Organic Synapses for Neuromorphic Electronics: From Brain-Inspired Computing to Sensorimotor Nervetronics.

Living organisms have a long evolutionary history that has provided them with functions and structures that enable them to survive in their environment. The goal of biomimetic technology is to emulate these traits of living things. Research in bioinspired electronics develops electronic sensors and motor systems that mimic biological sensory organs and motor systems and that are intended to be used in bioinspired applications such as humanoid robots, exoskeletons, and other devices that combine a living body and an electronic device. To develop bioinspired robotic and electronic devices that are compatible with the living body at the neuronal level and that are operated by mechanisms similar to those in a living body, researchers must develop biomimetic electronic sensors, motor systems, brains, and nerves. Artificial organic synapses have emulated the brain's plasticity with much simpler structures and lower fabrication cost than neurons based on silicon circuits, and with smaller energy consumption than traditional von Neumann computing methods. Organic synapses are promising components of future neuromorphic systems. In this Account, we review recent research trends of neuromorphic systems based on organic synapses, then suggest research directions. We introduce the device structures and working mechanisms of reported organic synapses and the brain's plasticity, which are mainly imitated to demonstrate the learning and memory function of the organic synapses. We also introduce recent reports on sensory synapses and sensorimotor nervetronics that mimic biological sensory and motor nervous systems. Sensory nervetronics can be used to augment the sensory functions of the living body and to comprise the sensory systems of biomimetic robots. Organic synapses can also be used to control biological muscles and artificial muscles that have the same working mechanism as biological muscle. Motor nervetronics would impart life-like motion to bioinspired robots. Chemical approaches may provide insights to guide development of new organic materials, device structures, and working mechanisms to improve synaptic responses of organic neuromorphic systems. For example, organic synapses can be applied to electronic and robotic skins and bioimplantable medical devices that use mechanically stable, self-healing, and biocompatible organic materials. Biochemical approaches may expand the plasticity of the brain and nervous system. We expect that organic neuromorphic systems will be vital components in bioinspired robotic and electronic applications, including biocompatible neural prosthetics, exoskeletons, humanoid soft robots, and cybernetics devices that are integrated with biological and artificial organs.

Structure-activity relationship-based screening of antibiotics against Gram-negative Acinetobacter baumannii.

To discover potent antibiotics against the Gram-negative bacteria, we performed a structure-activity relationship (SAR) study of YKsa-6, which was the most potent inhibitor of Staphylococcus aureus ß-ketoacyl acyl carrier protein III in our previous study. We identified and selected 11 candidates, and finally screened two active compounds, YKab-4 (4-[(3-chloro-4-methylphenyl)aminoiminomethyl]benzene-1,3-diol) and YKab-6 (4-[[3-(trifluoromethyl)phenyl]aminoiminomethyl]phenol) as inhibitors of Acinetobacter baumannii KAS III (abKAS III). They showed potent antimicrobial activities at 2 or 8 µg/mL, specifically against Acinetobacter baumannii and a strong binding affinity for abKAS III. From the homology modeling, we defined the three-dimensional (3D) structure of abKAS III for the first time and found that it had an extra loop region compared with common Gram-negative bacteria derived KAS IIIs. The docking study revealed that the hydroxyl groups of inhibitors formed extensive hydrogen bonds and the complicated hydrophobic and cation-stacking interactions are important to binding with abKAS III. We confirmed that the hydrophobicity of these compounds might be the essential factor for their antimicrobial activities against Gram-negative bacteria as well as their structural rigidity, a cooperative feature for retaining the hydrophobic interactions between abKAS III and its inhibitors. This study may provide an insight developing strategies for potent antibiotics against A. baumannii.

Probability-Based Recognition Framework for Underwater Landmarks Using Sonar Images †.

