Untethered Disposable Health Monitoring Electronic Patches with an Integrated Ag2O-Zn Battery, a AgInGa Current Collector, and Hydrogel Electrodes

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.

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.

Hydrogel-Based Bioelectronics and Their Applications in Health Monitoring

Flexible bioelectronics exhibit promising potential for health monitoring, owing to their soft and stretchable nature. However, the simultaneous improvement of mechanical properties, biocompatibility, and signal-to-noise ratio of these devices for health monitoring poses a significant challenge. Hydrogels, with their loose three-dimensional network structure that encapsulates massive amounts of water, are a potential solution. Through the incorporation of polymers or conductive fillers into the hydrogel and special preparation methods, hydrogels can achieve a unification of excellent properties such as mechanical properties, self-healing, adhesion, and biocompatibility, making them a hot material for health monitoring bioelectronics. Currently, hydrogel-based bioelectronics can be used to fabricate flexible bioelectronics for motion, bioelectric, and biomolecular acquisition for human health monitoring and further clinical applications. This review focuses on materials, devices, and applications for hydrogel-based bioelectronics. The main material properties and research advances of hydrogels for health monitoring bioelectronics are summarized firstly. Then, we provide a focused discussion on hydrogel-based bioelectronics for health monitoring, which are classified as skin-attachable, implantable, or semi-implantable depending on the depth of penetration and the location of the device. Finally, future challenges and opportunities of hydrogel-based bioelectronics for health monitoring are envisioned.

Nature-Driven Biocompatible Epidermal Electronic Skin for Real-Time Wireless Monitoring of Human Physiological Signals

Wearable bioelectronic patches are creating a transformative effect in the health care industry for human physiological signal monitoring. However, the use of such patches is restricted due to the unavailability of a proper power source. Ideal biodevices should be thin, soft, robust, energy-efficient, and biocompatible. Here, we report development of a flexible, lightweight, and biocompatible electronic skin-cum-portable power source for wearable bioelectronics by using a processed chicken feather fiber. The device is fabricated with a novel, breathable composite of biowaste chicken feather and organic poly(vinylidene fluoride) (PVDF) polymer, where the chicken feather fiber constitutes the "microbones" of the PVDF, enhancing its piezoelectric phase content, biocompatibility, and crystallinity. Thanks to its outstanding pressure sensitivity, the fabricated electronic skin is used for the monitoring of different human physiological signals such as body motion, finger and joint bending, throat activities, and pulse rate with excellent sensitivity. A wireless system is developed to remotely receive the different physiological signals as captured by the electronic skin. We also explore the capabilities of the device as a power source for other small electronics. The piezoelectric energy harvesting device can harvest a maximum output voltage of ∼28 V and an area power density of 1.4 μW·cm^-2 from the human finger imparting. The improved energy harvesting property of the device is related to the induced higher fraction of the electroactive phase in the composite. The easy process ability, natural biocompatibility, superior piezoelectric performance, high pressure sensitivity, and alignment toward wireless transmission of the captured data make the device a promising candidate for wearable bioelectronic patches and power sources.

Wearable Two-Dimensional Nanomaterial-Based Flexible Sensors for Blood Pressure Monitoring: A Review

Flexible sensors have been extensively employed in wearable technologies for physiological monitoring given the technological advancement in recent years. Conventional sensors made of silicon or glass substrates may be limited by their rigid structures, bulkiness, and incapability for continuous monitoring of vital signs, such as blood pressure (BP). Two-dimensional (2D) nanomaterials have received considerable attention in the fabrication of flexible sensors due to their large surface-area-to-volume ratio, high electrical conductivity, cost effectiveness, flexibility, and light weight. This review discusses the transduction mechanisms, namely, piezoelectric, capacitive, piezoresistive, and triboelectric, of flexible sensors. Several 2D nanomaterials used as sensing elements for flexible BP sensors are reviewed in terms of their mechanisms, materials, and sensing performance. Previous works on wearable BP sensors are presented, including epidermal patches, electronic tattoos, and commercialized BP patches. Finally, the challenges and future outlook of this emerging technology are addressed for non-invasive and continuous BP monitoring.

