Inkjet-printed stretchable and low voltage synaptic transistor array

Stretchable Neuromorphic Transistor That Combines Multisensing and Information Processing for Epidermal Gesture Recognition

We fabricated a nanowire-channel intrinsically stretchable neuromorphic transistor (NISNT) that perceives both tactile and visual information and emulates neuromorphic processing capabilities. The device demonstrated excellent stretching endurance of 1000 stretch cycles while retaining stable electrical properties. The device was then applied as a multisensitive afferent nerve that processes information in parallel. Compatible with skin deformation, the devices are attached to fingers to serve as conformal strain sensors and neuromorphic information-processing units for gesture recognition. The excitatory postsynaptic current in each device represents shape changes and is then analyzed using softmax activation processing of the neural network to recognize gestures. A multistage neural network that uses NISNT was used to further confirm the gestures. This work demonstrated an idea toward multisensory artificial nerves and neuromorphic systems.

Method for Inkjet-printing PEDOT:PSS polymer electrode arrays on piezoelectric PVDF-TrFE fibers

We present a method for printing conductive polymers onto P(VDF-TrFE) nanofibers to create all-polymer piezoelectric devices. Inkjet printing is an attractive fabrication approach for rapid prototyping of flexible electronics, but until now with limited applications in developing P(VDF-TrFE) nanofiber-based devices. We have demonstrated an approach to infill the void space within a piezoelectric nanofibrous matrix to allow for the inkjet printing of aqueous inks while avoiding leakage that typically leads to electrical shorting and without significant loss of voltage output. This was done using a diluted PDMS solution and a commercially available conductive ink. The 1 cm² devices showed a 254 mV/N sensitivity to impact as well as a sensitivity to bending. The device was shown to be able to detect breathing and pulse rate when placed superficially to the carotid and radial arteries. Using these techniques, flexible piezoelectric sensing can be done in an array format, shown with applications in foot movement sensing.

Beyond Color: The New Carbon Ink

For thousands of years, carbon ink has been used as a black color pigment for writing and painting purposes. However, recent discoveries of nanocarbon materials, including fullerenes, carbon nanotubes, graphene, and their various derivative forms, together with the advances in large-scale synthesis, are enabling a whole new generation of carbon inks that can serve as an intrinsically programmable materials platform for developing advanced functionalities far beyond color. The marriage between these multifunctional nanocarbon inks with modern printing technologies is facilitating and even transforming many applications, including flexible electronics, wearable and implantable sensors, actuators, and autonomous robotics. This review examines recent progress in the reborn field of carbon inks, highlighting their programmability and multifunctionality for applications in flexible electronics and stimuli-responsive devices. Current challenges and opportunities will also be discussed from a materials science perspective towards the advancement of carbon ink for new applications beyond color.

One-step preparation of Cr2O3-based inks with long-term dispersion stability for inkjet applications

Inkjet printing of functional materials has shown a wide range of applications in advertising, OLED display, printed electronics and other specialized utilities that require high-precision, mask-free, direct-writing deposition techniques. Nevertheless, the sedimentation risk of the refractory functional materials dispensed in inks hinders their further implementation. Herein, we present a bottom-up ink preparation strategy based on Cr₂O₃ by a one-step solvothermal method. The obtained ink remained stable under an equivalent natural sediment test for 2.5 years. The chemical composition of the solvothermal product was characterized, and the mechanism of the superior dispersion stability of Cr₂O₃ particles was analysed. These amorphous Cr₂O₃ particles were capped by ligands generated via low-temperature solvothermal reactions. Ethanol and acetylacetone covering the particle surfaces play an essential role in enhancing the solubility of Cr₂O₃ particles in the solvent forming the ultrastable colloidal ink. Moreover, this ink was successfully printed using a direct-write inkjet system JetLab®II on nylon fabrics, and the printed area of the fabrics shows a spectral correlation coefficient of 0.9043 to green leaves. Finally, we believe that the one-step bottom-up fabrication method of Cr₂O₃-based pigment inks may provide a general approach for preparing metal oxide-based pigment inks with long-term dispersion stability.