This paper proposes a probability-based framework for recognizing underwater landmarks using sonar images. Current recognition methods use a single image, which does not provide reliable results because of weaknesses of the sonar image such as unstable acoustic source, many speckle noises, low resolution images, single channel image, and so on. However, using consecutive sonar images, if the status-i.e., the existence and identity (or name)-of an object is continuously evaluated by a stochastic method, the result of the recognition method is available for calculating the uncertainty, and it is more suitable for various applications. Our proposed framework consists of three steps: (1) candidate selection, (2) continuity evaluation, and (3) Bayesian feature estimation. Two probability methods-particle filtering and Bayesian feature estimation-are used to repeatedly estimate the continuity and feature of objects in consecutive images. Thus, the status of the object is repeatedly predicted and updated by a stochastic method. Furthermore, we develop an artificial landmark to increase detectability by an imaging sonar, which we apply to the characteristics of acoustic waves, such as instability and reflection depending on the roughness of the reflector surface. The proposed method is verified by conducting basin experiments, and the results are presented.

Organic nanowire fabrication and device applications.

Organic nanowires (ONWs) are flexible, stretchable, and have good electrical properties, and therefore have great potential for use in next-generation textile and wearable electronics. Analysis of trends in ONWs supports their great potential for various stretchable and flexible electronic applications such as flexible displays and flexible photovoltaics. Numerous methods can be used to prepare ONWs, but the practical industrial application of ONWs has not been achieved because of the lack of reliable techniques for controlling and patterning of individual nanowires. Therefore, an "individually controllable" technique to fabricate ONWs is essential for practical device applications. In this paper, three types of fabrication methods of ONWs are reviewed: non-alignment methods, massive-alignment methods, and individual-alignment methods. Recent research on electronic and photonic device applications of ONWs is then reviewed. Finally, suggestions for future research are put forward.

Skin-Mountable Functional Electronic Materials for Bio-Integrated Devices.

Skin-mountable electronic materials are being intensively evaluated for use in bio-integrated devices that can mutually interact with the human body. Over the past decade, functional electronic materials inspired by the skin are developed with new functionalities to address the limitations of traditional electronic materials for bio-integrated devices. Herein, the recent progress in skin-mountable functional electronic materials for skin-like electronics is introduced with a focus on five perspectives that entail essential functionalities: stretchability, self-healing ability, biocompatibility, breathability, and biodegradability. All functionalities are advanced with each strategy through rational material designs. The skin-mountable functional materials enable the fabrication of bio-integrated electronic devices, which can lead to new paradigms of electronics combining with the human body.

Autonomous self-healing supramolecular polymer transistors for skin electronics.

Skin-like field-effect transistors are key elements of bio-integrated devices for future user-interactive electronic-skin applications. Despite recent rapid developments in skin-like stretchable transistors, imparting self-healing ability while maintaining necessary electrical performance to these transistors remains a challenge. Herein, we describe a stretchable polymer transistor capable of autonomous self-healing. The active material consists of a blend of an electrically insulating supramolecular polymer with either semiconducting polymers or vapor-deposited metal nanoclusters. A key feature is to employ the same supramolecular self-healing polymer matrix for all active layers, i.e., conductor/semiconductor/dielectric layers, in the skin-like transistor. This provides adhesion and intimate contact between layers, which facilitates effective charge injection and transport under strain after self-healing. Finally, we fabricate skin-like self-healing circuits, including NAND and NOR gates and inverters, both of which are critical components of arithmetic logic units. This work greatly advances practical self-healing skin electronics.

Development of 3D-Printed Self-Healing Capsules with a Separate Membrane and Investigation of Mechanical Properties for Improving Fracture Strength.

Studies on self-healing capsules embedded in cement composites to heal such cracks have recently been actively researched in order to improve the dimensional stability of concrete structures. In particular, capsule studies were mainly conducted to separately inject reactive healing solutions into different capsules. However, with this method, there is an important limitation in that the probability of self-healing is greatly reduced because the two healing solutions must meet and react. Therefore, we propose three-dimensional (3D) printer-based self-healing capsules with a membrane structure that allows two healing solutions to be injected into one capsule. Among many 3D printing methods, we used the fusion deposition modeling (FDM) to design, analyze, and produce new self-healing capsules, which are widely used due to their low cost, precise manufacturing, and high-speed. However, polylactic lactic acid (PLA) extruded in the FDM has low adhesion energy between stacked layers, which causes different fracture strengths depending on the direction of the applied load and the subsequent performance degradation of the capsule. Therefore, the isotropic fracture characteristics of the newly proposed four types of separated membrane capsules were analyzed using finite element method analysis. Additionally, capsules were produced using the FDM method, and the compression test was conducted by applying force in the x, y, and z directions. The isotropic fracture strength was also analyzed using the relative standard deviation (RSD) parameter. As a result, the proposed separated membrane capsule showed that the RSD of isotropic fracture strength over all directions fell to about 18% compared to other capsules.