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Recent progress in electrospun nanomaterials for wearables

Wearables have garnered significant attention in recent years not only as consumer electronics for entertainment, communications, and commerce but also for real-time continuous health monitoring. This has been spurred by advances in flexible sensors, transistors, energy storage, and harvesting devices to replace the traditional, bulky, and rigid electronic devices. However, engineering smart wearables that can seamlessly integrate with the human body is a daunting task. Some of the key material attributes that are challenging to meet are skin conformability, breathability, and biocompatibility while providing tunability of its mechanical, electrical, and chemical properties. Electrospinning has emerged as a versatile platform that can potentially address these challenges by fabricating nanofibers with tunable properties from a polymer base. In this article, we review advances in wearable electronic devices and systems that are developed using electrospinning. We cover various applications in multiple fields including healthcare, biomedicine, and energy. We review the ability to tune the electrical, physiochemical, and mechanical properties of the nanofibers underlying these applications and illustrate strategies that enable integration of these nanofibers with human skin.

Photo-degradable, tough and highly stretchable hydrogels

We present for the first time highly stretchable and tough hydrogels with controlled light-triggered photodegradation. A double-network of alginate/polyacrylamide (PAAm) is formed by using covalently and ionically crosslinked subnetworks. The ionic Ca²⁺ alginate interpenetrates a PAAm network covalently crosslinked by a bifunctional acrylic crosslinker containing the photodegradable o-nitrobenzyl (ONB) core instead of the commonly used methylene bisacrylamide (MBAA). Remarkably, due to the developed protocol, the change of the crosslinker did not affect the hydrogel's mechanical properties. The incorporation of photosensitive components in hydrogels allows external temporal control of their properties and tuneable degradation. Cell viability and cell proliferation assays revealed that hydrogels and their photodegradation products are not cytotoxic to the NIH3T3 cell line. In one example of application, we used these hydrogels for bio-potential acquisition in wearable electrocardiography. Surprisingly, these hydrogels showed a lower skin-electrode impedance, compared to the common medical grade Ag/AgCl electrodes. This work lays the foundation for the next generation of tough and highly stretchable hydrogels that are environmentally friendly and can find applications in a variety of fields such as health, electronics, and energy, as they combine excellent mechanical properties with controlled degradation.

Reversible polymer-gel transition for ultra-stretchable chip-integrated circuits through self-soldering and self-coating and self-healing

Integration of solid-state microchips into soft-matter, and stretchable printed electronics has been the biggest challenge against their scalable fabrication. We introduce, Pol-Gel, a simple technique for self-soldering, self-encapsulation, and self-healing, that allows low cost, scalable, and rapid fabrication of hybrid microchip-integrated ultra-stretchable circuits. After digitally printing the circuit, and placing the microchips, we trigger a Polymer-Gel transition in physically cross-linked block copolymers substrate, and silver liquid metal composite ink, by exposing the circuits to the solvent vapor. Once in the gel state, microchips penetrate to the ink and the substrate (Self-Soldering), and the ink penetrates to the substrate (Self-encapsulation). Maximum strain tolerance of ~1200% for printed stretchable traces, and >500% for chip-integrated soft circuits is achieved, which is 5x higher than the previous works. We demonstrate condensed soft-matter patches and e-textiles with integrated sensors, processors, and wireless communication, and repairing of a fully cut circuits through Pol-Gel.

Despite advances on fabrication of stretchable interconnects, realizing functional electronics with integrated solid-state technology (SST) remains a challenge. Here, the authors report a reversible Pol-Gel transition method for fabrication of liquid-metal based, chip-integrated, printed stretchable circuits.

