Flexible organohydrogel ionic skin with Ultra-Low temperature freezing resistance and Ultra-Durable moisture retention

Skin somatosensory system inspired gelatin-based organohydrogel electronic skin for infant hazard alarms

Multi-functional electronic skin (e-skin) demonstrates immense potential for applications in physiological monitoring and human-computer interaction. However, achieving reliable multi-modal sensing in complex environments and hazard alarms for infants remains a significant challenge. Drawing inspiration from the skin's somatosensory system, we herein present an innovative multi-modal sensing organohydrogel, fabricated by incorporating gelatin, laponite, and LiCl into a copolymer network within a glycerol/water binary solvent. The designed organohydrogel exhibits skin-like composition and modulus. Furthermore, the low-temperature induced triple-helix structure of gelatin and the presence of binary solvents, imparting excellent stretchability, thermosensitivity, and outstanding environmental tolerance. Remarkably, this organohydrogel is well-suited for gelatin-based e-skin applications, mimicking the skin's sensory functions to detect strain, pressure, temperature, and humidity. Furthermore, the gelatin-based e-skin can be integrated with a custom-designed data recognition and analysis system, enabling real-time monitoring of an infant's physiological and movement states. This capability proves particularly valuable for providing timely alerts in hazardous situations, including asphyxiation caused by chest pressure, fever, or excessive sweating. This work paves the way for the next generation of e-skin for human health monitoring and hazard detection.

Self-polymerized metal-phenolic ionogel with multifunctional properties towards theranostic wearable electronics

With the rapid development of wearable technology toward integrated diagnostics and therapy, wearable electronic materials are required to possess a range of properties, such as stretchable, compressible, conductive, anti-freezing, biocompatible, and antimicrobial properties. Metal-phenolic dual-network ionogel (MP-DN ionogel) was thus prepared by using FeIII-tannic acid and H2O2 as dual self-catalysis system to trigger the polymerization of hydrophilic ionic liquid monomer and hydrophobic acrylamide glycidyl ester monomer. The prepared ionogel showed well-rounded properties including high conductivity, good self-healing, anti-freezing (remains ice-free at -20 °C), anti-swelling, effective antibacterial property (anti-bacterial ratio > 99.9 %), and good cell and tissue biocompatibility. The ionogel exhibited the capability of recording electrocardiogram (ECG), electromyography (EMG), monitoring motion of finger bending and promoting wound healing. The present work provides a simple one-pot strategy to prepare multifunctional ionogels, to meet various application conditions for the next-generation theranostics wearable electronic devices. STATEMENT OF SIGNIFICANCE: 1. A dual-network ionogel with tuned mechanical properties was prepared using a simple one-pot method. 2. The ionogel exhibited superior conductivity, antifreeze, anti-swelling, good adhesion and antibacterial properties. 3. The prepared ionogel demonstrated good performance in rat ECG and EMG signal and high sensitivity to finger bending motions. 4. The ionogel could promote the healing of infected wounds. 5. Offer valuable guidance for the theranostic wearable electronics.

A flexible, antifreezing, and long-term stable cellulose ionic conductive hydrogel via one-step preparation for flexible electronic sensors

Ionic conductive hydrogels have attracted great attention due to their good flexibility and conductivity in flexible electronic devices. However, because of the icing and water loss problems, the compatibility issue between the mechanical properties and conductivity of hydrogel electrolytes over a wide temperature range remains extremely challenging to achieve. Although, antifreezing/water-retaining additives could alleviate these problems, the reduced performance and complex preparation methods seriously limit their development. In this work, a simple strategy without additives was provided to prepare an ionic conductive cellulose hydrogel (ICH) in one step through molten salt hydrate. The hydrogel featured controllable mechanical properties (0.19 MPa - 0.67 MPa), high ionic conductivity (78.96 mS/cm), excellent freezing resistance (-80 °C). More importantly, due to the existing metal salts component, the ICH exhibited long-term stability in water-retention ability (75.6 %, after 90 days) and ionic conductivity (85 %, after 90 days) over a wide working temperature range (-80 °C to 40 °C). Benefiting from these advantages, the ICH exhibited excellent electromechanical performance in human movement detection and movement direction identification, indicating a promising apply for flexible electronic device.