Nanomaterials and their applications on bio-inspired wearable electronics

Wearable electronics featuring conformal attachment, sensitive perception and intellectual signal processing have made significant progress in recent years. However, when compared with living organisms, artificial sensory devices showed undeniable bulky shape, poor adaptability, and large energy consumption. To make up for the deficiencies, biological examples provide inspirations of novel designs and practical applications. In the field of biomimetics, nanomaterials from nanoparticles to layered two-dimensional materials are actively involved due to their outstanding physicochemical properties and nanoscale configurability. This review focuses on nanomaterials related to wearable electronics through bioinspired approaches on three different levels, interfacial packaging, sensory structure, and signal processing, which comprehensively guided recent progress of wearable devices in leveraging both nanomaterial superiorities and biorealistic functionalities. In addition, opinions on potential development trend are proposed aiming at implementing bioinspired electronics in multifunctional portable sensors, health monitoring, and intelligent prosthetics.

Printable Smart Materials and Devices: Strategies and Applications

Smart materials are a kind of functional materials which can sense and response to environmental conditions or stimuli from optical, electrical, magnetic mechanical, thermal, and chemical signals, etc. Patterning of smart materials is the key to achieving large-scale arrays of functional devices. Over the last decades, printing methods including inkjet printing, template-assisted printing, and 3D printing are extensively investigated and utilized in fabricating intelligent micro/nano devices, as printing strategies allow for constructing multidimensional and multimaterial architectures. Great strides in printable smart materials are opening new possibilities for functional devices to better serve human beings, such as wearable sensors, integrated optoelectronics, artificial neurons, and so on. However, there are still many challenges and drawbacks that need to be overcome in order to achieve the controllable modulation between smart materials and device performance. In this review, we give an overview on printable smart materials, printing strategies, and applications of printed functional devices. In addition, the advantages in actual practices of printing smart materials-based devices are discussed, and the current limitations and future opportunities are proposed. This review aims to summarize the recent progress and provide reference for novel smart materials and printing strategies as well as applications of intelligent devices.

Ultralow-power flexible transparent carbon nanotube synaptic transistors for emotional memory

Emulating the biological behavior of the human brain with artificial neuromorphic devices is essential for the future development of human-machine interactive systems, bionic sensing systems and intelligent robotic systems. In this paper, artificial flexible transparent carbon nanotube synaptic transistors (F-CNT-STs) with signal transmission and emotional learning functions are realized by adopting the poly(vinyl alcohol) (PVA)/SiO2 proton-conducting electrolyte. Synaptic functions of biological synapses including excitatory and inhibitory behaviors are successfully emulated in the F-CNT-STs. Besides, synaptic plasticity such as spike-duration-dependent plasticity, spike-number-dependent plasticity, spike-amplitude-dependent plasticity, paired-pulse facilitation, short-term plasticity, and long-term plasticity have all been systematically characterized. Moreover, the F-CNT-STs also closely imitate the behavior of human brain learning and emotional memory functions. After 1000 bending cycles at a radius of 3 mm, both the transistor characteristics and the synaptic functions can still be implemented correctly, showing outstanding mechanical capability. The realized F-CNT-STs possess low operating voltage, quick response, and ultra-low power consumption, indicating their high potential to work in low-power biological systems and artificial intelligence systems. The flexible artificial synaptic transistor enables its potential to be generally applicable to various flexible wearable biological and intelligent applications.

Monolithic optical microlithography of high-density elastic circuits

Polymeric electronic materials have enabled soft and stretchable electronics. However, the lack of a universal micro/nanofabrication method for skin-like and elastic circuits results in low device density and limited parallel signal recording and processing ability relative to silicon-based devices. We present a monolithic optical microlithographic process that directly micropatterns a set of elastic electronic materials by sequential ultraviolet light-triggered solubility modulation. We fabricated transistors with channel lengths of 2 micrometers at a density of 42,000 transistors per square centimeter. We fabricated elastic circuits including an XOR gate and a half adder, both of which are essential components for an arithmetic logic unit. Our process offers a route to realize wafer-level fabrication of complex, high-density, and multilayered elastic circuits with performance rivaling that of their rigid counterparts.

