3R Electronics: Scalable Fabrication of Resilient, Repairable, and Recyclable Soft-Matter Electronics

Gallium-Carbon: A Universal Composite for Sustainable 3D Printing of Integrated Sensor-Heater-Battery Systems in Wearable and Recyclable Electronics

This study presents a novel three-dimensional (3D) printable gallium-carbon black-styrene isoprene styrene block copolymer (Ga-CB-SIS), offering a versatile solution for the rapid fabrication of stretchable and integrated sensor-heater-battery systems in wearable and recyclable electronics. The composite exhibits sinter-free characteristics, allowing for printing on various substrates, including heat-sensitive materials. Unlike traditional conductive inks, the Ga-CB-SIS composite, composed of gallium, carbon black, and styrene isoprene block copolymers, combines electrical conductivity, stretchability, and digital printability. By introducing carbon black as a filler material, the composite achieves promising electromechanical behavior, making it suitable for low-resistance heaters, batteries, and electrical interconnects. The fabrication process involves a simultaneous mixing and ball-milling technique, resulting in a homogeneous composition with a CB/Ga ratio of 4.3%. The Ga-CB-SIS composite showcases remarkable adaptability for digital printing on various substrates. Its self-healing property and efficient recycling technique using a deep eutectic solvent contribute to an environmentally conscious approach to electronic waste, with a high gallium recovery efficiency of ∼98%. The study's innovation extends to applications, presenting a fully digitally printed stretchable Ga-CB-SIS battery integrated with strain sensors and heaters, representing a significant leap in LM-based composites. This multifunctional and sustainable Ga-CB-SIS composite emerges as a key player in the future of wearable electronics, offering integrated circuits with sensing, heating, and energy storage elements.

Bulge-Free and Homogeneous Metal Line Jet Printing with StarJet Technology

The technology to jet print metal lines with precise shape fidelity on diverse substrates is gaining higher interest across multiple research fields. It finds applications in additively manufactured flexible electronics, environmentally friendly and sustainable electronics, sensor devices for medical applications, and fabricating electrodes for solar cells. This paper provides an experimental investigation to deepen insights into the non-contact printing of solder lines using StarJet technology, eliminating the need for surface activation, substrate heating, curing, or post-processing. Moreover, it employs bulk metal instead of conventional inks or pastes, leading to cost-effective production and enhanced conductivity. The effect of molten metal temperature, substrate temperature, standoff distance, and printing velocity was investigated on polymer foils (i.e., PET sheets). Robust printing parameters were derived to print uniform, bulge-free, bulk metal lines suitable for additive manufacturing applications. The applicability of the derived parameters was extended to 3D-printed PLA, TPU, PA-GF, and PETG substrates having a much higher surface roughness. Additionally, a high aspect ratio of approx. 16:1 wall structure has been demonstrated by printing multiple metal lines on top of each other. While challenges persist, this study contributes to advancing additively manufactured electronic devices, highlighting the capabilities of StarJet metal jet-printing technology.

Sustainable 3D printing by reversible salting-out effects with aqueous salt solutions

Achieving a simple yet sustainable printing technique with minimal instruments and energy remains challenging. Here, a facile and sustainable 3D printing technique is developed by utilizing a reversible salting-out effect. The salting-out effect induced by aqueous salt solutions lowers the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) solutions to below 10 °C. It enables the spontaneous and instant formation of physical crosslinks within PNIPAM chains at room temperature, thus allowing the PNIPAM solution to solidify upon contact with a salt solution. The PNIPAM solutions are extrudable through needles and can immediately solidify by salt ions, preserving printed structures, without rheological modifiers, chemical crosslinkers, and additional post-processing steps/equipment. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, which offers great potential for developing water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators.

3D Printed Gallium Battery with Outstanding Energy Storage: Toward Fully Printed Battery-on-the-Board Soft Electronics

The last decade observed rapid progress in soft electronics. Yet, the ultimate desired goal for many research fields is to fabricate fully integrated soft-matter electronics with sensors, interconnects, and batteries, at the ease of pushing a print button. In this work, an important step is taken toward this by demonstrating an ultra-stretchable thin-film Silver-Gallium (Ag-Ga) battery with an unprecedented combination of areal capacity and mechanical strain tolerance. The Biphasic Gallium-Carbon anode electrode demonstrates a record-breaking areal capacity of 78.7 mAh cm-2, and an exceptional stretchability of 170%, showing clear progress over state-of-the-art. The exceptional theoretical capacity of gallium, along with its natural liquid phase self-healing, and its dendrite-free operation permits excellent electromechanical cycling. All composites of the battery including liquid-metal-based current collectors, and electrodes are sinter-free and digitally printable at room temperature, enabling the use of a wide range of substrates, including heat-sensitive polymer films. Consequently, it is demonstrated for the first time multi-layer, and multi-material digital printing of complex battery-on-the-board stretchable devices that integrate printed sensor, multiple cells of printed battery, highly conductive interconnects, and silicone chips, and demonstrate a tailor-made patch for body-worn electrophysiological monitoring.