N-Doped Holey Graphene/Porous Carbon/Cellulose Nanofibers Electrode and Hydrogel Electrolyte for Low-temperature Zinc-ion Hybrid Supercapacitors

The susceptibility to freezing of the electrolyte and mismatched cathode make the aqueous zinc-ion hybrid supercapacitors (ZHSCs) have inferior electrochemical performance at low temperature. Herein, a novel freeze-tolerant hydrogel electrolyte (CEEZ) and matched graphene/porous carbon/cellulose nanofibers cathode (GPCN) are respectively fabricated via chemical cross-linking and a two-step process to assemble ZHSCs. The prepared electrode has a highly porous structure, abundant edge active sites, and increased interlayer spacing, which collectively reduces ion transport complexity and enhances the contact area with the electrolyte, promoting rapid ionic conduction pathways. For the CEEZ, the use of ethylene glycol reduces the saturated vapor pressure of water, thereby enhancing the frost resistance of the hydrogel electrolyte. Consequently, the ZHSCs assembled from GPCN, CEEZ, and Zn anode exhibit excellent specific capacitances of 1.11 F cm⁻2 (21.35 F cm⁻3) at 20 °C and 0.74 F cm⁻2 (14.23 F cm⁻3) at -20 °C. These results demonstrate the promising application potential of these ZHSCs in cold environments while maintaining impressive energy storage capabilities. This work provides valuable insights and a robust strategy for the design of high-performance low-temperature ZHSCs, enhancing their practical applicability in renewable energy storage systems.

Porous gradient hydrogel promotes skin regeneration by angiogenesis

The skin has a multilayered structure, and deep-seated injuries are exposed to external microbial invasion and in vivo microenvironmental destabilization. Here, a bilayer bionic skin scaffold (Bilayer SF) was developed based on methacrylated sericin protein to mimic the skin's multilayered structure and corresponding functions. The outer layer (SF@TA), which mimics the epidermal layer, was endowed with the function of resisting external bacterial and microbial invasion using a small pore structure and bio-crosslinking with tannic acid (TA). The inner layer (SF@DA@Gel), which mimics the dermal layer, was used to promote cellular growth using a large pore structure and introducing dopamine (DA) to regulate the wound microenvironment. This Bilayer SF showed good mechanical properties and structural stability, satisfactory antioxidant and promote cell proliferation and migration abilities. In vitro studies confirmed the antimicrobial properties of the outer layer and the pro-angiogenic ability of the inner layer. In vivo animal studies demonstrated that the bilayer scaffolds promoted collagen deposition, neovascularization, and marginal hair follicle formation, which might be a promising new bionic skin scaffold.

Lignin reinforced tough, adhesive, and recoverable protein organohydrogels for wearable strain sensing under sub-zero temperatures

Protein-based hydrogels with promising biocompatibility and biodegradability have attracted considerable interest in areas of epidermal sensing, whereas, which are still difficult to synchronously possess high mechanical strength, self-adhesion, and recoverability. Hence, the bio-polymer lignosulfonate-reinforced gluten organohydrogels (GOHLx) are fabricated through green and simple food-making processes and the following solvent exchange with glycerol/water binary solution. Ascribing to the uniform distribution of lignosulfonate in gluten networks, as well as the noncovalent interactions (e.g., H-bond) between them, the resultant GOHLx exhibit favorable conductivity (∼14.3 × 10-4 S m-1), toughness (∼711.0 kJ m-3), self-adhesion (a maximal lap-shear strength of ∼33.5 kPa), high sensitivity (GF up to ∼3.04), and durability (∼3000 cycles) toward shape deformation, which are suitable for the detection of both drastic (e.g., elbow and wrist bending) and subtle (e.g., swallowing and speaking) human movements even under -20 °C. Furthermore, the GOHLx is also biocompatible, degradable, and recoverable (by a simple kneading process). Thus, this work may pave a simple, green, and cheap way to prepare all-biomass-based, tough, sticky, and recoverable protein-based organohydrogels for epidermal strain sensing even in harsh environments.

