms of response time and amplitude of the transistor channel current. This ultimately leads to the computation of a model of the SLB-OECT system, able to depict complex ionic interactions between CPs and phospholipid membranes. In contrast to passive electrical modelling of the bilayers, this approach allows to characterize and exploit the biological tissue itself as a tool to tune the electrical properties of an organic electrochemical transistors, at the same time enhancing the biomimetic potential of the CP-based electronics.<br/>1. Mariano, A., Lubrano, C., Bruno, U., Ausilio, C., Dinger, N. B. & Santoro, F. Advances in Cell-Conductive Polymer Biointerfaces and Role of the Plasma Membrane. <i>Chem. Rev.</i> (2021).<br/>2. Friedlein, J. T., McLeod, R. R. & Rivnay, J. Device physics of organic electrochemical transistors. <i>Org. Electron.</i> <b>63,</b> 398–414 (2018).<br/>3. Tan, S. T. M., Giovannitti, A., Melianas, A., Moser, M., Cotts, B. L., Singh, D., McCulloch, I. & Salleo, A. High-Gain Chemically Gated Organic Electrochemical Transistor. <i>Adv. Funct. Mater.</i> <b>31,</b> 2010868 (2021).<br/>4. Pappa, A.-M., Liu, H.-Y., Traberg-Christensen, W., Thiburce, Q., Savva, A., Pavia, A., Salleo, A., Daniel, S. & Owens, R. M. Optical and Electronic Ion Channel Monitoring from Native Human Membranes. <i>ACS Nano</i> <b>14,</b> 12538–12545 (2020).<br/>5. Zhang, Y., Inal, S., Hsia, C.-Y., Ferro, M., Ferro, M., Daniel, S. & Owens, R. M. Supported Lipid Bilayer Assembly on PEDOT:PSS Films and Transistors. <i>Adv. Funct. Mater.</i> <b>26,</b> 7304–7313 (2016).