An organic artificial spiking neuron for in situ neuromorphic sensing and biointerfacing

The effective mimicry of neurons is key to the development of neuromorphic electronics. However, artificial neurons are not typically capable of operating in biological environments, which limits their ability to interface with biological components and to offer realistic neuronal emulation. Organic artificial neurons based on conventional circuit oscillators have been created, but they require many elements for their implementation. Here we report an organic artificial neuron that is based on a compact nonlinear electrochemical element. The artificial neuron can operate in a liquid and is sensitive to the concentration of biological species (such as dopamine or ions) in its surroundings. The system offers in situ operation and spiking behaviour in biologically relevant environments—including typical physiological and pathological concentration ranges (5–150 mM)—and with ion specificity. Small-amplitude (1–150 mV) electrochemical oscillations and noise in the electrolytic medium shape the neuronal dynamics, whereas changes in ionic (≥2% over the physiological baseline) and biomolecular (≥ 0.1 mM dopamine) concentrations modulate the neuronal excitability. We also create biohybrid interfaces in which an artificial neuron functions synergistically and in real time with epithelial cell biological membranes. An organic artificial neuron that is based on a compact nonlinear electrochemical element can operate in a liquid and responds to the concentration of biological species in its surroundings, allowing its behaviour to be modulated, for example, by interfacing with the membranes of living cells.

The effective mimicry of neurons is key to the development of neuromorphic electronics. However, artificial neurons are not typically capable of operating in biological environments, which limits their ability to interface with biological components and to offer realistic neuronal emulation. Organic artificial neurons based on conventional circuit oscillators have been created, but they require many elements for their implementation. Here we report an organic artificial neuron that is based on a compact nonlinear electrochemical element. The artificial neuron can operate in a liquid and is sensitive to the concentration of biological species (such as dopamine or ions) in its surroundings. The system offers in situ operation and spiking behaviour in biologically relevant environmentsincluding typical physiological and pathological concentration ranges (5-150 mM)-and with ion specificity. Small-amplitude (1-150 mV) electrochemical oscillations and noise in the electrolytic medium shape the neuronal dynamics, whereas changes in ionic (≥2% over the physiological baseline) and biomolecular (≥ 0.1 mM dopamine) concentrations modulate the neuronal excitability. We also create biohybrid interfaces in which an artificial neuron functions synergistically and in real time with epithelial cell biological membranes.
Neurons are the fundamental units of the nervous system and are used to transmit and process electrochemical signals. They operate in a liquid electrolyte and communicate with each other via gaps (synapses) between the axon of pre-synaptic neurons and the dendrite of post-synaptic neurons (Fig. 1a). Neuromorphic computing uses hardware-based implementations to mimic the behaviour of synapses and neurons 1 , for efficient brain-inspired computing. The approach could also be used to interface biology with electronics that share similar biocomputational primitives [2][3][4][5][6][7] . Synaptic phenomena-the gradual and activity-dependent coupling between neurons-are typically mapped onto memory devices, which can be binary, multistate or analogue. However, to emulate neuronal spiking and oscillatory dynamics, electronic oscillatory circuitry is required 8,9 . Neuron-like dynamics can be created with conventional microelectronics using oscillatory circuit topologies to mimic neuronal behaviours. For example, neuromorphic electronic circuits consisting of ring oscillators have been used for the implementation of mechanically flexible, skin-inspired electronics and neuro-inspired mechanoreceptors 10,11 . Many-element artificial neurons based on solid-state silicon or organic devices have also been reported 9,[11][12][13] . However, although these approaches can mimic specific aspects of neuronal behaviour, the integration of a large numbers of transistors and passive electronic Article https://doi.org/10.1038/s41928-022-00859-y Spin torque oscillators are magnetic nanodevices compatible with silicon technology 1,15 . Their nonlinearity and dynamics have been recently leveraged for spoken language and audible source recognition 15 . However, there is no viable route for biointerfacing with spin torque oscillators, as their oscillatory dynamics are too fast (around gigahertz frequencies) for interacting in real time with biological processes. Their operation also requires the presence of magnetic fields. Other approaches for spiking or oscillatory devices and circuits-including Mott-transition-based memristive devices 20,21 , ferroelectrics 22 , photonics 23 and two-dimensional materials 24 -have been developed, but all of them encounter similar problems. By omitting various aspects of actual biological wetware, artificial neurons based on electronics are insufficiently capable of emulating/handling the biosignal diversity and thus of operating in situ in biological environments.
