A stretchable, self-healing, okra polysaccharide-based hydrogel for fast-response and ultra-sensitive strain sensors

A flexible film sensor based on sodium alginate/silk nanofiber for real-time monitoring of the ambient temperature and personnel respiration in flame environment

Natural polymers are considered powerful candidate for preparing a new generation of intelligent sensors, however, their application in fire environment is limited restricted by the low flame retardancy and thermal stability. Herein, we developed a novel flexible film (SSGP) entirely composed of bio-mass, namely natural silk nanofibers, sodium alginate and phytic acid, via solution casting and self-assembly strategy. It exhibited reliable tensile properties (11.01 MPa), high flame retardancy (LOI reached 38 %), and thermal stability. Specifically, SSGP not only showed sensitive and reversible response at different temperatures (50-125 °C) with a GF of 10.8, but also triggered warning within 2.2 s of being exposed to a flame without being incinerated. Moreover, SSGP responded to humidity changes in the environment (22-94 % RH) and could be used for respiratory monitoring, with a response/recovery time of only 0.76 s/1.01 s. Additionally, SSGP based on natural polymers retain degradability (39.8 % within 30 days). Thus, SSGP had the potential to be integrated as a core component into sensor for real-time monitoring of ambient temperature and respiration of personnel in steelworks, fire rescue, battlefield and other flame environment. This study may provide new insights into the application of natural polymers in the flexible multifunctional sensor.

Progress in the isolation, purification, structural characteristics and biological functions of polysaccharides from okra (Abelmoschus esculentus (L.) Moench): A review

Abelmoschus esculentus (L.) Moench (A. esculentus L.), commonly known as okra, is a famous tonic traditional Chinese medicine which have drawn significant interest because of its excellent edible and medicinal properties. A. esculentus L. polysaccharides (APs) are the main bioactive substances of A. esculentus L. In the past decades, various methods (e.g. triple-phase partitioning and -assisted extraction etc.) have shown promise in extraction, and diverse techniques (e.g. ethanol precipitation and chromatography etc) have already been in use in purification. Although techniques for the isolation and purification of APs are constantly evolving to better cope with their complex structural features, these techniques still need to be innovated. APs are heteropolysaccharides with complex structures, exhibiting various biological activities, such as antioxidant, anti-inflammatory and antidepressant effects. However, the structure-activity relationships (SARs) and mechanisms between their higher structures and pharmacological effects need to be further investigated. In view of this, this review provides a comprehensive overview of recent advances in APs extraction, purification, structural characterization and bioactivity, discusses structure-activity relationships, and outlines the current applications, challenges and prospect of APs. The aim of this review is to provide valuable insights and information to facilitate the use of APs in food and pharmaceutical applications.

High performance breathable conductive hydrogel sensor based on sodium alginate and polyacrylamide with cross-linked dual network structures

Conductive hydrogels have attracted immense interest in the field of flexible electronic devices and biosensors. However, issues such as poor breathability, insufficient adhesion and water retention properties still remain and limit their applications as wearable sensors. To address these issues, a porous conductive hydrogel (SA/PAM/CTAB-GO) with high breathability (5.97 mg.cm-2·h-1), favorable conductivity (2.85 S/m), desirable adhesion (1.39 KPa) and moisture retention properties was synthesized by polymerization and crosslinking using sodium alginate and polyacrylamide as gel substrates, cetyltrimethylammonium bromide modified graphene oxide as foam stabilizer and Ca2+ as crosslinking agent. The hydrogel exhibited excellent mechanical properties, water retention characteristics, and high breathability comparable to cotton. Moreover, the SA/PAM/CTAB-GO hydrogel presented excellent sensing sensitivity, fast response ability (225 ms), and favorable endurance, which monitored human motions sensitively and realized the accurate, sensitive and reliable detection of human ECG and EMG signals. The hydrogel sensor was applied in human-computer interaction and sensitively controlled robot arms and virtual characters. The design of dual network mechanism of calcium crosslinking and foam porosity method improved the sensing properties, breathability, adhesion, and stability of the hydrogel making it possesses high performance as wearable sensor and has promising application prospect in flexible electronics.