Rapid custom prototyping of soft poroelastic biosensor for simultaneous epicardial recording and imaging

The growing need for the implementation of stretchable biosensors in the body has driven rapid prototyping schemes through the direct ink writing of multidimensional functional architectures. Recent approaches employ biocompatible inks that are dispensable through an automated nozzle injection system. However, their application in medical practices remains challenged in reliable recording due to their viscoelastic nature that yields mechanical and electrical hysteresis under periodic large strains. Herein, we report sponge-like poroelastic silicone composites adaptable for high-precision direct writing of custom-designed stretchable biosensors, which are soft and insensitive to strains. Their unique structural properties yield a robust coupling to living tissues, enabling high-fidelity recording of spatiotemporal electrophysiological activity and real-time ultrasound imaging for visual feedback. In vivo evaluations of custom-fit biosensors in a murine acute myocardial infarction model demonstrate a potential clinical utility in the simultaneous intraoperative recording and imaging on the epicardium, which may guide definitive surgical treatments.

Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities

Conducting polymers (CPs) have been attracting great attention in the development of (bio)electronic devices. Most of the current devices are rigid two-dimensional systems and possess uncontrollable geometries and architectures that lead to poor mechanical properties presenting ion/electronic diffusion limitations. The goal of the article is to provide an overview about the additive manufacturing (AM) of conducting polymers, which is of paramount importance for the design of future wearable three-dimensional (3D) (bio)electronic devices. Among different 3D printing AM techniques, inkjet, extrusion, electrohydrodynamic, and light-based printing have been mainly used. This review article collects examples of 3D printing of conducting polymers such as poly(3,4-ethylene-dioxythiophene), polypyrrole, and polyaniline. It also shows examples of AM of these polymers combined with other polymers and/or conducting fillers such as carbon nanotubes, graphene, and silver nanowires. Afterward, the foremost applications of CPs processed by 3D printing techniques in the biomedical and energy fields, that is, wearable electronics, sensors, soft robotics for human motion, or health monitoring devices, among others, will be discussed.

Interface Design for Stretchable Electronic Devices

Stretchable electronics has emerged over the past decade and is now expected to bring form factor-free innovation in the next-generation electronic devices. Stretchable devices have evolved with the synthesis of new soft materials and new device architectures that require significant deformability while maintaining the high device performance of the conventional rigid devices. As the mismatch in the mechanical stiffness between materials, layers, and device units is the major challenge for stretchable electronics, interface control in varying scales determines the device characteristics and the level of stretchability. This article reviews the recent advances in interface control for stretchable electronic devices. It summarizes the design principles and covers the representative approaches for solving the technological issues related to interfaces at different scales: i) nano- and microscale interfaces between materials, ii) mesoscale interfaces between layers or microstructures, and iii) macroscale interfaces between unit devices, substrates, or electrical connections. The last section discusses the current issues and future challenges of the interfaces for stretchable devices.

Trends and recent development of the microelectrode arrays (MEAs)

In this paper, we reviewed the history of microelectrode arrays (MEAs), compared different microfabrication techniques applied to modern MEAs in terms of their material characters, device properties and application scenarios. Then we discussed the biocompatibility of different MEAs as well as corresponding strategy of improvement. At last, we analyzed the growing trend of MEAs' technical route, expected application of MEAs in the field of Electrical impedance tomography (EIT).

Solution-Processed Flexible Biomemristor Based on Gold-Decorated Chitosan

The main requirements for skin-attachable memory devices are flexibility and biocompatibility. We represent a flexible, transparent, and biocompatible resistive switching random access memory (ReRAM) based on gold-decorated chitosan for future flexible and wearable electronics. The device with an Ag/Au-chitosan/Au cross-bar structure shows nonvolatile ReRAM properties. This fabricated Au-chitosan-based biocompatible ReRAM (bioReRAM) shows reliable bipolar memory performance with mechanical flexibility. The device shows essential memory characterizations including long data retention and hundreds of switching cycles. The origin of the resistance switching properties is related to trap-assisted space-charge-limited conduction in the high-resistance state and formation/annihilation of a conductive filament in the low-resistance state. This transparent bioReRAM is a viable candidate for flexible and biodegradable nanoelectronic devices.

