Physically Transient Resistive Switching Memory Based on Silk Protein

An Artificial Universal Tactile Nociceptor Based on 2D Polymer Film Memristor Arrays with Tunable Resistance Switching Behaviors

Nociceptor is an important receptor in the organism's sensory system; it can perceive harmful stimuli and send signals to the brain in order to protect the body in time. The injury degree of nociceptor can be divided into three stages: self-healing injury, treatable injury, and permanent injury. However, the current studies on nociceptor simulation are limited to the self-healing stage due to the limitation of the untunable resistance switching behavior of memristors. In this study, we constructed Al/2DPTPAK+TAPB/Ag memristor arrays with adjustable memory behaviors to emulate the nociceptor of biological neural network of all three stages. For this purpose, a PDMS/AgNWs/ITO/PET pressure sensor was assembled to mimic the tactile perception of the skin. The memristor arrays can not only simulate all the response of nociceptor, i.e., the threshold, relaxation, no adaptation, and sensitization with the self-healing injury, but can also simulate the treatable injury and the permanent injury. These behaviors are both demonstrated with a single memristor and in the form of pattern mapping of the memristor array.

Recent Development of Implantable Chemical Sensors Utilizing Flexible and Biodegradable Materials for Biomedical Applications

Implantable chemical sensors built with flexible and biodegradable materials exhibit immense potential for seamless integration with biological systems by matching the mechanical properties of soft tissues and eliminating device retraction procedures. Compared with conventional hospital-based blood tests, implantable chemical sensors have the capability to achieve real-time monitoring with high accuracy of important biomarkers such as metabolites, neurotransmitters, and proteins, offering valuable insights for clinical applications. These innovative sensors could provide essential information for preventive diagnosis and effective intervention. To date, despite extensive research on flexible and bioresorbable materials for implantable electronics, the development of chemical sensors has faced several challenges related to materials and device design, resulting in only a limited number of successful accomplishments. This review highlights recent advancements in implantable chemical sensors based on flexible and biodegradable materials, encompassing their sensing strategies, materials strategies, and geometric configurations. The following discussions focus on demonstrated detection of various objects including ions, small molecules, and a few examples of macromolecules using flexible and/or bioresorbable implantable chemical sensors. Finally, we will present current challenges and explore potential future directions.

Noncytotoxic WORM Memory Using Lysozyme with Ultrahigh Stability for Transient and Sustainable Electronics Applications

Biocompatibility and transient nature of electronic devices have been the matter of attention in recent times due to their immense potential for sustainable solutions toward hazardous e-wastes. In order to fulfill the requirement of high-density data-storage devices due to explosive growth in digital data, a resistive switching (RS)-based memory device could be the promising alternative to the present Si-based electronics. In this research work, we employed a biocompatible enzymatic protein lysozyme (Lyso) as the active layer to design a RS memory device having a device structure Au/Lyso/ITO. Interestingly the device showed transient, WORM memory behavior. It has been observed that the WORM memory performance of the device was very good with high memory window (2.78 × 102), data retention (up to 300 min), device yield (∼73.8%), read cyclability, as well as very high device stability (experimentally >700 days, extrapolated to 3000 days). Bias-induced charge trapping followed by conducting filament formation was the key behind such switching behavior. Transient behavior analysis showed that electronic as well as optical behaviors completely disappeared after 10 s dissolution of the device in luke warm water. Cytotoxicity of the as-prepared device was tested by challenging two environmentally derived bacteria, S. aureus and P. aeruginosa, and was found to have no biocidal effects. Hence, the device would cause no harm to the microbial flora when it is discarded. As a whole, this work suggests that Lyso-based WORM memory device could play a key role for the design of transient WORM memory device for sustainable electronic applications.

Multilevel Reset Dependent Set of a Biodegradable Memristor with Physically Transient

The electronic device, with its biocompatibility, biodegradability, and ease of fabrication process, shows great potential to embed into health monitoring and hardware data security systems. Herein, polyvinylpyrrolidone (PVP) biopolymer is presented as an active layer, electrochemically active magnesium (Mg) as a metal electrode, and chitosan-based substrate (CHS) to fabricate biocompatible and biodegradable physically transient neuromorphic device (W/Mg/PVP/Mg/CHS). The I-V curve of device is non-volatile bipolar in nature and shows a unique compliance-induced multilevel RESET-dependent-SET behavior while sweeping the compliance current from a few microamperes to milliamperes. Non-volatile and stable switching properties are demonstrated with a long endurance test (100 sweeps) and retention time of over 104  s. The physically transient memristor (PTM) has remarkably high dynamic RON /ROFF (ON/OFF state resistance) ratio (106 Ω), and when placed in deionized (DI) water, the device is observed to completely dissolve within 10 min. The pulse transient measurements demonstrate the neuromorphic computation capabilities of the device in the form of excitatory post synaptic current (EPSC), potentiation, depression, and learning behavior, which resemble the biological function of neurons. The results demonstrate the potential of W/Mg/PVP/Mg/CHS device for use in future healthcare and physically transient electronics.

Bubble-blowing-inspired sub-micron thick freestanding silk films for programmable electronics

Thin film electronics that are capable of deforming and interfacing with nonplanar surfaces have attracted widespread interest in wearable motion detection or physiological signal recording due to their light weight, low stiffness, and high conformality. However, it is still a challenge to fabricate freestanding thin film substrates or matrices with only sub-micron thickness in a simple way, especially for those materials with metastable conformations, like regenerated silk protein. Herein, we developed a dip-coating method for the fabrication of sub-micron thick freestanding silk films inspired by blowing soap bubbles. Using a closed-loop frame to dip-coat in a concentrated silk fibroin aqueous solution, the substrate-free silk films with a thickness as low as hundreds of nanometres (∼150 nm) can be easily obtained after solvent evaporation. The silk films have extremely smooth surfaces (R_q < 3 nm) and can be tailored with different geometric shapes. The naturally dried silk films possess random coil dominated uncrystallized secondary structures, exhibiting high modulation ability and adaptability, which can be conformally attached on wrinkled skin or wrapped on human hair. Considering the methodological advantages and the unique properties of the obtained sub-micron thick silk films, several thin film based programmable electronics including transient/durable circuits, skin electrodes, transferred skin light-emitting devices and injectable electronics are successfully demonstrated after being deposited with gold or conducting polymer layers. This research provides a new avenue for preparing freestanding thin polymer films, showing great promise for developing thin film electronics in wearable and biomedical applications.