Modeling the Temperature-Dependent Size Change of Polydisperse Nano-objects using a Deep Generative Model

Modern microscopy techniques can be used to investigate soft nano-objects at the nanometer scale. However, time-consuming microscopy measurements combined with low numbers of observable polydisperse objects often limit the statistics. We propose a method for identifying the most representative objects from their respective point clouds. These point cloud data are obtained, for example, through the localization of single emitters in super-resolution fluorescence microscopy. External stimuli, such as temperature, can cause changes in the shape and properties of adaptive objects. Due to the demanding and time-consuming nature of super-resolution microscopy experiments, only a limited number of temperature steps can be performed. Therefore, we propose a deep generative model that learns the underlying point distribution of temperature-dependent microgels, enabling the reliable generation of unlimited samples with an arbitrary number of localizations. Our method greatly cuts down the data collection effort across diverse experimental conditions, proving invaluable for soft condensed matter studies.

Ultrasensitive Soft Sensor from Anisotropic Conductive Biphasic Liquid Metal-Polymer Gels

Subtle vibrations, such as sound and ambient noises, are common mechanical waves that can transmit energy and signals for modern technologies such as robotics and health management devices. However, soft electronics cannot accurately distinguish ultrasmall vibrations owing to their extremely small pressure, complex vibration waveforms, and high noise susceptibility. This study successfully recognizes signals from subtle vibrations using a highly flexible anisotropic conductive gel (ACG) based on biphasic liquid metals. The relationships between the anisotropic structure, subtle vibrations, and electrical performance are investigated using rheological-electrical experiments. The refined anisotropic design successfully realized low-cost flexible electronics with ultrahigh sensitivity (Gauge Factor: 12787), extremely low detection limit (strain: 0.01%), and excellent frequency recognition accuracy (>99%), significantly surpassing those of current flexible sensors. The ultrasensitive flexible electronics in this study are beneficial for diverse advanced technologies such as acoustic engineering, wearable electronics, and intelligent robotics.

Liquid metal assisted fabrication of MXene-based films: Toward superior electromagnetic interference shielding and thermal management

The development of thin and flexible films that possess both electromagnetic interference (EMI) shielding and thermal management capabilities has always been an intriguing pursuit, but itisnevertheless a crucialproblemtoaddress. Inspired by the deformability of liquid metal (LM) and film forming capacity of MXene, here we present a series of ternary compositing films prepared via cellulose nanofiber (CNF) assisted vacuum filtration technology. Originating from the highly conductive LM/MXene network, the MLMC film presents a maximum EMI shielding effectiness (EMI SE) of 78 dB at a tiny thickness of 45 μm, together with a high specific EMI SE of 3046 dB mm-1. Meanwhile, these compositing films also deliver excellent flexibility and mechanical reliability, showing no obvious decline in EMI shielding performance even after 1000 bending and 500 folding cycles, respectively. Moreover, notable anisotropic thermal conductive property was successfully achieved, allowing for a highly desirable in-plane thermal conductivity of 7.8 W m-1 K-1. This accomplishment also yielded an exceptional electro-thermal conversion capacity, enabling efficient low-voltage (3 V) heating capabilities. These captivating features are expected to greatly drive the widespread adoption of LM-based films in future flexible electronic and wearable technologies.

Towards High Efficiency and Rapid Production of Room-Temperature Liquid Metal Wires Compatible with Electronic Prototyping Connectors

We present in this work new methodologies to produce, refine, and interconnect room-temperature liquid-metal-core thermoplastic elastomer wires that have extreme extendibility (>500%), low production time and cost at scale, and may be integrated into commonly used electrical prototyping connectors like a Japan Solderless Terminal (JST) or Dupont connectors. Rather than focus on the development of a specific device, the aim of this work is to demonstrate strategies and processes necessary to achieve scalable production of liquid-metal-enabled electronics and address several key challenges that have been present in liquid metal systems, including leak-free operation, minimal gallium corrosion of other electrode materials, low liquid metal consumption, and high production rates. The ultimate goal is to create liquid-metal-enabled rapid prototyping technologies, similar to what can be achieved with Arduino projects, where modification and switching of components can be performed in seconds, which enables faster iterations of designs. Our process is focused primarily on fibre-based liquid metal wires contained within thermoplastic elastomers. These fibre form factors can easily be integrated with wearable sensors and actuators as they can be sewn or woven into fabrics, or cast within soft robotic components.