A flexible multifunctional sensor based on in situ reduction of Ag nanoparticles by yam polysaccharides

Developing flexible multifunctional sensors that combine humidity, temperature, and strain sensing properties is a challenge. In this paper, PVA/YPs/H3PO4/AgNPs (PYHA) flexible composite films loaded with Ag nanoparticles (AgNPs) were synthesized through in situ reduction and solution casting using polyvinyl alcohol (PVA), yam polysaccharide (YPs), phosphoric acid (H3PO4), and silver nitrate (AgNO3) as raw materials, which exhibited sensitivity to humidity, temperature, and strain. The prepared PYHA humidity sensor was capable of generating stable electrical signals through adsorption and desorption over the relative humidity (RH) range of 35-95 %. Furthermore, the humidity sensor displayed minimal hysteresis (2.17 % RH) and excellent linearity (R2 = 0.973) during respiratory rate monitoring in different body states. As a temperature sensor, the PYHA sensor was capable of sensing human body temperature, exhibiting strong temperature sensitivity (TCR = -1.058 % °C-1) and maintaining excellent linearity (R2 = 0.994) ranging from 35 to 95 °C in temperature. Moreover, the PYHA flexible strain sensor boasted an extensive strain detection range (1-320 %) and swift response/reply time (0.8/1.1 s), and detected physiological signals due to large movements of body joints and weak changes in facial expressions. Therefore, the designed PYHA multifunctional sensor has a promising use in flexible wearable.

Flame-retardant bamboo fiber-based films for high-performance fire sensors

Natural polymers play a non-negligible role in the development of green and sustainable sensors. However, their poor flame retardancy and deficient thermal stability inevitably restrict their application in sensors for monitoring the fire full-process. In this work, a facile and eco-friendly method for the large-scale preparation of flexible films composed of bamboo fiber (BF), carboxymethyl cellulose (CC), and graphene (GN) by solution casting was put forward. Iron ions and phytic acid (PA) self-assemble on its surface, eventually resulting in a film (referred to as BCGP) that exhibits high strength and high flame retardancy. Specifically, the strength of the film reaches 3.92 MPa, and reliable strength still be ensured even after long-term soaking. Furthermore, the limiting oxygen index of the film is as high as 33 %, the vertical burning test attains the V-0 level. As a sensor, the film promptly triggers an alarm within 3.6 s of contact with flames, and its response remains sensitive even after repeated scorching. The temperature response encompasses a wide range of 50 to 200 °C, and the gauge factor is as high as 1572.

Hydroxypropyl methylcellulose reinforced collagen/PVA composite hydrogel wound dressing with self-adaptive, hemostasis and antibacterial ability for wound healing

Nowadays, hydrogels have been extensively and increasingly applied in the field of wound dressings in terms of their high biocompatibility and adjustable functionality. Bacterial infection can serious hinder wound healing the incomplete contact between hydrogel and wound increased the risk of infection. Thus, hydrogel wound dressings with antibacterial and can self-adapt to wound are urgently desired and still a challenge. Herein, a series of hydroxypropyl methylcellulose (HPMC) reinforced hydrogel wound dressings (CPH hydrogels) based on collagen/polyvinyl alcohol (COL/PVA) were designed and constructed, which with polyhexamethylene biguanide (PHMB) as antibacterial agent and borax as crosslinking agent. The dynamic cross-linking of reversible borate ester bond and hydrogen bonds endowed the CPH hydrogels with high adaptability and dynamic self-healing properties. The hydrogel exhibited good blood and cell biocompatibility, which with hemolysis rate lower than 5.0 % and cell survival rate higher than 90 %. The hydrogel also possessed excellent antibacterial ability for both E. coli and S. aureus by destroying the integrity of bacterial membrane. More importantly, the hydrogel with the formulation of 5 % COL, 5 %/PVA, 4 % borax and 1.75 % HPMC successfully achieved in promoting wound healing and accelerated collagen deposition, which provided a promising candidate to serve as wound dressing for wound healing.

Natural polysaccharides-based smart sensors for health monitoring, diagnosis and rehabilitation: A review

With the rapid growth of multi-level health needs, precise and real-time health sensing systems have become increasingly pivotal in personal health management and disease prevention. Natural polysaccharides demonstrate immense potential in healthcare sensors by leveraging their superior biocompatibility, biodegradability, environmental sustainability, as well as diverse structural designs and surface functionalities. This review begins by introducing a variety of natural polysaccharides, including cellulose, alginates, chitosan, hyaluronic acid, and starch, and analyzing their structural and functional distinctions, which offer extensive possibilities for sensor design and construction. Further, we summarize several principal sensing mechanisms, such as piezoresistivity, piezoelectricity, capacitance, triboelectricity, and hygroelectricity, which provide a theoretical and technological foundation for developing high-performance healthcare sensing devices. Additionally, the review discusses the most recent applications of natural polysaccharide-based sensors in diverse healthcare contexts, including human body motion tracking, respiratory and heartbeat monitoring, electrophysiological signal recording, body temperature variation detection, and biomarker analysis. Finally, prospective development directions are proposed, such as the integration of artificial intelligence for real-time data analysis, the design of multifunctional devices that combine sensing with therapeutic functionalities, and advancements in remote monitoring and smart wearable technologies. This review aims to provide valuable insights into the development of next-generation healthcare sensors and propose novel research directions for personalized medicine and remote health management.