Organic electrochemical devices based on organic mixed ionicelectronic conductors offer an alternative approach to neuromorphic components results in bulky biomimetic circuits that are not suitable for direct, in situ biointerfacing.
Volatile and nonlinear devices based on memristors or spin torque oscillators can be used to increase the integration density and emulating neuronal dynamics 14,15 . Metal-oxide memristive devices based on metal-insulator transitions exhibit negative differential resistance phenomena that are suitable for the emulation of neuronal dynamics 14 . Such devices with a negative differential resistance are locally active, and therefore, electrical input stimuli trigger voltage or current spikes in analogy with biological neurons. Artificial neurons based on memristive devices with a negative differential resistance have the potential for high integration density 1,16 . Nevertheless, the intrinsic sensitivity of solid-state memristive devices to moisture prevents in situ biointerfacing in biologically relevant host environments 17 . Although memristive arrays have been used for pre-and post-acquisition biosignal processing, they have not been used for in situ biointerfacing 18 Fig. 1 | A biological neuron and the OAN. a, Simplified schematic of a biological neuron. Action potentials, the basic cell-to-cell communication events, are generated by rapid transmembrane ion exchanges through ion channels, and they propagate across the axon. In myelinated cells, alternate myelin/non-myelin domains (nodes of Ranvier) contribute to the fast and long-range action potential propagation. Biological neurons are immersed in an electrochemical environment, such as an aqueous electrolyte. This extracellular space is a common reservoir containing various biological carriers for signalling and processing (ions, biomolecules and so on). Noise is also present in this environment. Ionic channels on the membrane endow neurons with ionic/ molecular specificity and recognition. b, Circuit diagram of the OAN. The main part is an OEND that displays S-shaped negative differential resistance (S-NDR) phenomena and is responsive to ionic and biomolecular species common to biological environments. The OEND consists of two OECTs, namely, T 1 and T 2 , that are connected via the R 1 = 5 kΩ and R 2 = 10 kΩ resistors in a cascade-like configuration with feedback. The OAN is formed when the OEND is connected to an RC element (R = 10 kΩ; C = 6 nF to 10 μF) and voltage source V in . Here V out and I out are the resulting output voltage and current, respectively, of the OEND under the influence of V in . G, gate; S, source; D, drain. c, Schematic of the OECT that forms the OEND. The channel of the OECT consists of an organic mixed ionic-electronic conductor (OMIEC), such as PEDOT:PSS. d, Sensing mechanism in PEDOT:PSS. Ionic or polyatomic ions interact with PEDOT:PSS and modulate the doping level and hole conductivity of PEDOT:PSS, which results in a change in the OECT drain current and threshold voltage.
Article https://doi.org/10.1038/s41928-022-00859-y electronics 25 . Organic electronics can operate in close proximity to biology due to their soft nature and the ability to directly interact with ions in aqueous electrolytes 3,26,27 . Organic synaptic transistors have developed rapidly, showing outstanding analogue memory phenomena with low-voltage operation and linearity in weight update (in contrast, nonlinear phenomena are required for the implementation of neuronal dynamics) [28][29][30][31] . In addition, organic artificial synaptic devices have demonstrated neurotransmitter-mediated plasticity when coupled with dopaminergic cells 32,33 and have also been interfaced with non-electrogenic cells 34 . However, such devices are passive elements and therefore are not able to emulate neuronal dynamics. Recently, organic electrochemical neurons have been reported 13 . However, this approach is based on many-element conventional oscillatory circuitry and does not possess the complexity in neuronal dynamics that nonlinear elements can potentially display. The integration of neuromorphic electronics with biology requires artificial synapses that can interface with biological ones, as well as artificial spiking neurons that can operate and respond to local biological signals in situ, that is, in the wet biological environment.