Solution-processed electronics for artificial synapses

Artificial synaptic devices and systems have become hot topics due to parallel computing, high plasticity, integration of storage, and processing to meet the challenges of the traditional Von Neumann computers. Currently, two-terminal memristors and three-terminal transistors have been mainly developed for high-density storage with high switching speed and high reliability because of the adjustable resistivity, controllable ion migration, and abundant choices of functional materials and fabrication processes. To achieve the low-cost, large-scale, and easy-process fabrication, solution-processed techniques have been extensively employed to develop synaptic electronics towards flexible and highly integrated three-dimensional (3D) neural networks. Herein, we have summarized and discussed solution-processed techniques in the fabrication of two-terminal memristors and three-terminal transistors for the application of artificial synaptic electronics mainly reported in the recent five years from the view of fabrication processes, functional materials, electronic operating mechanisms, and system applications. Furthermore, the challenges and prospects were discussed in depth to promote solution-processed techniques in the future development of artificial synapse with high performance and high integration.

Printed carbon nanotube thin-film transistors: progress on printable materials and the path to applications

Printing technologies have attracted significant attention owing to their potential use in the low-cost manufacturing of custom or large-area flexible electronics. Among the many printable electronic materials that have been explored, semiconducting carbon nanotubes (CNTs) have shown increasing promise based on their exceptional electrical and mechanical properties, relative stability in air, and compatibility with several printing techniques to form semiconducting thin films. These attractive attributes make printed CNT thin films promising for applications including, but not limited to, sensors and display backplanes - at the heart of which is electronics' most versatile device: the transistor. In this review, we present a summary of recent advancements in the field of printed carbon nanotube thin-film transistors (CNT-TFTs). In addition to an introduction of different printing techniques, together with their strengths and limitations, we discuss key aspects of ink/material selection and processing of various device components, including the CNT channels, contacts, and gate insulators. It is clear that printed CNT-TFTs are rapidly advancing, but there remain challenges, which are discussed along with current techniques to resolve them and future developments towards practical applications from these devices. There has been interest in low-cost, printable transistors for many years and the CNT-TFTs show great promise for delivering, but will not become a reality without further research advancement.

Ultra-high drivability, high-mobility, low-voltage and high-integration intrinsically stretchable transistors

Realizing intrinsically stretchable transistors with high current drivability, high mobility, small feature size, low power and the potential for mass production is essential for advancing stretchable electronics a critical step forward. However, it is challenging to realize these requirements simultaneously due to the limitations of the existing fabrication technologies when integrating intrinsically stretchable materials into transistors. Here, we propose a removal-transfer-photolithography method (RTPM), combined with adopting poly(urea-urethane) (PUU) as a dielectric, to realize integratable intrinsically stretchable carbon nanotube thin-film transistors (IIS-CNT-TFTs). The realized IIS-CNT-TFTs achieve excellent electrical and mechanical properties simultaneously, showing high field-effect-mobility up to 221 cm2 V-1 s-1 and high current density up to 810 μA mm-1 at a low driving voltage of -1 V, which are both the highest values for intrinsically stretchable transistors today to the best of our knowledge. At the same time, the transistors can survive 2000 cycles of repeated stretching by 50%, indicating their promising applicability to stretchable circuits, displays, and wearable electronics. The achieved intrinsically stretchable thin-film transistors show higher electrical performance, higher stretching durability, and smaller feature size simultaneously compared with the state-of-the-art works, providing a novel solution to integratable intrinsically stretchable electronics. Besides, the proposed RTPM involves adopting removable sacrificial layers to protect the PDMS substrate and PUU dielectric during the photolithography and patterning steps, and finally removing the sacrificial layers to improve the electrical and mechanical performance. This method is generally applicable to further enhance the performance of the existing transistors and devices with a similar structure in soft electronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Facile Approach to Fabricating Stretchable Organic Transistors with Laser-Patterned Ag Nanowire Electrodes

