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The rising flexible and intelligent electronics greatly facilitate the noninvasive and timely tracking of physiological information in telemedicine healthcare. Meticulously building bionic-sensitive moieties is vital for designing efficient electronic skin with advanced cognitive functionalities to pluralistically capture external stimuli. However, realistic mimesis, both in the skin's three-dimensional interlocked hierarchical structures and synchronous encoding multistimuli information capacities, remains a challenging yet vital need for simplifying the design of flexible logic circuits. Herein, we construct an artificial epidermal device by in situ growing Cu3(HHTP)2 particles onto the hollow spherical Ti3C2Tx surface, aiming to concurrently emulate the spinous and granular layers of the skin's epidermis. The bionic Ti3C2Tx@Cu3(HHTP)2 exhibits independent NO2 and pressure response, as well as novel functionalities such as acoustic signature perception and Morse code-encrypted message communication. Ultimately, a wearable alarming system with a mobile application terminal is self-developed by integrating the bimodular senor into flexible printed circuits. This system can assess risk factors related with asthmatic, such as stimulation of external NO2 gas, abnormal expiratory behavior and exertion degrees of fingers, achieving a recognition accuracy of 97.6% as assisted by a machine learning algorithm. Our work provides a feasible routine to develop intelligent multifunctional healthcare equipment for burgeoning transformative telemedicine diagnosis. Highlights: A smart wearable alarming system integrated artificial epidermal device for pluralistically identifying asthmatic attack risk factors, achieving a 97.6% classification accuracy as assisted by machine learning algorithm. A meticulous mimicry both in the advanced structural attributes and encoding information abilities of the skin was adopted to design a novel artificial epidermal device by integrating conductive Cu3(HHTP)2 coupled with spherical Ti3C2Tx. The bioinspired Ti3C2Tx@Cu3(HHTP)2 sensors can independently perceive NO2 gas and pressure-triggered stimuli. [ABSTRACT FROM AUTHOR]

Metal micro/nanoparticle ink-based printed circuits have shown promise for promoting the scalable application of flexible electronics due to enabling superhigh metallic conductivity with cost-effective mass production. However, it is challenging to activate printed metal-particle patterns to approach the intrinsic conductivity without damaging the flexible substrate, especially for high melting-point metals. Here, we report a pressure-constrained sonication activation (PCSA) method of the printed flexible circuits for more than dozens of metal (covering melting points from room temperature to 3422 °C) and even nonmetallic inks, which is integrated with the large-scale roll-to-roll process. The PCSA-induced synergistic heat-softening and vibration-bonding effect of particles can enable multilayer circuit interconnection and join electronic components onto printed circuits without solder within 1 s at room temperature. We demonstrate PCSA-based applications of 3D flexible origami electronics, erasable and foldable double-sided electroluminescent displays, and custom-designed and large-area electronic textiles, thus indicating its potential for universality in flexible electronics. It is challenging to activate the flexible printed metal circuit to approach the intrinsic conductivity with minimal damage to the substrate. Here, the authors report a large-scale pressure-constrained sonication activation method for various metals and non-metallic inks. [ABSTRACT FROM AUTHOR]

It remains a huge challenge to efficiently recycle the printed circuit boards (PCBs) because of the diversity and complexity of the PCBs in terms of material composition. The recycling of PCBs highly depends on the design of the polymers that make up the polymer substrates and conductive pastes. Herein, closed‐loop recyclable plastics for use as the polymer substrates and for the preparation of the conductive pastes are fabricated by dynamically cross‐linking phenylboronic acid‐functionalized poly(methylmethacrylate) (PMMA‐B) and poly(urea‐urethane) (PUU‐B) with boroxines. The closed‐loop recyclable plastic, which is denoted as PMMA‐PUU exhibits a breaking strength of 71.0 MPa and Young's modulus of 1.8 GPa. Because of the dynamic nature of boroxines, the PMMA‐PUU0.6 plastic can be depolymerized in the N, N‐dimethylacetamide/ethanol mixture solvent. The depolymerized PMMA and PUU can be efficiently recovered in high yields and purity through their solubility differences in selective solvents. Conductive pastes suitable for printing of electrical circuits are fabricated through complexation of Ag particles with PMMA‐PUU. PCBs composed of PMMA‐PUU substrates and PMMA‐PUU‐based conductive pastes are flexible, healable, and recyclable. The closed‐loop recycling of the PCBs enables the collection of highly purified Ag particles, PMMA‐B, and PUU‐B monomers even when the PCBs are mixed with polymer wastes. [ABSTRACT FROM AUTHOR]

Traditional circuit board fabrication schemes are not directly applicable to the production of flexible, multi‐material circuits. This article reports a technique, microfluidic directed material patterning, which combines soft microfluidic stamps and low‐temperature solution‐phase deposition to generate multi‐material circuits on flexible, non‐planar polymeric supports. Specifically, metallic and semiconductive traces are combined on commodity plastic films to yield functional photosensitive circuits that can be used in the spectrophotometric detection and concentration measurement of microdroplets on 3D “e‐plates.” The photoresistive material cadmium sulfide is used in these circuits because it is suitable for visible light detection and it can be deposited directly from aqueous solutions following established bath deposition procedures. This method can produce colorimetric devices capable of quantifying micromolar concentrations of Allura Red in microdroplets of Kool‐Aid. This technique presents the opportunity for producing single‐use or low‐use disposable/recyclable devices for flexible 3D sensors and detectors following a convenient, low‐waste fabrication scheme. The general capabilities of this approach, in terms of substrate geometry and device layout (e.g., the number, area, and pattern of photoresistive elements), can be applied to the design and manufacture of more intricate, multiplexed devices supportive of advanced and/or specialized functions that go beyond those reported in this initial demonstration. [ABSTRACT FROM AUTHOR]

Electrical and mechanical integration approaches are essential for emerging hybrid electronics that must robustly bond rigid electrical components with flexible circuits and substrates. However, flexible polymeric substrates and circuits cannot withstand the high temperatures used in traditional electronic processing. This constraint requires new strategies to create flexible materials that simultaneously achieve high electrical conductivity, strong adhesion, and processibility at low temperature. Here, an electrically conductive adhesive is introduced that is flexible, electrically conductive (up to 3.25×105 S m−1) without sintering or high temperature post‐processing, and strongly adhesive to various materials common to flexible and stretchable circuits (fracture energy 350