In this Article, we report an organic artificial spiking neuron with electrobiochemical degrees of control that enable local and in situ neuromorphic sensing and biointerfacing. The organic artificial neuron (OAN) can operate in a liquid and shows inherent biosensing primitives. It consists of a compact nonlinear electrochemical element that exhibits negative differential resistance that is sensitive to the biological environment that hosts the OAN. The OAN responds to ionic species commonly found in the extracellular space, and its spiking response is sensitive to typical physiological and pathological ionic concentration ranges (5-150 mM). Small-amplitude (1-150 mV) electrochemical oscillations and noise in the electrolytic medium shape the neuronal firing properties. Therefore, the artificial neuron exhibits spiking properties (which are stable for >10 5 spiking cycles) that depend on the local ionic, biomolecular or neurotransmitter species of the aqueous environment-behaviour that is analogous to a biological neuron that is surrounded by the extracellular space containing various biological carriers for signalling and processing (Fig. 1a). In particular, in situ changes in ionic (≥2% increase) and biomolecular (≥0.1 mM of dopamine) concentrations modulate the neuronal excitability and trigger spikes. Ion-specific oscillations for sodium (Na + ) and potassium (K + ) are also possible, providing a pathway for emulating ion channel dynamics. Furthermore, we create a biohybrid interface in which artificial neurons synergistically function with membranes of epithelial cells and where the biological membrane barrier modulates the spiking properties of the artificial neuron in real time.

Organic artificial spiking neuron
The OAN consists of a compact nonlinear building block made of only two organic electrochemical transistors (OECTs), namely, T 1 and T 2 (Fig. 1b). Both OECTs are p-type transistors: T 1 is a depletion-mode transistor, whereas T 2 is an enhancement-mode transistor. The mixed ionicelectronic conductor poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrene sulfonate) (PSS) and poly (2- ) are used for the T 1 and T 2 channel, respectively 35,36 . The electrical characteristics of T 1 and T 2 are presented in Supplementary Fig. 1. The OECTs operate in aqueous environments and are sensitive to ionic species and polyatomic ions (Fig. 1c). For instance, the channel of T 1 consists of the organic mixed ionic-electronic conductor PEDOT:PSS (ref. 26 ). Both channel and gate of an OECT are in direct contact with the electrolyte. In the case of a p-type OECT, when a positive gate voltage V G is applied, cations drift into the polymeric channel and reduce the hole concentration, and consequently, drain current I D is lowered. When a negative V G is applied, cations are removed from and anions drift into the polymeric channel, the hole concentration increases, and this results in a larger I D . In OECTs, ions can penetrate the bulk of the polymer and the volumetric nanoscale ionic-electronic charge compensation results in a large current modulation. This high gate voltage to drain current modulation yields a high transconductance, which is the hallmark of OECTs. Another key feature of OECTs is the dependence of I D on the ion concentration in the electrolyte. More precisely, the fixed charges in the ion-conducting phase of the polymeric channel are electrostatically compensated by the mobile ions provided by the electrolyte. This Donnan equilibrium results in a concentration-dependent voltage drop at the polymer/electrolyte interface, which is, in turn, mirrored by the OECT threshold voltage. As shown in Fig. 1d, ionic or polyatomic cations (anions) interact with PEDOT, a hole conductor, and PSS, an ionic conductor, and decrease (increase) the doping level of PEDOT, resulting in a decrease (increase) in I D and the OECT threshold voltage (Fig. 1d). Here T 1 and T 2 are connected in the cascade-like configuration with feedback resistors R 1 and R 2 , obtaining a two-terminal organic electrochemical nonlinear device (OEND) (Fig. 1b).