Stretchable electronics are poised to revolutionize personal healthcare and robotics, where they enable distributed and conformal sensors. Transistors are fundamental building blocks of electronics, and there is a need to produce stretchable transistors using low-cost and scalable fabrication techniques. Here, we introduce a facile fabrication approach using laser patterning and transfer printing to achieve high-performance, solution-processed intrinsically stretchable organic thin-film transistors (OTFTs). The device consists of Ag nanowire (NW) electrodes, where the source and drain electrodes are patterned using laser ablation. The Ag NWs are then partially embedded in a poly(dimethylsiloxane) (PDMS) matrix. The electrodes are combined with a PDMS dielectric and polymer semiconductor, where the layers are individually transfer printed to complete the OTFT. Two polymer semiconductors, DPP-DTT and DPP-4T, are considered and show stable operation under the cyclic strain of 20 and 40%, respectively. The OTFTs maintain electrical performance by adopting a buckled structure after the first stretch-release cycle. The conformability and stretchability of the OTFT is also demonstrated by operating the transistor while adhered to a finger being flexed. The ability to pattern highly conductive Ag NW networks using laser ablation to pattern electrodes as well as interconnects provides a simple strategy to produce complex stretchable OTFT-based circuits.

Synaptic transistors with aluminum oxide dielectrics enabling full audio frequency range signal processing

The rapid evolution of the neuromorphic computing stimulates the search for novel brain-inspired electronic devices. Synaptic transistors are three-terminal devices that can mimic the chemical synapses while consuming low power, whereby an insulating dielectric layer physically separates output and input signals from each other. Appropriate choice of the dielectric is crucial in achieving a wide range of operation frequencies in these devices. Here we report synaptic transistors with printed aluminum oxide dielectrics, improving the operation frequency of solution-processed synaptic transistors by almost two orders of magnitude to 50 kHz. Fabricated devices, yielding synaptic response for all audio frequencies (20 Hz to 20 kHz), are employed in an acoustic response system to show the potential for future research in neuro-acoustic signal processing with printed oxide electronics.

Inkjet Printing of Synthesized Melanin Nanoparticles as a Biocompatible Matrix for Pharmacologic Agents

Melanin is a natural biopigment that is produced by melanocytes and can be found in most living organisms. The unique physical and chemical properties of melanin render it potentially useful for numerous applications, particularly those in which a biocompatible functional material is required. Herein, we introduce one important technology in which melanin can be utilized: a drug delivery system in terms of a biocompatible matrix. However, extracting melanin from different biological sources is costly and time-consuming and introduces variabilities in terms of chemical structure, properties, and functions. Hence, a functionally reproducible system is hard to achieve using biologically extracted melanin. Here we report the synthesis of melanin nanoparticles of controlled uniform sizes and chemical characteristics. The optical, chemical, and structural characteristics of synthesized nanoparticles were characterized by optical confocal photoluminescence (PL) imaging, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Zeta potentiometry. The melanin nanoparticles have 100 nm size and a narrow size distribution. The advantage of a nanoparticle structure is its enhanced surface-to-volume ratio compared to bulk pigments, which is important for applications in which controlling the microscopic surface area is essential. Using the inkjet printing technique, we developed melanin thin films with minimum ink waste and loaded them with methylene blue (our representative drug) to test the drug-loading ability of the melanin nanoparticles. Inkjet printing allowed us to create smooth uniform films with precise deposition and minimum ink-waste. The spectroscopic analysis confirmed the attachment of the "drug" onto the melanin nanoparticles as a matrix. Hence, our data identify melanin as a material system to integrate into drug release applications.

High-Performance Nonvolatile Organic Photonic Transistor Memory Devices using Conjugated Rod-Coil Materials as a Floating Gate

A novel approach for using conjugated rod-coil materials as a floating gate in the fabrication of nonvolatile photonic transistor memory devices, consisting of n-type Sol-PDI and p-type C10-DNTT, is presented. Sol-PDI and C10-DNTT are used as dual functions of charge-trapping (conjugated rod) and tunneling (insulating coil), while n-type BPE-PDI and p-type DNTT are employed as the corresponding transporting layers. By using the same conjugated rod in the memory layer and transporting channel with a self-assembled structure, both n-type and p-type memory devices exhibit a fast response, a high current contrast between "Photo-On" and "Electrical-Off" bistable states over 10⁵ , and an extremely low programing driving force of 0.1 V. The fabricated photon-driven memory devices exhibit a quick response to different wavelengths of light and a broadband light response that highlight their promising potential for light-recorder and synaptic device applications.