Flexible Transient Resistive Memory Based on Biodegradable Composites

Physical transient electronics have attracted more attention as the basis for building green electronics and biomedical devices. However, there are difficulties in selecting materials for the fabricated devices to take into account both biodegradability and high performance. In this paper, a physically transient resistive random-access memory (RRAM) device was fabricated by using egg protein and graphene quantum dot composites as active layers. The sandwich structure composed of Al/EA:GQD/ITO shows a good write-once-multiple-read memory characteristic, and the introduced GQD improves the switching current ratio of the device. By using the sensitivity of GQDs to ultraviolet light, the logic operation of the "OR gate" is completed. Furthermore, the device exhibits a physical transient behavior and good biodegradability due to the dissolution behavior in deionized water. These results suggest that the device is a favorable candidate for the construction of memory elements for transient electronic systems.

Biopolymer based artificial synapses enable linear conductance tuning and low-power for neuromorphic computing

Neuromorphic computing is considered a promising method for resolving the traditional von Neumann bottleneck. Natural biomaterial-based artificial synapses are popular units for constructing neuromorphic computing systems while suffering from poor linearity and limited conduction states. In this work, a AgNO₃ doped iota-carrageenan (ι-car) based memristor is proposed to resolve the non-linear limitation. The memristor presents linear conductance tuning with a higher endurance (∼10⁴), more enriched conduction states (>2000), and much lower power consumption (∼3.6 μW) than previously reported biomaterial-based analog memristors. AgNO₃ is doped to ι-car to suppress the formation of Ag filaments, thereby eliminating uneven Joule heating. Using deep learning of hand-written digits as an application, a doping-enhanced recognition accuracy (93.8%) is achieved, close to that of an ideal synaptic device (95.7%). This work verifies the feasibility of using biopolymers for future high-performance computational and wearable/implantable electronic applications.

Water-Soluble Polythiophene-Conjugated Polyelectrolyte-Based Memristors for Transient Electronics

The key to protect sensitive information stored in electronic memory devices from disclosure is to develop transient electronic devices that are capable of being destroyed quickly in an emergency. By using a highly water-soluble polythiophene-conjugated polyelectrolyte PTT-NMI⁺Br^- as an active material, which was synthesized by the reaction of poly[thiophene-alt-4,4-bis(6-bromohexyl)-4H-cyclopenta(1,2-b:5,4-b')dithiophene] with N-methylimidazole, a flexible electronic device, Al/PTT-NMI⁺Br^-/ITO-coated PET (ITO: indium tin oxide; PET: polyethylene terephthalate), is successfully fabricated. This device shows a typical nonvolatile rewritable resistive random access memory (RRAM) effect at a sweep voltage range of ±3 V and a history-dependent memristive switching performance at a small sweep voltage range of ±1 V. Both the learning/memorizing functions and the synaptic potentiation/depression of biological systems have been emulated. The switching mechanism for the PTT-NMI⁺Br^--based electronic device may be highly associated with ion migration under bias. Once water is added to this device, it will be destructed rapidly within 20 s due to the dissolution of the active layer. This device is not only a typical transient device but can also be used for constructing conventional memristors with long-term stability after electronic packaging. Furthermore, the soluble active layer in the device can be easily recycled from its aqueous solution and reused for fabricating new transient memristors. This work offers a train of new thoughts for designing and constructing a neuromorphic computing system that can be quickly destroyed with water in the near future.

Applications of biomemristors in next generation wearable electronics

With the rapid development of mobile internet and artificial intelligence, wearable electronic devices have a great market prospect. In particular, information storage and processing of real-time collected data are an indispensable part of wearable electronic devices. Biomaterial-based memristive systems are suitable for storage and processing of the obtained information in wearable electronics due to the accompanying merits, i.e. sustainability, lightweight, degradability, low power consumption, flexibility and biocompatibility. So far, many biomaterial-based flexible and wearable memristive devices were prepared by spin coating or other technologies on a flexible substrate at room temperature. However, mechanical deformation caused by mechanical mismatch between devices and soft tissues leads to the instability of device performance. From the current research and practical application, the device will face great challenges when adapting to different working environments. In fact, some interesting studies have been performed to address the above issues while they were not intensively highlighted and overviewed. Herein, the progress in wearable biomemristive devices is reviewed, and the outlook and perspectives are provided in consideration of the existing challenges during the development of wearable biomemristive systems.

Natural Polymer in Soft Electronics: Opportunities, Challenges, and Future Prospects

Pollution caused by nondegradable plastics has been a serious threat to environmental sustainability. Natural polymers, which can degrade in nature, provide opportunities to replace petroleum-based polymers, meanwhile driving technological advances and sustainable practices. In the research field of soft electronics, regenerated natural polymers are promising building blocks for passive dielectric substrates, active dielectric layers, and matrices in soft conductors. Here, the natural-polymer polymorphs and their compatibilization with a variety of inorganic/organic conductors through interfacial bonding/intermixing and surface functionalization for applications in various device modalities are delineated. Challenges that impede the broad utilization of natural polymers in soft electronics, including limited durability, compromises between conductivity and deformability, and limited exploration in controllable degradation, etc. are explicitly inspected, while the potential solutions along with future prospects are also proposed. Finally, integrative considerations on material properties, device functionalities, and environmental impact are addressed to warrant natural polymers as credible alternatives to synthetic ones, and provide viable options for sustainable soft electronics.

A Hierarchically Encoded Data Storage Device with Controlled Transiency

In the era of information explosion, high-security and high-capacity data storage technology attracts more and more attention. Physically transient electronics, a form of electronics that can physically disappear with precisely controlled degradation behaviors, paves the way for secure data storage. Herein, the authors report a silk-based hierarchically encoded data storage device (HEDSD) with controlled transiency. The HEDSD can store electronic, photonic, and optical information simultaneously by synergistically integrating a resistive switching memory (ReRAM), a terahertz metamaterial device, and a diffractive optical element, respectively. These three data storage units have shared materials and structures but diverse encoding mechanisms, which increases the degree of complexity and capacity of stored information. Silk plays an important role as a building material in the HEDSD thanks to its excellent mechanical, optical, and electrical properties and controlled transiency as a naturally extracted protein. By controlling the degradation rate of storage units of the silk-based HEDSD, different degradation modes of the HEDSD, and multilevel information encryption/decryption have been realized. Compared with the conventional memory devices, as-reported silk-based HEDSD can store multilevel complex information and realize multilevel information encryption and decryption, which is highly desirable to fulfill the future demands of secure memory systems and implantable storage devices.

