Myelin Sheath-Inspired Hydrogel Electrode for Artificial Skin and Physiological Monitoring.

Significant advancements in hydrogel-based epidermal electrodes have been made in recent years. However, inherent limitations, such as adaptability, adhesion, and conductivity, have presented challenges, thereby limiting the sensitivity, signal-to-noise ratio (SNR), and stability of the physiological-electrode interface. In this study, we propose the concept of myelin sheath-inspired hydrogel epidermal electronics by incorporating numerous interpenetrating core-sheath-structured conductive nanofibers within a physically cross-linked polyelectrolyte network. Poly(3,4-ethylenedioxythiophene)-coated sulfonated cellulose nanofibers (PEDOT:SCNFs) are synthesized through a simple solvent-catalyzed sulfonation process, followed by oxidative self-polymerization and ionic liquid (IL) shielding steps, achieving a low electrochemical impedance of 42 Ω. The physical associations within the composite hydrogel network include complexation, electrostatic forces, hydrogen bonding, π-π stacking, hydrophobic interaction, and weak entanglements. These properties confer the hydrogel with high stretchability (770%), superconformability, self-adhesion (28 kPa on pigskin), and self-healing capabilities. By simulating the saltatory propagation effect of the nodes of Ranvier in the nervous system, the biomimetic hydrogel establishes high-fidelity epidermal electronic interfaces, offering benefits such as low interfacial contact impedance, significantly increased SNR (30 dB), as well as large-scale sensor array integration. The advanced biomimetic hydrogel holds tremendous potential for applications in electronic skin (e-skin), human-machine interfaces (HMIs), and healthcare assessment devices.

Monodisperse polyhydroxyalkanoate nanoparticles as self-sticky and bio-resorbable tissue adhesives.

Monodisperse nanoparticles of biodegradable polyhydroxyalkanoates (PHAs) polymers, copolymers of 3-hydroxybutyrate (3HB) and 4-hydroxybutyrate (4HB), are synthesized using a membrane-assisted emulsion encapsulation and evaporation process for biomedical resorbable adhesives. The precise control over the diameter of these PHA particles, ranging from 100 nm to 8 µm, is achieved by adjusting the diameter of emulsion or the PHA concentration. Mechanical properties of the particles can be tailored based on the 3HB to 4HB ratio and molecular weight, primarily influenced by the level of crystallinity. These monodisperse PHA particles in solution serve as adhesives for hydrogel systems, specifically those based on poly(N, N-dimethylacrylamide) (PDMA). Semi-crystalline PHA nanoparticles exhibit stronger adhesion energy than their amorphous counterparts. Due to their self-adhesiveness, adhesion energy increases even when those PHA nanoparticles form multilayers between hydrogels. Furthermore, as they degrade and are resorbed into the body, the PHA nanoparticles demonstrate efficacy in in vivo wound closure, underscoring their considerable impact on biomedical applications.

Janus Membrane-Based Wearable pH Sensor with Sweat Absorption, Gas Permeability, and Self-Adhesiveness.

Wearable biomedical sensors have enabled noninvasive and continuous physiological monitoring for daily health management and early detection of chronic diseases. Among biomedical sensors, wearable pH sensors attracted significant interest, as pH influences most biological reactions. However, conformable pH sensors that have sweat absorption ability, are self-adhesive to the skin, and are gas permeable remain largely unexplored. In this study, we present a pioneering approach to this problem by developing a Janus membrane-based pH sensor with self-adhesiveness on the skin. The sensor is composed of a hydrophobic polyurethane-polydimethylsiloxane porous hundreds nanometer-thick substrate and a hydrophilic poly(vinyl alcohol)-poly(acrylic acid) porous nanofiber layer. This Janus membrane exhibits a thickness of around 10 µm, providing a conformable adhesion to the skin. The simultaneous realization of solution absorption, gas permeability, and self-adhesiveness makes it suitable for long-term continuous monitoring without compromising the comfort of the wearer. The pH sensor was tested successfully for continuous monitoring for 7.5 h, demonstrating its potential for stable analysis of skin health conditions. The Janus membrane-based pH sensor holds significant promise for comprehensive skin health monitoring and wearable biomedical applications.

Self-adhesive wearable poly (vinyl alcohol)-based hybrid biofuel cell powered by human bio-fluids.