Flexible Carbon Nanotube Synaptic Transistor for Neurological Electronic Skin Applications

There is an increasing interest in the development of memristive or artificial synaptic devices that emulate the neuronal activities for neuromorphic computing applications. While there have already been many reports on artificial synaptic transistors implemented on rigid substrates, the use of flexible devices could potentially enable an even broader range of applications. In this paper, we report artificial synaptic thin-film transistors built on an ultrathin flexible substrate using high carrier mobility semiconducting single-wall carbon nanotubes. The synaptic characteristics of the flexible synaptic transistor including long-term/short-term plasticity, spike-amplitude-dependent plasticity, spike-width-dependent plasticity, paired-pulse facilitation, and spike-time-dependent plasticity have all been systematically characterized. Furthermore, we have demonstrated a flexible neurological electronic skin and its peripheral nerve with a flexible ferroelectret nanogenerator (FENG) serving as the sensory mechanoreceptor that generates action potentials to be processed and transmitted by the artificial synapse. In such neurological electronic skin, the flexible FENG sensor converts the tactile input (magnitude and frequency of force) into presynaptic action potential pulses, which are then passed to the gate of the synaptic transistor to induce change in its postsynaptic current, mimicking the modulation of synaptic weight in a biological synapse. Our neurological electronic skin closely imitates the behavior of actual human skin, and it allows for instantaneous detection of force stimuli and offers biological synapse-like behavior to relay the stimulus signals to the next stage. The flexible sensory skin could potentially be used to interface with skeletal muscle fibers for applications in neuroprosthetic devices.

Laser writing of nitrogen-doped silicon carbide for biological modulation

Conducting or semiconducting materials embedded in insulating polymeric substrates can be useful in biointerface applications; however, attainment of this composite configuration by direct chemical processes is challenging. Laser-assisted synthesis has evolved as a fast and inexpensive technique to prepare various materials, but its utility in the construction of biophysical tools or biomedical devices is less explored. Here, we use laser writing to convert portions of polydimethylsiloxane (PDMS) into nitrogen-doped cubic silicon carbide (3C-SiC). The dense 3C-SiC surface layer is connected to the PDMS matrix via a spongy graphite layer, facilitating electrochemical and photoelectrochemical activity. We demonstrate the fabrication of arbitrary two-dimensional (2D) SiC-based patterns in PDMS and freestanding 3D constructs. To establish the functionality of the laser-produced composite, we apply it as flexible electrodes for pacing isolated hearts and as photoelectrodes for local peroxide delivery to smooth muscle sheets.

Stable and Biocompatible Carbon Nanotube Ink Mediated by Silk Protein for Printed Electronics

Ink-based processes, which enable scalable fabrication of flexible devices based on nanomaterials, are one of the practical approaches for the production of wearable electronics. However, carbon nanotubes (CNTs), which possess great potential for flexible electronics, are facing challenges for use in inks due to their low dispersity in most solvents and suspicious cytotoxicity. Here, a stable and biocompatible CNT ink, which is stabilized by sustainable silk sericin and free from any artificial chemicals, is reported. The ink shows stability up to months, which can be attributed to the formation of sericin-CNT (SSCNT) hybrid through non-covalent interactions. It is demonstrated that the SSCNT ink can be used for fabricating versatile circuits on textile, paper, and plastic films through various techniques. As proofs of concept, electrocardiogram electrodes, breath sensors, and electrochemical sensors for monitoring human health and activity are fabricated, demonstrating the great potential of the SSCNT ink for smart wearables.