Transient electronics with degradability or dissolvability on demand possess remarkable characteristics for wearable, bioelectronic, and implanted electronic devices. While synthetic materials have made significant progress in degradability and stretchability, they suffer from intrinsic limitations, including low degradation rates, poor mechanical compliance, and high production cost, which restrict the growth of transient electronic devices. Herein, a versatile fabrication strategy is presented for transient electronics that combine a resilient biopolymer substrate (gelatin biogels) with liquid metal (Galinstan). The circuits made from gelatin biogel‐based and liquid metal are mechanically resilient and durable, exhibiting good compliance with irregular and deformable surfaces, and thus can withstand long‐term exposure to dynamic deformation (about 60 000 cycles at a strain of 50%). More importantly, gelatin biogels are naturally derived and endowed with thermally reversible gelling, which enables on‐demand rapid dissolution and recycling of the materials. As a proof‐of‐concept, transient circuit‐based capacitive sensors, which can be degraded within 20 s, are fabricated for monitoring finger touching, morse code generation, and electrical touchpad. Such a strategy provides a guideline for designing transient electronics with a combination of biopolymer gels and liquid metal, and this approach is anticipated to find applications in human–machine interfaces and wearable devices. [ABSTRACT FROM AUTHOR]

Papertronics introduce a sustainable, cost‐effective revolution in electronics, especially for the Internet of Things. This research overcomes the traditional challenges of paper's porosity, which has impeded electronic component fabrication and performance. A novel approach that harnesses paper's natural capillary action, combined with hydrophobic wax patterning, to achieve precise vertical integration of electronic components is introduced. This method marks a significant departure from conventional surface deposition techniques. This study demonstrates the successful creation of tunable resistors, capacitors, and field‐effect transistors, embedded within a single sheet of paper. Contrary to previous assumptions that impeded the use of paper, its rough and porous texture as a strategic advantage, facilitating the precise fabrication of intricate electronic components is leveraged. Machine learning algorithms play an important role in predicting and enhancing the performance of these papertronic components. This innovation facilitates the development of compact printed circuit boards with increased circuit density, enabling the integration of diverse analog and digital circuits in either single or multi‐layer paper formats. The resulting papertronic systems exceed performance benchmarks, offering eco‐friendly disposal through biodegradability or incineration. These breakthroughs establish papertronics as a feasible, eco‐friendly alternative in the electronics industry, permitting widespread adoption and continuous innovation in sustainable electronic solutions. [ABSTRACT FROM AUTHOR]

Flexible electronics, also referred to as printable electronics, represent an interesting technology for implementing electronic circuits via depositing electronic devices onto flexible substrates, boosting their possible applications. Among all flexible electronics, interdigitated electrodes (IDEs) are currently being used for different sensor applications since they offer significant benefits beyond their functionality as capacitors, like the generation of high output voltage, fewer fabrication steps, convenience of application of sensitive coatings, material imaging capability and a potential of spectroscopy measurements via electrical excitation frequency variation. This review examines the role of IDEs in printed and flexible electronics since they are progressively being incorporated into a myriad of applications, envisaging that the growth pattern will continue in the next generations of flexible circuits to come. [ABSTRACT FROM AUTHOR]

To meet the demand for higher performance and wearability, integrated circuits are developing towards having multilayered structures and greater flexibility. However, traditional circuit fabrication methods using etching and lamination processes are not compatible with flexible substrates. As a non-contact printing method in additive manufacturing, electrohydrodynamic printing possesses advantages such as environmental friendliness, sub-micron manufacturing, and the capability for flexible substrates. However, the interconnection and insulation of different conductive layers become significant challenges. This study took composite silver ink as a conductive material to fabricate a circuit via electrohydrodynamic printing, applied polyimide spraying to achieve interlayer insulation, and drilled micro through-holes to achieve interlayer interconnection. A 200 × 200 mm2 ten-layer flexible circuit was thus prepared. Furthermore, we combined a finite element simulation with reliability experiments, and the prepared ten-layer circuit was found to have excellent bending resistance and thermal cycling stability. This study provides a new method for the manufacturing of low-cost, large-sized, multilayer flexible circuits, which can improve circuit performance and boost the development of printed electronics. [ABSTRACT FROM AUTHOR]

Due to the high application value in intelligent robots, tactile sensors with large sensing area and multi‐dimensional sensing ability have attracted the attention of researchers in recent years. Inspired by bionics of hairs on human skin, a flexible tactile sensor based on magnetic cilia array is developed, showing extremely high sensitivity and stability. The upper layers of the sensor are multiple magnetic cilia containing magnetic particles, while the lower layer is a serpentine flexible circuit board with a magnetic sensor array. When magnetic cilia are bent under force, the magnetic sensor array can detect changes in the magnetic field, thereby the magnitude and direction of external force can be obtained. The proposed sensor has a resolution of 0.2 mN with a working range of 0–19.5 mN and can distinguish the direction of external force. The large sensing area and short response time make this sensor suitable for sliding tactile detection, and experiments show that the sensor can be also applied in object recognition with a success accuracy of 97%. In addition to the shape of objects, the sensor can identify whether there is magnetism inside objects, making it of significant value in intelligent robots and modern medicine. [ABSTRACT FROM AUTHOR]

In this study, the optimal forming parameters for printing flexible circuits using aerosol jet printing technology are explored through numerical simulation and experiments. The printhead during the deposition process is numerically simulated. By employing the controlled variable method, the process parameters such as gas flow rate, working distance, nozzle diameter, and printing speed are selected to investigate their effects on the morphology of the printed lines. Accordingly, single-factor experiments are designed to validate the printing of flexible circuits on both planar and curved substrates. Laser micro-sintering is utilized to improve the conductivity of the printed lines and ultimately fabricate flexible strain sensors. Under the sheath gas flow rate of 400 sccm, carrier gas flow rate of 100 sccm, working distance of 3 mm, nozzle diameter of 500 μm, and printing speed of 10 mm/s, the optimal morphology of the printed lines is achieved with low linewidth characteristics. The variations in the focal ratio, working distance, nozzle diameter, and printing speed significantly affect the minimum feature line width and morphology of the printed lines. [ABSTRACT FROM AUTHOR]

