A Single vOECT Architecture Integrating Bio-Hybrid Synapses and Spiking Circuits for Replicating Neuron Specialisation

Signal communication mechanisms within the human body rely on the transmission and modulation of action potentials. Replicating these interdependent functions with artificial neurons and biohybrid synapses is essential to merge neuromorphic circuits and biological systems¹. Traditional brain-inspired systems such as spiking neural networks (SNNs)^2,3 lack ionic conduction (interaction with aqueous electrolytes) which limits their level of bio-realism and impede a two-way communication with the central nervous system. Although biohybrid synapses have replicated essential biological functions such as association and conditioning, they are limited to operate within a restrict range of biorelevant molecules and still suffer from molecular crosstalk when interfaced to a complex biological scenario^1,4,5. On the other hand, anti-ambipolar OECTs has recently allowed to design HH spiking circuits with unprecedent stability and neural characteristic fidelity⁶. However, a seamless communication with the central nervous system requires an unsurpassed level of bio-realism that only OMIECs and their manipulation promise with devices that capture neurotransmitter signaling at physiological levels (synapses) but also closely match the frequency, amplitude and activation threshold of spikes (neurons), attaining a bio-plausible power consumption and replicating neurons specialisation. Here, we developed a novel vertical electrochemical transistor vOECT realised by connecting in series two vOECTs: a layer of PEDOT:PSS is electrodeposited on the top electrode of a BBL-based antiambipolar vOECT.. The top PEDOT:PSS vOECT can work as a bio-hybrid synapse, and due to the vertical architecture it operates in “physiological conditions” replicating dopamine-mediated plasticity functions in the nM range. We analyzed the PEDOT:PSS film formation with in-situ techniques, i.e. monitoring the film formation and growth with optical spectroscopy, EQCM and GIWAXS. We provide fundamental insights on the film formation mechanisms and the relationship cycle number/ structure / performance thus achieving a full control of the optoelectronic characteristics of this device. Moreover, we studied the film formation in case the alternative stabilizing agent NaClO4 is employed, instead of the conventional PSSNa, providing further structure/property understanding. As such, we achieved a full control of the PEDOT-based vOECTs threshold voltage and response time, by selecting the stabilizing agent and the number of deposition cycles. Hence, being the PEDOT vOECT connected to the top electrode of the antiambipolar BBL vOECT, we realized a single architecture which replicates the neural Na⁺ channel, allowing to translate physiological changes in the concentrations of dopamine and serotonin in the electrolyte environment directly into spikes encoded signals since we also integrated the whole device into a single-chip Hodgkin-Huxley spiking circuit. As such, we developed physiological bio-hybrid synapses intimately connected to their neurons demonstrating different sensitivity to electroactive neurotransmitters and different neuronal threshold potentials depending on the morphology of the electrodeposited PEDOT, replicating the high specialization of the central nervous system neurons. This system constitutes a building block for programmable neural pathways, for locally executing bio-inspired pre-processing functions, thus allowing for a seamless and dynamic communication with the nervous system.

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8 Tropp, J. et al. Matter 6, 3132–3164 (2023)