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

Highly Stretchable Conductive Covalent Coacervate Gels for Electronic Skin

Highly stretchable electrically conductive hydrogels have been extensively researched in recent years, especially for applications in strain and pressure sensing, electronic skin, and implantable bioelectronic devices. Herein, we present a new cross-linked complex coacervate approach to prepare conductive hydrogels that are both highly stretchable and compressive. The gels involve a complex coacervate between carboxylated nanogels and branched poly(ethylene imine), whereby the latter is covalently cross-linked by poly(ethylene glycol) diglycidyl ether (PEGDGE). Inclusion of graphene nanoplatelets (Gnp) provides electrical conductivity as well as tensile and compressive strain-sensing capability to the hydrogels. We demonstrate that judicious selection of the molecular weight of the PEGDGE cross-linker enables the mechanical properties of these hydrogels to be tuned. Indeed, the gels prepared with a PEGDGE molecular weight of 6000 g/mol defy the general rule that toughness decreases as strength increases. The conductive hydrogels achieve a compressive strength of 25 MPa and a stretchability of up to 1500%. These new gels are both adhesive and conformal. They provide a self-healable electronic circuit, respond rapidly to human motion, and can act as strain-dependent sensors while exhibiting low cytotoxicity. Our new approach to conductive gel preparation is efficient, involves only preformed components, and is scalable.

Soft Stretchable Conductive Carboxymethylcellulose Hydrogels for Wearable Sensors

Hydrogels that have a capability to provide mechanical modulus matching between time-dynamic curvilinear tissues and bioelectronic devices have been considered tissue-interfacing ionic materials for stably sensing physiological signals and delivering feedback actuation in skin-inspired healthcare systems. These functionalities are totally different from those of elastomers with low ionic conductivity and higher stiffness. Despite such remarkable progress, their low conductivity remains limited in transporting electrical charges to internal or external terminals without undesired information loss, potentially leading to an unstable biotic-abiotic interfaces in the wearable electronics. Here, we report a soft stretchable conductive hydrogel composite consisting of alginate, carboxymethyl cellulose, polyacrylamide, and silver flakes. This composite was fabricated via sol-gel transition. In particular, the phase stability and low dynamic modulus rates of the conductive hydrogel were confirmed through an oscillatory rheological characterization. In addition, our conductive hydrogel showed maximal tensile strain (≈400%), a low deformations of cyclic loading (over 100 times), low resistance (≈8.4 Ω), and a high gauge factor (≈241). These stable electrical and mechanical properties allowed our composite hydrogel to fully support the operation of a light-emitting diode demonstration under mechanical deformation. Based on such durable performance, we successfully measured the electromyogram signals without electrical malfunction even in various motions.

Adhesive and high-sensitivity modified Ti3C2TX (MXene)-based organohydrogels with wide work temperature range for wearable sensors

Hydrogel-based wearable sensors have gained great interest on account of their huge application in human-machine interfaces, electronic skin, and healthcare monitoring. However, there are still challenges in designing hydrogel-based sensors with high stability in a wide temperature range, superior adhesion, and excellent sensitivity. Herein, sensors based on oxidized sodium alginate (OSA)/polyacrylamide (PAm)/polydopamine-Ti₃C₂T_X (PMXene) /glycerol/water (Gly/H₂O) organohydrogels were designed. The organohydrogels exhibited excellent mechanical properties (elongation at break of 1037%, tensile strength of 0.17 MPa), predominant self-healing ability (self-healing efficiency of 91%), as well as high sensing stability in a wide temperature range (from -20 to 60°C). The introduction of PDA (polydopamine) and viscous glycerin (Gly) provide organohydrogels with superior adhesion. Organohydrogels sensors demonstrated high sensitivity (Gauge Factor, GF = 2.2) due to the combination of ionic and electron conduction. Sensors could stably detect human movement under different strain levels at high and low temperatures, providing a new solution for wearable sensors in extreme conditions.

Robust, anti-freezing and conductive bonding of chitosan-based double-network hydrogels for stable-performance flexible electronic

Unstable hydrogel-substrate interfaces and defunctionalization at low temperature severely restrict versatile applications of hydrogel-based systems. Herein, various chitosan-polyacrylamide double-network (CS-PAM DN) ionic hydrogels were chemically linked with diverse substrates to construct robust and anti-freezing hydrogel-substrate combination, wherein the destructible CS physical network rendered effective energy dissipation mechanism to significantly enhanced the cohesion of hydrogels and the covalent linkage between PAM network with substrate surface strongly improved the interfacial adhesion. The synergistic effects enabled the CS-PAM DN hydrogels to be tightly bonded on diverse metals and inorganics. Impressively, the hydrogel-substrate combinations were freezing tolerant to well-maintain high interfacial toughness at low temperature. Notably, due to the high toughness and conductivity of hydrogel-metal interface, the hydrogel-metal combination can be utilized as a multi-model flexible sensor to detect strain and pressure within broad temperature range. This work may provide a platform for construction and emerging application of robust, anti-freezing and stable-performance hydrogel-based systems.