The current-voltage V(I) and voltage-current I(V) characteristics of the OEND are displayed in Fig. 2a. The OEND is accessed either in the V(I) or I(V) mode with current I or voltage V as the independent variable, respectively. An S-shaped negative differential resistance is accessible only in the V(I) mode, by applying current at the single-valued negative differential resistance characteristic. In the I(V) mode, the OEND   Fig. 3). The time response τ OEND of the OEND element for square-wave input pulses is τ OEND ≈ 11 ms ( Supplementary Fig. 3). When the OEND is coupled to an RC element (Fig. 1b) forming an OAN, its response bifurcates, producing voltage or current oscillations ( Fig. 2; load-line analysis is shown in Supplementary Fig. 4). These spike-based oscillations represent the 'action potentials' of the OAN ( Supplementary Fig. 5). Figure 2b displays the current response I out of the OAN for different values of capacitor C. The firing frequency f fir can be finely tuned between 6 and 40 Hz by changing C, a range that is consistent with physiological levels of instantaneous firing rates in biological neurons 37 . The firing frequency range can be further extended by modifying C or by using high-frequency-response OECTs 38 . In the case of polymer-based capacitors, capacitances in the range of nanofarads to microfarads can be reached with micrometre-scale PEDOT:PSS-based capacitors 26 . The parametric oscillatory response in the I−V plane and the instantaneous power dissipation P inst of the OAN is shown in Supplementary Fig. 6. The instantaneous power dissipation is given by where V in , V out and I out refer to the input voltage, output voltage and output current of the whole OAN, respectively. The calculation includes all the OAN components (for example, transistors, resistors and capacitor). The mean power dissipation P mean = 143 μW. As shown in Fig. 2c and Supplementary Fig. 7, the OAN behaves as a voltage-controlled oscillator and f fir is modulated by increasing ΔV in (ΔV in = V in − V th , where V th is the OAN oscillation threshold) within the oscillation window. For ΔV in = 0-70 mV, the relative increase in firing frequency is Δf fir /f min ≅ 18%, with a voltage-controlled oscillator sensitivity of Δf fir /f min /ΔV in ≅ 260% per volt.
The amplitude and window of the current or voltage oscillations can be precisely designed ( Supplementary Fig. 8), for instance, by engineering the threshold voltage of transistors T 1 and T 2 . In this case, the oscillatory window is shifted to a lower voltage level when decreasing the threshold voltage of T 1 by doping PEDOT:PSS with the amine-based molecular dopant N-methyl-2,2′-diaminodiethylamine (ref. 39 ), and P mean is decreased from 143 to 24 μW. For a capacitor with C = 6 nF and by neglecting the static power dissipation, the energy consumption per spike is E spike = 57 nJ. The amplitude profile of the current oscillations can also be engineered by varying R 1 and R 2 of the OAN (the effect of R 1 and R 2 on the nonlinear properties of the OEND is shown in Supplementary Fig. 2). It should be noted that doping causes permanent threshold-voltage shifts and dynamic reconfigurability can be induced by introducing synaptic transistors instead of volatile ones. In the case of PEDOT:PSS, the threshold voltage can be tuned on the fly by changing the ion concentration 40 , a phenomenon that is exploited here to obtain an ion-concentration-dependent firing frequency. The OAN displays fully consistent stability when continuously operated at various amplitude and frequency conditions, as the firing response is practically unaffected for >10 5 spiking cycles (Fig. 2d). The OAN stability as a function of the number of spikes is also evaluated. The amplitude of I out reduces by ~2.8 × 10 −5 % per spike. As a result, after 10 6 spikes, the OAN current amplitude is equal to approximately 71% of the initial amplitude.
The OAN shows the key characteristics observed in the spiking response of biological neurons. The OAN operates in a liquid, a property that is reminiscent of the extracellular environment of biological neurons in the cerebrospinal fluid. The excitability of the OAN, that is, the tendency of a neuron to fire spikes, can be modulated by the presence of electrochemical oscillations transmitted by means of ionic fluxes in the electrolytic medium. Figure 3a shows that the in-liquid electrochemical oscillations shape the firing properties of the OAN, mimicking the characteristic features of biological neurons 41 .
An increase of only a few millivolts at the potential of the electrolytic medium elicits spikes with high temporal precision ( Supplementary  Fig. 9), and a forced bursting activity is phase locked with the ionic signal in the electrolyte. During the time window of the input signal that is above the OAN threshold voltage V th , the OAN fires and therefore the input phase coordinates firing. Variation in the electrolyte potential as small as 1-2 mV at the very edge of the OAN threshold V th results in stochastic firing, thus reproducing the behaviour observed in biological neurons 42 . Such small variations in the electrolytic medium potential are in the same range of the biopotentials of the extracellular electrolytic space (microvolts to millivolts) 43 . The OAN threshold allows for additional bioplausible behaviours, such as in-liquid all-or-nothing spiking and subthreshold oscillations (Supplementary Figs. 9 and 10, respectively). The spiking properties of the OAN for input voltage pulses is shown in Supplementary Fig. 9.