A low-power stretchable neuromorphic nerve with proprioceptive feedback.

By relaying neural signals from the motor cortex to muscles, devices for neurorehabilitation can enhance the movement of limbs in which nerves have been damaged as a consequence of injuries affecting the spinal cord or the lower motor neurons. However, conventional neuroprosthetic devices are rigid and power-hungry. Here we report a stretchable neuromorphic implant that restores coordinated and smooth motions in the legs of mice with neurological motor disorders, enabling the animals to kick a ball, walk or run. The neuromorphic implant acts as an artificial efferent nerve by generating electrophysiological signals from excitatory post-synaptic signals and by providing proprioceptive feedback. The device operates at low power (~1/150 that of a typical microprocessor system), and consists of hydrogel electrodes connected to a stretchable transistor incorporating an organic semiconducting nanowire (acting as an artificial synapse), connected via an ion gel to an artificial proprioceptor incorporating a carbon nanotube strain sensor (acting as an artificial muscle spindle). Stretchable electronics with proprioceptive feedback may inspire the further development of advanced neuromorphic devices for neurorehabilitation.

Spiral NeuroString: High-Density Soft Bioelectronic Fibers for Multimodal Sensing and Stimulation.

Bioelectronic fibers hold promise for both research and clinical applications due to their compactness, ease of implantation, and ability to incorporate various functionalities such as sensing and stimulation. However, existing devices suffer from bulkiness, rigidity, limited functionality, and low density of active components. These limitations stem from the difficulty to incorporate many components on one-dimensional (1D) fiber devices due to the incompatibility of conventional microfabrication methods (e.g., photolithography) with curved, thin and long fiber structures. Herein, we introduce a fabrication approach, ‶spiral transformation″, to convert two-dimensional (2D) films containing microfabricated devices into 1D soft fibers. This approach allows for the creation of high density multimodal soft bioelectronic fibers, termed Spiral NeuroString (S-NeuroString), while enabling precise control over the longitudinal, angular, and radial positioning and distribution of the functional components. We show the utility of S-NeuroString for motility mapping, serotonin sensing, and tissue stimulation within the dynamic and soft gastrointestinal (GI) system, as well as for single-unit recordings in the brain. The described bioelectronic fibers hold great promises for next-generation multifunctional implantable electronics.

Organic Neuroelectronics: From Neural Interfaces to Neuroprosthetics.

Requirements and recent advances in research on organic neuroelectronics are outlined herein. Neuroelectronics such as neural interfaces and neuroprosthetics provide a promising approach to diagnose and treat neurological diseases. However, the current neural interfaces are rigid and not biocompatible, so they induce an immune response and deterioration of neural signal transmission. Organic materials are promising candidates for neural interfaces, due to their mechanical softness, excellent electrochemical properties, and biocompatibility. Also, organic nervetronics, which mimics functional properties of the biological nerve system, is being developed to overcome the limitations of the complex and energy-consuming conventional neuroprosthetics that limit long-term implantation and daily-life usage. Examples of organic materials for neural interfaces and neural signal recordings are reviewed, recent advances of organic nervetronics that use organic artificial synapses are highlighted, and then further requirements for neuroprosthetics are discussed. Finally, the future challenges that must be overcome to achieve ideal organic neuroelectronics for next-generation neuroprosthetics are discussed.

Stretchable PPG sensor with light polarization for physical activity-permissible monitoring.

Skin-attachable sensors, which represent the ultimate form of wearable electronic devices that ensure conformal contact with skin, suffer from motion artifact limitations owing to relative changes in position between the sensor and skin during physical activities. In this study, a polarization-selective structure of a skin-conformable photoplethysmographic (PPG) sensor was developed to decrease the amount of scattered light from the epidermis, which is the main cause of motion artifacts. The motion artifacts were suppressed more than 10-fold in comparison with those of rigid sensors. The developed sensor-with two orthogonal polarizers-facilitated successful PPG signal monitoring during wrist angle movements corresponding to high levels of physical activity, enabling continuous monitoring of daily activities, even while exercising for personal health care.