Lignin and Keratin-Based Materials in Transient Devices and Disposables: Recent Advances Toward Materials and Environmental Sustainability

Rising concerns and the associated negative implications of pollution from e-waste and delayed decomposition and mineralization of component materials (e.g., plastics) are significant environmental challenges. Hence, concerted pursuit of accurate and efficient control of the life cycle of materials and subsequent dematerialization in target environments has become essential in recent times. The emerging field of transient technology will play a significant role in this regard to help overcome current environmental challenges by enabling the use of novel approaches and new materials with unique functionalities to produce devices and materials such as disposable diagnostic devices, flexible solar panels, and foldable displays that are more ecologically benign, low-cost, and sustainable. The prerequisites for materials employed in transient devices and disposables include biodegradability, biocompatibility, and the inherent ability to mineralize or dissipate in target environments (e.g., body fluids) in a short lifetime with net-zero impact. Biomaterials such as lignin and keratin are well-known to be among the most promising environmentally benign, functional, sustainable, and industrially applicable resources for transient devices and disposables. Consequently, considering the current environmental concerns, this work focuses on the advances in applying lignin and keratin-based materials in short-life electronics and single-use consumables, current limitations, future research outlook toward materials, and environmental sustainability.

NiO-Based Electronic Flexible Devices

Personal, portable, and wearable electronics have become items of extensive use in daily life. Their fabrication requires flexible electronic components with high storage capability or with continuous power supplies (such as solar cells). In addition, formerly rigid tools such as electrochromic windows find new utilizations if they are fabricated with flexible characteristics. Flexibility and performances are determined by the material composition and fabrication procedures. In this regard, low-cost, easy-to-handle materials and processes are an asset in the overall production processes and items fruition. In the present mini-review, the most recent approaches are described in the production of flexible electronic devices based on NiO as low-cost material enhancing the overall performances. In particular, flexible NiO-based all-solid-state supercapacitors, electrodes electrochromic devices, temperature devices, and ReRAM are discussed, thus showing the potential of NiO as material for future developments in opto-electronic devices.

Silk Microneedle Patch Capable of On-Demand Multidrug Delivery to the Brain for Glioblastoma Treatment

Glioblastoma (GBM) is the most common and aggressive primary brain tumor. Surgery followed by chemotherapy and radiotherapy remains the standard treatment strategy for GBM patients. However, challenges still exist when surgery is difficult or impossible to remove the tumor completely. Herein, the design, fabrication and application of a heterogenous silk fibroin microneedle (SMN) patch is reported for circumventing the blood-brain barrier and releasing multiple drugs directly to the tumor site for drug combination treatment. The biocompatible and biodegradable SMN patch can dissolve slowly over time, allowing the sustained release of multiple drugs at different doses. Furthermore, it can be triggered remotely to induce rapid drug delivery at a designated stage after implantation. In the GBM mouse models, two clinically relevant chemotherapeutic agents (thrombin and temozolomide) and targeted drug (bevacizumab) are loaded into the SMN patch with individually controlled release profiles. The drugs are spatiotemporally and sequentially delivered for hemostasis, anti-angiogenesis, and apoptosis of tumor cells. Device application is non-toxic and results in decreased tumor volume and increased survival rate in mice. The SMN patch with on-demand multidrug delivery has potential applications for the combined administration of therapeutic drugs for the clinical treatment of brain tumors when other methods are insufficient.

Research Progress of Biomimetic Memristor Flexible Synapse

With the development of the Internet of things, artificial intelligence, and wearable devices, massive amounts of data are generated and need to be processed. High standards are required to store and analyze this information. In the face of the explosive growth of information, the memory used in data storage and processing faces great challenges. Among many types of memories, memristors have received extensive attentions due to their low energy consumption, strong tolerance, simple structure, and strong miniaturization. However, they still face many problems, especially in the application of artificial bionic synapses, which call for higher requirements in the mechanical properties of the device. The progress of integrated circuit and micro-processing manufacturing technology has greatly promoted development of the flexible memristor. The use of a flexible memristor to simulate nerve synapses will provide new methods for neural network computing and bionic sensing systems. In this paper, the materials and structure of the flexible memristor are summarized and discussed, and the latest configuration and new materials are described. In addition, this paper will focus on its application in artificial bionic synapses and discuss the challenges and development direction of flexible memristors from this perspective.

Recent Advances in Self-Powered Piezoelectric and Triboelectric Sensors: From Material and Structure Design to Frontier Applications of Artificial Intelligence

The development of artificial intelligence and the Internet of things has motivated extensive research on self-powered flexible sensors. The conventional sensor must be powered by a battery device, while innovative self-powered sensors can provide power for the sensing device. Self-powered flexible sensors can have higher mobility, wider distribution, and even wireless operation, while solving the problem of the limited life of the battery so that it can be continuously operated and widely utilized. In recent years, the studies on piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) have mainly concentrated on self-powered flexible sensors. Self-powered flexible sensors based on PENGs and TENGs have been reported as sensing devices in many application fields, such as human health monitoring, environmental monitoring, wearable devices, electronic skin, human–machine interfaces, robots, and intelligent transportation and cities. This review summarizes the development process of the sensor in terms of material design and structural optimization, as well as introduces its frontier applications in related fields. We also look forward to the development prospects and future of self-powered flexible sensors.

Natural Acidic Polysaccharide-Based Memristors for Transient Electronics: Highly Controllable Quantized Conductance for Integrated Memory and Nonvolatile Logic Applications

As a leading candidate for further memory and computing applications, memristors are being developed in an important direction of transient electronics. Herein, wafer-scale acidic polysaccharide thin films are reported as promising materials for memristors with remarkable transient characteristics. The memristor shows freestanding and lightweight features, and can be fully dissolved in deionized water within 3.5 s. More importantly, the ion-confinement capability of acidic polysaccharides where the cations can interact with the ionizable acid groups enables atomic manipulation of conductive filament. As a result, (i) a single device can produce 16 highly controllable and independent quantized conductance (QC) states with quasi-nonvolatile and nonvolatile characteristics and (ii) QC switching can be performed with ultrafast speed (2-5 ns) and low energy consumption (0.6-16 pJ). These remarkable features make the memristor promising for fast, low-power, and high-density memory and computing applications. Based on QC switching, the encoding/decoding and nonvolatile basic Boolean logic are designed and implemented. More importantly, "stateful" material implication logic which is promising for future in-memory computing is demonstrated with QC switching. These results significantly advance acidic polysaccharides to develop nanodevices with quantum effects.