Due to its finite response time, the OAN displays a stimulusresponse delay (Fig. 3b), as well as behaves as a temporal integrator (Fig. 3c). The stimulus-response delay in biological neurons, known as spike latency, can provide a rapid and efficient neural coding scheme beyond simple rate coding, as latency can be a faster differentiator than the mean firing rates 44 . We further investigate this delay and reproduce the spike latency characteristic of biological neurons. Figure 3b shows that the stimulus-firing phase difference Δφ is modulated by the input voltage difference ΔV in (time-domain response is shown in Supplementary Fig. 11). Stronger stimulation induces shorter latencies, as observed in biological sensory systems 45 . Due to the finite and relatively slow time constant of biomembranes, biological neurons temporally integrate inputs and display firing under certain interstimulus timing conditions 46 . This integration lowers the timing precision and offers temporal buffering windows for ongoing neuronal inputs, ensuring the consolidation and stability of neuronal sequences 46 . Analogous to biological neurons, the OAN integrates time, buffers input stimuli and fires for short non-overlapping interstimulus intervals (Fig. 3c).
Biological environments are characterized by seemingly random fluctuations at a range of spatiotemporal scales, and therefore, neurons are constantly operating under noisy conditions 47 . This noise couples with neuronal dynamics, influencing neuronal excitability and firing properties. Although counterintuitive, noise can be beneficial for neuronal communication and processing. For instance, noise can enable the transmission of weak subthreshold signals, smoothens subthreshold-to-threshold nonlinearities or even enhances the communication efficiency by increasing the signal-to-noise ratio or by altering the neuronal coding schemes 47 . The noise-induced activity of the OAN is presented in Fig. 3d. The OAN is biased with a d.c. input voltage at the subthreshold regime, and white noise of variable amplitude (V pp = 5−150 mV) is injected in the electrolytic medium to emulate extracellular noise/fluctuations. As the amplitude of the noise increases, a gradual transition from tonic (V pp = 0−25 mV) to irregular firing (V pp = 50 mV) is observed. For low noise levels, the frequency of tonic firing remains practically constant, that is, f fir ≅ 6.5-7.0 Hz. As the noise level increases, packets of spikes are observed. Recurrence plots of the interspike intervals and spike-to-spike amplitudes (Supplementary Fig. 12) indicate a change in the coding scheme from tonic to noise-induced bursting activity, as well as the resilience of spiking against noise in the electrolytic medium.

In situ spike-based neuromorphic sensing
It is estimated that the extracellular electrolytic space occupies a volume fraction of ~15-30% of the brain tissue. This extracellular space is an aqueous electrolyte comprising various ionic species (mostly Na + , K + , Cl − and Ca 2+ ) and represents a reservoir by maintaining homoeostatic balance of the ion concentrations under physiological conditions. In mammalian cells, the range of physiological concentrations for Na + is c ext = 130−150 mM and c int = 10−15 mM and for K + , c ext = 3−12 mM and c int = 150−160 mM, where c ext and c int are the Article https://doi.org/10.1038/s41928-022-00859-y extracellular and intracellular concentrations, respectively 48 . Although homoeostatically balanced, these ion concentrations can change in different spatiotemporal scales. Minute concentration variations in the proximal extracellular space of a neuron are induced during an action potential (fluctuations of approximately 5% for Na + and 20% for K + from physiological baselines) 49 . Many pathological conditions (for example, spreading depression, epilepsy and migraines) are manifested as homoeostatic imbalance; for instance, extracellular Na + and K + concentrations can decrease to 60 mM and increase to roughly 55 mM, respectively 50 . Moreover, the extracellular medium contributes to intercellular, non-synaptic communication via variations in the extracellular electric fields (for example, ephaptic coupling) 51 , neurotransmitter spillover (such as diffusion to adjacent synapses) 52 or via extracellular-mediated diffusion of neuromodulators 53 .
The OAN, due to its in-liquid operation, exhibits firing properties that depend on the ionic concentration of the electrolyte host. As the electrolyte concentration increases, the ionic conductivity of the electrolyte also increases; thus, T 1 (or T 2 ) has a lower response time and f fir increases 54 . Figure 4a shows the dependence of the firing waveform on the NaCl concentration (in an aqueous solution), c NaCl . Here f fir increases from 25 to >50 Hz for c NaCl = 80-150 mM, a range that is on par with common physiological and pathological extracellular Na + concentrations (Fig. 4b). This results in a relative increase in the firing frequency of Δf fir /f min ≅ 110% and agrees with the behaviour of biological neurons in which f fir increases with the extracellular Na + concentration 55 . As further confirmation, Supplementary Fig. 13 shows that the OAN also operates under the common physiological/pathological range of extracellular K + concentrations in an aqueous solution, namely, c KCl = 5-50 mM; furthermore, in this case, f fir increases with the K + concentration. The responsiveness of the OAN at biophysically relevant ionic concentration ranges is essential for the in situ operation of the OAN with biological membranes and neurons.
