Solvent-Free and Skin-Like Supramolecular Ion-Conductive Elastomers with Versatile Processability for Multifunctional Ionic Tattoos and On-Skin Bioelectronics

Viscoelastic Adhesive, Super-Conformable, and Semi-Flowable Liquid Metal Eutectogels for High-Fidelity Electrophysiological Monitoring

Integrating gels with human skin through wearables provides unprecedented opportunities for health monitoring technology and artificial intelligence. However, most conductive hydrogels, organogels, and ionogels lack essential environmental stability, biocompatibility, and adhesion for reliable epidermal sensing. In this study, we have developed a liquid metal eutectogel simultaneously possessing superior viscoelasticity, semiflowability, and mechanical rigidity for low interfacial skin impedance, high skin adhesion, and durability. Liquid metal particles (LMPs) are employed to generate free radicals and gallium ions to accelerate the polymerization of acrylic acid monomers in a deep eutectic solvent (DES), obtaining highly viscoelastic polymer networks via physical cross-linking. In particular, graphene oxide (GO) is utilized to encapsulate the LMPs through a sonication-assisted electrostatic assembly to stabilize the LMPs in DES, which also enhances the mechanical toughness and regulates the rheological properties of the eutectogels. Our optimized semi-flowable eutectogel exhibits viscous fluid behavior at low shear rates, facilitating a highly conformable interface with hairy skin. Simultaneously, it demonstrates viscoelastic behavior at high shear rates, allowing for easy peel-off. These distinctive attributes enable the successful applications of on-skin adhesive strain sensing and high-fidelity human electrophysiological (EP) monitoring, showcasing the versatility of these ionically conductive liquid metal eutectogels in advanced personal health monitoring.

Engineering robust and transparent dual-crosslinked hydrogels for multimodal sensing without conductive additives

Conductive hydrogels are pivotal for the advancement of flexible sensors, electronic skin, and healthcare monitoring systems, facilitating transformative innovations. However, issues such as inadequate intrinsic compatibility, mismatched mechanical properties, and limited stability curtail their potential, resulting in compromised device efficacy and performance degradation. In this research, we engineered functional hydrogels featuring a dual-crosslinked network composed of (PA/PVA)-P(AM-AA) to address these challenges. This design eliminates the need for conductive additives, thereby enhancing intrinsic compatibility. Notably, the hydrogels exhibit exceptional mechanical properties, with high tensile strength (∼700 %), Young's modulus (∼5.33 MPa), increased strength (∼2.46 MPa) and toughness (∼6.59 MJ m-3). They also achieve a compressive strength of ∼7.33 MPa at 80 % maximal compressive strain and maintain about 89 % transparency. Moreover, flexible sensors derived from these hydrogels demonstrate enhanced multimodal sensing capabilities, including temperature, strain, and pressure detection, enabling precise monitoring of human movements. The integration of multiple hydrogels into a three-dimensional sensor array facilitates detailed spatial pressure distribution mapping. By strategically applying dual-crosslinked network engineering and eliminating conductive additives, we have streamlined the design and manufacturing of hydrogels to meet the rising demand for high-performance wearable sensors.

Lanternarene-Based Self-Sorting Double-Network Hydrogels for Flexible Strain Sensors

Conductive flexible hydrogels have attracted immense attentions recently due to their wide applications in wearable sensors. However, the poor mechanical properties of most conductive polymer limit their utilizations. Herein, a double network hydrogel is fabricated via a self-sorting process with cationic polyacrylamide as the first flexible network and the lantern[33]arene-based hydrogen organic framework nanofibers as the second rigid network. This hydrogel is endowed with good conductivity (0.25 S m-1) and mechanical properties, such as large Young's modulus (31.9 MPa), fracture elongation (487%) and toughness (6.97 MJ m-3). The stretchability of this hydrogel is greatly improved after the kirigami cutting, which makes it can be used as flexible strain sensor for monitoring human motions, such as bending of fingers, wrist and elbows. This study not only provides a valuable strategy for the construction of double network hydrogels by lanternarene, but also expands the application of the macrocycle hydrogels to flexible electronics.