New Silk Road: From Mesoscopic Reconstruction/Functionalization to Flexible Meso-Electronics/Photonics Based on Cocoon Silk Materials

Two of the key questions to be addressed are whether and how one can turn cocoon silk into fascinating materials with different electronic and optical functions so as to fabricate the flexible devices. In this review, a comprehensive overview of the unique strategy of mesoscopic functionalization starting from silk fibroin (SF) materials to the fabrication of various meso flexible SF devices is presented. Notably, SF materials with novel and enhanced properties can be achieved by mesoscopically reconstructing the hierarchical structures of SF materials. This is based on rerouting the refolding process of SF molecules by meso-nucleation templating. As-acquired functionalized SF materials can be applied to fabricate bio-compatible/degradable flexible/implantable meso-optical/electronic devices of various types. Consequently, functionalized SF can be fabricated into optical elements, that is, nonlinear photonic and fluorescent components, and make it possible to construct silk meso-electronics with high-performance. These advances enable the applications of SF-material based devices in the areas of physical and biochemical sensing, meso-memristors, transistors, brain electrodes, and energy generation/storage, applicable to on-skin long-term monitoring of human physiological conditions, and in-body sensing, information processing, and storage.

Bio-memristors based on silk fibroin

Bio-memristors constitute candidates for the next generation of non-volatile storage and bionic synapses due to their biocompatibility, environmental benignity, sustainability, flexibility, degradability, and impressive memristive performance. Silk fibroin (SF), a natural and abundant biomaterial with excellent mechanical, optical, electrical, and structure-adjustable properties as well as being easy to process, has been utilized and shown to have potential in the construction of bio-memristors. Here, we first summarize the fundamental mechanisms of bio-memristors based on SF. Then, the latest achievements and developments of pristine and composited SF-based memristors are highlighted, followed by the integration of memristive devices. Finally, the challenges and insights associated with SF-based bio-memristors are presented. Advances in SF-based bio-memristors will open new avenues in the design and integration of high-performance bio-integrated systems and facilitate their application in logic operations, complex circuits, and neural networks.

Immune Response to Silk Sericin-Fibroin Composites: Potential Immunogenic Elements and Alternatives for Immunomodulation

The unique properties of silk proteins (SPs), particularly silk sericin (SS) and silk fibroin (SF), have attracted attention in the design of scaffolds for tissue engineering over the past decades. Since SF has good mechanical properties, while SS displays bioactivity, scaffolds combining both proteins should exhibit complementary properties enhancing the potential of these materials. Unfortunately, SS-SF composites can generate chronic immune responses and their immunogenic element is not completely clear. The potential of SS-SF composites in tissue engineering, elements which may contribute to their immunogenicity, and alternatives for their preparation and design, to modulate the immune response and take advantage of their useful properties, are discussed in this review. It is known that SS can enhance β-sheet formation in SF, which may act as hydrophobic regions with a strong affinity for adsorption proteins inducing the chronic recruitment of inflammatory cells. Therefore, tailoring the exposure of hydrophobic regions at the scaffold surface should represent a viable strategy to modulate the immune response. This can be achieved by coating SS-SF composites with SS or other hydrophilic polymers, to take advantage of their antibiofouling properties. Research is still needed to realize the full potential of these composites for tissue engineering.

Low-Power and Tunable-Performance Biomemristor Based on Silk Fibroin

Biomemristors have attracted significant attention because of their potential applications in logic operations, nonvolatile memory, and synaptic emulators, thus leading to the urgent need to improve memristive performance. In this work, a silk fibroin (SF)-based memristor, integrated with both low power and low operating current simultaneously, has been reported. Doping the SF with Ag and an ethanol-based post-treatment promote microcrystal formation in the bulk of the SF. This induces carrier transport along fixed, short paths and results in a low set voltage, low operating current, and high memristive stability. Such performances can greatly reduce power consumption and heat generation, beneficial for the accuracy and durability of memristor devices. The memristive mechanism of SF-based memristors with different Ag contents is the space-charge-limited conduction (SCLC) mechanism. In addition, the nonlinear transmission property of SF-based memristors suggests useful applications in bioelectronics.

Recent progress in silk fibroin-based flexible electronics

With the rapid development of the Internet of Things (IoT) and the emergence of 5G, traditional silicon-based electronics no longer fully meet market demands such as nonplanar application scenarios due to mechanical mismatch. This provides unprecedented opportunities for flexible electronics that bypass the physical rigidity through the introduction of flexible materials. In recent decades, biological materials with outstanding biocompatibility and biodegradability, which are considered some of the most promising candidates for next-generation flexible electronics, have received increasing attention, e.g., silk fibroin, cellulose, pectin, chitosan, and melanin. Among them, silk fibroin presents greater superiorities in biocompatibility and biodegradability, and moreover, it also possesses a variety of attractive properties, such as adjustable water solubility, remarkable optical transmittance, high mechanical robustness, light weight, and ease of processing, which are partially or even completely lacking in other biological materials. Therefore, silk fibroin has been widely used as fundamental components for the construction of biocompatible flexible electronics, particularly for wearable and implantable devices. Furthermore, in recent years, more attention has been paid to the investigation of the functional characteristics of silk fibroin, such as the dielectric properties, piezoelectric properties, strong ability to lose electrons, and sensitivity to environmental variables. Here, this paper not only reviews the preparation technologies for various forms of silk fibroin and the recent progress in the use of silk fibroin as a fundamental material but also focuses on the recent advanced works in which silk fibroin serves as functional components. Additionally, the challenges and future development of silk fibroin-based flexible electronics are summarized. (1) This review focuses on silk fibroin serving as active functional components to construct flexible electronics. (2) Recent representative reports on flexible electronic devices that applied silk fibroin as fundamental supporting components are summarized. (3) This review summarizes the current typical silk fibroin-based materials and the corresponding advanced preparation technologies. (4) The current challenges and future development of silk fibroin-based flexible electronic devices are analyzed.

Artificial Skin Perception

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.