Synthesis of micrometer-sized poly(vinyl acetate) particles through microsuspension iodine transfer polymerization: effect of iodine species in a water medium

Schematic of the behaviors of iodine species in the microsuspension polymerization of vinyl acetate (VAc) in an aqueous medium.

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(β-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.

Mesoporous nanomaterial-assisted hydrogel double network composite for mixed-mode liquid chromatography

By introducing functional groups such as quaternary amine groups, sulfonic acid groups, triazine groups, and other mespore nanomaterials into the hydrogel, better separation effect of some organic framework materials has been obtained. Due to a reasonable design and preparation strategy, the hydrogel composite-modified silica can be used in the selective separation of various analytes such as pesticides, alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides/bases, benzoic acids, antibiotics, and carbohydrates. Through the exploration of chromatographic retention behavior, it is proved that the column can be used in mixed-mode liquid chromatography. The intra-day relative standard deviation for retention time of this new stationary phase is 0.12-0.16% (n = 10), and the inter-day relative standard deviation is less than 0.39% (n = 5). This new stationary phase can also be used for separation in complex samples. The limit of detection (LOD) for chlorotoluron in farm irrigation water is 0.21 µg/L and the linear range is 2-250 µg/L. After optimizing the chromatographic conditions, the highest efficiency of the hydrogel column in RPLC and HILIC modes has reached 32,400 plates/m (chlorobenzuron) and 41,300 plates/m (galactose). This new type of hydrogel composite is a porous network material with flexible functional design and simple preparation method and its application has been expanded in liquid chromatography separation successfully. The hydrogel composed of triallyl cyanate cross-linking agent and 3-(2-(methacryloyloxy) ethyl) dimethylamine) propane-1-sulfonate (SBMA) monomer which were co-modified on the surface of mesoporous silica with MOF-919 for separation in mixed-mode liquid chromatography.

Bio-based, self-adhesive, and self-healing ionogel with excellent mechanical properties for flexible strain sensor

Bio-based ionogels with versatile properties are highly desired for practical applications. Herein, we designed a novel self-healing, anti-freezing, and self-adhesive ionogel with excellent sensor capability. The ionogel was obtained by cross-linking amino groups (chitosan) and aldehyde groups (dextran oxide) to form Schiff-base bonds in the ionic liquids (EMIMOAc) with TA. Ionogels inherited the superior electrical conductivity of ionic liquids (IG2, 1.1 mS cm-1). Due to the dynamic reaction of Schiff-base bonds, the obtained IG2 possessed self-healing properties (self-healing efficiency = 89%). The presence of TA also provided the ionogel with excellent self-adhesive properties (IG2/TA, adhesive strength to hogskin = 8.05 kPa). Owing to the low freezing point and low vapor pressure of ionic liquids, ionogels were endowed with anti-freeze properties and resistance to solvent volatility. Moreover, the ionogel can act as a strain sensor, and exhibited excellent sensitivity and sensing performance. Our work provided a green and effective method in preparation of the high performance ionogel sensor, which could accommodate future practical industrial applications.

Self-Healing Mechanism and Conductivity of the Hydrogel Flexible Sensors: A Review

Sensors are devices that can capture changes in environmental parameters and convert them into electrical signals to output, which are widely used in all aspects of life. Flexible sensors, sensors made of flexible materials, not only overcome the limitations of the environment on detection devices but also expand the application of sensors in human health and biomedicine. Conductivity and flexibility are the most important parameters for flexible sensors, and hydrogels are currently considered to be an ideal matrix material due to their excellent flexibility and biocompatibility. In particular, compared with flexible sensors based on elastomers with a high modulus, the hydrogel sensor has better stretchability and can be tightly attached to the surface of objects. However, for hydrogel sensors, a poor mechanical lifetime is always an issue. To address this challenge, a self-healing hydrogel has been proposed. Currently, a large number of studies on the self-healing property have been performed, and numerous exciting results have been obtained, but there are few detailed reviews focusing on the self-healing mechanism and conductivity of hydrogel flexible sensors. This paper presents an overview of self-healing hydrogel flexible sensors, focusing on their self-healing mechanism and conductivity. Moreover, the advantages and disadvantages of different types of sensors have been summarized and discussed. Finally, the key issues and challenges for self-healing flexible sensors are also identified and discussed along with recommendations for the future.