Changes in ionic concentration gradients between the intracellular and extracellular medium of biological neurons alter their excitability/ threshold, and firing can be initiated by varying these concentrations 56 .
As an example, Fig. 4c shows that small variations (~2-10%) in NaCl concentration over a biologically relevant baseline (100 mM NaCl) can increase the OAN excitability and induce firing. Both f fir and time delay between the increase in concentration and neuronal excitation Δt fir correlate with the ion concentration ( Supplementary Fig. 14). Moreover, the behaviour shown in Fig. 4c is in agreement with the excitation profiles for similar extracellular Na + concentrations of the Hodgkin-Huxley neuron model (Supplementary Fig. 14).
Dopamine is a modulatory neurotransmitter that regulates essential brain functions including cognition, learning, motivation, motor control, mood regulation and addiction 57 . At the cellular level, dopamine can impact neuronal excitability in a multitude of ways (resulting in excitatory or inhibitory and time-and concentration-dependent Article https://doi.org/10.1038/s41928-022-00859-y effects), both via synaptic and non-synaptic activation of dopamine receptors 58 . For example, dopamine has a net excitatory effect on primate pyramidal neurons 59 . As shown in Fig. 4d, the presence of dopamine in the electrolyte increases the excitability of the OAN and initiates tonic firing. Excitation is observed in shorter time delays Δt fir for higher dopamine concentrations, whereas a slight decrease in f fir is observed (Supplementary Fig. 15). The behaviour shown in Fig. 4d highlights that the initiation of spikes can be biochemically triggered, a property that resembles the biological neuronal signalling phenomena. It should be noted that the OAN also exhibits biorealistic diversity in signalling, as dopamine-mediated inhibition can be induced by altering the OAN biasing scheme (Supplementary Fig. 15).
The ion channels of biological membranes pass inward and outward ionic currents, a hallmark of neuronal signalling 60 . The dysregulation of these processes is the consequence of a large number of channelopathies that can lead to serious pathological conditions such as cystic fibrosis and myotonia congenita 61 . Another layer of biophysical realism is added to the OAN response by implementing on-chip selectivity and specificity characteristics, akin to biological ion channels. Figure 4e demonstrates that the OAN function directly incorporates aspects of the molecular machinery that are responsible for the selective processing of the biological carriers of information. OANs displaying ion-specific (Na + or K + ) oscillatory activities are realized by incorporating ionophore-based selective membranes 62 at the channel/ electrolyte interface of T 1 (Fig. 1b). As an example, a K + -selective OAN shows oscillations in the case of the KCl electrolyte, with f fir increasing with the ion-selected concentration, but does not show oscillations in a control experiment with NaCl electrolyte (Supplementary Fig. 16). The selectivity library can be further extended with membranes that are selective to other biologically relevant ions such as Na + , Ca 2+ and ammonium (NH 4 + ) (refs. 63,64 ).

Biohybrid neuron
The OAN is capable of direct biointerfacing in a biologically relevant environment, and a biohybrid neuron is formed by incorporating a biomembrane between the gate and channel of T 1 (Fig. 5). The system consists of a biological and artificial compartment. As a relevant biomembrane model system, the prototypical epithelial cell line Caco-2, which is a model of the intestinal epithelial barrier and widely used for in vitro toxicology and drug delivery studies [65][66][67] , is incorporated with the OAN. The biomembrane comprises epithelial cells joined with tight junctions, thus forming a natural barrier for ion passage (Fig. 5a). The biohybrid neuron functions in situ and in real time (Fig. 5b). Initially, the OAN is operated in a plain cell culture medium; in the absence of the biomembrane barrier, electrochemical oscillations are sustained at f fir ≅ 12 Hz. A barrier functionality is induced with the incorporation of the biomembrane. This biomembrane barrier blocks the gate-to-channel ion passage and suppresses the oscillations of the biohybrid neuron, with f fir ≅ 0 Hz. The addition of a chemical agent that attacks the biomembrane's tight junctions-here the toxin hydrogen neurotransmitter-induced excitability. e, Ion-selective OANs induce ion-specific oscillations, emulating the dynamics of biological ion channels in liquid and on chip (channel conductance g K , g Na ). As an example, the OAN is selective to K + ions and insensitive to Na + interfering ions. In all the experiments (ion concentration changes, exposure to dopamine and incorporation of ion-selective membranes), the sensing device is T 1 and the same electrolyte conditions (100 mM NaCl) are maintained for T 2 .