A Battery-Like Self-Selecting Biomemristor from Earth-Abundant Natural Biomaterials

Using the earth-abundant natural biomaterials to manufacture functional electronic devices meets the sustainable requirement of green electronics, especially for the practical application of memristors in data storage and neuromorphic computing. However, the sneak currents flowing though the unselected cells in a large-scale cross-bar memristor array is one of the major problems which need to be tackled. The self-selecting memristors can solve the problem to develop compact and concise integrated circuits. Here, a sustainable natural biomaterial (anthocyanin, C₁₅H₁₁O₆) extracted from plant tissue is demonstrated for ions and electron transport. The capacitive-coupled memristive behavior of as-prepared bioelectronic device can be significantly modulated by diethylmethyl(2-methoxyethyl)ammoium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) ionic liquid (IL). Furthermore, graphene was inserted into biomaterial matrix to manipulate the memristive effects by graphene protonation. This results in a battery-like self-selective memristive effect. This phenomenon is explained by a physical model and density functional theory (DFT) based first-principles calculations. Finally, the self-selective behavior was applied in 0T-1R array configuration, which indicates the battery-like self-selecting biomemristor has potential applications in the brain-inspired computing, data storage systems, and high-density device integration.

Charge transport in pyroprotein-based electronic yarns

Pyroprotein-based carbon materials produced by heat-treating silk proteins have many potential applications in electronic devices, such as electronic textiles. To further develop potential electronic devices using these pyroproteins, the charge transport mechanism has to be verified. However, the electrical characteristics of the pyroproteins have not been reported yet. In this study, the temperature-dependent charge transport behavior of pyroprotein-based electronic yarns prepared from commercial silks (e-CS yarns) is investigated with respect to various heat treatment temperatures (HTT, 800, 1000, 1200, and 1400 °C). The linear current-voltage properties are shown at a low bias of 100 nA from 9 K to 300 K. The temperature-dependent resistivity of the e-CS yarns can be clearly described by the crossover of 3-dimensional Mott variable range hopping and fluctuation-induced tunneling conduction at the crossover temperature (Tc). These Tc factors are significantly different, due to the structural modulation of the e-CS yarns depending on the HTT, and are characterized by Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. This study is expected to provide a better understanding of the electrical properties of pyroproteins.

Advances of RRAM Devices: Resistive Switching Mechanisms, Materials and Bionic Synaptic Application

Resistive random access memory (RRAM) devices are receiving increasing extensive attention due to their enhanced properties such as fast operation speed, simple device structure, low power consumption, good scalability potential and so on, and are currently considered to be one of the next-generation alternatives to traditional memory. In this review, an overview of RRAM devices is demonstrated in terms of thin film materials investigation on electrode and function layer, switching mechanisms and artificial intelligence applications. Compared with the well-developed application of inorganic thin film materials (oxides, solid electrolyte and two-dimensional (2D) materials) in RRAM devices, organic thin film materials (biological and polymer materials) application is considered to be the candidate with significant potential. The performance of RRAM devices is closely related to the investigation of switching mechanisms in this review, including thermal-chemical mechanism (TCM), valance change mechanism (VCM) and electrochemical metallization (ECM). Finally, the bionic synaptic application of RRAM devices is under intensive consideration, its main characteristics such as potentiation/depression response, short-/long-term plasticity (STP/LTP), transition from short-term memory to long-term memory (STM to LTM) and spike-time-dependent plasticity (STDP) reveal the great potential of RRAM devices in the field of neuromorphic application.

Flexible and fully biodegradable resistance random access memory based on a gelatin dielectric

The increased public concerns on healthcare, the environment and sustainable development inspired the development of biodegradable and biocompatible electronics that could be used as degradable electronics in implants. In this work, a fully biodegradable and flexible resistance random access memory (RRAM) was developed with low-cost biomaterial gelatin as the dielectric layer and the biodegradable polymer poly(lactide-coglycolide) acid (PLGA) as the substrate. PLGA can be synthesized by a simple solution process, and the PLGA substrate can be peeled off the handling substrate for operation once the devices are fabricated. The fabricated memory devices exhibited reliable nonvolatile resistive switching characteristics with a long retention time over 10⁴ s and a near-constant on/off resistance ratio of 10² even after 200 bending cycles, showing the promising potential for application in flexible electronics. Degradation of the devices in deionized water and in phosphate buffered saline (PBS) solution showed that the whole devices can be completely degraded in water. The dissolution time of the metals and the gelatin layer was a few days, while that for PLGA is about 6 months, and can be modified by changing the synthesis conditions of the film, thus allowing the development of biodegradable electronics with designed dissolution time.

Degradable and Dissolvable Thin-Film Materials for the Applications of New-Generation Environmental-Friendly Electronic Devices

The environmental pollution generated by electronic waste (e-waste), waste-gas, and wastewater restricts the sustainable development of society. Environmental-friendly electronics made of degradable, resorbable, and compatible thin-film materials were utilized and explored, which was beneficial for e-waste dissolution and sustainable development. In this paper, we present a literature review about the development of various degradable and disposable thin-films for electronic applications. The corresponding preparation methods were simply reviewed and one of the most exciting and promising methods was discussed: Printing electronics technology. After a short introduction, detailed applications in the environment sensors and eco-friendly devices based on these degradable and compatible thin-films were mainly reviewed, finalizing with the main conclusions and promising perspectives. Furthermore, the future on these upcoming environmental-friendly electronic devices are proposed and prospected, especially on resistive switching devices, showing great potential applications in artificial intelligence (AI) and the Internet of Thing (IoT). These resistive switching devices combine the functions of storage and computations, which can complement the off-shelf computing based on the von Neumann architecture and advance the development of the AI.

Interfacial synthesis of a large-area coordination polymer membrane for rewritable nonvolatile memory devices

The facile synthesis of large-area coordination polymer membranes with controlled nanoscale thicknesses is critical towards their applications in information storage electronics. Here, we have reported a facile and substrate-independent interfacial synthesis method for preparing a large-area two-dimensional (2D) coordination polymer membrane at the air–liquid interface. The prepared high-quality 2D membrane could be transferred onto an indium tin oxide (ITO) substrate to construct a nonvolatile memory device, which showed reversible switching with a high ON/OFF current ratio of 10³, good stability and a long retention time. Our discovery of resistive switching with nonvolatile bistability based on the substrate-independent growth of the 2D coordination polymer membrane holds significant promise for the development of solution-processable nonvolatile memory devices with a miniaturized device size.