[Display omitted] The flexible electronics is a fast-moving, emerging interdisciplinary research that involves the use of organic/inorganic functional materials, electronic component designs, and their integration fabrication. Here, we review recent advancements in flexible piezoelectronics and 3D printing preparation, including (i) flexible piezoelectric materials suitable for 3D printing, their piezoelectric properties, and a comparison among them, (ii) 3D-printed flexible piezoelectronics with increasing capabilities in sensing, energy harvesting, and electronic display, and (iii) 3D printing technology and preparation methods, such as inkjet printing, thermal or laser-assisted direct writing printing (DIW), and fused deposition molding (FDM), which enable rapid design and manufacture of new flexible electronic devices. We also summarize the design methods of flexible piezoelectric devices based on 3D printing technology, and discuss their extensive applications in smart wearable electronics, flexible circuits, the Internet of Things, and software robots. Flexible piezoelectronics is a rapidly advancing field that is poised to have a significant impact on the modern industrial structure and human life. Finally, we prospect for the future development of 3D printing flexible piezoelectronics and the challenges that researchers may encounter. [ABSTRACT FROM AUTHOR]

Ionic circuits using ions as charge carriers have demonstrated great potential for flexible and bioinspired electronics. The emerging ionic thermoelectric (iTE) materials can generate a potential difference by virtue of selective thermal diffusion of ions, which provide a new route for thermal sensing with the merits of high flexibility, low cost, and high thermopower. Here, ultrasensitive flexible thermal sensor arrays based on an iTE hydrogel consisting of polyquaternium‐10 (PQ‐10), a cellulose derivative, as the polymer matrix and sodium hydroxide (NaOH) as the ion source are reported. The developed PQ‐10/NaOH iTE hydrogel achieves a thermopower of 24.17 mV K−1, which is among the highest values reported for biopolymer‐based iTE materials. The high p‐type thermopower can be attributed to thermodiffusion of Na+ ions under a temperature gradient, while the movement of OH− ions is impeded by the strong electrostatic interaction with the positively charged quaternary amine groups of PQ‐10. Flexible thermal sensor arrays are developed through patterning the PQ‐10/NaOH iTE hydrogel on flexible printed circuit boards, which can perceive spatial thermal signals with high sensitivity. A smart glove integrated with multiple thermal sensor arrays is further demonstrated, which endows a prosthetic hand with thermal sensation for human–machine interaction. [ABSTRACT FROM AUTHOR]

The Internet of Things (IoT) is gaining more and more popularity and it is establishing itself in all areas, from industry to everyday life. Given its pervasiveness and considering the problems that afflict today's world, that must be carefully monitored and addressed to guarantee a future for the new generations, the sustainability of technological solutions must be a focal point in the activities of researchers in the field. Many of these solutions are based on flexible, printed or wearable electronics. The choice of materials therefore becomes fundamental, just as it is crucial to provide the necessary power supply in a green way. In this paper we want to analyze the state of the art of flexible electronics for the IoT, paying particular attention to the issue of sustainability. Furthermore, considerations will be made on how the skills required for the designers of such flexible circuits, the features required to the new design tools and the characterization of electronic circuits are changing. [ABSTRACT FROM AUTHOR]

Flexible electronic devices require metal interconnects to facilitate the flow of electrical signals among the device components, ensuring its proper functionality. There are multiple factors to consider when designing metal interconnects for flexible electronics, including their conductivity, flexibility, reliability, and cost. This article provides an overview of recent endeavors to create flexible electronic devices through different metal interconnect approaches, with a focus on materials and structural aspects. Additionally, the article discusses emerging flexible applications, such as e-textiles and flexible batteries, as essential considerations. [ABSTRACT FROM AUTHOR]

The design and fabrication of biopolymer‐incorporated flexible electronics have attracted immense interest in healthcare systems, degradable implants, and electronic skin. However, the application of these soft bioelectronic devices is often hampered by their intrinsic drawbacks, such as poor stability, inferior scalability, and unsatisfactory durability. Herein, for the first time, using wool keratin (WK) as a structural biomaterial and natural mediator to fabricate soft bioelectronics is presented. Both theoretical and experimental studies reveal that the unique features of WK can endow carbon nanotubes (CNTs) with excellent water dispersibility, stability, and biocompatibility. Therefore, well‐dispersed and electroconductive bio‐inks can be prepared via a straightforward mixing process of WK and CNTs. The as‐obtained WK/CNTs inks can be directly exploited to design versatile and high‐performance bioelectronics, such as flexible circuits and electrocardiogram electrodes. More impressively, WK can also be a natural mediator to connect CNTs and polyacrylamide chains to fabricate a strain sensor with enhanced mechanical and electrical properties. With conformable and soft architectures, these WK‐derived sensing units can be further assembled into an integrated glove for real‐time gesture recognition and dexterous robot manipulations, suggesting the great potential of the WK/CNT composites for wearable artificial intelligence. [ABSTRACT FROM AUTHOR]

The past several years have witnessed a rapid development of intelligent wearable devices. However, despite the splendid advances, the creation of flexible human–machine interfaces that synchronously possess multiple sensing capabilities, wearability, accurate responsivity, sensitive detectivity, and fast recyclability remains a substantial challenge. Herein, a convenient yet robust strategy is reported to craft flexible transient circuits via stencil printing liquid metal conductor on the water‐soluble electrospun film for human–machine interaction. Due to the inherent liquid conductor within porous substrate, the circuits feature high‐resolution, customized patterning viability, attractive permeability, excellent electroconductivity, and superior mechanical stability. More importantly, such circuits display appealing noncontact proximity capabilities while maintaining compelling tactile sensing performance, which is unattainable by traditional systems with compromised contact sensing. As such, the flexible circuit is utilized as wearable sensors with practical multifunctionality, including information transfer, smart identification, and trajectory monitoring. Furthermore, an intelligent human–machine interface composed of the flexible sensors is fabricated to realize specific goals such as wireless object control and overload alarm. The transient circuits are quickly and efficiently recycled toward high economic and environmental values. This work opens vast possibilities of generating high‐quality flexible and transient electronics for advanced applications in soft and intelligent systems. [ABSTRACT FROM AUTHOR]