Direct Printing of Asymmetric Electrodes for Improving Charge Injection/Extraction in Organic Electronics

Engineering the energy levels of organic conducting materials can be useful for developing high-performance organic field-effect transistors (OFETs), whose electrodes must be well controlled to facilitate easy charge carrier transport from the source to drain through an active channel. However, symmetric source and drain electrodes that have the same energy levels are inevitably unfavorable for either charge injection or charge extraction. In this study, asymmetric source and drain electrodes are simply prepared using the electrohydrodynamic (EHD)-jet printing technique after the careful work function engineering of organic conducting material composites. Two types of additives effectively tune the energy levels of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-based composites. These solutions are alternately patterned using the EHD-jet printing process, where the use of an electric field makes fine jet control that enables to directly print asymmetric electrodes. The asymmetric combination of EHD-printed electrodes helps in obtaining advanced charge transport properties in p-type and n-type OFETs, as well as their organic complementary inverters. This strategy is believed to provide useful guidelines for the facile patterning of asymmetric electrodes, enabling the desirable properties of charge injection and extraction to be achieved in organic electronic devices.

Pencil-paper on-skin electronics

Pencils and papers are ubiquitous in our society and have been widely used for writing and drawing, because they are easy to use, low-cost, widely accessible, and disposable. However, their applications in emerging skin-interfaced health monitoring and interventions are still not well explored. Herein, we report a variety of pencil-paper-based on-skin electronic devices, including biophysical (temperature, biopotential) sensors, sweat biochemical (pH, uric acid, glucose) sensors, thermal stimulators, and humidity energy harvesters. Among these devices, pencil-drawn graphite patterns (or combined with other compounds) serve as conductive traces and sensing electrodes, and office-copy papers work as flexible supporting substrates. The enabled devices can perform real-time, continuous, and high-fidelity monitoring of a range of vital biophysical and biochemical signals from human bodies, including skin temperatures, electrocardiograms, electromyograms, alpha, beta, and theta rhythms, instantaneous heart rates, respiratory rates, and sweat pH, uric acid, and glucose, as well as deliver programmed thermal stimulations. Notably, the qualities of recorded signals are comparable to those measured with conventional methods. Moreover, humidity energy harvesters are prepared by creating a gradient distribution of oxygen-containing groups on office-copy papers between pencil-drawn electrodes. One single-unit device (0.87 cm²) can generate a sustained voltage of up to 480 mV for over 2 h from ambient humidity. Furthermore, a self-powered on-skin iontophoretic transdermal drug-delivery system is developed as an on-skin chemical intervention example. In addition, pencil-paper-based antennas, two-dimensional (2D) and three-dimensional (3D) circuits with light-emitting diodes (LEDs) and batteries, reconfigurable assembly and biodegradable electronics (based on water-soluble papers) are explored.

Visible light communication with efficient far-red/near-infrared polymer light-emitting diodes

Visible light communication (VLC) is a wireless technology that relies on optical intensity modulation and is potentially a game changer for internet-of-things (IoT) connectivity. However, VLC is hindered by the low penetration depth of visible light in non-transparent media. One solution is to extend operation into the "nearly (in)visible" near-infrared (NIR, 700-1000 nm) region, thus also enabling VLC in photonic bio-applications, considering the biological tissue NIR semitransparency, while conveniently retaining vestigial red emission to help check the link operativity by simple eye inspection. Here, we report new far-red/NIR organic light-emitting diodes (OLEDs) with a 650-800 nm emission range and external quantum efficiencies among the highest reported in this spectral range (>2.7%, with maximum radiance and luminance of 3.5 mW/cm2 and 260 cd/m2, respectively). With these OLEDs, we then demonstrate a "real-time" VLC setup achieving a data rate of 2.2 Mb/s, which satisfies the requirements for IoT and biosensing applications. These are the highest rates ever reported for an online unequalised VLC link based on solution-processed OLEDs.

A New Frontier of Printed Electronics: Flexible Hybrid Electronics

The performance and integration density of silicon integrated circuits (ICs) have progressed at an unprecedented pace in the past 60 years. While silicon ICs thrive at low-power high-performance computing, creating flexible and large-area electronics using silicon remains a challenge. On the other hand, flexible and printed electronics use intrinsically flexible materials and printing techniques to manufacture compliant and large-area electronics. Nonetheless, flexible electronics are not as efficient as silicon ICs for computation and signal communication. Flexible hybrid electronics (FHE) leverages the strengths of these two dissimilar technologies. It uses flexible and printed electronics where flexibility and scalability are required, i.e., for sensing and actuating, and silicon ICs for computation and communication purposes. Combining flexible electronics and silicon ICs yields a very powerful and versatile technology with a vast range of applications. Here, the fundamental building blocks of an FHE system, printed sensors and circuits, thinned silicon ICs, printed antennas, printed energy harvesting and storage modules, and printed displays, are discussed. Emerging application areas of FHE in wearable health, structural health, industrial, environmental, and agricultural sensing are reviewed. Overall, the recent progress, fabrication, application, and challenges, and an outlook, related to FHE are presented.