Tough soldering for stretchable electronics by small-molecule modulated interfacial assemblies

The rapid-developing soft robots and wearable devices require flexible conductive materials to maintain electric functions over a large range of deformations. Considerable efforts are made to develop stretchable conductive materials; little attention is paid to the frequent failures of integrated circuits caused by the interface mismatch of soft substrates and rigid silicon-based microelectronics. Here, we present a stretchable solder with good weldability that can strongly bond with electronic components, benefiting from the hierarchical assemblies of liquid metal particles, small-molecule modulators, and non-covalently crosslinked polymer matrix. Our self-solder shows high conductivity (>2×105  S  m-1), extreme stretchability (~1000%, and >600% with chip-integrated), and high toughness (~20 MJ m-3). Additionally, the dynamic interactions within our solder's surface and interior enable a range of unique features, including ease of integration, component substitution, and circuit recyclability. With all these features, we demonstrated an application as thermoforming technology for three-dimensional (3D) conformable electronics, showing potential in reducing the complexity of microchip interfacing, as well as scalable fabrication of chip-integrated stretchable circuits and 3D electronics.

A biomimetic closed-loop recyclable, long-term durable, extreme-condition resistant, flame-retardant nanocoating synthesized by reversible flocculation assembly

Flame-retardant coatings have attracted increasing attention in mitigating the fire threat of flammable polymer materials. Their durable application inevitably provides high resistance to various complex environments, however, discarded stable materials will turn into another man-made waste disaster. The paradigm shift toward a sustainable future is to combine durability and recyclability of coatings. Herein, we demonstrate a biomimetic coating that reversibly captures active flame-retardant nanomaterials by flocculation assembly using anionic polyacrylamide covering the polyurethane foam surface. Strong hydrogen bonding and microstructural interlocking provide the coating with high durability under complex harsh conditions (underwater, chemical exposure, hydrothermal aging, long-term external extrusion, etc.). Meanwhile, the disassembly/reorganization of the coating can be easily repeated in response to pH stimulation with a recycling rate of 97%. The experiments and theoretical calculations reveal the mechanism of the reversible flocculation assembly. This biomimetic strategy of responsive flocculation assembly opens the way for functional coatings with integrated durability and recyclability.

Recyclable Thin-Film Soft Electronics for Smart Packaging and E-Skins

Despite advances in soft, sticker-like electronics, few efforts have dealt with the challenge of electronic waste. Here, this is addressed by introducing an eco-friendly conductive ink for thin-film circuitry composed of silver flakes and a water-based polyurethane dispersion. This ink uniquely combines high electrical conductivity (1.6 × 105 S m-1 ), high resolution digital printability, robust adhesion for microchip integration, mechanical resilience, and recyclability.  Recycling is achieved with an ecologically-friendly processing method to decompose the circuits into constituent elements and recover the conductive ink with a decrease of only 2.4% in conductivity. Moreover, adding liquid metal enables stretchability of up to 200% strain, although this introduces the need for more complex recycling steps. Finally, on-skin electrophysiological monitoring biostickers along with a recyclable smart package with integrated sensors for monitoring safe storage of perishable foods are demonstrated.

Ultra-robust, Highly Stretchable, and Conductive Nanocomposites with Self-healable Asymmetric Structures Prepared by a Simple Green Method

Flexible conductive polymer nanocomposites based on silver nanowires (AgNWs) have been extensively studied to develop the next generation of flexible electronic devices. Fiber materials with high strength and large stretching are an important part of high-performance wearable electronics. However, manufacturing conductive composites with both high mechanical strength and good stability remains challenging. In addition, the process of effectively dispersing conductive fillers into substrates is relatively complex, which greatly hampers its widespread application. Here, a simple green self-assembly preparation method in water is reported. The AgNW is evenly dispersed in aqueous, i.e., water-borne polyurethane (WPU) with water as the solvent, and a AgNW/WPU conductive nanocomposite film with an asymmetric structure is formed by self-assembly in one step. The film has high strength (≈49.2 MPa) and high strain (≈910%), low initial resistance (99.9 mΩ/sq), high conductivity (9968.1 S/cm), and excellent self-healing (93%) and adhesion. With good self-healing performance, fibers with a conductive filler spiral structure are formed. At the same time, the application of the conductive composite material with an asymmetric structure in intelligent wearability is demonstrated.