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Liquid metals (LMs) and conjugated polymers are both critically important materials in modern technologies including soft robotics, printable circuits, and stretchable electronics. Although broadly applied, the joint use of LMs and conjugated polymers are rarely explored due to the notoriously interfacial challenges of the high surface tension of LMs and the unfavorable rigid backbones of conjugated polymer chains. This work successfully introduced conjugated polymers (poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate, (PEDOT:PSS)) into the system of liquid metals to provide a highly stable, aqueous, and electrically conductive ink (LM‐PEDOT:PSS inks, LMPInks) for flexible electronics. The PSS segments act as a strong ionic bridge to effectively stabilize core‐shell structure of the PEDOT and LMs in the homogenous ink. Different from previous insulated LM‐inks, LMPInk is aqueous and more importantly, electrically conductive without posttreatments due to the intrinsically conductive PEDOT:PSS. Interestingly, with low content of LM particles (<2 wt%), the printed circuits inherit the valuable transparency of PEDOT:PSS (transmission >92% in visible region), which has never been observed in LM‐based organic systems. Furthermore, such LMPInks show distinguished electrical robustness, which can automatically recover their initial electrical conductivity even after severe physical damage. With the above phenomenal advantages, various flexible circuits on different substrates are successfully provided. [ABSTRACT FROM AUTHOR]

The flexible circuit pattern is the key component of flexible electronics. A near-field electrohydrodynamic direct-writing (NFEDW) method was proposed to fabricate the flexible circuit pattern. The prepared poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/graphene/single-walled carbon nanotubes mixed solution was deposited on the flexible substrate to fabricate the flexible circuit patterns, which could improve the fabricated quality of flexible circuit patterns. The influence of the multi-parameters on the pattern width of flexible circuit was analysed by the direct-writing experiment method. In order to explore the mapping law of multi-field coupling parameters on the pattern width of the direct-writing flexible circuit, a multivariable grey regulation method with a small number of test sample data was proposed by combining a small number of experiments and optimization design, and a multi-field coupling parameter regulation model of the NFEDW method was constructed. The multi-parameters with the expected width of the flexible electronic pattern could be effectively controlled. The experimental results show that the average correlation error between the actual measured pattern width and the predicted width of the prediction model is 4.04%. The proposed optimization method can effectively improve the fabrication efficiency and reduce the manufacturing cost of the flexible circuit with the expected pattern width. [ABSTRACT FROM AUTHOR]

The integration of structural electronics in injection-molded parts is a challenging step. The films—comprising of laminated stacks with electronics—are exposed to shear stresses and elevated temperatures by the molten thermoplastic. Hence, molding settings have a significant impact on the successful, damage-free manufacturing of such parts. In this paper, test films with polycarbonate (PC) sheets as outer and two different thermoplastic polyurethanes (TPUs) as middle layers incorporating conductive tracks on a flexible printed circuit board (flexPCB) are manufactured and overmolded with PC. Parameter studies investigating the influence of the melt temperature, mold temperature, injection speed and used TPU layer were performed. The molded parts were inspected visually and compared with a numerical simulation using injection molding software. A shear distortion factor for the TPU layer was derived based on the simulations that linked the shear stresses with the injection time and the softening (melting) of the TPU. The distortion of the films was found to reduce with higher melt temperature, lower mold temperature and faster injection speed. Films using the TPU with the higher melting temperature yielded significantly better results. Moreover, distortion on the films reduced with the increasing distance to the gate and a larger cavity thickness was found to be beneficial. All those relations could be correlated with the shear distortion factor. [ABSTRACT FROM AUTHOR]

High-throughput printing-based fabrication has emerged as a key enabler of flexible electronics given its unique capability for low-cost integration of circuits based on printed thin film transistors (TFTs). Research in printing inorganic metal oxides has revealed the potential for fabricating oxide TFTs with an unmatched combination of high electron mobility and optical transparency. Here, we highlight recent developments in ink chemistry, printing physics, and material design for high-mobility metal oxide transistors. We consider ongoing challenges for this field that include lowering process temperatures, achieving high speed and high resolution printing, and balancing device performance with the need for high mechanical flexibility. Finally, we provide a roadmap for overcoming these challenges with emerging synthetic strategies for fabricating 2D oxides and complementary TFT circuits for flexible electronics. [ABSTRACT FROM AUTHOR]

Aircraft structural health monitoring (SHM) technology is a revolutionary technology based on sensor networks, which has the advantages of ensuring flight safety, extending structural life, and reducing maintenance costs. Due to the large size of aircraft structures and large deformation of morphing wings, the sensor networks integrated into the aircraft structures should be large-area, expandable, and flexible. Island-interconnect structure design in flexible electronics is expected to realize such sensor networks, with the help of large-area and low-cost flexible printed circuit (FPC) process. Mechanical analysis can guide the design of island-interconnect sensor networks. However, few studies have been reported to analyze the mechanical properties of whole island-interconnect sensor networks. This paper proposes a simulation analysis method based on the finite element method for the mechanical analysis of expandable and flexible sensor networks. This method mainly includes two parts: extracting the buckling modes through linear buckling analysis and obtaining the deformation states and mechanical properties through postbuckling behavior analysis. It is applied on the mechanical analysis of island-interconnect sensor networks with different geometry designs, including serpentine, fractal, and irregular island-interconnect sensor networks. Tensile experiments of island-interconnect sensor networks manufactured by the FPC process show that the deformation of experimental results is basically consistent with that of simulation results. In conclusion, the proposed simulation analysis method is appliable for the mechanical analysis of serpentine, fractal, and irregular island-interconnect structure networks, guiding the structural design of expandable and flexible sensor networks for large-area SHM of aircraft. [ABSTRACT FROM AUTHOR]

With the growing number of flexible electronics applications, environmentally benign ways of mass-producing graphene electronics are sought. In this study, we present a scalable mechanochemical route for the exfoliation of graphite in a planetary ball mill with melamine to form melamine-intercalated graphene nanosheets (M-GNS). M-GNS morphology was evaluated, revealing small particles, down to 14 nm in diameter and 0.4 nm thick. The M-GNS were used as a functional material in the formulation of an inkjet-printable conductive ink, based on green solvents: water, ethanol, and ethylene glycol. The ink satisfied restrictions regarding stability and nanoparticle size; in addition, it was successfully inkjet printed on plastic sheets. Thermal and photonic post-print processing were evaluated as a means of reducing the electrical resistance of the printed features. Minimal sheet resistance values (5 kΩ/sq for 10 printed layers and 626 Ω/sq for 20 printed layers) were obtained on polyimide sheets, after thermal annealing for 1 h at 400 °C and a subsequent single intense pulsed light flash. Lastly, a proof-of-concept simple flexible printed circuit consisting of a battery-powered LED was realized. The demonstrated approach presents an environmentally friendly alternative to mass-producing graphene-based printed flexible electronics. [ABSTRACT FROM AUTHOR]

