W. S. Dong

Controlling Ion Conductance and Channels to Achieve Synaptic-like Frequency Selectivity

Abstract Enhancing ion conductance and controlling transport pathway in organic electrolyte could be used to modulate ionic kinetics to handle signals. In a Pt/Poly(3-hexylthiophene-2,5-diyl)/Polyethylene+LiCF3SO3/Pt hetero-junction, the electrolyte layer handled at high temperature showed nano-fiber microstructures accompanied with greatly improved salt solubility. Ions with high mobility were confined in the nano-fibrous channels leading to the semiconducting polymer layer, which is favorable for modulating dynamic doping at the semiconducting polymer/electrolyte interface by pulse frequency. Such a device realized synaptic-like frequency selectivity, i.e., depression at low frequency stimulation but potentiation at high-frequency stimulation.

Simulation of synaptic short-term plasticity using Ba(CF3SO3)2-doped polyethylene oxide electrolyte film.

The simulation of synaptic plasticity using new materials is critical in the study of brain-inspired computing. Devices composed of Ba(CF3SO3)2-doped polyethylene oxide (PEO) electrolyte film were fabricated and with pulse responses found to resemble the synaptic short-term plasticity (STP) of both short-term depression (STD) and short-term facilitation (STF) synapses. The values of the charge and discharge peaks of the pulse responses did not vary with input number when the pulse frequency was sufficiently low(~1 Hz). However, when the frequency was increased, the charge and discharge peaks decreased and increased, respectively, in gradual trends and approached stable values with respect to the input number. These stable values varied with the input frequency, which resulted in the depressed and potentiated weight modifications of the charge and discharge peaks, respectively. These electrical properties simulated the high and low band-pass filtering effects of STD and STF, respectively. The simulations were consistent with biological results and the corresponding biological parameters were successfully extracted. The study verified the feasibility of using organic electrolytes to mimic STP.

Effect of heavy-ion on frequency selectivity of semiconducting polymer/electrolyte heterojunction

Heavy ion Nd3+ is introduced into the electrolyte layer to study frequency selectivity of a semiconducting polymer/electrolyte double-layer cell. This cell exhibits long-term depression under low-frequency stimulations and potentiation under high-frequency stimulations by positive triangular pulses, suggesting a conventional learning protocol, i.e., spike-rate-dependent plasticity. The frequency selectivity depends significantly on the input shape due to large ionic size and mass. The input threshold of the frequency selectivity is around the voltage inducing a negative differential resistance (VNDR) influenced by the loading rate. The typical value of VNDR is 0.3 V for a loading rate of 100 V s−1, but VNDR disappears when the loading rate exceeds 1000 V s−1. Besides, the frequency selectivity has not been observed under rectangular pulse input. Moreover, the possibility of bidirectional signal transfer has been tested simply by anti-connecting two individual cells. Our study suggests the possibility to realize signal pruning and synthetizing by changing ionic types.

Spatial summation of the short-term plasticity of a pair of organic heterogeneous junctions

Recent studies have found that responses to electrical stimulations in organic semiconductor and/or electrolyte heterogeneous junctions possess features in common with synaptic plasticity in neural networks. Spatial summation of short-term plasticity was then studied using a pair of such junctions, i.e., Pt/Mg-doped polyethylene oxide (PEO)/Pt and Pt/Mg-doped PEO/poly(3-hexylthiophene-2,5-diyl) (P3HT)/Pt devices. The former displayed short-term depression for charging peaks and short-term facilitation (STF) for discharging peaks, while the latter displayed STF for both the charging and discharging peaks. A simple integration of parallel connection showed that the system displayed frequency selectivity in the weight modification of the charging peaks, i.e., it facilitated below a frequency threshold but depressed at a higher frequency. The frequency threshold varied with input numbers from about 60 Hz to 100 Hz. In contrast, only STF was observed in the weight modifications of the discharging peaks. In addition, the weight modification could be linearly summed from those of the two source devices though the absolute peak currents could not. Our study demonstrates that synaptic computation are feasible for parallel connection system, depending on both input frequency and linear summation of weight modifications. Finally, we suggest that directional selectivity might be realized using the parallel system.

Diverse Synaptic Plasticity Induced by the Interplay of Ionic Polarization and Doping at Salt-Doped Electrolyte/Semiconducting Polymer Interface

Pt/Ca2+–polyethylene oxide/polymer poly[3-hexylthiophene-2,5-diyl]/Pt devices were fabricated, and their pulse responses were studied. The discharging peak, represented by the postsynaptic current (PSC), first increases and then decreases with increasing input number in a pulse train. The weight of the PSC decreased for low-frequency stimulations but increased for high-frequency stimulations. However, the peak of the negative differential resistance during the charging process varied following the opposite trend. These behaviors suggested the ability for transferring the signal bidirectionally, confirming the equivalence between the ionic kinetics of our device and the transmitter kinetics of one kind of synapse. A facilitation (F)–depression (D) interplay model corresponding to the ionic polarization and doping interplay at the electrolyte/semiconducting polymer interface was adopted to successfully mimic the weight modification of the PSC. The simulation results showed that the observed synaptic plasticity was caused by the great disparity between the recovery time constants of F and D (τF and τD). Moreover, such an interplay could inspire the features of responses to post-tetanic stimulations. Our study suggested a means to realize synaptic computation.

Excitatory post-synaptic current and synaptic plasticity of semiconducting polymer/electrolyte system

There are two important resemblances between bio-synapse and semiconducting polymer/electrolyte hetero-junction. The first is that ionic migration at the interface would modulate the ionic movement. The second is in the structure that the ionic channel form in hetero-junction to conduct ion migration. These two aspects suggested the latter could be used in simulating synaptic plasticity. Therefore, we adopted conventional measurement method used in neuroscience studying synaptic plasticity. We have successfully mimicked frequency selectivity using Pt/P3HT/PEO+Li+/Pt cell, which responded in depression at LFS and in potentiation at HFS. The ionic dynamic was controlled by the ionic type and the interface. Moreover, we have found that long term memory (plasticity) existed in Cs and K doped PEO, which might be used as band-pass filtering.

Influence of ionic size to the pulse responses of semiconducting polymer/electrolyte hetero-junctions

Pt/Poly[2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV)/polyethylene oxide (PEO) + Alkali ions/Ag devices were fabricated. Microstructure analysis indicated that Cs and K ions were doped initially into the semiconducting polymer after fabrication, which suggests resemblance to the resting state of bio-synapse. The pulse responses of Cs-doped device potentiated monotonically with increased frequency. K-doped device had a threshold frequency at about 60 Hz, and it responded in potentiation at frequency higher than this threshold. The special filtering effects should relate to the ionic states in nanoscale in both semiconducting polymer and electrolyte. Thus, these two types of device could be used in signal computing.