Liquid-free ionic conductive elastomers with high mechanical properties and ionic conductivity for multifunctional sensors and triboelectric nanogenerators

Liquid-free ionic conductive elastomers (ICEs) are ideal materials for constructing flexible electronic devices by avoiding the limitations of liquid components. However, developing all-solid-state ionic conductors with high mechanical strength, high ionic conductivity, excellent healing, and recyclability remains a great challenge. Herein, a series of liquid-free polyurethane-based ICEs with a double dynamic crosslinked structure are reported. As a result of interactions between multiple dynamic bonds (multi-level hydrogen bonds, disulfide bonds, and dynamic D-A bonds) and lithium-oxygen bonds, the optimal ICE exhibited a high mechanical strength (1.18 MPa), excellent ionic conductivity (0.14 mS cm-1), desirable healing capacity (healing efficiency >95%), and recyclability. A multi-functional wearable sensor based on the novel ICE enabled real-time and rapid detection of various human activities and enabled recognizing writing signals and encrypted information transmission. A triboelectric nanogenerator based on the novel ICE exhibited an excellent open-circuit voltage of 464 V, a short-circuit current of 16 μA, a transferred charge of 50 nC, and a power density of 720 mW m-2, enabling powering of small-scale electronic products. This study provides a feasible strategy for designing flexible sensor products and healing, self-powered devices, with promising prospects for application in soft ionic electronics.

Facile Multiple Graded Wrinkle Construction Strategy for Vastly Boosting the Sensing Performance of Ionic Skins

The construction of surface microstructures (e.g., micropyramids and wrinkles) has been proven as the most effective means to boost the sensitivity of ionic skins (I-skins). However, the single-scale micronano patterns constructed by the common fabrication strategy generally lead to a limited pressure-response range. Here, a convenient repeated stretching/coordinating/releasing strategy is developed to controllably construct multiple graded wrinkles on the polyelectrolyte hydrogel-based I-skins for increasing their sensitivity over a broad pressure range. We find that the small wrinkles allow for high sensitivity yet small pressure detection range, while the large wrinkles can reduce structural stiffening to generate large pressure-response range but incur limited sensitivity. The multiple graded wrinkles can combine the merits of both the small and large wrinkles to simultaneously improve the sensitivity and broaden the pressure-response range. In particular, the sensing performance of multiple-wrinkle-based I-skins substantially outperforms the superposition of the sensing performance of different single-wrinkle-based I-skins. As a proof of concept, the triple-wrinkle-based I-skins can provide an extremely high sensitivity of 17,309 kPa-1 and an ultrawide pressure detection range of 0.38 Pa to 372 kPa. The approach and insight contribute to the future development of I-skins with a broader pressure-response range and higher sensitivity.

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.

Solid-state, liquid-free ion-conducting elastomers: rising-star platforms for flexible intelligent devices

Soft ionic conductors have emerged as a powerful toolkit to engineer transparent flexible intelligent devices that go beyond their conventional counterparts. Particularly, due to their superior capacities of eliminating the evaporation, freezing and leakage issues of the liquid phase encountered with hydrogels, organohydrogels and ionogels, the emerging solid-state, liquid-free ion-conducting elastomers have been largely recognized as ideal candidates for intelligent flexible devices. However, despite their extensive development, a comprehensive and timely review in this emerging field is lacking, particularly from the perspective of design principles, advanced manufacturing, and distinctive applications. Herein, we present (1) the design principles and intriguing merits of solid-state, liquid-free ion-conducting elastomers; (2) the methods to manufacture solid-state, liquid-free ion-conducting elastomers with preferential architectures and functions using advanced technologies such as 3D printing; (3) how to leverage solid-state, liquid-free ion-conducting elastomers in exploiting advanced applications, especially in the fields of flexible wearable sensors, bioelectronics and energy harvesting; (4) what are the unsolved scientific and technical challenges and future opportunities in this multidisciplinary field. We envision that this review will provide a paradigm shift to trigger insightful thinking and innovation in the development of intelligent flexible devices and beyond.