Article https://doi.org/10.1038/s41928-022-00859-y peroxide-lowers the barrier of ion passage and results in a gradual recovery of oscillations. This demonstration shows that the biomembrane dynamics change the excitability of the OAN in real time, and this is directly reflected on the firing response of the biohybrid neuron. A more detailed schematic of the biohybrid neuron and a description of the biomembrane disruption mechanism are shown in Supplementary  Fig. 17. Such biohybrid systems can be used as controllable in vitro models for basic research, such as to understand the underlying mechanisms of neuronal signalling, as well as a platform for studying the barrier integrity of biological tissues under various physiopathological conditions or the influence of external physicochemical cues (toxins, neuromodulators and so on). It should also be mentioned that interfacing the OAN with biological neurons requires that both domains have similar dimensions. Therefore, careful design of the OAN is necessary, as the device dimensions play a critical role in the spiking response of the OAN. The impact of the device dimension on the OAN spiking response with simulations is shown in Supplementary Fig. 18.

Conclusions
We have reported an OAN based on a nonlinear electrochemical element. Inspired by the properties of biological neurons functioning in wet surroundings, the OAN can mimic the biological sensitivity to ionic and biomolecular species in a surrounding aqueous environment. The artificial neuron exhibits nonlinear phenomena that depend on the composition of biophysically relevant host environments. We experimentally validated its operation with various electrolytes, including common aqueous electrolytes, buffered solutions and cell culture media. We also created biohybrid interfaces in which the OAN was modulated by the biological membrane of epithelial cells in situ and in real time. A comparison with the state-of-the-art technology is provided in Supplementary Tables 1 and 2. Neuronal excitability, dynamics and spiking properties depend on the electrolytic potential and noise, as well as on the local concentrations of specific ionic and biomolecular species. Therefore, just as in biological neurons-where sensing and actuation is merged and happens locally in the same surroundings-sensing (for example, neurotransmitters) and actuation/communication (via spiking, oscillating or other behaviours) are inherently embedded in the device operation, and this can enable tighter closed-loop control of biological substrates.
The operation in close physical, functional and temporal proximity with biology can enable the real-time interaction between artificial and biological rhythmicity, for example, the development of new strategies for understanding, restoring and augmenting biorhythmic processes.
In contrast to conventional organic ring oscillators that consist of multiple transistors, only two transistors are required for the OAN. This compactness means that the OAN can potentially be merged into a single device-a challenging venture for many-element implementations. Negative-differential-resistance-based ionoelectronics can lead to much richer dynamics compared with conventional electronics. For practical applications, the integration density and variability of soft matter devices should be further developed and improved. In addition, although the OAN is externally powered in this work, biofuel-powered and self-sustainable oscillators could be developed that emulate certain metabolic pathways of biological neurons 68 . Non-synaptic modes of neuronal communication that are found in biological networks could be introduced with global electrolytes 69,70 . Furthermore, synaptic capabilities can be introduced at the function of the OAN circuit, by incorporating organic synaptic transistors 28,31,71,72 . Finally, in the case of dopamine detection, latent 'memory time windows' can form the basis for on-chip learning phenomena, such as biomolecular reward prediction error coding 73 .