Advanced function, design and application of skin substitutes for skin regeneration

The development of skin substitutes aims to replace, mimic, or improve the functions of human skin, regenerate damaged skin tissue, and replace or enhance skin function. This includes artificial skin, scaffolds or devices designed for treatment, imitation, or improvement of skin function in wounds and injuries. Therefore, tremendous efforts have been made to develop functional skin substitutes. However, there is still few reports systematically discuss the relationship between the advanced function and design requirements. In this paper, we review the classification, functions, and design requirements of artificial skin or skin substitutes. Different manufacturing strategies for skin substitutes such as hydrogels, 3D/4D printing, electrospinning, microfluidics are summarized. This review also introduces currently available skin substitutes in clinical trials and on the market and the related regulatory requirements. Finally, the prospects and challenges of skin substitutes in the field of tissue engineering are discussed.

An Environment-Tolerant Ion-Conducting Double-Network Composite Hydrogel for High-Performance Flexible Electronic Devices

High-performance ion-conducting hydrogels (ICHs) are vital for developing flexible electronic devices. However, the robustness and ion-conducting behavior of ICHs deteriorate at extreme temperatures, hampering their use in soft electronics. To resolve these issues, a method involving freeze-thawing and ionizing radiation technology is reported herein for synthesizing a novel double-network (DN) ICH based on a poly(ionic liquid)/MXene/poly(vinyl alcohol) (PMP DN ICH) system. The well-designed ICH exhibits outstanding ionic conductivity (63.89 mS cm-1 at 25 °C), excellent temperature resistance (- 60-80 °C), prolonged stability (30 d at ambient temperature), high oxidation resistance, remarkable antibacterial activity, decent mechanical performance, and adhesion. Additionally, the ICH performs effectively in a flexible wireless strain sensor, thermal sensor, all-solid-state supercapacitor, and single-electrode triboelectric nanogenerator, thereby highlighting its viability in constructing soft electronic devices. The highly integrated gel structure endows these flexible electronic devices with stable, reliable signal output performance. In particular, the all-solid-state supercapacitor containing the PMP DN ICH electrolyte exhibits a high areal specific capacitance of 253.38 mF cm-2 (current density, 1 mA cm-2) and excellent environmental adaptability. This study paves the way for the design and fabrication of high-performance multifunctional/flexible ICHs for wearable sensing, energy-storage, and energy-harvesting applications.

Strong and Tough Antifreezing Hydrogel Sensor via the Synergy of Coordination and Hydrogen Bonds

Hydrogel sensors are fascinating as flexible sensors and electronic skin due to their excellent biocompatibility and structure controllability. However, developing conductive hydrogels possessing both excellent mechanical and antifreezing properties for environmental-adaptive sensors remains a challenge. Herein, a strategy of combining betaine and metal ions to construct poly(acrylic acid) (PAA)-based high-conductive hydrogels has been reported. PAA-Al3+/betaine hydrogels with high toughness and antifreezing property were prepared by a one-step UV curing method. Their high toughness is attributed to the coordination of metal ions with the carboxylic groups in PAA, the interaction of betaine with PAA, and the formation of hydrogen bonds between them and water molecules. Moreover, the significant antifreezing property is due to the reduction of free water in the hydrogel. This, in turn, is attributed to the hydration of metal ions and the synergistic hydrogen bonding between betaine and water. The experiments demonstrate that the hydrogel has excellent mechanical property, high conductivity, superior transparency, antiswelling property, antipuncture as well as shape memory properties, and especially, low cytotoxicity. It can be used as a sensor for motion detection and information recognition. This work provides new insights into the application of flexible sensors and human-machine interfaces in multienvironmental conditions.