Advancement of wearable microelectronics demands their power source with continuous energy supply, skin-integration and miniaturization. In light of poly (vinyl alcohol) (PVA) hydrogel with nontoxicity, good biocompatibility and low cost, an advanced wearable PVA-based hybrid biofuel cells (HBFCs) with high self-adhesiveness was developed. Through the reaction between PVA molecules and succinic anhydride (SAA), the carboxylated PVA (PVA/SAA) was obtained, and by incorporation with PDA as crosslinker, the self-adhesive PVA/SAA-DA hydrogel electrolytes formed by dual covalent and hydrogen bonding. With increasing SAA and PDA content, the pore size decreased, and a uniform and dense network formed, endowing the hydrogel with a relatively high absorption capacity of PBS solution of lactate as cell fuel. Meanwhile the various functional groups of hydrogel, including catechol, quinone, amino and hydroxyl groups, contributed to impressive tissue adhesion strength against pigskin under dry and wet conditions. The PVA/SAA-DA hydrogel displayed high conductive property, and the integrated PVA-based HBFC generated open circuit voltage of 0.50 V and maximum power density of 128.76 µW/cm2 in 20 mM lactate solution, which was optimized to be 0.57 V/224.85 µW/cm2 when the pore size was enlarged. The power retention reached above 70% in one week, showing long-term stability of HBFC. The PVA-based HBFC was further adhered to human skin without extra adhesive tapes to scavenge human sweat as biofuel, and the maximum power density reached 85.34 µW/cm2, while by connected with a DC-DC converter, the HBFC could power watch, exhibiting promising application potentials as wearable electronic device to provide bioelectricity.

Super Water-Storage Self-Adhesive Gel for Solar Vapor Generation and Collection.

Water treatment consumes lots of energy from fossil fuels nowadays, and the emission of CO2 enhances the temperature on earth, resulting in more and more hazards. Thus, clean water production enabled by green energy without CO2 emission is attracting more and more attention. Herein, we propose a novel solar evaporation system achieving both solar evaporation and water storage with two different unique hydrogels based on a three-dimensional (3D) printing technique. The hydrogel absorber demonstrates an ultrahigh absorptance (98.2%) of solar light, while the water-storage hydrogel absorbs more than 100 times its own weight of water, demonstrating super water-storage performance with strong self-adhesiveness. The solar vapor generation rate can be as high as 3.14 kg·m-2·h-1, with a solar evaporation efficiency up to 91.2% irradiated by 1.43 sun. Furthermore, our environmentally friendly solar evaporation system achieves ultrahigh water purification efficiency of 99.99% for salt, heavy ions, and acid/alkaline with remarkable stability and durability. Our solar evaporation system promises long-lasting applications for the hydrological cycle enabled by solar energy, such as seawater desalination, sterilization, wastewater purification, and so on.

Highly Adhesive, Stretchable, and Antifreezing Hydrogel with Excellent Mechanical Properties for Sensitive Motion Sensors and Temperature-/Humidity-Driven Actuators.

Conductive hydrogels as flexible wearable devices have attracted considerable attention due to their mechanical flexibility and intelligent sensing. How to endow more and better performance, such as high self-adhesion, stretchability, and wide application temperature range for traditional hydrogels and flexible sensors is a challenge. Herein, a stretchable, self-adhesive, and antifreezing conductive hydrogel with multiple networks and excellent mechanical properties was prepared by a two-step method for its application in sensitive motion sensors and temperature-/humidity-driven actuators. First, quaternary chitosan (QCS) was introduced into the network of an acrylamide (AM) and 1-vinyl imidazole (VI) copolymer initiated by UV-photoinitiated radical polymerization. Then, the double-network hydrogel was immersed in a FeCl3 solution to fabricate the P(AAm-co-VI)/QCS-Fe3+ ionic hydrogel with multiple physical networks. The properties of the hydrogel were controllable and adjustable. The toughness of the ionic hydrogel could reach up to 654.4 kJ/m3, the fracture strength could reach 253.1 kPa, and the compressive strength reached 8.4 MPa at an 80% compression strain. The multiple physical networks improved the mechanical properties and the quick resilience of the hydrogel. A large amount of FeCl3 in the network greatly enhanced the ionic conductivity. Meanwhile, hydrogen bonds with water molecules inhibit the formation of ice crystals between zero water molecules and enhance the freezing resistance of P(Aam-co-VI)/QCS hydrogels. The active group on the QCS chain provided adhesiveness to various substrates for hydrogels. The P(AAm-co-VI)/QCS-Fe3+ hydrogel-based sensor showed high sensitivity, which can detect human movement and pulse, with a gauge factor of 2.37. Finally, due to the different dehydration rates of the P(AAm-co-VI)/QCS-Fe3+ and P(AAm-co-VI)/QCS hydrogel, a double-layer temperature/humidity-driven actuator was fabricated, expanding the application of conductive hydrogels.

Rapid self-healing and self-adhesive chitosan-based hydrogels by host-guest interaction and dynamic covalent bond as flexible sensor.