Polyphenol-sodium alginate supramolecular injectable hydrogel with antibacterial and anti-inflammatory capabilities for infected wound healing

Injectable hydrogel has attracted appealing attention for skin wound treatment. Although multifunctional injectable hydrogels can be prepared by introducing bioactive ingredients with antibacterial and anti-inflammatory capabilities, their preparation remains complicated. Herein, a polyphenol-based supramolecular injectable hydrogel (PBSIH) based on polyphenol gallic acid and biological macromolecule sodium alginate is developed as a wound dressing to accelerate wound healing. We show that such PBSIH can be rapidly formed within 15 s by mixing the sodium alginate and gallic acid solutions based on the hydrogen bonding and hydrophobic interactions. The PBSIH shows excellent cytocompatibility, antibacterial, and antioxidant properties, which enhance infected wound healing by inhibiting bacterial infection and alleviating inflammation after treatment of 11 days. Moreover, we show that the preparative strategies of injectable supramolecular hydrogels can be extended to other polyphenols, including protocatechuic and tannic acids. This study provides a facile yet highly effective method to design injectable polyphenol- sodium alginate hydrogel for wound dressing based on naturally bioactive ingredients.

High-Toughness and High-Strength Solvent-Free Linear Poly(ionic liquid) Elastomers

Solvent-free elastomers, unlike gels, do not suffer from solvent evaporation and leakage in practical applications. However, it is challenging to realize the preparation of high-toughness (with both high stress and strain) ionic elastomers. Herein, high-toughness linear poly(ionic liquid) (PIL) elastomers are constructed via supramolecular ionic networks formed by the polymerization of halometallate ionic liquid (IL) monomers, without any chemical crosslinking. The obtained linear PIL elastomers exhibit high strength (16.5 MPa), Young's modulus (157.49 MPa), toughness (130.31 MJ m-3 ), and high crack propagation insensitivity (fracture energy 243.37 kJ m-2 ), owing to the enhanced intermolecular noncovalent interactions of PIL chains. Furthermore, PIL elastomer-based strain, pressure, and touch sensors have shown high sensitivity. The linear noncovalent crosslinked network endows the PIL elastomers with self-healing and recyclable properties, and broad application prospects in the fields of flexible sensor devices, health monitoring, and human-machine interaction.

Self-compliant ionic skin by leveraging hierarchical hydrogen bond association

Robust interfacial compliance is essential for long-term physiological monitoring via skin-mountable ionic materials. Unfortunately, existing epidermal ionic skins are not compliant and durable enough to accommodate the time-varying deformations of convoluted skin surface, due to an imbalance in viscosity and elasticity. Here we introduce a self-compliant ionic skin that consistently works at the critical gel point state with almost equal viscosity and elasticity over a super-wide frequency range. The material is designed by leveraging hierarchical hydrogen bond association, allowing for the continuous release of polymer strands to create topological entanglements as complementary crosslinks. By embodying properties of rapid stress relaxation, softness, ionic conductivity, self-healability, flaw-insensitivity, self-adhesion, and water-resistance, this ionic skin fosters excellent interfacial compliance with cyclically deforming substrates, and facilitates the acquisition of high-fidelity electrophysiological signals with alleviated motion artifacts. The presented strategy is generalizable and could expand the applicability of epidermal ionic skins to more complex service conditions.

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.

Recent advances in wearable iontronic sensors for healthcare applications

Iontronic sensors have garnered significant attention as wearable sensors due to their exceptional mechanical performance and the ability to maintain electrical performance under various mechanical stimuli. Iontronic sensors can respond to stimuli like mechanical stimuli, humidity, and temperature, which has led to exploration of their potential as versatile sensors. Here, a comprehensive review of the recent researches and developments on several types of iontronic sensors (e.g., pressure, strain, humidity, temperature, and multi-modal sensors), in terms of their sensing principles, constituent materials, and their healthcare-related applications is provided. The strategies for improving the sensing performance and environmental stability of iontronic sensors through various innovative ionic materials and structural designs are reviewed. This review also provides the healthcare applications of iontronic sensors that have gained increased feasibility and broader applicability due to the improved sensing performance. Lastly, outlook section discusses the current challenges and the future direction in terms of the applicability of the iontronic sensors to the healthcare.