Implantable electronic neural interfaces typically take the form of probes and are made with rigid materials such as silicon and metals. These have advantages such as compatibility with integrated microchips, simple implantation and high-density nanofabrication but tend to be lacking in terms of biointegration, biocompatibility and durability due to their mechanical rigidity. This leads to damage to the device or, more importantly, the brain tissue surrounding the implant. Flexible polymer-based probes offer superior biocompatibility in terms of surface biochemistry and better matched mechanical properties. Research which aims to bring the fabrication of electronics on flexible polymer substrates to the nano-regime is remarkably sparse, despite the push for flexible consumer electronics in the last decade or so. Cleanroom-based nanofabrication techniques such as photolithography have been used as pattern transfer methods by the semiconductor industry to produce single nanometre scale devices and are now also used for making flexible circuit boards. There is still much scope for miniaturizing flexible electronics further using photolithography, bringing the possibility of nanoscale, non-invasive, high-density flexible neural interfacing. This work explores the fabrication challenges of using photolithography and complementary techniques in a cleanroom for producing flexible electronic neural probes with nanometre-scale features. This article is part of the theme issue 'Advanced neurotechnologies: translating innovation for health and well-being'. [ABSTRACT FROM AUTHOR]

Organic thin‐film transistors (OTFTs) exhibit great potential applications in flexible logic circuits, sensors, nonvolatile memory, etc. The vertical phase separation (VPS) structure, deriving from the blends of small molecules and polymers, can produce a high‐quality crystalline semiconducting layer and an intrinsic and trap‐free semiconductor/insulator interface. OTFTs with a VPS structure provide a promising route to accelerate their commercialization due to these desirable features including excellent device performance, good uniformity and reproducibility, environmental and mechanical stability, low‐cost and large‐area fabrication. Additionally, VPS OTFTs also exhibit a good balance between electronical properties and stretchability for intrinsic stretchable OTFTs. Herein, a comprehensive review is given on VPS structure for flexible and stretchable OTFTs. First, the evolution history of VPS structures and its formation mechanism are summarized. Based on the understanding of the VPS structure, the optimization strategies of VPS structures for high‐performance OTFTs are presented. Subsequently, the large‐area fabrication techniques of VPS structures, as well as the potential applications in flexible and stretchable OTFTs, are discussed. Finally, conclusions and outlook are provided for the further development of VPS structure in OTFTs. This review would promote the understanding and development of VPS structures in OTFTs, accelerating its applications in flexible and stretchable electronics. [ABSTRACT FROM AUTHOR]

The nanofibrillated cellulose paper (nanocellulose paper or nanopaper), which is flexible, transparent, ultrasmooth, and biodegradable, has emerged as a new substrate material for the next generation of paper‐based flexible electronics. Herein a visible light‐induced printing technique for depositing highly conductive silver (Ag) patterns on nanopaper is reported. The optical Ag printing process is simple to implement at room temperature and only requires nontoxic, low‐cost aqueous chemical solutions and an inexpensive light projection setup. The abundant carboxyl groups on the nanopaper enable efficient absorption of Ag+ ions on the nanopaper surface for light‐induced reduction of Ag+ into a thin film of densely packed silver nanoparticles (AgNPs). Chemical annealing of the deposited AgNPs further enhances the conductivity of the printed Ag patterns. The mechanical and electrical properties of the printed Ag patterns on nanopaper are characterized, and the application of the optical Ag printing technique fabricating the nanopaper‐based flexible circuits and electrochemical biosensors is also demonstrated. The optical printing technique will enable new designs and applications of nanopaper‐based flexible electronics. [ABSTRACT FROM AUTHOR]

Recently, p-type tin monoxide (SnO) thin-film transistors (TFTs) have gained interest for all-oxide complementary metal–oxide semiconductor (CMOS) circuits for flexible electronics and back-end-of-line integration with Si. However, most SnO TFTs demonstrated so far exhibit a low on– off-current ratio and limited field effect mobility. To understand and improve SnO TFT performance, it is necessary to quantitatively characterize the subbandgap defects in SnO and at its dielectric interface. In this work, we establish numerical models which provide this understanding. By concurrent fitting of simulated ${I}_{\text{D}}$ – ${V}_{\text{GS}}$ and effective mobility versus ${V}_{\text{GS}}$ to experimentally measured data, we can accurately extract SnO defect state profiles. Using the extracted simulation models, we then identify ways to improve SnO TFT performance. Specifically, a reduction in the density of shallow, bulk Gaussian acceptor states, is necessary to reduce the off-current, while reducing the contact resistance and interface donor-like tail state density can improve the SnO field effect mobility. [ABSTRACT FROM AUTHOR]

Printed flexible hybrid electronics (FHE) is finding an increasing number of applications in the fields of displays, sensors, actuators and in energy harvesting and storage. The technology involves the printing of conductive and insulating patterns as well as mounting electronic devices and circuits on flexible substrate materials. Typical plastic substrates in use are, for example, non-renewable-based poly(ethylene terephthalate) (PET) or poly(imides) (PI) with high thermal and dimensional stability, solvent resistance and mechanical strength. The aim of this study was to assess whether renewable-based plastic materials can be applied on sheet-to-sheet (S2S) screen-printing of conductive silver patterns. The selected materials were biaxially oriented (BO) bio-based PET (Bio-PET BO), poly(lactic acid) (PLA BO), cellulose acetate propionate (CAP BO) and regenerated cellulose film, NatureFlex™ (Natureflex). The biaxial orientation and annealing improved the mechanical strength of Bio-PET and PLA to the same level as the reference PET (Ref-PET). All renewable-based substrates showed a transparency comparable to the Ref-PET. The printability of silver ink was good with all renewable-based substrates and printed pattern resistance on the same level as Ref-PET. The formation of the printed pattern to the cellulose-based substrates, CAP BO and Natureflex, was very good, showing 10% to 18% lower resistance compared to Ref-PET and obtained among the bio-based substrates the smallest machine and transverse direction deviation in the S2S printing process. The results will open new application possibilities for renewable-based substrates, and also potentially biodegradable solutions enabled by the regenerated cellulose film and PLA. [ABSTRACT FROM AUTHOR]

