PEDOT:PSS Wire: A\nTwo-Terminal Synaptic Device for\nOperation in Electrolyte and Saline Solutions

Synaptic devices, which are designed to emulate the synaptic\nfunctions\nof neurons, have recently gained attention as key elements in the\ndevelopment of neuromorphic hardware. To date, most synaptic devices\nutilizing conductive polymer materials, particularly poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)\n(PEDOT:PSS), have been designed as three-terminal devices. Nevertheless,\na recent study revealed that a single PEDOT:PSS wire can function\nas a two-terminal synaptic device through additional polymerization,\nwhich creates asymmetry in the wire diameter between the anode and\ncathode. Owing to its high biocompatibility, PEDOT is considered a\npromising candidate for use in clinical information-processing devices.\nHowever, previous studies examined the synaptic function of PEDOT:PSS\nonly in PSS solutions. Therefore, the performance of PEDOT:PSS wires\nin other solutions, such as physiological saline solutions, remains\nunknown. Herein, we investigated the effects of operating environmental\nconditions (including phosphate-buffered saline (PBS)) on the synaptic\nfunctions of the asymmetric PEDOT:PSS wire. Our results indicate that\nthe synaptic conductance change in the PEDOT:PSS wire occurred in\nall investigated aqueous electrolyte solutions. Moreover, we revealed\nthe relationship between the synaptic conductance change behavior\nand the molecular properties of the electrolyte ions. Furthermore,\nthe waveform of the conductance change can be controlled by adjusting\nthe conditions for wire asymmetrization. These results demonstrate\nthat the PEDOT:PSS wire exhibits a synaptic conductance change, yielding\na waveform suitable for machine learning, even under wet conditions\n(i.e., in any electrolyte solution, including PBS). Therefore, PEDOT:PSS\nwire is a promising material for two-terminal synaptic devices applicable\nin clinical studies.