In Situ Formation of Conductive Epidermal Electrodes Using a Fully Integrated Flexible System and Injectable Photocurable Ink

Light-Mediated Polymerization Catalyzed by Carbon Nanomaterials

Carbon nanomaterials, specifically carbon dots and carbon nitrides, play a crucial role as heterogeneous photoinitiators in both radical and cationic polymerization processes. These recently introduced materials offer promising solutions to the limitations of current homogeneous systems, presenting a novel approach to photopolymerization. This review highlights the preparation and photocatalytic performance of these nanomaterials, emphasizing their application in various polymerization techniques, including photoinduced i) free radical, ii) RAFT, iii) ATRP, and iv) cationic photopolymerization. Additionally, it discusses their potential in addressing contemporary challenges and explores prospects in this field. Moreover, carbon nitrides, in particular, exhibit exceptional oxygen tolerance, underscoring their significance in radical polymerization processes and allowing their applications such as 3D printing, surface modification of coatings, and hydrogel engineering.

Wireless Technologies in Flexible and Wearable Sensing: From Materials Design, System Integration to Applications

Wireless and wearable sensors attract considerable interest in personalized healthcare by providing a unique approach for remote, noncontact, and continuous monitoring of various health-related signals without interference with daily life. Recent advances in wireless technologies and wearable sensors have promoted practical applications due to their significantly improved characteristics, such as reduction in size and thickness, enhancement in flexibility and stretchability, and improved conformability to the human body. Currently, most researches focus on active materials and structural designs for wearable sensors, with just a few exceptions reflecting on the technologies for wireless data transmission. This review provides a comprehensive overview of the state-of-the-art wireless technologies and related studies on empowering wearable sensors. The emerging functional nanomaterials utilized for designing unique wireless modules are highlighted, which include metals, carbons, and MXenes. Additionally, the review outlines the system-level integration of wireless modules with flexible sensors, spanning from novel design strategies for enhanced conformability to efficient transmitting data wirelessly. Furthermore, the review introduces representative applications for remote and noninvasive monitoring of physiological signals through on-skin and implantable wireless flexible sensing systems. Finally, the challenges, perspectives, and unprecedented opportunities for wireless and wearable sensors are discussed.

Miniaturized Implantable Fluorescence Probes Integrated with Metal-Organic Frameworks for Deep Brain Dopamine Sensing

Continuously monitoring neurotransmitter dynamics can offer profound insights into neural mechanisms and the etiology of neurological diseases. Here, we present a miniaturized implantable fluorescence probe integrated with metal-organic frameworks (MOFs) for deep brain dopamine sensing. The probe is assembled from physically thinned light-emitting diodes (LEDs) and phototransistors, along with functional surface coatings, resulting in a total thickness of 120 μm. A fluorescent MOF that specifically binds dopamine is introduced, enabling a highly sensitive dopamine measurement with a detection limit of 79.9 nM. A compact wireless circuit weighing only 0.85 g is also developed and interfaced with the probe, which was later applied to continuously monitor real-time dopamine levels during deep brain stimulation in rats, providing critical information on neurotransmitter dynamics. Cytotoxicity tests and immunofluorescence analysis further suggest a favorable biocompatibility of the probe for implantable applications. This work presents fundamental principles and techniques for integrating fluorescent MOFs and flexible electronics for brain-computer interfaces and may provide more customized platforms for applications in neuroscience, disease tracing, and smart diagnostics.

E-Tattoos: Toward Functional but Imperceptible Interfacing with Human Skin

The human body continuously emits physiological and psychological information from head to toe. Wearable electronics capable of noninvasively and accurately digitizing this information without compromising user comfort or mobility have the potential to revolutionize telemedicine, mobile health, and both human-machine or human-metaverse interactions. However, state-of-the-art wearable electronics face limitations regarding wearability and functionality due to the mechanical incompatibility between conventional rigid, planar electronics and soft, curvy human skin surfaces. E-Tattoos, a unique type of wearable electronics, are defined by their ultrathin and skin-soft characteristics, which enable noninvasive and comfortable lamination on human skin surfaces without causing obstruction or even mechanical perception. This review article offers an exhaustive exploration of e-tattoos, accounting for their materials, structures, manufacturing processes, properties, functionalities, applications, and remaining challenges. We begin by summarizing the properties of human skin and their effects on signal transmission across the e-tattoo-skin interface. Following this is a discussion of the materials, structural designs, manufacturing, and skin attachment processes of e-tattoos. We classify e-tattoo functionalities into electrical, mechanical, optical, thermal, and chemical sensing, as well as wound healing and other treatments. After discussing energy harvesting and storage capabilities, we outline strategies for the system integration of wireless e-tattoos. In the end, we offer personal perspectives on the remaining challenges and future opportunities in the field.