Stable nonvolatile memory devices with a high ON/OFF current ratio have been realized based on a large-area two-dimensional coordination polymer membrane.

Silk-Based Advanced Materials for Soft Electronics

Soft bioelectronics that could be integrated with soft and curvilinear biological tissues/organs have attracted multidisciplinary research interest from material scientists, electronic engineers, and biomedical scientists. Because of their potential human health-related applications, soft bioelectronics require stringent demands for biocompatible components. Silk, as a kind of well-known ancient natural biopolymer, shows unique combined merits such as good biocompatibility, programmable biodegradability, processability into various material formats, and large-scale sustainable production. Such unique merits have made silk popular for intensive design and study in soft bioelectronics over the past decade. Due to the development of fabrication techniques in material processing and progress in research, silk has been engineered into a variety of advanced materials including silk fibers/textiles, nanofibers, films, hydrogels, and aerogels. Natural and regenerated silk materials can also be transformed into intrinsically nitrogen-doped and electrically conductive carbon materials, due to their unique molecular structure and high nitrogen content. The rich morphologies and varied processing options for silk materials can furnish transformed carbon materials with well-designed structures and properties. The favorable and unique material merits of silk materials and silk-derived carbon materials offer potential applications in soft electronics. Based on commercial silk fibers/textiles and the availability of re-engineered silk materials with versatile technological formats, functional soft electronics have been explored with silk as flexible biosupports/biomatrixes or active components. These soft systems include bioresorbable electronics, ultraconformal bioelectronics, transient electronics, epidermal electronics, textile electronics, conformal biosensors, flexible transistors, and resistive switching memory devices. Silk-derived carbon materials with rationally designed morphologies and structures have also been developed as active components for wearable sensors, electronic skins, and flexible energy devices, which provide novel concepts and opportunities for soft electronics. In this Account, we highlight the unique hierarchical and chemical structure of natural silk fibers, the fabrication strategies for processing silk into materials with versatile morphologies and into electrically conductive carbon materials, as well as recent progress in the development of silk-based advanced materials (silk materials and silk-derived carbon materials) for soft bioelectronics. The design and functionality of soft electronics developed with commercial silk fibers/textiles, re-engineered silk materials, and silk-derived carbon materials as biosubstrate/matrix and active components is introduced in detail. We further discuss future challenges and prospects for developing silk-based soft bioelectronics for wearable healthcare systems. By leveraging the unique advantages of silk-based advanced materials, the design and construction strategy for flexible electronics, as well as the potential of flexible electronics for conformable and intimate association with human tissues/organs, silk-based soft bioelectronics should have a significant impact on diverse healthcare fields.

Softening gold for elastronics

Gold, one of the noble metals, has played a significant role in human society throughout history. Gold's excellent electrical, optical and chemical properties make the element indispensable in maintaining a prosperous modern electronics industry. In the emerging field of stretchable electronics (elastronics), the main challenge is how to utilize these excellent material properties under various mechanical deformations. This review covers the recent progress in developing "softening" gold chemistry for various applications in elastronics. We systematically present material synthesis and design principles, applications, and challenges and opportunities ahead.

Wafer-Scale Multilayer Fabrication for Silk Fibroin-Based Microelectronics

Silk fibroin is an excellent candidate for biomedical implantable devices because of its biocompatibility, controllable biodegradability, solution processability, flexibility, and transparency. Thus, fibroin has been widely explored in biomedical applications as biodegradable films as well as functional microstructures. Although there exists a large number of patterning methods for fibroin thin films, multilayer micropatterning of fibroin films interleaved with metal layers still remains a challenge. Herein, we report a new wafer-scale multilayer microfabrication process named aluminum hard mask on silk fibroin (AMoS), which is capable of micropatterning multiple layers composed of both fibroin and inorganic materials (e.g., metal and dielectrics) with high-precision microscale alignment. To the best of our knowledge, our AMoS process is the first demonstration of wafer-scale multilayer processing of both silk fibroin and metal micropatterns. In the AMoS process, aluminum deposited on fibroin is first micropatterned using conventional ultraviolet (UV) photolithography, and the patterned aluminum layer is then used as a mask to pattern fibroin underneath. We demonstrate the versatility of our fabrication process by fabricating fibroin microstructures with different dimensions, passive electronic components composed of both fibroin and metal layers, and functional fibroin microstructures for drug delivery. Furthermore, because one of the crucial advantages of fibroin is biocompatibility, we assess the biocompatibility of our fabrication process through the culture of highly susceptible primary neurons. Because the AMoS process utilizes conventional UV photolithography, the principal advantages of our process are multilayer fabrication with high-precision alignment, high resolution, wafer-scale large area processing, no requirement for chemical modification of the protein, and high throughput and thus low cost, all of which have not been feasible with silk fibroin. Therefore, the proposed fabrication method is a promising candidate for batch fabrication of functional fibroin microelectronics (e.g., memristors and organic thin film transistors) for next-generation implantable biomedical applications.

Biodegradable Natural Pectin-Based Flexible Multilevel Resistive Switching Memory for Transient Electronics

Transient electronics that can physically vanish in solution can offer opportunities to address the ecological challenges for dealing with the rapidly growing electronic waste. As one important component, it is desirable that memory devices combined with the transient feature can also be developed as secrecy information storage systems besides the above advantage. Resistive switching (RS) memory is one of the most promising technologies for next-generation memory. Herein, the biocompatible pectin extracted from natural orange peel is introduced to fabricate RS memory devices (Ag/pectin/indium tin oxides (ITO)), which exhibit excellent RS characteristics, such as forming free characteristic, low operating voltages (≈1.1 V), fast switching speed (<70 ns), long retention time (>10⁴ s), and multilevel RS behaviors. The device performance is not degraded after 10⁴ bending cycles, which will be beneficial for flexible memory applications. Additionally, instead of using acid solution, the Ag/pectin/ITO memory device can be dissolved rapidly in deionized water within 10 min thanks to the good solubility arising from ionization of its carboxylic groups, which shows promising application for green electronics. The present biocompatible memory devices based on natural pectin suggest promising material candidates toward enabling high-density secure information storage systems applications, flexible electronics, and green electronics.

