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.

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Flexible Neuromorphic Electronics for Computing, Soft Robotics, and Neuroprosthetics

Skin is the largest organ, with the functionalities of protection, regulation, and sensation. The emulation of human skin via flexible and stretchable electronics gives rise to electronic skin (e-skin), which has realized artificial sensation and other functions that cannot be achieved by conventional electronics. To date, tremendous progress has been made in data acquisition and transmission for e-skin systems, while the implementation of perception within systems, that is, sensory data processing, is still in its infancy. Integrating the perception functionality into a flexible and stretchable sensing system, namely artificial skin perception, is critical to endow current e-skin systems with higher intelligence. Here, recent progress in the design and fabrication of artificial skin perception devices and systems is summarized, and challenges and prospects are discussed. The strategies for implementing artificial skin perception utilize either conventional silicon-based circuits or novel flexible computing devices such as memristive devices and synaptic transistors, which enable artificial skin to surpass human skin, with a distributed, low-latency, and energy-efficient information-processing ability. In future, artificial skin perception would be a new enabling technology to construct next-generation intelligent electronic devices and systems for advanced applications, such as robotic surgery, rehabilitation, and prosthetics.

Artificial Skin Perception.

Transparent photovoltaic technologies: Current trends towards upscaling

Emerging flexible artificial sensory systems using neuromorphic electronics have been considered as a promising solution for processing massive data with low power consumption. The construction of artificial sensory systems with synaptic devices and sensing elements to mimic complicated sensing and processing in biological systems is a prerequisite for the realization. To realize high-efficiency neuromorphic sensory systems, the development of artificial flexible synapses with low power consumption and high-density integration is essential. Furthermore, the realization of efficient coupling between the sensing element and the synaptic device is crucial. This Review presents recent progress in the area of neuromorphic electronics for flexible artificial sensory systems. We focus on both the recent advances of artificial synapses, including device structures, mechanisms, and functions, and the design of intelligent, flexible perception systems based on synaptic devices. Additionally, key challenges and opportunities related to flexible artificial perception systems are examined, and potential solutions and suggestions are provided.

Flexible Artificial Sensory Systems Based on Neuromorphic Devices.

Integrated energy conversion and storage devices: interfacing solar cells, batteries and supercapacitors

The development of electronic devices possessing the functionality of a nociceptor is a crucial step toward electronic receptors that can transfer the external stimuli to the internal nerve system. Of the various materials that have been used to realize artificial nociceptors, biopolymers have the advantages of being abundant, inexpensive, biocompatible, and flexible. In this study, nociceptor behaviors are demonstrated by the flexible Ag/carboxymethyl ι-carrageenan/ITO/PET forming-free memristors for the electronic receptors. The flexible carboxymethyl ι-carrageenan-based memristor showed threshold switching characteristics with a high ION/IOFF ratio of ∼104 and good switching endurance (>1.5 × 105 cycles). It also showed high bending endurance over 1000 cycles when measured in both the flat and the maximum bending conditions. More importantly, it differs from other common sensory receptors with its key features and functions, including threshold, relaxation, allodynia and hyperalgesia behaviors. Such nociceptive behaviors are attributed to the formation and spontaneous rupture of the Ag filament with diffusive dynamics. Finally, we built a pressure sensory alarm system by using our artificial nociceptor devices.

Flexible artificial nociceptor using a biopolymer-based forming-free memristor.

Memristive synapses from biomaterials are promising for building flexible and implantable artificial neuromorphic systems due to their remarkable mechanical and biological properties. However, these biological devices have relatively poor memristive switching characteristics, and thus fail to meet the requirement of neuromorphic networks for high learning accuracy. Here, memristive synapses based on carrageenan nanocomposites that possess desirable characteristics are demonstrated. These devices show highly reproducible analog resistive switching behaviors with 250 conductance states, low write noise, good write linearity, high retention of more than 104 s and endurance for at least 106 pulses. The enhanced switching properties are attributed to controllable and confined conductive filament growth, owing to the synergistic effect of self-assembled silver nanocluster doping and nanocone-shaped electrode contact. Moreover, the devices exhibit excellent reliability after 1000 bending cycles. Simulations including the non-ideal factors prove that the synaptic device array can operate with an online learning accuracy of 94.3%. These findings enable broader applications of biomaterials in flexible memristive devices and neuromorphic systems.

Memristive synapses with high reproducibility for flexible neuromorphic networks based on biological nanocomposites

Inorganic perovskite has attracted great interest due to its excellent optoelectronic properties. There are much less low band gap halide perovskite semiconductors, and CsPbCl3 is one of a wide band gap semiconductor in the perovskite family. In this study, a 0.5-mm CsPbCl3 perovskite single crystal with tetragonal structure and a direct band gap of 2.86 ± 0.3 eV is synthesized by flash evaporation of CsCl-PbCl2 solution. An ultraviolet photodetector based on a CsPbCl3 single crystal is fabricated, showing a photoresponse in a wide wavelength range of 280–435 nm, with a maximum responsivity of 0.272 A/W at 410 nm. Rise and decay response times of the device are less than 28.4 and 2.7 ms, respectively. The good performance of this CsPbCl3 photodetector indicates promising applications in the field of UV optoelectronic devices.

Shusheng Pan

Papers

Flexible artificial nociceptor using a biopolymer-based forming-free memristor.

Memristive synapses with high reproducibility for flexible neuromorphic networks based on biological nanocomposites

High sensitivity and rapid response ultraviolet photodetector of a tetragonal CsPbCl 3 perovskite single crystal

A sub-500 mV monolayer hexagonal boron nitride based memory device

Ultrahigh detectivity ultraviolet photodetector based on orthorhombic phase CsPbI3 microwire using temperature self-regulating solar reactor