Boost of the Bio-memristor Performance for Artificial Electronic Synapses by Surface Reconstruction.

Biomaterial-based memristors (bio-memristors) are often adopted to emulate biological synapse functions and applied to construct neural computing networks in brain-inspired chip systems. However, the randomness of conductive filament formation in bio-memristors inhibits their switching performance by causing the dispersion of the device-switching parameters. In this case, a facile porous silk fibroin (p-SF) memristor was obtained through a protein surface reconstruction strategy, in which the size of the hole can be adjusted by the density of hybrid nanoseeds. The porous SF memristors exhibit greatly enhanced electrical characteristics, including uniform I-V cycles, centralized distribution of the switching voltages, and both high and low resistances, compared to devices without pores. The results of three-dimensional (3D) simulations based on classical density functional theory (cDFT) suggest that the reconstructed pores in the SF layers guide the formation and fracture of Ag filaments under an electric field and enhance the overall conductivity by separating Ag+ ion and electron diffusion pathways. Ag+ ions are predicted to preferentially diffuse through pores, whereas electrons diffuse through the SF network. Interestingly, the device conductance can be bidirectionally modulated gradually by positive and negative voltages, can faithfully simulate short-term and long-term plasticity, and can even realize the triplet-spike-timing-dependent plasticity (triplet-STDP) rule, which can be used for pattern recognition in biological systems. The simulation results reveal that a memristor network of this type has an accuracy of ∼95.78% in memory learning and the capability of pattern learning. This work provides a facile technology route to improve the performance of bionic-material memristors.

Stretchable and conformable synapse memristors for wearable and implantable electronics.

Stretchable and conformable synapse memristors that can emulate the behaviour of the biological neural system and well adhere onto the curved surfaces simultaneously are desirable for the development of imperceptible wearable and implantable neuromorphic computing systems. Previous synapse memristors have been mainly limited to rigid substrates. Herein, a stretchable and conformable memristor with fundamental synaptic functions including potentiation/depression characteristics, long/short-term plasticity (STP and LTP), "learning-forgetting-relearning" behaviour, and spike-rate-dependent and spike-amplitude-dependent plasticity is demonstrated based on highly elastic Ag nanoparticle-doped thermoplastic polyurethanes (TPU : Ag NPs) and polydimethylsiloxane (PDMS). The memristor can be well operated even at 60% strain and can be well conformed onto the curved surfaces. The formed conductive filament (CF) obtained from the movement of Ag nanoparticle clusters under the locally enhanced electric field gives rise to resistance switching of our memristor. These results indicate a feasible strategy to realize stretchable and conformable synaptic devices for the development of new-generation artificial intelligence computers.

Gap junctions.

In vertebrates and invertebrates, signaling among neurons is most commonly mediated by chemical synapses. At these synapses neurotransmitter released by presynaptic neurons is detected by receptors on the postsynaptic neurons, leading to an influx of ions through the receptors themselves or through channels activated by intracellular signaling downstream of the receptors. But neurons can communicate with each other in a more direct way, by passing signals composed of small molecules and ions through pores called gap junctions. Gap junctions that transmit electrical signals are called electrical synapses. Unlike most chemical synapses, electrical synapses interact through axon-to-axon or dendrite-to-dendrite contacts. Found throughout the nervous system, they are probably best known for linking the relatively few inhibitory, GABAergic, neurons into large, effective networks within vertebrate brains. They are particularly important early in development before the formation of most chemical synapses, but recent work shows gap junctions play important roles in the adult nervous system, too. Gap junctions are sometimes thought to be mere passageways between cells. But, as recent work shows, their properties can be complex and surprising. Gap junctions help generate, propagate, and regulate neural oscillations, can filter electrical signals, and can be modulated in a variety of ways. Here we discuss recent work highlighting the diversity and importance of gap junctions throughout the nervous system.

Thrombospondins as key regulators of synaptogenesis in the central nervous system.

Thrombospondins (TSPs) are a family of large, oligomeric multidomain glycoproteins that participate in a variety of biological functions as part of the extracellular matrix (ECM). Through their associations with a number of binding partners, TSPs mediate complex cell-cell and cell-matrix interactions in such diverse processes as angiogenesis, inflammation, osteogenesis, cell proliferation, and apoptosis. It was recently shown in the developing central nervous system (CNS) that TSPs promote the formation of new synapses, which are the unique cell-cell adhesions between neurons in the brain. This increase in synaptogenesis is mediated by the interaction between astrocyte-secreted TSPs and their neuronal receptor, calcium channel subunit α2δ-1. The cellular and molecular mechanisms that underlie induction of synaptogenesis via this interaction are yet to be fully elucidated. This review will focus on what is known about TSP and synapse formation during development, possible roles for TSP following brain injury, and what the previously established actions of TSP in other biological tissues may tell us about the mechanisms underlying TSP's functions in CNS synaptogenesis.

The diverse functional roles and regulation of neuronal gap junctions in the retina.

Electrical synaptic transmission through gap junctions underlies direct and rapid neuronal communication in the CNS. The diversity of functional roles that electrical synapses have is perhaps best exemplified in the vertebrate retina, in which gap junctions are formed by each of the five major neuron types. These junctions are dynamically regulated by ambient illumination and by circadian rhythms acting through light-activated neuromodulators such as dopamine and nitric oxide, which in turn activate intracellular signalling pathways in the retina.The networks formed by electrically coupled neurons are plastic and reconfigurable, and those in the retina are positioned to play key and diverse parts in the transmission and processing of visual information at every retinal level.