A sustainable resistive switching memory device based on organic keratin extracted from hair

It is the consensus of researchers that the reuse of natural resources is an effective way to solve the problems of environmental pollution, waste and overcapacity. Moreover, compared with the case of inorganic materials, the renewability of natural biomaterials has great prominent advantages. In this study, keratin, which was first extracted from hair due to its high content in hair, was chosen as a functional layer for the fabrication of a resistance switching device with the Ag/keratin/ITO structure; in this device, a stable resistive switching memory behavior with good retention characteristic was observed. Via mechanism analysis, it is expected that there is hopping conduction at low biases, and the formation of a conductive filament occurs at high biases. Furthermore, our device exhibited a stable switching behavior with different conductive materials (Ti and FTO) as bottom electrodes, and the influence of Ag and graphite conductive nanoparticles (NPs) doped into the keratin layer on the switching performance of the device was also investigated. This study not only suggests that keratin is a potential biomaterial for the preparation of memory devices, but also provides a promising route for the fabrication of bio-electronic devices with non-toxicity, degradability, sustainability etc.

This study suggests that keratin is a potential biomaterial for the preparation of memory devices with non-toxicity, degradability and sustainability.

Physically Transient Field-Effect Transistors Based on Black Phosphorus

Black phosphorus (BP) has shown great potential as a semiconductor material beyond graphene and MoS₂ because of its intrinsic band gap and high mobility. Moreover, the biocompatibility of the final biodegradation products of BP has led to extensive research on biomedical applications. Herein, physically transient field-effect transistors (FETs) based on black phosphorus have been demonstrated using peptide insulator as a gate dielectric layer. The fabricated devices show high hole mobility up to 468 cm² V^-1 s^-1 and on-off current ratio over 10³. The combined use of black phosphorus, peptide, and molybdenum provides rapid disappearance of the devices within 36 h. Dissolution kinetics and cytotoxicity of black phosphorus are assessed to clarify its availability to be applied in transient electronics. This work provides transient FETs with high degradability and high performance based on biocompatible black phosphorus.

A bio-inspired physically transient/biodegradable synapse for security neuromorphic computing based on memristors

Physically transient electronic devices that can disappear on demand have great application prospects in the field of information security, implantable biomedical systems, and environment friendly electronics. On the other hand, the memristor-based artificial synapse is a promising candidate for new generation neuromorphic computing systems in artificial intelligence applications. Therefore, a physically transient synapse based on memristors is highly desirable for security neuromorphic computing and bio-integrated systems. Here, this is the first presentation of fully degradable biomimetic synaptic devices based on a W/MgO/ZnO/Mo memristor on a silk protein substrate, which show remarkable information storage and synaptic characteristics including long-term potentiation (LTP), long-term depression (LTD) and spike timing dependent plasticity (STDP) behaviors. Moreover, to emulate the apoptotic process of biological neurons, the transient synapse devices can be dissolved completely in phosphate-buffered saline solution (PBS) or deionized (DI) water in 7 min. This work opens the route to security neuromorphic computing for smart security and defense electronic systems, as well as for neuro-medicine and implantable electronic systems.

Enabling Transient Electronics with Degradation on Demand via Light-Responsive Encapsulation of a Hydrogel-Oxide Bilayer

Physically transient electronics, which can disappear under certain conditions in aqueous solutions or biofluids, has attracted increasing attention because of its potential applications as "green" electronics and biomedical devices. Till now, the excitation of the transient process is achieved by passive dissolution of the encapsulation layer, which has a very limited control over the process. Here, we report a novel light-triggered encapsulation strategy via a bilayer of a light-responsive hydrogel and oxide to control the degradation on demand in aqueous environment. The hydrogel serving as a barrier between the environment and oxide limited the water's movement and penetration, leading to improved stable operation time. More importantly, the light-responsive hydrogel underwent a gel-to-solution transition upon applying ultraviolet (UV) light. The drastic change of the water movement enabled a transient process triggered on demand. Via this encapsulation scheme, we demonstrated fully soluble resistors and resistive random access memory devices with the UV light-triggered transient process. This work provides a new pathway to design transient devices with controllable degradation to meet various requirements of green electronics and biomedical devices.

Phototunable Biomemory Based on Light-Mediated Charge Trap

Phototunable biomaterial-based resistive memory devices and understanding of their underlying switching mechanisms may pave a way toward new paradigm of smart and green electronics. Here, resistive switching behavior of photonic biomemory based on a novel structure of metal anode/carbon dots (CDs)-silk protein/indium tin oxide is systematically investigated, with Al, Au, and Ag anodes as case studies. The charge trapping/detrapping and metal filaments formation/rupture are observed by in situ Kelvin probe force microscopy investigations and scanning electron microscopy and energy-dispersive spectroscopy microanalysis, which demonstrates that the resistive switching behavior of Al, Au anode-based device are related to the space-charge-limited-conduction, while electrochemical metallization is the main mechanism for resistive transitions of Ag anode-based devices. Incorporation of CDs with light-adjustable charge trapping capacity is found to be responsible for phototunable resistive switching properties of CDs-based resistive random access memory by performing the ultraviolet light illumination studies on as-fabricated devices. The synergistic effect of photovoltaics and photogating can effectively enhance the internal electrical field to reduce the switching voltage. This demonstration provides a practical route for next-generation biocompatible electronics.

Transient and flexible polymer memristors utilizing full-solution processed polymer nanocomposites

Building transient and flexible memristors is a promising strategy for developing emerging memory technologies. Here, a transient and flexible memristor based on a polymer nanocomposite, with a configuration of silver nanowire (AgNW)/citric acid quantum dot (CA QD)-polyvinyl pyrrolidone (PVP)/AgNW, is fabricated using a full-solution process method. The obtained device exhibits reversible resistive switching behavior and a dynamic random access memory (DRAM) storage feature, with the significant merits of a high ON/OFF ratio, low switching voltage, excellent reproducibility and desirable high flexibility, indicating outstanding memory characteristics such as low misreading, low power operation and low cost potential. Moreover, an operating mechanism of charge trapping/de-trapping of the quantum dots in the polymer matrix has been proposed. Importantly, the memristor can be disintegrated in water within 30 minutes, showing that it is a promising candidate for transient memories. This work paves a new way for potential use of this material in transient electronics, implanted electronics, data storage security and flexible electronic systems.