A matched and self‐powered supply for a variety of wearable electronics remains a critical challenge. Here, a fully self‐powered early warning glove (SEWG) is developed via unique elastic‐arched triboelectric nanogenerator (EA‐TENG) and flexible printed circuit technology. The EA‐TENG generates electricity from a continuous touch lightly by hand or body, as a sustainable self‐powered source that can be utilized to complete one charging process by simply five times slapping. The SEWG has the advantage that can really deliver real‐time no‐contact electrostatic monitoring when no external power is supplied, and can effectively avoid the electric shock accident in 220/380 V electric maintenance work. Quantitative analysis of the mechanical and electronic characteristics of the SEWG demonstrated its advantages in terms of stability and reliability, with consistently well performance over more than 80,000 working cycles. It is anticipated that the special design of integrated self‐powered system could produce more implications in the field of wearable/flexible electronics and human‐machine interaction. [ABSTRACT FROM AUTHOR]

Dynamic multilead monitoring of cardiovascular health conditions is challenging due to the complexity of constructing a flexible system that covers large surface areas and synchronizes measurement results from different leads. A flexible 12‐lead electrocardiography system that can simultaneously conduct electrocardiogram sensing and 3‐axis acceleration monitoring is developed. The system is fabricated through a modularized printing process, resulting in individual modules that can be assembled reversibly through magnetic connectors. Different levels of stretchability and adjustable length of interconnect can accommodate different body statures and deformations. A wireless flexible circuit with high adaptability can deform through individually deformable interconnectors. The system is used to monitor seven people for more than 2 h, offering comparable accuracy as the commercial system. The accuracy values of R‐peak detection at 12 leads during jogging are higher than 85%, showing improved stability and excellent tolerance to motion. The system is later employed for 24 h monitoring to judge the activities and heart health conditions. All experimental results indicate that the system can be readily applied to accommodate large‐scale manufacturing and large population health monitoring. [ABSTRACT FROM AUTHOR]

Next‐generation flexible electronics must achieve multifunctionality, environmental friendliness and antibacterial activity. Accordingly, organisms in nature are interconnected. Inspired by the honeycomb multilayer porous structure, a wood‐nanotechnology‐derived flexible membrane circuit is created to meet the abovementioned requirements. The flexible wood (FW) matrix is made of natural balsa wood that underwent a simple top‐down chemical treatment. The multiwalled carbon nanotube (MWCNT) acts as a "bridge" between the FW matrix with a porous array structure and the active material (silver nanoparticles (Ag NPs) and poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS). Due to the three‐dimensional porous microstructure and highly conductive surface inherited by the wood‐nanotechnology‐derived flexible membrane (FW/MWCNT/Ag/PEDOT:PSS), it shows a high area capacitance (266.7 mF cm−2 at 20 mV s−1) and a good long‐term cycling stability 84.3% capacitance retention after 5000 cycles at 5 mA cm−2 when used as a supercapacitor electrode. In addition, it shows an excellent specific electromagnetic shielding efficiency (up to 970 dB cm2 g−1), proving its application potential in the field of electromagnetic shielding. Because of the biocidal property of Ag NPs, the FW/MWCNT/Ag/PEDOT:PSS shows a remarkable antibacterial effect on Escherichia coli and Staphylococcus aureus. This strategy provides a new opportunity for researchers to design biomass‐based integrated electronic materials. [ABSTRACT FROM AUTHOR]

Paper electronics offer an environmentally sustainable option for flexible and wearable systems and perfectly fit the available printing technologies for high manufacturing efficiency. As the heart of energy‐consuming devices, paper‐based batteries are required to be compatible with printing processes with high fidelity. Herein, hydrogel reinforced cellulose paper (HCP) is designed to serve as the separator and solid electrolyte for paper batteries. The HCP can sustain higher strain than pristine papers and are biodegradable in natural environment within four weeks. Zinc‐metal (Ni and Mn) batteries printed on the HCP present remarkable volumetric energy density of ≈26 mWh cm–3, and also demonstrate the feature of cuttability and compatibility with flexible circuits and devices. As a result, self‐powered electronic system could be constructed by integrating printed paper batteries with solar cells and light‐emitting diodes. The result highlights the feasibility of hydrogel reinforced paper for ubiquitous flexible and eco‐friendly electronics. [ABSTRACT FROM AUTHOR]

Despite the increasingly significant role of flexible electronics in information, energy, and medical treatment, their integration with a special‐shaped interface remains an unresolved challenge. The traditional transfer method, as a core technology of device integration, is still unsuitable for thinned chips and 3D sensors. Solid‐contact elastomer stamp sometimes causes cracks while non‐contact method such as sacrificial layer method fails to achieve precise positioning transfer. Herein, the authors present liquid droplet stamp transfer printing (LSTP) with a high yield ratio which allows flexible devices to be transferred form silicon wafer to complex special‐shaped interfaces. Following the transfer scheme, the regulation of interface force is demonstrated with different thin‐film patterns. Besides, the liquid droplet stamp is designed as an efficient tool to transfer thinned inorganic flexible chips. A thinned micro light emitting diode, extensively used in large‐scale manufacturing of flexible circuits, is transferred and lighted successfully. In addition, a new method to fabricate 3D sensors is proposed with the liquid droplet stamp, which provides a new way of manufacturing wearable antenna and reconfigurable devices. Consequently, the LSTP has great potential for future sophisticated and system‐level flexible devices transfer printing and plays a vital role in the research of 3D flexible electronics. [ABSTRACT FROM AUTHOR]

