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.

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

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properties of biological synapses and perform parallel operations, they require larger energy than a biological synapse. Therefore, development of an artificial synapse with energy consumption on the level of a biological synapse remains an open problem.Organic-inorganic halide perovskite (OHPs) may provide a material to solve this problem, because of their low activation energy of ion migration. Moreover, various structural modulation of polycrystalline films is possible with facile solution processing so that organic parts in the OHPs can control the ion migration and electrical conduction. OHPs have an ABX 3 crystal structure; the A-site cation is located at the center of a BX 6 octahedral cage, and the B-site metal cation is surrounded by the six nearest-neighbor X-site halide anions. [7] OHPs have a significant hysteresis property that is caused by ion migration or space charges, or both, which may enable gradual modulation of conductance in OHP. [8] Two-terminal artificial synapses based on 3D methylammonium (MA) lead halide perovskite (MAPbX 3 , X = Br, I) films showed synaptic responses that are caused by ion migration in the OHP layer. [9,10] Ion migration in 3D OHP film is induced by relatively low activation energy and a low energy consumption of ≈20 fJ per synaptic event was achieved in the synaptic devices. However, the energy consumption could be further reduced to the energy level of biological synapses when the ion migration was controlled by engineering the structure of OHP films could be optimally done.In this work, we introduce 2D and quasi-2D OHP films into artificial synapses to enable control of ion migration and resultant synaptic responses. For this purpose, we replaced the small MA ion with a bulky phenethylammonium (PEA) ion in their crystalline structures. To prepare 2D, quasi-2D, and 3D OHP films, we controlled the stoichiometric ratio of PEA and MA cations to induce self-assembly of a layered structure. This replacement of an MA cation with PEA cation suppresses ion migration in the out-of-plane direction of the OHP films. [11][12][13] Thereby, the activation energy E A of ion migration is increased, so ion migration and excitatory postsynaptic current (EPSC) can be reduced. Also, energy consumption of the device is reduced to ≈0.7 fJ per synaptic event, which is comparable to that of biological synapses. Memory retention of artificial The hysteretic behavior of organic-inorganic halide perovskites (OHPs) are exploited for application in neuromorphic electronics. Artificial synapses with 2D and quasi-2D perovskite are demonstrated that have a bulky organic cation (phenethylammonium (PEA)) to form structures of (PEA) 2 MA n-1 Pb n Br 3n+1 . The OHP films have morphological properties that depend on their structure dimensionality (i.e., n value), and artificial synapses fabricated from them show synaptic responses such as short-term plasticity, paired-pulse facilitation, and long-term plasticity. The operation mechanism of OHP artificial synapses are also analyzed depending on the dimen...

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