Fabrication of OENDs and OANs
Standard microscope glass slides (75 mm × 25 mm) were cleaned in a sonicated bath, first in soap solution (Micro-90 (Sigma-Aldrich)) and then in a 1:1 (vol/vol) solvent mixture of acetone and isopropanol. Gold electrodes for source and drain electrodes were photolithographically patterned (with positive Microposit S1813 photoresist (DOW)) on the cleaned glass slides. A chromium layer was used to improve the adhesion of gold. Each glass slide contains a series of circuit blocks consisting of T 1 and T 2 OECTs. The channel dimensions of T 1 and T 2 are W 1 × L 1 = 50 μm × 20 μm and W 2 × L 2 = 50 μm × 10 μm, respectively. The OECTs are separately gated with Ag/AgCl electrodes via aqueous electrolytes. Two layers of parylene C (SCS Coatings) were deposited. Soap (Micro-90 soap solution, 1% vol/vol in deionized water) was used for separation between the parylene C layers to enable the peel-off of the upper parylene C layer. The lower parylene C layer insulates the I out versus time and the corresponding time-frequency analysis of the biohybrid neuron is displayed. The presence of an ion-blocking epithelial biomembrane between the gate and channel of T 1 suppresses the neuronal excitability and spiking that is initially observed in the plain cell culture medium. Toxin-mediated opening of the biomembrane by attacking its tight junctions results in the recovery of the spiking activity over time. In the biohybrid neuron experiment, the sensing device is T 1 (that is, incorporation of the biomembrane) and keeping the same electrolyte conditions in T 2 (100 mM NaCl).
Article https://doi.org/10.1038/s41928-022-00859-y gold electrodes. A promoter (Silane A-174 (γ-methacryloxypropyl trimethoxysilane), Sigma-Aldrich) was added to the lower parylene C layer to enhance adhesion. In the second photolithography step using the positive photoresist AZ 9260 MicroChemicals (Cipec Spécialités), the channel dimensions of T 1 and T 2 are defined. Reactive ion etching (O 2 /CF 4 plasma, 160 W for 16 min with O 2 flow rate of 50 s.c.c.m. and CHF 3 flow rate of 5 s.c.c.m.) was used to define the channels of T 1 and T 2 throughout the photoresist mask. The channel of T 1 is made with the organic mixed ionic-electronic conductor polymer PEDOT:PSS (Clevios PH 1000) mixed with 5.0 wt% ethylene glycol, 0.1 wt% dodecyl benzene sulfonic acid and 1.0 wt% (3-glycidyloxypropyl)trimethoxysilane. The film was spin coated in two steps at 1,500 rpm and 650 rpm for 1 min and annealed at 120 °C for 1 min in between. The devices were subsequently baked at 140 °C for 1 hour. For the implementation of T 2 , the semiconducting polymer p(g2T-TT) was synthesized according to another work 36 . Here p(g2T-TT) was dissolved in chloroform (3 mg ml -1 ) inside a N 2 -filled glovebox and spin coated in ambient conditions at 1,000 rpm for 1 min resulting in a thickness of 40 nm. The devices were baked at 60 °C for 1 min. The sacrificial upper parylene C layer was peeled off to confine the polymer to the inside of the channel regions. Excess soap was rinsed off with deionized water. A schematic of the OAN is shown in Supplementary Fig. 19.

Electrical characterization of OENDs and OANs
The current versus voltage characteristics of the individual OECTs and OENDs were obtained using a Keithley 2400 semiconductor parameter analyser. Ag/AgCl were used as the gate electrodes with 100 mM NaCl electrolyte solution, unless otherwise stated. The nonlinear characteristics of the OENDs were obtained by enforcing current when the corresponding voltage was measured or by enforcing voltage when the corresponding current was measured. The OEND was coupled with RC and V in elements (external components) to complete the OAN, which exhibits neuronal dynamics when connected to voltage source V in (Fig. 1b). The voltage oscillation was directly recorded at the V out terminal using an Agilent infiniiVision digital oscilloscope. The output current I out of the OEND is measured by measuring the voltage V 0 across resistor R M with a differential amplifier (based on an INA122 integrated circuit) and a digital oscilloscope (Supplementary Fig. 20). To characterize the OAN response in the presence of different electrobiochemical signals, a Tektronix AFG1022 arbitrary function generator was used for the input voltage. The arbitrary function generator and semiconductor parameter analyser were used together to generate an arbitrary noise signal for the noise-induced neuronal characterization, phase-locking measurements and characterization of the various neuromorphic behaviours. The spectrogram of the short-time Fourier transform was performed using OriginPro 2016 with a Hanning-type window.

Data availability
The data that support the findings of this study are available from the corresponding authors on reasonable request. Source data are provided with this paper.