Paschalis Gkoupidenis

Neuromorphic Functions in PEDOT:PSS Organic Electrochemical Transistors.

UNLABELLED Depressive short-term synaptic plasticity functions are implemented with a simple polymer poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) ( PEDOT PSS) organic electrochemical transistor device. These functions are a first step toward the realization of organic-based neuroinspired platforms with spatiotemporal information processing capabilities.

Synaptic plasticity functions in an organic electrochemical transistor

Synaptic plasticity functions play a crucial role in the transmission of neural signals in the brain. Short-term plasticity is required for the transmission, encoding, and filtering of the neural signal, whereas long-term plasticity establishes more permanent changes in neural microcircuitry and thus underlies memory and learning. The realization of bioinspired circuits that can actually mimic signal processing in the brain demands the reproduction of both short- and long-term aspects of synaptic plasticity in a single device. Here, we demonstrate the implementation of neuromorphic functions similar to biological memory, such as short- to long-term memory transition, in non-volatile organic electrochemical transistors (OECTs). Depending on the training of the OECT, the device displays either short- or long-term plasticity, therefore, exhibiting non von Neumann characteristics with merged processing and storing functionalities. These results are a first step towards the implementation of organic-based neuromorphic circuits.

Neuromorphic device architectures with global connectivity through electrolyte gating.

Information processing in the brain takes place in a network of neurons that are connected with each other by an immense number of synapses. At the same time, neurons are immersed in a common electrochemical environment, and global parameters such as concentrations of various hormones regulate the overall network function. This computational paradigm of global regulation, also known as homeoplasticity, has important implications in the overall behaviour of large neural ensembles and is barely addressed in neuromorphic device architectures. Here, we demonstrate the global control of an array of organic devices based on poly(3,4ethylenedioxythiophene):poly(styrene sulf) that are immersed in an electrolyte, a behaviour that resembles homeoplasticity phenomena of the neural environment. We use this effect to produce behaviour that is reminiscent of the coupling between local activity and global oscillations in the biological neural networks. We further show that the electrolyte establishes complex connections between individual devices, and leverage these connections to implement coincidence detection. These results demonstrate that electrolyte gating offers significant advantages for the realization of networks of neuromorphic devices of higher complexity and with minimal hardwired connectivity.

Orientation selectivity in a multi-gated organic electrochemical transistor

Neuromorphic devices offer promising computational paradigms that transcend the limitations of conventional technologies. A prominent example, inspired by the workings of the brain, is spatiotemporal information processing. Here we demonstrate orientation selectivity, a spatiotemporal processing function of the visual cortex, using a poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) organic electrochemical transistor with multiple gates. Spatially distributed inputs on a gate electrode array are found to correlate with the output of the transistor, leading to the ability to discriminate between different stimuli orientations. The demonstration of spatiotemporal processing in an organic electronic device paves the way for neuromorphic devices with new form factors and a facile interface with biology.

Orientation selectivity with organic photodetectors and an organic electrochemical transistor

Neuroinspired device architectures offer the potential of higher order functionalities in information processing beyond their traditional microelectronic counterparts. Here we demonstrate a neuromorphic function of orientation selectivity, which is inspired from the visual system, with a combination of organic photodetectors and a multi-gated organic electrochemical transistor based on poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The device platform responds preferably to different orientations of light bars, a behaviour that resembles orientation selectivity of visual cortex cells. These results pave the way for organic-based neuromorphic devices with spatially correlated functionalities and potential applications in the area of organic bioelectronics.

Neuromorphic devices based on organic mixed conductors (Conference Presentation)

Neuromorphic devices and architectures offer novel ways of data manipulation and processing, especially in data intensive applications. At a single device level, various forms of neuroplasticity have been emulated over the past years, mainly with inorganic devices. The implementation of neuroplasticity functions with these devices also enabled applications at a circuit level related to machine learning such as feature or pattern recognition. Although the field of organic-based neuromorphic devices and circuits is still at its infancy, organic materials may offer attractive features for neuromorphic engineering. Over the past years for example, a few simple neuromorphic functions have been demonstrated with biological substances and bioelectronic devices. In this work various neuromorphic devices will be presented that are based on organic mixed conductors, materials that are traditionally used in organic bioelectronics. A prominent example of a device in bioelectronics that exploits mixed conductivity phenomena is the organic electrochemical transistor (OECT). Devices based on OECTs show volatile and tunable dynamics suitable for the emulation of short-term synaptic plasticity functions. Chemical synthesis allows for the introduction of non-volatile phenomena suitable for long-term memory functions. The device operation in common electrolyte permits the definition of spatially distributed multiple inputs at a single device level. The presence of a global electrolyte in an array of devices also allows for the homeostatic or global control of the array. Global electrical oscillations can be used as global clocks that frequency-lock the local activity of individual devices in analogy to the global oscillations in the brain. Finally, “soft” interconnectivity through the electrolyte can be defined, a feature that paves the way for parallel interconnections between devices with minimal hard-wired connections.

Organic neuromorphic devices based on electrochemical concepts (Conference Presentation)

Neuroinspired device architectures offer the potential of higher order functionalities in information processing beyond their traditional microelectronic counterparts. In the actual neural environment, neural processing takes place in a complex and interwoven network of neurons and synapses. In addition, this network is immersed in a common electrochemical environment and global parameters such as ionic concentrations and concentrations of various hormones regulate the overall behaviour of the network. Here, various concepts of organic neuromorphic devices are presented based on organic electrochemical transistors (OECTs). Regarding the implementation of neuromorphic devices, the key properties of the OECT that resemble the neural environment are also presented. These include the operation in liquid electrolyte environment, low power consumption and the ability of formation of massive interconnections through the electrolyte continuum. Showcase examples of neuromorphic functions with OECTs are demonstrated, including short-, long-term plasticity and spatiotemporal or distributed information processing.