Polyvinyl alcohol/polyacrylamide double-network hydrogel-based semi-dry electrodes for robust electroencephalography recording at hairy scalp for noninvasive brain-computer interfaces

Objective.Reliable and user-friendly electrodes can continuously and real-time capture the electroencephalography (EEG) signals, which is essential for real-life brain-computer interfaces (BCIs). This study develops a flexible, durable, and low-contact-impedance polyvinyl alcohol/polyacrylamide double-network hydrogel (PVA/PAM DNH)-based semi-dry electrode for robust EEG recording at hairy scalp.Approach.The PVA/PAM DNHs are developed using a cyclic freeze-thaw strategy and used as a saline reservoir for semi-dry electrodes. The PVA/PAM DNHs steadily deliver trace amounts of saline onto the scalp, enabling low and stable electrode-scalp impedance. The hydrogel also conforms well to the wet scalp, stabilizing the electrode-scalp interface. The feasibility of the real-life BCIs is validated by conducting four classic BCI paradigms on 16 participants.Main results.The results show that the PVA/PAM DNHs with 7.5 wt% PVA achieve a satisfactory trade-off between the saline load-unloading capacity and the compressive strength. The proposed semi-dry electrode exhibits a low contact impedance (18 ± 8.9 kΩ at 10 Hz), a small offset potential (0.46 mV), and negligible potential drift (1.5 ± 0.4μV min^-1). The temporal cross-correlation between the semi-dry and wet electrodes is 0.91, and the spectral coherence is higher than 0.90 at frequencies below 45 Hz. Furthermore, no significant differences are present in BCI classification accuracy between these two typical electrodes.Significance.Based on the durability, rapid setup, wear-comfort, and robust signals of the developed hydrogel, PVA/PAM DNH-based semi-dry electrodes are a promising alternative to wet electrodes in real-life BCIs.

A moisture self-regenerative, ultra-low temperature anti-freezing and self-adhesive polyvinyl alcohol/polyacrylamide/CaCl2/MXene ionotronics hydrogel for bionic skin strain sensor

Ignited by the concept of bionics, hydrogel-based bionic skin sensors have received more and more attention and been widely used in health monitoring, robots, implantable prostheses and human-machine interfaces. However, there still remain some challenges to be urgently solved for hydrogel-based bionic skin sensors, such as the water evaporation and the defects of single conductive mechanism of electronic skin or ionic skin. Herein, we prepared a polyvinyl alcohol/polyacrylamide/CaCl₂/MXene (PPCM) ionotronics hydrogel with moisture self-regenerative, highly sensitive, ultra-low temperature anti-freezing (-50 °C) and self-adhesive features and applied it as bionic skin strain sensor. The introduction of MXene and CaCl₂ endows the PPCM hydrogel with both electron and ion conductive channels, which effectively compensates for the defects of single electronic skin or ionic skin. Importantly, the addition of CaCl₂ into the PPCM hydrogel offers it the moisture self-regenerative ability, holding the long-term water retention. The water in the PPCM hydrogel can still be kept in a stable state after continuous use for 70 days at room temperature, thus ensuring the long-term stability of the hydrogel-based sensor. Such a moisture self-regenerative ability should be an important feature for intelligentizing the hydrogel-based bionic skin for practical applications.

A Review on Thermal Properties of Hydrogels for Electronic Devices Applications

Hydrogels, as a series of three-dimensional, crosslinked, hydrophilic network polymers, exhibit extraordinary properties in softness, mechanical robustness and biocompatibility, which have been extensively utilized in various fields, especially for electronic devices. However, since hydrogels contain plenty of water, the mechanical and electrochemical properties are susceptible to temperature. The thermal characteristics of hydrogels can significantly affect the performance of flexible electronic devices. In this review, recent research on the thermal characteristics of hydrogels and their applications in electronic devices is summarized. The focus of future work is also proposed. The thermal stability, thermoresponsiveness and thermal conductivity of hydrogels are discussed in detail. Anti-freezing and anti-drying properties are the critical points for the thermal stability of hydrogels. Methods such as introducing soluble ions and organic solvents into hydrogels, forming ionogels, modifying polymer chains and incorporating nanomaterials can improve the thermal stability of hydrogels under extreme environments. In addition, the critical solution temperature is crucial for thermoresponsive hydrogels. The thermoresponsive capacity of hydrogels is usually affected by the composition, concentration, crosslinking degree and hydrophilic/hydrophobic characteristics of copolymers. In addition, the thermal conductivity of hydrogels plays a vital role in the electronics applications. Adding nanocomposites into hydrogels is an effective way to enhance the thermal conductivity of hydrogels.