Supra-Binary Polarization in a Ferroelectric Nanowire.

The prediction and observation of supra-binary polarization in a ferroelectric nanowire (FNW) covered with a semicylindrical gate that provides an anisotropic electric field in the FNW are reported. There are gate-voltage-driven transitions between four polarization states in the FNW's cross-section, dubbed vertical-up, vertical-down, radial-in, and radial-out. They are determined by the interplay between the spatial depolarization energy and the free energy induced by an anisotropic external electric field, in clear distinction from the conventional film-based binary ferroelectricity. When the FNW is mounted on a biased graphene nanoribbon (GNR), these transitions induce exotic current-voltage hysteresis in the FNW-GNR transistor. This discovery suggests new operating mechanisms of ferroelectric devices. In particular, it enables intrinsic quaternary-digit information manipulation in parallel to the bit manipulation employed in conventional data storage.

Standalone real-time health monitoring patch based on a stretchable organic optoelectronic system.

Skin-like health care patches (SHPs) are next-generation health care gadgets that will enable seamless monitoring of biological signals in daily life. Skin-conformable sensors and a stretchable display are critical for the development of standalone SHPs that provide real-time information while alleviating privacy concerns related to wireless data transmission. However, the production of stretchable wearable displays with sufficient pixels to display this information remains challenging. Here, we report a standalone organic SHP that provides real-time heart rate information. The 15-µm-thick SHP comprises a stretchable organic light-emitting diode display and stretchable organic photoplethysmography (PPG) heart rate sensor on all-elastomer substrate and operates stably under 30% strain using a combination of stress relief layers and deformable micro-cracked interconnects that reduce the mechanical stress on the active optoelectronic components. This approach provides a rational strategy for high-resolution stretchable displays, enabling the production of ideal platforms for next-generation wearable health care electronics.

Transparent Flexible Nanoline Field-Effect Transistor Array with High Integration in a Large Area.

Transparent flexible transistor array requests large-area fabrication, high integration, high manufacturing throughput, inexpensive process, uniformity in transistor performance, and reproducibility. This study suggests a facile and reliable approach to meet the requirements. We use the Al-coated polymer nanofiber patterns obtained by electrohydrodynamic (EHD) printing as a photomask. We use the lithography and deposition to produce highly aligned nanolines (NLs) of metals, insulators, and semiconductors on large substrates. With these NLs, we demonstrate a highly integrated NL field-effect transistor (NL-FET) array (105/(4 × 4 in2), 254 pixel-per-inch) made of pentacene and indium zinc oxide semiconductor NLs. In addition, we demonstrate a NL complementary inverter (NL-CI) circuit consisting of pentacene and fullerene NLs. The NL-FET array shows high transparency (∼90%), flexibility (stable at 2.5 mm bending radius), uniformity (∼90%), and high performances (mobility = 0.52 cm2/(V s), on-off ratio = 7.0 × 106). The NL-CI circuit also shows high transparency, flexibility, and typical switching characteristic with a gain of 21. The reliable large-scale fabrication of the various NLs proposed in this study is expected to be applied for manufacturing transparent flexible nanoelectronic devices.

Retina-Inspired Carbon Nitride-Based Photonic Synapses for Selective Detection of UV Light.

Photonic synapses combine sensing and processing in a single device, so they are promising candidates to emulate visual perception of a biological retina. However, photonic synapses with wavelength selectivity, which is a key property for visual perception, have not been developed so far. Herein, organic photonic synapses that selectively detect UV rays and process various optical stimuli are presented. The photonic synapses use carbon nitride (C3 N4 ) as an UV-responsive floating-gate layer in transistor geometry. C3 N4 nanodots dominantly absorb UV light; this trait is the basis of UV selectivity in these photonic synapses. The presented devices consume only 18.06 fJ per synaptic event, which is comparable to the energy consumption of biological synapses. Furthermore, in situ modulation of exposure to UV light is demonstrated by integrating the devices with UV transmittance modulators. These smart systems can be further developed to combine detection and dose-calculation to determine how and when to decrease UV transmittance for preventive health care.

Flexible Neuromorphic Electronics for Computing, Soft Robotics, and Neuroprosthetics.