Improving the Performance of Paper-Based Dipole Antennas by Electromagnetic Flux Concentration

One of the essential issues in modern advanced materials science is to design and manufacture flexible devices, in particular in the framework of the Internet of Things (IoT), to improve integration into applications. An antenna is an essential component of wireless communication modules and, in addition to flexibility, compact dimensions, printability, low cost, and environmentally friendlier production strategies, also represent relevant functional challenges. Concerning the antenna's performance, the optimization of the reflection coefficient and maximum range remain the key goals. In this context, this work reports on screen-printed paper@Ag-based antennas and optimizes their functional properties, with improvements in the reflection coefficient (S₁₁) from -8 to -56 dB and maximum transmission range from 208 to 256 m, with the introduction of a PVA-Fe₃O₄@Ag magnetoactive layer into the antenna's structure. The incorporated magnetic nanostructures allow the optimization of the functional features of antennas with possible applications ranging from broadband arrays to portable wireless devices. In parallel, the use of printing technologies and sustainable materials represents a step toward more sustainable electronics.

Liquid Metal Patterning and Unique Properties for Next-Generation Soft Electronics

Room-temperature liquid metal (LM)-based electronics is expected to bring advancements in future soft electronics owing to its conductivity, conformability, stretchability, and biocompatibility. However, various difficulties arise when patterning LM because of its rheological features such as fluidity and surface tension. Numerous attempts are made to overcome these difficulties, resulting in various LM-patterning methods. An appropriate choice of patterning method based on comprehensive understanding is necessary to fully utilize the unique properties. Therefore, the authors aim to provide thorough knowledge about patterning methods and unique properties for LM-based future soft electronics. First, essential considerations for LM-patterning are investigated. Then, LM-patterning methods-serial-patterning, parallel-patterning, intermetallic bond-assisted patterning, and molding/microfluidic injection-are categorized and investigated. Finally, perspectives on LM-based soft electronics with unique properties are provided. They include outstanding features of LM such as conformability, biocompatibility, permeability, restorability, and recyclability. Also, they include perspectives on future LM-based soft electronics in various areas such as radio frequency electronics, soft robots, and heterogeneous catalyst. LM-based soft devices are expected to permeate the daily lives if patterning methods and the aforementioned features are analyzed and utilized.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Triple crosslinking conductive hydrogels with digitally printable and outstanding mechanical stability for high-resolution conformable bioelectronics

Soft, conductive, and stretchable hydrogels offer a broad variety of applications, including skin-interfacing electrodes, biomonitoring patches, and electrostimulation. Despite rapid developments over the last decades, a combination of good electrical and mechanical properties, low-cost fabrication, and biocompatibility is yet to be demonstrated. Also, the current methods for deposition and patterning of these hydrogels are manual, and there is a need toward autonomous and digital fabrication techniques. In this work, we demonstrate a novel Gallium (Ga) embedded sodium-alginate-polyacrylamide-LAPONITE® (Ga-SA-PAAM-La) hydrogel, that is ultra-stretchable (Maximum strain tolerance of∼985%), tough (toughness ∼30 kJ m^-3), bio-adhesive (adhesion energy ∼216 J m^-2), conductive, and digitally printable. Ga nanoparticles are used as radical initiators. By adjusting the sonication parameters, we control the solution viscosity and curing time, thus allowing us to prepare pre-polymers with the desired properties for casting, or digital printing. These hydrogels benefit from a triple-network structure due to the role of Ga droplets as crosslinkers besides BIS (N,N'-methylene-bis-acrylamide) and LAPONITE®, thus resulting in tough composite hydrogels. The inclusion of LAPONITE® into the hydrogel network improved its electrical conductivity, adhesion, digital printability, and its mechanical properties, (>6× compared to the same hydrogel without LAPONITE®). As electrodes in the electrocardiogram, the signal-to-noise ratio was surprisingly higher than the medical-grade Ag/AgCl electrodes, which are applied for monitoring muscles, heart, respiration, and body joint angle through EMG, ECG, and bioimpedance measurements. The results obtained prove that such digitally printed conductive and tough hydrogels can be used as potential electrodes and sensors in practical applications in the next generation of printed wearable computing devices.

Metal-organic framework (MOF) facilitated highly stretchable and fatigue-resistant ionogels for recyclable sensors

Ionogel-based flexible sensors are widely applied in wearable biomedical devices and soft robots. However, the abandoned ionic sensors are rapidly turning into e-waste. Here, we harness the porosity and the coordination of metal sites of metal-organic frameworks (MOFs) to develop physically crosslinked ionogels, which are composed of polymer chains that coordinate with the MOF metal sites. The covalent crosslinking of the host material transformed into reversible bond interactions that significantly enhance the mechanical properties of the MOF-ionogels. The obtained ionogels can endure an 11 000% stretch and exhibit Young's modulus and toughness of 58 MPa and 25 MJ m^-3, respectively. In addition, the fracture energy is as high as 125 kJ m^-2, outperforming most reported ionogels. Furthermore, the UiO-66-ionogels are fully recyclable and both the mechanical and electrical properties can be restored. The results of this work provide a new vision for the development of future "green" sensors.