Solvent-Triggered, Ultra-Adhesive, Conductive, and Biocompatible Transition Gels for Wearable Devices

The development of robust adhesive, conductive, and flexible materials has garnered significant attention in the realm of human-machine interface and electronic devices. Conventional preparation methods to achieve these exceptional properties rely on incorporating highly polar raw materials, multiple components, or solvents. However, the overexposure of functional groups and the inherent toxicity of organic solvents often render gels non-stick or potentially biocompatible making them unsuitable for human-contact devices. In this study, a straightforward three-step strategy is devised for preparing responsive adhesive gels without complex components. Structurally conductive poly(N-(2-hydroxyethyl)-acrylamide-co-p-styrene sulfonate hydrate) (PHEAA-NaSS) gels are synthesized by integrating ionic and hydrophilic networks with distinct solvent effects. Initially, the in-suit formed PHEAA-NaSS networks are activated by dimethyl sulfoxide, which substantially increases intramolecular hydrogen bonding and enhances the matrix stretchability and interfacial adhesion. Subsequently, ethanol exchange reduced solvent impact and led to a compact network that limited surface exposure of ionic and hydrophilic groups, resulting in nonstick, robust for convenient storage. Finally, upon contacting with water, the network demonstrates rehydration, resulting in favorable adhesion, biocompatibility, and conductivity. The proposed PHEAA-NaSS/W gels can stably and reliably capture joint motion and electrophysiological signals. Furthermore, this uncomplicated gel preparation method is also applicable to other electrolyte monomers.

A 10-micrometer-thick nanomesh-reinforced gas-permeable hydrogel skin sensor for long-term electrophysiological monitoring

Hydrogel-enabled skin bioelectronics that can continuously monitor health for extended periods is crucial for early disease detection and treatment. However, it is challenging to engineer ultrathin gas-permeable hydrogel sensors that can self-adhere to the human skin for long-term daily use (>1 week). Here, we present a ~10-micrometer-thick polyurethane nanomesh-reinforced gas-permeable hydrogel sensor that can self-adhere to the human skin for continuous and high-quality electrophysiological monitoring for 8 days under daily life conditions. This research involves two key steps: (i) material design by gelatin-based thermal-dependent phase change hydrogels and (ii) robust thinness geometry achieved through nanomesh reinforcement. The resulting ultrathin hydrogels exhibit a thickness of ~10 micrometers with superior mechanical robustness, high skin adhesion, gas permeability, and anti-drying performance. To highlight the potential applications in early disease detection and treatment that leverage the collective features, we demonstrate the use of ultrathin gas-permeable hydrogels for long-term, continuous high-precision electrophysiological monitoring under daily life conditions up to 8 days.

Lignosulfonate sodium assisted PEDOT-based all-gel supercapacitors with enhanced supercapacitance and wide temperature tolerance

Conducting polymer hydrogels are typically employed in all-gel supercapacitors; however, Poly[3,4-ethylene-dioxythiophene] (PEDOT)-based hydrogel supercapacitors still suffer from low capacitance because of the low packing density of PEDOT in the electrodes. Here, we demonstrate lignosulfonate sodium (LS) as an excellent template to synthesize various LS-PEDOT conductive nanofillers for high mass-loading LS-PEDOT/PAAM hydrogel electrodes. Then, the optimum LS-PEDOT/PAAM electrode was assembled with a redox-active LS/PAAM/Fe3+ hydrogel electrolyte to form sandwich-structured all-gel supercapacitors, which could deliver a high specific capacitance of 672.5 mF/cm2 and an energy efficiency of 60 μWh/cm2, which are three times higher than the 220 mF/cm2 and 19.5 μWh/cm2 of the device without Fe3+ at the same condition. Such a device shows excellent temperature tolerance from -30 to 100 °C. Besides, the LS-PEDOT/PAAM electrode has excellent photothermal conversion effects under simulated solar illumination. The sluggish electrochemical performance of the SC under low temperatures could be significantly boosted by ~50 % under simulated solar light. All of these findings demonstrate that the capacitance performance of the PEDOT-based hydrogel device is successfully improved not only at room temperature but also under subzero conditions.

Skin-Integrated Electrodes Based on Room-Temperature Curable, Highly Conductive Silver/Polydimethylsiloxane Composites

The quality of electrophysiological (EP) signals heavily relies on the electrode's contact with the skin. However, motion or exposure to water can easily destabilize this connection. In contrast to traditional methods of attaching electrodes to the skin surface, this study introduces a skin-integration strategy inspired by the skin's intergrown structure. A highly conductive and room-temperature curable composite composed of silver microflakes and polydimethylsiloxane (Ag/PDMS) is applied to the skin. Before curing, the PDMS oil partially diffuse into the stratum corneum (SC) layer of the skin. Upon curing, the composite solidifies into an electrode that seamlessly integrated with the skin, resembling a natural extension. This skin-integration strategy offers several advantages. It minimizes motion artifacts resulting from relative electrode-skin displacement, significantly reduces interface impedance (67% of commercial Ag/AgCl gel electrodes at 100 Hz) and withstands water flushes due to its hydrophobic nature. These advantages pave the way for promising advancements in EP signal recording, particularly during motion and underwater conditions.