Recent Advances in Mechanical Reinforcement of Zwitterionic Hydrogels

As a nonspecific protein adsorption material, a strong hydration layer provides zwitterionic hydrogels with excellent application potential while weakening the interaction between zwitterionic units, leading to poor mechanical properties. The unique anti-polyelectrolyte effect in ionic solution further restricts the application value due to the worsening mechanical strength. To overcome the limitations of zwitterionic hydrogels that can only be used in scenarios that do not require mechanical properties, several methods for strengthening mechanical properties based on enhancing intermolecular interaction forces and polymer network structure design have been extensively studied. Here, we review the works on preparing tough zwitterionic hydrogel. Based on the spatial and molecular structure design, tough zwitterionic hydrogels have been considered as an important candidate for advanced biomedical and soft ionotronic devices.

Porous polyvinyl alcohol/graphene oxide composite film for strain sensing and energy-storage applications

In this study, a flexible porous polyvinyl alcohol (PVA)/graphene oxide (GO) composite film was developed and tested for flexible strain sensing and energy-storage applications. Morphology and mechanical properties were studied; tensile strength and Young's modulus increased by 225% and 86.88%, respectively, at 0.5 wt% GO. The PVA/GO film possesses exceptional sensing ability to various mechanical strains, such as tension, compression, bending, and torsion. For example, the gauge factor of the PVA/GO film as a tensile-strain sensor was measured as 2.46 (246%). Under compression loads, the PVA/GO composite film showed piezoresistive and capacitive strain-sensing characteristics. Under 5 kPa of compression load, the relative resistance increased by 81% with a 100 msec response time; the relative capacitance increased by 160% with a 120 msec response time. The PVA/GO strain sensor exhibited high durability and reliability over 20 × 10³cycles of tensile strain and bending at 3.33 Hz. Moreover, the PVA/GO composite film showed good electrochemical properties due to its porous structure; the maximum capacitance was 124.7 F g^-1at 0.5 wt% GO. After 20 × 10³charging-discharging cycles, the capacitance retention rate was 94.45%, representing high stable capacitance performance. The results show that electrically conductive porous PVA nanocomposite films are promising candidates for strain sensing and energy-storage devices.

Antifreezing and Nondrying Sensors of Ionic Hydrogels with a Double-Layer Structure for Highly Sensitive Motion Monitoring

Freezing and dehydration together with interfacial failure are capable of causing the functional reduction of hydrogels for sensing applications. Herein, we develop a multifunctional bilayer that consists of a mussel-inspired adhesive layer and a functionally ionic layer that is composed of sodium p-styrene sulfonate (SSS) and an ionic liquid of [BMIM]Cl. The adhesive layer enables the strong adhesion of the bilayer to the surface of the skin. The introduction of ionic elements of SSS-[BMIM]Cl not only provides the bilayer with sensing adaptability in a wide temperature range of -25 to 75 °C, but also endows it with elastic, stretchable, self-healing, and conductive features. These mechanical properties are utilized to assemble a wearable sensor that has unprecedented sensitivity and reusability in monitoring human motions, including stretching, pulsing, frowning, and speaking. It is thus expected that the concept in this work would provide a promising route to design soft sensing devices that can work in a wide temperature range.

