Self-Assembled Monolayer and Nanoparticles Coenhanced Fragmented Silver Nanowire Network Memristor.

Silver nanowire (AgNW) networks with self-assembled structures and synaptic connectivity have been recently reported for constructing neuromorphic memristors. However, resistive switching at the cross-point junctions of the network is unstable due to locally enhanced Joule heating and the Gibbs-Thomson effect, which poses an obstacle to the integration of threshold switching and memory function in the same AgNW memristor. Here, fragmented AgNW networks combined with Ag nanoparticles (AgNPs) and mercapto self-assembled monolayers (SAMs) are devised to construct memristors with stable threshold switching and memory behavior. In the above design, the planar gaps between NW segments are for resistive switching, the AgNPs act as metal islands in the gaps to reduce threshold voltage (Vth) and holding voltage (Vhold), and the SAMs suppress surface atom diffusion to avoid Oswald ripening of the AgNPs, which improves switching stability. The fragmented NW-NP/SAM memristors not only circumvent the side effects of conventional NW-stacked junctions to provide durable threshold switching at >Vth but also exhibit synaptic characteristics such as long-term potentiation at ultralow voltage (≪Vth). The combination of NW segments, nanoparticles, and SAMs blazes a new trail for integrating artificial neurons and synapses in AgNW network memristors.

Theoretical simulation of the selective stimulation of axons in different areas of a nerve bundle by multichannel near-infrared lasers.

Damaged nerve function can be repaired by applying external stimuli, and the selective stimulation of nerve fibers is the highest goal of nerve functional repair. This paper proposes a method of using multichannel near-infrared lasers to achieve the selective stimulation of axons in different areas in a mixed nerve bundle. An exposed bullfrog sciatic nerve was considered the object of study to realize the selective stimulation. A model was established by using COMSOL Multiphysics to simulate the temperature distribution of nerves under multichannel near-infrared laser stimulation. The results of this model showed that by changing the distance between the laser fiber and the nerve (d) or the power of the 4 lasers (P), the axons at different parts of the nerve bundle may be selectively stimulated. If only the axons located in the center are selected to be activated, it is necessary to set the d and P value in the four directions to the same value. If only axons on the nerve edge are selected for activation, we can reduce the d value of the nearest laser (or increase P) and increase the d value of lasers in other directions (or decrease P). If only axons in the shallow area below the surface between the two lasers are selected for activation, it is necessary to reduce the d value of the laser in two directions close there (or increase P) and increase the d value of the laser in the other two directions (or decrease P). If only the axons of the superficial region on the coordinate axis are activated, the d value of the laser in the farthest direction can be increased (or decrease P) and the d value of the other three lasers can be reduced (or increase P). Moreover, the results of animal experiments further verify the feasibility of our method to realize selective activation of the axons.

Selective stimulation of bullfrog sciatic nerve by gold nanorod assisted combined electrical and near-infrared stimulation.

Selective stimulation of the nervous system is an important way to improve the therapeutic efficacy and minimize side effects. This paper introduces an improved method using combined electrical and near-infrared stimulation to realize selective excitation and inhibition of different sciatic nerve branches. Both the electrical stimulation and the near-infrared laser are added to the main trunk of the sciatic nerve, and gold nanorods are injected into the light irradiation point of the nerve to increase the absorption of light. Two cuff recording electrodes are added to the two sciatic nerve branches, respectively. The compound nerve action potential recorded by the cuff electrode is transmitted to the physiological signal instrument. In the experiment, selective activation and inhibition of the two nerve branches are achieved by adjusting the electrical stimulation parameters, the light stimulus parameters and the location of the light. These results demonstrate that combined electrical and near-infrared stimulation, which can effectively activate or suppress the different nerve fibers in the nerve fiber bundle, is suitable for selective regulation of peripheral nerve. Meanwhile, the photoelectric combined stimulation can reduce both the electrical energy and light energy needed for the stimulation, and reduce the electrical damage and light damage to the nerve.

Gold nanorod-assisted near-infrared stimulation of bullfrog sciatic nerve.

Infrared neural stimulation (INS) is a new and developing approach for neural repair, with the advantages of being non-contact, spatially precise, and artifact-free. However, the disadvantage of infrared light is that it is difficult to stimulate deep tissue because of its weak penetrating power. Therefore, this paper introduces an improved method using near-infrared laser to stimulate bullfrog sciatic nerves because of its strong penetrating power. Meanwhile, gold nanorods (Au NRs) are injected into the nerve to increase the absorption of light. The mechanism is the instantaneous temperature rise caused by the absorption of infrared light by Au NRs. The compound muscle action potential (CMAP) associated with the irradiated sciatic nerve is recorded by a multi-channel physiological signal instrument. The peak to peak amplitude (Vpp) of CMAP for sciatic nerves injected with Au NRs increases significantly compared to the CMAP for control nerves without Au NRs. These results demonstrate INS by labeling nerves with nanoparticle exhibiting latent capacity to increase the efficiency, spatial resolution, and the neural responsivity, and especially, can increase the penetration depth and reduce the requisite radiant exposure level.

Model study of combined electrical and near-infrared neural stimulation on the bullfrog sciatic nerve.

This paper implemented a model study of combined electrical and near-infrared (808 nm) neural stimulation (NINS) on the bullfrog sciatic nerve. The model includes a COMSOL model to calculate the electric-field distribution of the surrounding area of the nerve, a Monte Carlo model to simulate light transport and absorption in the bullfrog sciatic nerve during NINS, and a NEURON model to simulate the neural electrophysiology changes under electrical stimulus and laser irradiation. The optical thermal effect is considered the main mechanism during NINS. Therefore, thermal change during laser irradiation was calculated by the Monte Carlo method, and the temperature distribution was then transferred to the NEURON model to stimulate the sciatic nerve. The effects on thermal response by adjusting the laser spot size, energy of the beam, and the absorption coefficient of the nerve are analyzed. The effect of the ambient temperature on the electrical stimulation or laser stimulation and the interaction between laser irradiation and electrical stimulation are also studied. The results indicate that the needed stimulus threshold for neural activation or inhibition is reduced by laser irradiation. Additionally, the needed laser energy for blocking the action potential is reduced by electrical stimulus. Both electrical and laser stimulation are affected by the ambient temperature. These results provide references for subsequent animal experiments and could be of great help to future basic and applied studies of infrared neural stimulation (INS).

A simulation study of the combined thermoelectric extracellular stimulation of the sciatic nerve of the Xenopus laevis: the localized transient heat block.

The electrical behavior of the Xenopus laevis nerve fibers was studied when combined electrical (cuff electrodes) and optical (infrared laser, low power sub-5 mW) stimulations are applied. Assuming that the main effect of the laser irradiation on the nerve tissue is the localized temperature increase, this paper analyzes and gives new insights into the function of the combined thermoelectric stimulation on both excitation and blocking of the nerve action potentials (AP). The calculations involve a finite-element model (COMSOL) to represent the electrical properties of the nerve and cuff. Electric-field distribution along the nerve was computed for the given stimulation current profile and imported into a NEURON model, which was built to simulate the electrical behavior of myelinated nerve fiber under extracellular stimulation. The main result of this study of combined thermoelectric stimulation showed that local temperature increase, for the given electric field, can create a transient block of both the generation and propagation of the APs. Some preliminary experimental data in support of this conclusion are also shown.