Transient security transistors self-supported on biodegradable natural-polymer membranes for brain-inspired neuromorphic applications

Transient electronics, a new generation of electronics that can physically or functionally vanish on demand, are very promising for future "green" security biocompatible electronics. At the same time, hardware implementation of biological synapses is highly desirable for emerging brain-like neuromorphic computational systems that could look beyond the conventional von Neumann architecture. Here, a hardware-security physically-transient bidirectional artificial synapse network based on a dual in-plane-gate Al-Zn-O neuromorphic transistor was fabricated on free-standing laterally-coupled biopolymer electrolyte membranes (sodium alginate). The excitatory postsynaptic current, paired-pulse-facilitation, and temporal filtering characteristics from high-pass to low-pass transition were successfully mimicked. More importantly, bidirectional dynamic spatiotemporal learning rules and neuronal arithmetic were also experimentally demonstrated using two lateral in-plane gates as the presynaptic inputs. Most interestingly, excellent physically-transient behavior could be achieved with a superfast water-soluble speed of only ∼120 seconds. This work represents a significant step towards future hardware-security transient biocompatible intelligent electronic systems.

Physically Transient Threshold Switching Device Based on Magnesium Oxide for Security Application

Transient memristors are prospective candidates for both secure memory systems and biointegrated electronics, which are capable to physically disappear at a programmed time with a triggered operation. However, the sneak current issue has been a considerable obstacle to achieve high-density transient crossbar array of memristors. To solve this problem, it is necessary to develop a transient switch device to turn the memory device on and off controllably. Here, a dissolvable and flexible threshold switching (TS) device with a vertically crossed structure is introduced, which exhibits a high selectivity of 10⁷ , steep turn-on slope of <8 mV dec^-1 , and fast ON/OFF switch speed within 50/25 ns. Triggered failure could be achieved after soaking the device in deionized water for 8 min at room temperature. Furthermore, a water-assisted transfer printing method is used to fabricate flexible and transient TS device arrays for bioresorbable systems, in which none of any significant degradation is observed under a bending radius of 2 mm. Integrating the selector with a transient memristor is capable of 10⁷ Gb memory implementation, indicating that the transient TS device could provide great opportunities to achieve highly integrated transient memory arrays.

Analysis of the Bipolar Resistive Switching Behavior of a Biocompatible Glucose Film for Resistive Random Access Memory

Resistive random access memory (RRAM) devices are fabricated through a simple solution process using glucose, which is a natural biomaterial for the switching layer of RRAM. The fabricated glucose-based RRAM device shows nonvolatile bipolar resistive switching behavior, with a switching window of 10³ . In addition, the endurance and data retention capability of glucose-based RRAM exhibit stable characteristics up to 100 consecutive cycles and 10⁴ s under constant voltage stress at 0.3 V. The interface between the top electrode and the glucose film is carefully investigated to demonstrate the bipolar switching mechanism of the glucose-based RRAM device. The glucose based-RRAM is also evaluated on a polyimide film to verify the possibility of a flexible platform. Additionally, a cross-bar array structure with a magnesium electrode is prepared on various substrates to assess the degradability and biocompatibility for the implantable bioelectronic devices, which are harmless and nontoxic to the human body. It is expected that this research can provide meaningful insights for developing the future bioelectronic devices.

Plasticizing Silk Protein for On-Skin Stretchable Electrodes

Soft and stretchable electronic devices are important in wearable and implantable applications because of the high skin conformability. Due to the natural biocompatibility and biodegradability, silk protein is one of the ideal platforms for wearable electronic devices. However, the realization of skin-conformable electronic devices based on silk has been limited by the mechanical mismatch with skin, and the difficulty in integrating stretchable electronics. Here, silk protein is used as the substrate for soft and stretchable on-skin electronics. The original high Young's modulus (5-12 GPa) and low stretchability (<20%) are tuned into 0.1-2 MPa and > 400%, respectively. This plasticization is realized by the addition of CaCl₂ and ambient hydration, whose mechanism is further investigated by molecular dynamics simulations. Moreover, highly stretchable (>100%) electrodes are obtained by the thin-film metallization and the formation of wrinkled structures after ambient hydration. Finally, the plasticized silk electrodes, with the high electrical performance and skin conformability, achieve on-skin electrophysiological recording comparable to that by commercial gel electrodes. The proposed skin-conformable electronics based on biomaterials will pave the way for the harmonized integration of electronics into human.

Transient Light Emitting Devices Based on Soluble Polymer Composites

Building transient electronics are promising and emerging strategy to alleviate the pollution issues from electronic waste (e-waste). Although a variety of transient devices comprising organic and inorganic materials have been described, transient light emitting devices are still elusive but highly desirable because of the huge amount of lighting and display related consumer electronics. Here we report a simple and efficient fabrication of large-area flexible transient alternating current electroluminescent (ACEL) device through a full-solution processing method. Using transparent flexible AgNW-polymer as both electrodes, the devices exhibit high flexibility and both ends side light emission, with the features of controlled size and patterned structure. By modulating the mass ratio of blue and yellow phosphors, the emission color is changed from white to blue. Impressively, the fabricated ACEL device can be thoroughly dissolved in water within 30 min. Our strategy combining such advances in transient light emitting devices with simple structure, widely used materials, full solution process, and high solubility will demonstrate great potential in resolving the e-waste from abandoned light-emitting products and expand the opportunities for air-stable flexible light emitting devices.

Full imitation of synaptic metaplasticity based on memristor devices

Neuromorphic engineering is a promising technology for developing new computing systems owing to the low-power operation and the massive parallelism similarity to the human brain. Optimal function of neuronal networks requires interplay between rapid forms of Hebbian plasticity and homeostatic mechanisms that adjust the threshold for plasticity, termed metaplasticity. Metaplasticity has important implications in synapses and is barely addressed in neuromorphic devices. An understanding of metaplasticity might yield new insights into how the modification of synapses is regulated and how information is stored by synapses in the brain. Here, we propose a method to imitate the metaplasticity inhibition of long-term potentiation (MILTP) for the first time based on memristors. In addition, the metaplasticity facilitation of long-term potentiation (MFLTP) and the metaplasticity facilitation of long-term depression (MFLTD) are also achieved. Moreover, the mechanisms of metaplasticity in memristors are discussed. Additionally, the proposed method to mimic the metaplasticity is verified by three different memristor devices including oxide-based resistive memory (OxRAM), interface switching random access memory, and conductive bridging random access memory (CBRAM). This is a further step toward developing fully bio-realistic artificial synapses using memristors. The findings in this study will deepen our understanding of metaplasticity, as well as provide new insight into bio-realistic neuromorphic engineering.