Flexible neuromorphic electronics that emulate biological neuronal systems constitute a promising candidate for next-generation wearable computing, soft robotics, and neuroprosthetics. For realization, with the achievement of simple synaptic behaviors in a single device, the construction of artificial synapses with various functions of sensing and responding and integrated systems to mimic complicated computing, sensing, and responding in biological systems is a prerequisite. Artificial synapses that have learning ability can perceive and react to events in the real world; these abilities expand the neuromorphic applications toward health monitoring and cybernetic devices in the future Internet of Things. To demonstrate the flexible neuromorphic systems successfully, it is essential to develop artificial synapses and nerves replicating the functionalities of the biological counterparts and satisfying the requirements for constructing the elements and the integrated systems such as flexibility, low power consumption, high-density integration, and biocompatibility. Here, the progress of flexible neuromorphic electronics is addressed, from basic backgrounds including synaptic characteristics, device structures, and mechanisms of artificial synapses and nerves, to applications for computing, soft robotics, and neuroprosthetics. Finally, future research directions toward wearable artificial neuromorphic systems are suggested for this emerging area.

Water Passivation of Perovskite Nanocrystals Enables Air-Stable Intrinsically Stretchable Color-Conversion Layers for Stretchable Displays.

Conventional organic light-emitting devices without an encapsulation layer are susceptible to degradation when exposed to air, so realization of air-stable intrinsically-stretchable display is a great challenge because the protection of the devices against penetration of moisture and oxygen is even more difficult under stretching. An air-stable intrinsically-stretchable display that is composed of an intrinsically-stretchable electroluminescent device (SELD) integrated with a stretchable color-conversion layer (SCCL) that contains perovskite nanocrystals (PeNCs) is proposed. PeNCs normally decay when exposed to air, but they become resistant to this decay when dispersed in a stretchable elastomer matrix; this change is a result of a compatibility between capping ligands and the elastomer matrix. Counterintuitively, the moisture can efficiently passivate surface defects of PeNCs, to yield significant increases in both photoluminescence intensity and lifetime. A display that can be stretched up to 180% is demonstrated; it is composed of an air-stable SCCL that down-converts the SELD's blue emission and reemits it as green. The work elucidates the basis of moisture-assisted surface passivation of PeNCs and provides a promising strategy to improve the quantum efficiency of PeNCs with the aid of moisture, which allows PeNCs to be applied for air-stable stretchable displays.

Stretchable self-healable semiconducting polymer film for active-matrix strain-sensing array.

Skin-like sensory devices should be stretchable and self-healable to meet the demands for future electronic skin applications. Despite recent notable advances in skin-inspired electronic materials, it remains challenging to confer these desired functionalities to an active semiconductor. Here, we report a strain-sensitive, stretchable, and autonomously self-healable semiconducting film achieved through blending of a polymer semiconductor and a self-healable elastomer, both of which are dynamically cross-linked by metal coordination. We observed that by controlling the percolation threshold of the polymer semiconductor, the blend film became strain sensitive, with a gauge factor of 5.75 × 105 at 100% strain in a stretchable transistor. The blend film is also highly stretchable (fracture strain, >1300%) and autonomously self-healable at room temperature. We proceed to demonstrate a fully integrated 5 × 5 stretchable active-matrix transistor sensor array capable of detecting strain distribution through surface deformation.

Stretchable organic optoelectronic sensorimotor synapse.

Emulation of human sensory and motor functions becomes a core technology in bioinspired electronics for next-generation electronic prosthetics and neurologically inspired robotics. An electronic synapse functionalized with an artificial sensory receptor and an artificial motor unit can be a fundamental element of bioinspired soft electronics. Here, we report an organic optoelectronic sensorimotor synapse that uses an organic optoelectronic synapse and a neuromuscular system based on a stretchable organic nanowire synaptic transistor (s-ONWST). The voltage pulses of a self-powered photodetector triggered by optical signals drive the s-ONWST, and resultant informative synaptic outputs are used not only for optical wireless communication of human-machine interfaces but also for light-interactive actuation of an artificial muscle actuator in the same way that a biological muscle fiber contracts. Our organic optoelectronic sensorimotor synapse suggests a promising strategy toward developing bioinspired soft electronics, neurologically inspired robotics, and electronic prostheses.