Flexible electronics have found a wide variety of applications, especially in biomedical engineering, where their intrinsic safety and conformability provide for configurations of sensors and actuators that can accommodate complex anatomies and movement. Current scalable techniques in the manufacture of flexible electronics are limited by the challenges of working with novel, intrinsically stretchable materials, or using cost‐intensive clean room fabrication techniques needed to create strain accommodating geometries. Recently, a technique that allows for scalable postprocessing of flexible printed circuit boards (flex PCBs) to convert them to stretchable configurations is described. Herein, a mechanism for understanding this process based on a thermal masking phenomenon is described. Thermal simulation provides an understanding of the phenomenon and various aspects of the process. Copper traces are modeled along with Kapton insulating layer on ANSYS workbench and a Gaussian waveform is introduced to simulate a laser beam. An inhouse laser cutter is used to calibrate the simulations and an accurate predictive model is used to determine working conditions, resolution, and other parameters that affect the standardized usage of this process for various thicknesses of copper and Kapton that can be produced by most mass manufacturers. This process provides a path for cost‐effective manufacture of stretchable electronics. [ABSTRACT FROM AUTHOR]

Electrolyte-gated transistors (EGTs) hold great promise for next-generation printed logic circuitry, biocompatible integrated sensors, and neuromorphic devices. However, EGT-based complementary circuits with high voltage gain and ultralow driving voltage (<0.5 V) are currently unrealized, because achieving balanced electrical output for both the p- and n-type EGT components has not been possible with current materials. Here we report high-performance EGT complementary circuits containing p-type organic electrochemical transistors (OECTs) fabricated with an ion-permeable organic semiconducting polymer (DPP-g2T) and an n-type electrical double-layer transistor (EDLT) fabricated with an ion-impermeable inorganic indium–gallium–zinc oxide (IGZO) semiconductor. Adjusting the IGZO composition enables tunable EDLT output which, for In:Ga:Zn = 10:1:1 at%, balances that of the DPP-g2T OECT. The resulting hybrid electrolyte-gated inverter (HCIN) achieves ultrahigh voltage gains (>110) under a supply voltage of only 0.7 V. Furthermore, NAND and NOR logic circuits on both rigid and flexible substrates are realized, enabling not only excellent logic response with driving voltages as low as 0.2 V but also impressive mechanical flexibility down to 1-mm bending radii. Finally, the HCIN was applied in electrooculographic (EOG) signal monitoring for recording eye movement, which is critical for the development of wearable medical sensors and also interfaces for human–computer interaction; the high voltage amplification of the present HCIN enables EOG signal amplification and monitoring in which a small ∼1.5 mV signal is amplified to ∼30 mV. [ABSTRACT FROM AUTHOR]

Flexible phased arrays potentially enable diverse applications not permitted by rigid systems; however, they introduce ambiguity in antenna element positions. If this position ambiguity can be overcome, flexible arrays can perform the full suite of array functions: beam steering, wavefront engineering, and beam focusing. Furthermore, shape reconstructions of arrays can be used for applications beyond beamforming. We propose a framework to reconstruct the shape of a flexible array that only uses mutual coupling measurements and does not require additional sensors or functionalities in the system. We discuss the approach, a two-step algorithm, which is highly modular and can be implemented in a variety of phased array systems. To demonstrate the accuracy of the approach, we present results from two passive 2.5-GHz phased array setups using dipole and patch antennas, as well as a 10-GHz (active) integrated circuit flexible phased array, and demonstrate the accuracy of the approach in this system. In all cases, the algorithm reconstructs the antenna shape accurately, with average position errors of approximately 6% of the wavelength. This article can serve as the beginning of the broad study of shape reconstruction algorithms and their applications. [ABSTRACT FROM AUTHOR]

Widely used flexible electronics are vulnerable to liquid infiltration during working, and the superhydrophobic surface has an available protective effect. In this work, a flow field‐induced one‐step electrodeposition method is used to fabricate superhydrophobic films. The main components are cerium myristate (CeM) and carboxylated multi‐walled carbon nanotubes (MWCNTs‐COOH). The morphology of the films changes a lot due to the addition of MWCNTs‐COOH from granular structure to network structure. The faster rotation of solution makes the deposition of MWCNTs‐COOH more uniform without vertical patterned stripes and smoother surfaces. The films' deionized water contact angles are not affected, but the sliding angles decline distinctly because of the smoother surface. Besides, two competing flow fields and electric fields will degrade the wettability of the films electrodeposited for a short time at 10 V. Electrochemical performance tests and abrasion tests are employed to examine if the superhydrophobic films can withstand abrasions, impacts, and electrochemical corrosion. The lowest electric resistivity of the films reaches 56.1 ± 4.3 mΩ mm. In the end, a light‐emitting diode bulb connected to a simple, flexible circuit prepared by a selective mask method can glow, which also proves its potential for applications in flexible electronics. [ABSTRACT FROM AUTHOR]

Accurate circuit simulation reflecting physical and electrical stress is of importance in indium gallium zinc oxide (IGZO)-based flexible electronics. In particular, appropriate modeling of threshold voltage (VT) changes in different bias and bending conditions is required for reliability-aware simulation in both device and circuit levels. Here, we present SPICE compatible compact modeling of IGZO transistors and inverters having an atomic layer deposition (ALD) Al2O3 gate insulator on a polyethylene terephthalate (PET) substrate. Specifically, the modeling was performed to predict the behavior of the circuit using stretched exponential function (SEF) in a bending radius of 10 mm and operating voltages ranging between 4 and 8 V. The simulation results of the IGZO circuits matched well with the measured values in various operating conditions. It is expected that the proposed method can be applied to process improvement or circuit design by predicting the direct current (DC) and alternating current (AC) responses of flexible IGZO circuits. [ABSTRACT FROM AUTHOR]

• Developing a wireless charging flexible in-situ optical sensing for food monitoring. • Realizing the non-destructive and in-situ wireless monitoring for food quality. • Building a partial least squares regression based prediction model for food monitoring. • Improving the visualization and transparency of food in supply chain. Food safety is an important public health issue, real-time monitoring and identification of food freshness is essential to ensure food quality and consumer health. This study presents a wireless charging flexible in-situ optical sensing system (FIOS). FIOS integrates visible (VIS)/near-infrared (NIR) spectral sensors, electronic components, and microcontrollers into flexible circuits by flexible electronic technology, and obtains energy through wireless charging to solve the problem of battery replacement and frequent charging of electronic systems. FIOS exhibits excellent flexibility and bendability which can be attached well to the surface of the food for efficient, accurate, non-destructive, and in-situ monitoring. A prediction model between food quality and spectral data was established by chemometrics to achieve accurate prediction of food quality. It can obtain the spectral information of the food in real-time and transmit the data wirelessly to a cloud platform, which enables the visualization of food quality and facilitates quality traceability. FIOS is characterized by simple craft, low cost, and real-time monitoring, and can be widely deployed as an Internet of Things (IoT) node in food transportation, storage, and sales to improve the transparency of food at all stages and ensure the quality and safety of food. The FIOS has the advantages of portable and non-destructive monitoring, which improves the accuracy of food quality monitoring and provides a new technological program in the field of food quality monitoring. It is expected to promote the development of food quality and safety monitoring in the future and provide consumers with a more reliable guarantee of food quality. [ABSTRACT FROM AUTHOR]