Casein micelles embedded composite organohydrogel as potential wound dressing

Excellent mechanical and tissue adhesive properties, long-lasting environmental suitability and reliable biocompatibility are essential factors for the hydrogels to be applied as wound dressing in the clinical fields. Based on the self-assembly micelle structures, a new type of casein micelles (CEs)/polyvinyl alcohol (PVA) GW (glycerol-water) organohydrogel was designed and synthesized by a simple one-pot method. Through a unique "load sharing" effect, the CEs which own suitable adhesion abilities and drug loading capacities simultaneously were embedded into the PVA networks by rich hydrogen bonds, so that to obtain the composite organ hydrogel with not only excellent adhesive abilities, but also enhanced mechanical properties. Benefited from the unique GW binary solvent system, the organohydrogel showed long-lasting moisture lock-in capacity and extreme temperature tolerance (in the range of --20 °C ~ 60 °C). Particularly, after loading the model antibacterial drugs (allicin) within the CEs, the as-developed CEs/PVA GW gel exhibited a prominent long-lasting (>100 h) antibacterial properties (>90%). Furthermore, the organohydrogel was confirmed with prominent biocompatibility to support fibroblast cell proliferation and migration. This work proposed a new strategy to build CEs-based gel system, which have a great potential application in terms of prevent bacterial infection, accelerate tissue proliferation and wound healing.

Freeze-Tolerant Hydrogel Electrolyte with High Strength for Stable Operation of Flexible Zinc-Ion Hybrid Supercapacitors

Constructing ionic conductive hydrogels with diversified properties is crucial for portable zinc-ion hybrid supercapacitors (ZHSCs). Herein, a freeze-tolerant hydrogel electrolyte (AF PVA-CMC/Zn(CF₃ SO₃ )₂ ) is developed by forming a semi-interpenetrating anti-freezing polyvinyl alcohol-carboxymethyl cellulose (AF PVA-CMC) network filled with the ethylene glycol (EG)-containing Zn(CF₃ SO₃ )₂ aqueous solution. The semi-interpenetrating AF PVA-CMC/Zn(CF₃ SO₃ )₂ possesses enhanced mechanical properties, realizes the uniform zinc deposition, and impedes the dendrite growth. Notably, the interaction between PVA and EG suppresses the ice crystal formation and prevents freezing at -20 °C. Due to these advantages, the designed hydrogel owns high ionic conductivity of 1.73/0.75 S m^-1 at 20/-20 °C with excellent tensile/compression strength at 20 °C. Impressively, the flexible AF quasi-solid-state ZHSC employing the hydrogel electrolyte achieves a superior energy density at 20/-20 °C (87.9/60.7 Wh kg^-1 ). It maintains nearly 84.8% of the initial capacity after 10 000 cycles and a low self-discharge rate (1.77 mV h^-1 ) at 20 °C, together with great tolerance to corrosion. Moreover, this device demonstrates a stable electrochemical performance at -20 °C under deformation. The obtained results provide valuable insights for constructing durable hydrogel electrolytes in cold environments.

Highly Transparent, Self-Healing, and Self-Adhesive Double Network Hydrogel for Wearable Sensors

Hydrogel-based flexible electronic devices are essential in future healthcare and biomedical applications, such as human motion monitoring, advanced diagnostics, physiotherapy, etc. As a satisfactory flexible electronic material, the hydrogel should be conductive, ductile, self-healing, and adhesive. Herein, we demonstrated a unique design of mechanically resilient and conductive hydrogel with double network structure. The Ca²⁺ crosslinked alginate as the first dense network and the ionic pair crosslinked polyzwitterion as the second loose network. With the synthetic effect of these two networks, this hydrogel showed excellent mechanical properties, such as superior stretchability (1,375%) and high toughness (0.57 MJ/m³). At the same time, the abundant ionic groups of the polyzwitterion network endowed our hydrogel with excellent conductivity (0.25 S/m). Moreover, due to the dynamic property of these two networks, our hydrogel also performed good self-healing performance. Besides, our experimental results indicated that this hydrogel also had high optical transmittance (92.2%) and adhesive characteristics. Based on these outstanding properties, we further explored the utilization of this hydrogel as a flexible wearable strain sensor. The data strongly proved its enduring accuracy and sensitivity to detect human motions, including large joint flexion (such as finger, elbow, and knee), foot planter pressure measurement, and local muscle movement (such as eyebrow and mouth). Therefore, we believed that this hydrogel had great potential applications in wearable health monitoring, intelligent robot, human-machine interface, and other related fields.