Polyacrylamide/Copper-Alginate Double Network Hydrogel Electrolyte with Excellent Mechanical Properties and Strain-Sensitivity

The double network (DN) hydrogel has attracted great attention due to its wide applications in daily life. However, synthesis DN hydrogel with excellent mechanical properties is still a big challenge. Here, polyacrylamide/copper-alginate double network (PAM/Cu-alg DN) hydrogel electrolyte is successfully synthesized by radiation-induced polymerization and cross-linking process of acrylamide with N, N'-methylene-bis-acrylamide and subsequent cupric ion (Cu²⁺ ) crosslinking of alginate. The content of sodium alginate, absorbed dose, and the concentration of Cu²⁺ are investigated in detail for improving the overall properties of PAM/Cu-alg DN hydrogel electrolyte. The PAM/Cu-alg DN hydrogel electrolyte synthesizes by radiation technique and Cu²⁺ crosslinking shows superior mechanical properties with a tensile strength of 2.25 ± 0.02 MPa, excellent energy dissipation mechanism, and the high ionic conductivity of 4.08 ± 0.17 mS cm^-1 . PAM/Cu-alg DN hydrogel is characterized with attenuated total reflection Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy analyses and the reason for the improvement of mechanical properties is illustrated. Furthermore, PAM/Cu-alg DN hydrogel electrolyte exhibits excellent strain-sensitivity, cyclic stability, and durability. This work paves for the new way for the preparation of DN hydrogel electrolytes with excellent properties.

Self-Healing, Self-Adhesive Silk Fibroin Conductive Hydrogel as a Flexible Strain Sensor

Flexible and wearable hydrogel strain sensors have attracted tremendous attention for applications in human motion and physiological signal monitoring. However, it is still a great challenge to develop a hydrogel strain sensor with certain mechanical properties and tensile deformation capabilities, which can be in conformal contact with the target organ and also have self-healing properties, self-adhesive capability, biocompatibility, antibacterial properties, high strain sensitivity, and stable electrical performance. In this paper, an ionic conductive hydrogel (named PBST) is rationally designed by proportionally mixing polyvinyl alcohol (PVA), borax, silk fibroin (SF), and tannic acid (TA). SF can not only be a reinforcement to introduce an energy dissipation mechanism into the dynamically cross-linked hydrogel network to stabilize the non-Newtonian behavior of PVA and borax but it can also act as a cross-linking agent to combine with TA to reduce the dissociation of TA on the hydrogel network, improving the mechanical properties and viscoelasticity of the hydrogel. The combination of SF and TA can improve the self-healing ability of the hydrogel and realize the adjustable viscoelasticity of the hydrogel without sacrificing other properties. The obtained hydrogel has excellent stretchability (strain > 1000%) and shows good conformal contact with human skin. When the hydrogel is damaged by external strain, it can rapidly self-repair (mechanical and electrical properties) without external stimuli. It shows adhesiveness and repeatable adhesiveness to different materials (steel, wood, PTFE, glass, iron, and cotton fabric) and biological tissues (pigskin) and is easy to peel off without residue. The obtained PBST conductive hydrogel also has a wide strain-sensing range (>650%) and reliable stability. The hydrogel adhered to the skin surface can monitor large strain movements such as in finger joints, wrist joints, knee joints, and so on and detect swallowing, smiling, facial bulging and calming, and other micro-deformation behaviors. It can also distinguish physical signals such as light smile, big laugh, fast and slow breathing, and deep and shallow breathing. Therefore, the PBST conductive hydrogel material with multiple synergistic functions has great potential as a flexible wearable strain sensor. The PBST hydrogel has antibacterial properties and good biocompatibility at the same time, which provides a safety guarantee for it as a flexible wearable strain sensor. This work is expected to provide a new way for people to develop ideal wearable strain sensors.

Injectable Nanocomposite Hydrogels for Cancer Therapy

Hydrogel is a kind of 3D polymer network with strong swelling ability in water and appropriate mechanical and biological properties, which make it feasible to maintain bioactive substances and has promising applications in the fields of biomaterials, soft machines, and artificial tissues. Unfortunately, traditional hydrogels prepared by chemical crosslinking have poor mechanical properties and limited functions, which limit their further application. In recent years, with the continuous development of nanoparticle research, more and more studies have combined nanoparticles with hydrogels to make up for the shortcomings of traditional hydrogels. In this article, the types and functions of hydrogels and nanomaterials are introduced first, as well as the functions and applications of injectable nanocomposite hydrogels (INHs), then the latest progress of INHs for cancer treatment is reviewed, some existing problems are summarized, and the application prospect of NHs is prospected.

Fabrication of a solid sensor based on a phenazine derivative film for enhancing the sensing properties of biogenic amine and applying for monitoring shrimp freshness

In practical applications, fixing a fluorescent sensor on a solid composite film matrix can settle the boundaries of keeping the sensor in the solution state.