A sensor used to monitor tissue deformation requires good flexibility, stretchability, self-adhesion, cyto-compatibility, and antibacterial property. Here, we prepared hydrogel sensor based on O-carboxymethyl chitosan (O-CMCS) and poly(vinyl alcohol) (PVA) for monitoring human and organ motions. Based on the host-guest complexing of poly(ß-cyclodextrin) with diamantane, a cross-linker containing multiple aldehyde groups was prepared for cross-linking with O-CMCS through Schiff base linkages. Borax was used as the second cross-linker to cross-link PVA through dynamic borate ester bonds. Carbon nanotubes (CNTs) were added into the hydrogels to improve their electrical conductivity and mechanical properties. The obtained hydrogel exhibited rapid self-healing ability with healing efficiency as high as 97%-103% (in 15 s), good adhesion to human skin and wet organ, good antibacterial property, cyto-compatibility, and stretchability. Furthermore, the hydrogel sensor can monitor the respiratory movement of porcine lungs and the beating of rat hearts.

A Multifunctional, Self-Healing, Self-Adhesive, and Conductive Sodium Alginate/Poly(vinyl alcohol) Composite Hydrogel as a Flexible Strain Sensor.

Hydrogel-based wearable devices have attracted tremendous interest due to their potential applications in electronic skins, soft robotics, and sensors. However, it is still a challenge for hydrogel-based wearable devices to be integrated with high conductivity, a self-healing ability, remoldability, self-adhesiveness, good mechanical strength and high stretchability, good biocompatibility, and stimulus-responsiveness. Herein, multifunctional conductive composite hydrogels were fabricated by a simple one-pot method based on poly(vinyl alcohol) (PVA), sodium alginate (SA), and tannic acid (TA) using borax as a cross-linker. The composite hydrogel network was built by borate ester bonds and hydrogen bonds. The obtained hydrogel exhibited pH- and sugar-responsiveness, high stretchability (780% strain), and fast self-healing performance with healing efficiency (HE) as high as 93.56% without any external stimulus. Additionally, the hydrogel displayed considerable conductive behavior and stable changes of resistance with high sensitivity (gauge factor (GF) = 15.98 at a strain of 780%). The hydrogel was further applied as a strain sensor for monitoring large and tiny human motions with durable stability. Significantly, the healed hydrogel also showed good sensing behavior. This work broadens the avenue for the design and preparation of biocompatible polymer-based hydrogels to promote the application of hydrogel sensors with comfortable wearing feel and high sensitivity.

Adhesive, Stretchable, and Transparent Organohydrogels for Antifreezing, Antidrying, and Sensitive Ionic Skins.

As a flexible wearable device, hydrogel-based sensors have attracted widespread attention in soft electronics. However, the application of traditional hydrogels at extreme temperatures or for a long-term stability still remain a challenge because of the existence of water. Herein, we reported an antifreezing and antidrying organohydrogel with high transparency (over 85% transmittance), high stretchability (up to 1200%), and robust adhesiveness to various substrates, which consist of polyacrylic acid, gelatin, AlCl3+, and tannic acid in a water/glycerin binary solvent as the dispersion medium. As the binary solvent easily forms strong hydrogen bonds with water molecules, organohydrogels exhibited excellent tolerance for drying and freezing. The organohydrogels maintained conductivity, adhesion, and stable sensitivity after a long-term storage or at subzero temperature (-14 °C). Moreover, the organohydrogel-based wearable sensors with a gauge factor of 2.5 (strain, 0-100%) could detect both large-scale movements and subtle motions. Therefore, the multifunctional organohydrogel-wearable sensors with antifreezing and antidrying properties have promising potential for human-machine interfaces and healthcare monitoring under a broad range of environmental conditions.

Multiple Weak H-Bonds Lead to Highly Sensitive, Stretchable, Self-Adhesive, and Self-Healing Ionic Sensors.

Herein, we demonstrate a ternary ionic hydrogel sensor consisting of tannic acid, sodium alginate, and covalent cross-linked polyacrylamide as skin-mountable and wearable sensors. Based on the multiple weak H-bonds and synergistic effects between the three components, the as-prepared hybrid hydrogel exhibits ultrastretchability with high elasticity, good self-healing, excellent conformability, and high self-adhesiveness to diverse substrates both in air and underwater. More importantly, the ternary hydrogel exhibits high strain sensitivity especially under subtle strains with a gauge factor of 2.0, which is close to the theoretical value of the ionic hydrogel sensors; an extremely large workable range of strain (0.05-2100%); and a low operating voltage 0.07 V. Consequently, the sensor demonstrates superior sensing performance for real-time monitoring of the large and subtle human motions, including limb motions, swallowing, smiling, and wrist pulse. Therefore, it is believed that the STP hydrogel has great potential applications in health monitoring, smart wearable devices, and soft robots.