• An all-cellulose ionogel was prepared using directly cracked wood. • The ionogel exhibits superior performance in both mechanical properties and ionic conductivity. • The ionogel shows excellent self-healing properties and is fully biodegradable. • Flexible printed electronic circuits, sensors and flexible supercapacitors are well served by the ionogel. For thousands of years, wood has been utilized as a natural, abundant, and sustainable material for structural construction and furniture. To enhance its value, we have developed a simple process that can directly convert low-value wood (including cracked wood, chips, shives, and other wood residues) into high-performance all-cellulose ionogels. This process involves delignification, in situ dissolution of cellulose, and self-assembly of molecular cellulose chains. The resulting ionogel exhibits excellent properties such as high ion conductivity (∼60 mS/cm), mechanical strength (∼6 MPa), and self-healing capability (∼10 min). Notably, it also demonstrates exceptional freeze tolerance, withstanding temperatures as low as −50 °C while maintaining high ion conductivity (∼6.4 mS/cm). The ionogel is a versatile substrate for flexible electronic circuits. We have developed a sensor utilizing this ionogel that is breathable, flexible, and highly responsive to temperature, humidity, and strain. Furthermore, we have used it as a gel electrolyte to create supercapacitors with exceptional performance even in low-temperature conditions. The ionic liquid can be reused, with an all-cellulose framework that breaks down naturally in moist soil within 15 days. This sustainable method of producing high-performance ionogels from low-value wood has immense potential in shaping the next generation of soft, intelligent devices. [ABSTRACT FROM AUTHOR]

The effects of fluorination on amorphous indium‐gallium‐zinc oxide thin‐film transistors fabricated on polyimide (PI) at a maximal temperature of 300 °C were investigated. An excimer laser was used to lift‐off the PI from a glass‐based carrier substrate. The resulting TFTs exhibit more positive threshold voltage, steeper subthreshold swing, reduced apparent short‐channel effects, and tighter distribution of device parameters. Various test circuit blocks and a 4‐stage 15T1C gate driver were constructed to demonstrate the beneficial effects of fluorination on circuit applications. [ABSTRACT FROM AUTHOR]

Flexible and stretchable circuit is the key element for the upcoming flexible hybrid electronics in addition to high performance silicon based active devices and passive components. The circuitry and structure design of the circuit carrier have to be able to withstand the stress and tolerate biometric signal variation induced by external force. These circuit carriers can be fabricated by use of the flexible display technologies and facilities. In this study, we integrated ICs, passive components and sensors to form a system upon flexible/plastic circuit carrier. Bendable and/or stretchable smart textiles and patches were successful demonstrated with the functionality of electromyography (EMG) and electrocardiography (ECG). [ABSTRACT FROM AUTHOR]

Wearable bio-sensing devices are considered promising for ubiquitous heath monitoring. To accurately read out small bio-signals, the development of high-performance flexible front-end circuits is crucial. Oxide semiconductors are considered one of the most promising active channel materials for on-polymeric-foil electronics. In this article, a high-gain flexible complementary metal–oxide–semiconductor (CMOS) inverter with a beta ratio of 1, a desirable feature for device miniaturization, was demonstrated by monolithically integrating a top-gated n-type indium gallium zinc oxide (IGZO) thin-film transistor (TFT) and a bottom-gated p-type SnO TFT on a 5.5-μm-thick polyimide substrate. The influence of atomic layer deposited (ALD)-HfO2 capping layer on the properties of oxide active channels has been analyzed. The flexible inverter exhibited a high static voltage gain of 370 V/V and balanced noise margins (noise margin high of 4.8 V, noise margin low of 4.7 V) for a supply voltage of 10 V. In addition, the voltage transfer characteristics remained virtually unchanged when the device was subjected to an outward or an inward bending radius of 2.5 mm, indicating the complementary oxide-TFT-based inverter is practical for flexible electronics applications. [ABSTRACT FROM AUTHOR]

Thin‐film transistor circuits on flexible substrates hold major promise for next‐generation human–machine interface systems. However, a major bottleneck is the reliability of the interconnect, which is prone to open‐circuit faults due to mechanical, electrical, and environmental stresses. Here, self‐healing interconnects in thin‐film transistor circuits are demonstrated on flexible substrates resulting in the restoration of >99% of the prefault current. The active material for self‐healing is a dispersion of conductive particles in an insulating fluid that is contained over the interconnect. Healing is triggered by the electric field that appears in the open gap during the occurrence of the fault. The engineering of the active material is discussed; self‐healing circuits are demonstrated and analyzed; and methods to package and integrate the self‐healing feature are discussed with the process flow. This work sets a new benchmark for reliable inkjet‐printed thin‐film transistor circuits on flexible substrates. [ABSTRACT FROM AUTHOR]

Printed conductors are the main building blocks of printed flexible electronic circuits. With the advancement and growing demand for printed and flexible electronics, researchers are focused on assessing the mechanical reliability of interconnects. Even more important is, often, the electrical performance or the current‐carrying capacity of the interconnects. A mechanically reliable interconnect has limited use if it cannot meet the current‐carrying requirements. However, current‐carrying capacity curves and circuit design guidelines such as those available for conventional copper conductors have not yet been established for printed interconnects, i.e., there is a technological gap in this domain. The present work focuses on assessing the maximum allowed current through an aerosol jet printed silver nanoparticle‐based conductor on a 75 μm thick polyimide substrate. The temperature coefficient of resistance for the printed ink is experimentally determined. The effect of printed ink microstructure on the temperature coefficient of resistance is also investigated. Standard curves are generated that would guide designers to determine the trace width required for an allowed temperature increase. Furthermore, the effect of sintering temperature on the current‐carrying capability is evaluated. [ABSTRACT FROM AUTHOR]