Nanomaterial Patterning in 3D Printing

The synergistic integration of nanomaterials with 3D printing technologies can enable the creation of architecture and devices with an unprecedented level of functional integration. In particular, a multiscale 3D printing approach can seamlessly interweave nanomaterials with diverse classes of materials to impart, program, or modulate a wide range of functional properties in an otherwise passive 3D printed object. However, achieving such multiscale integration is challenging as it requires the ability to pattern, organize, or assemble nanomaterials in a 3D printing process. This review highlights the latest advances in the integration of nanomaterials with 3D printing, achieved by leveraging mechanical, electrical, magnetic, optical, or thermal phenomena. Ultimately, it is envisioned that such approaches can enable the creation of multifunctional constructs and devices that cannot be fabricated with conventional manufacturing approaches.

Low-cost and customizable inkjet printing for microelectrodes fabrication

Microelectrodes for detection of chemicals present several advantages over conventional sized electrodes. However, rapid and low-cost fabrication of microelectrodes is challenging due to high complexity of patterning equipment. We present the development of a low-cost, customizable inkjet printer for printing nanomaterials including carbon nanotubes for the fabrication of microelectrodes. The achieved spatial resolution of the inkjet printer is less than 20 µm, which is comparable to advanced commercially available inkjet printers, with the advantage of being low-cost and easily replicated.

Pyrazine-Flanked Diketopyrrolopyrrole (DPP): A New Polymer Building Block for High-Performance n-Type Organic Thermoelectrics

n-Doped conjugated polymers usually show low electrical conductivities and low thermoelectric power factors, limiting their applications in n-type organic thermoelectrics. Here, we report the synthesis of a new diketopyrrolopyrrole (DPP) derivative, pyrazine-flanked DPP (PzDPP), with the deepest LUMO level in all the reported DPP derivatives. Based on PzDPP, a donor-acceptor copolymer, P(PzDPP-CT2), is synthesized. The polymer displays a deep LUMO energy level and strong interchain interaction with a short π-π stacking distance of 3.38 Å. When doped with n-dopant N-DMBI, P(PzDPP-CT2) exhibits high n-type electrical conductivities of up to 8.4 S cm^-1 and power factors of up to 57.3 μW m^-1 K^-2. These values are much higher than previously reported n-doped DPP polymers, and the power factor also ranks the highest in solution-processable n-doped conjugated polymers. These results suggest that PzDPP is a promising high-performance building block for n-type organic thermoelectrics and also highlight that, without sacrificing polymer interchain interactions, efficient n-doping can be realized in conjugated polymers with careful molecular engineering.

Direct Patterning of Highly Conductive PEDOT:PSS/Ionic Liquid Hydrogel via Microreactive Inkjet Printing

The gelation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has gained popularity for its potential applications in three dimensions, while possessing tissue-like mechanical properties, high conductivity, and biocompatibility. However, the fabrication of arbitrary structures, especially via inkjet printing, is challenging because of the inherent gel formation. Here, microreactive inkjet printing (MRIJP) is utilized to pattern various 2D and 3D structures of PEDOT:PSS/IL hydrogel by in-air coalescence of PEDOT:PSS and ionic liquid (IL). By controlling the in-air position and Marangoni-driven encapsulation, single droplets of the PEDOT:PSS/IL hydrogel as small as a diameter of ≈260 μm are fabricated within ≈600 μs. Notably, this MRIJP-based PEDOT:PSS/IL has potential for freeform patterning while maintaining identical performance to those fabricated by the conventional spin-coating method. Through controlled deposition achieved via MRIJP, PEDOT:PSS/IL can be transformed into different 3D structures without the need for molding, potentially leading to substantial progress in next-generation bioelectronics devices.