Well-rounded devices: the fabrication of electronics on curved surfaces - a review

With the arrival of the internet of things and the rise of wearable computing, electronics are playing an increasingly important role in our everyday lives. Until recently, however, the rigid angular nature of traditional electronics has prevented them from being integrated into many of the organic, curved shapes that interface with our bodies (such as ergonomic equipment or medical devices) or the natural world (such as aerodynamic or optical components). In the past few years, many groups working in advanced manufacturing and soft robotics have endeavored to develop strategies for fabricating electronics on these curved surfaces. This is their story. In this work, we describe the motivations, challenges, methodologies, and applications of curved electronics, and provide a outlook for this promising field.

Energy Harvesting and Storage with Soft and Stretchable Materials

This review highlights various modes of converting ambient sources of energy into electricity using soft and stretchable materials. These mechanical properties are useful for emerging classes of stretchable electronics, e-skins, bio-integrated wearables, and soft robotics. The ability to harness energy from the environment allows these types of devices to be tetherless, thereby leading to a greater range of motion (in the case of robotics), better compliance (in the case of wearables and e-skins), and increased application space (in the case of electronics). A variety of energy sources are available including mechanical (vibrations, human motion, wind/fluid motion), electromagnetic (radio frequency (RF), solar), and thermodynamic (chemical or thermal energy). This review briefly summarizes harvesting mechanisms and focuses on the materials' strategies to render such devices into soft or stretchable embodiments.

Bi-Phasic Ag-In-Ga-Embedded Elastomer Inks for Digitally Printed, Ultra-Stretchable, Multi-layer Electronics

A bi-phasic ternary Ag-In-Ga ink that demonstrates high electrical conductivity, extreme stretchability, and low electromechanical gauge factor (GF) is introduced. Unlike popular liquid metal alloys such as eutectic gallium-indium (EGaIn), this ink is easily printable and nonsmearing and bonds strongly to a variety of substrates. Using this ink and a simple extrusion printer, the ability to perform direct writing of ultrathin, multi-layer circuits that are highly stretchable (max. strain >600%), have excellent conductivity (7.02 × 10⁵ S m^-1), and exhibit only a modest GF (0.9) related to the ratio of percent increase in trace resistance with mechanical strain is demonstrated. The ink is synthesized by mixing optimized quantities of EGaIn, Ag microflakes, and styrene-isoprene block copolymers, which functions as a hyperelastic binder. When compared to the same composite without EGaIn, the Ag-In-Ga ink shows over 1 order of magnitude larger conductivity, up to ∼27× lower GF, and ∼5× greater maximum stretchability. No significant change over the resistance of the ink was observed after 1000 strain cycles. Microscopic analysis shows that mixing EGaIn and Ag microflakes promotes the formation of AgIn₂ microparticles, resulting in a cohesive bi-phasic ink. The ink can be sintered at room temperature, making it compatible with many heat-sensitive substrates. Additionally, utilizing a simple commercial extrusion based printer, the ability to perform stencil-free, digital printing of multi-layer stretchable circuits over various substrates, including medical wound-dressing adhesives, is demonstrated for the first time.

Domiciliary Hospitalization through Wearable Biomonitoring Patches: Recent Advances, Technical Challenges, and the Relation to Covid-19

This article reviews recent advances and existing challenges for the application of wearable bioelectronics for patient monitoring and domiciliary hospitalization. More specifically, we focus on technical challenges and solutions for the implementation of wearable and conformal bioelectronics for long-term patient biomonitoring and discuss their application on the Internet of medical things (IoMT). We first discuss the general architecture of IoMT systems for domiciliary hospitalization and the three layers of the system, including the sensing, communication, and application layers. In regard to the sensing layer, we focus on current trends, recent advances, and challenges in the implementation of stretchable patches. This includes fabrication strategies and solutions for energy storage and energy harvesting, such as printed batteries and supercapacitors. As a case study, we discuss the application of IoMT for domiciliary hospitalization of COVID 19 patients. This can be used as a strategy to reduce the pressure on the healthcare system, as it allows continuous patient monitoring and reduced physical presence in the hospital, and at the same time enables the collection of large data for posterior analysis. Finally, based on the previous works in the field, we recommend a conceptual IoMT design for wearable monitoring of COVID 19 patients.