A rewritable and shape memory hydrogel doped with fluorescein-functionalized ZIF-8 for information storage and fluorescent anti-counterfeiting

The emergence of stimuli-responsive fluorescence anti-counterfeiting technology has garnered increasing attention in the era of intelligent internet. Smart fluorescent hydrogels combine the characteristics of luminous materials with the unique structure of hydrogels, offering the potential for dynamic reversible erasing and multi-tiered data encryption. In this work, a fluorescent hydrogel was constructed by zeolitic imidazolate framework-8 loaded with fluorescein and then mixed with polyvinyl alcohol hydrogel, sodium carboxymethyl cellulose and borax, which could be used for image hiding in visible light. The reversible bonds cross-linked fluorescent hydrogel was stretchable and self-healing with a three-dimensional network structure. The hydrogel presented bright green fluorescence under 365 nm UV light, which was quenched by adding copper ions. Meanwhile, the imprint of the hydrogel could be cleared by L-Cysteine and repeatedly recorded information many times. The alkali-induced shape memory capability was further utilized to achieve multi-tiered data encryption by deforming it to a 3D-specific shape through folding. The rewritable and multi-dimensional encrypted hydrogel is expected to improve data security and reduce resource consumption.

Self-healing PVA/Chitosan/MXene triple network hydrogel for strain and temperature sensors

Conductive hydrogels have attracted intensive attention for their promising applications in flexible electronics, sensors, and electronic skins. However, extremely poor adaptability under cold or dry environmental conditions along with inferior repairability seriously hinders the development of hydrogels in wearable electronics. Here, a triple network conductive hydrogel (PBCPA-MXene) was prepared by proportionally mixing polyvinyl alcohol (PVA), borax, chitosan (CS), phytic acid (PA), and MXene. The prepared triple network hydrogels composed of robust chitosan polysaccharide as the first network, tough PVA biopolymer gel as the second network, and MXene nanosheets as the third network. Facilitated by triple networks, multiple hydrogen bonds, and electrostatic interactions of CS and PA, the obtained hydrogels not only exhibited outstanding mechanical properties (tensile strain of ∼1580 %, stress of ∼280 kPa) and electrical properties (∼ 2.72 S/m), but also possessed excellent self-healing, self-adhesion, anti-freezing and anti-drying properties. This work presents a strategy for the development of biopolysaccharide hydrogels for applications in the field of sensors.

A slime-inspired phycocyanin/κ-carrageenan-based hydrogel bandage with ultra-stretchability, self-healing, antioxidative, and antibacterial activity for wound healing

Hydrogels have attracted extensive attention as wound dressing owing to their excellent multifunctionality, flexibility, and biocompatibility. Due to the frequent movement and stretching of skin as well as complex surface of wound, traditional wound dressings have difficulty to adapt to motion and irregular wounds. Furthermore, excessive reactive oxygen species (ROS) and bacterial infection can induce delayed wound healing. To this end, we developed a set of versatile phycocyanin-based dual network hydrogels (PPC hydrogels) with polyvinyl alcohol (PVA) and κ-carrageenan (CRG) as substrate via forming of borate ester bonds, hydrogen bonds, and electrostatic interaction. The PPC hydrogels not only possessed adaptivity, ultra-stretchability (7036.12 %), efficient self-healing and injectability, but also possessed antioxidative and antibacterial capacities conferred by C-phycocyanin (PC) and rhein. Moreover, the hydrogels also exhibited excellent hemostatic ability and high biocompatibility. More remarkably, the PPC-I hydrogel could accelerate wound healing by effect of anti-inflammation (downregulating TNF-α and IL-6) and promoting collagen deposition and angiogenesis (upregulating CD31), which may be utilized as hydrogel bandages and applied to motion and irregular wounds, thereby promising the application prospect of the hydrogels as wound dressing.

Ultrastretchable and highly sensitive ionic conductive hydrogel for environmentally resistant all-in-one human-motion sensors

Conductive hydrogels have been considered ideal candidate materials for fabricating human-motion sensors due to their combination properties of electronic and tissue-like soft nature and the similar functions of human skin with mechanical and sensory properties. However, the perfect integration of multiple functionalities such as environmentally tolerant, stretchable, self-adhesive, self-healing, transparent, high sensitivity, and rapid response in one system (all-in-one) is still a significant challenge. Herein, a novel ionic conductive hydrogel platform with excellent comprehensive performance through multiple dynamic interactions was prepared by employing [BMIm]BF4/glycerol/water ternary solvent system. The dynamic hydrogen bonds, coordination bonds, and electrostatic interaction within the network endows the hydrogel excellent mechanical performance. The synchronous effect of ionic liquids and glycerol realized the high ionic conductivity, transparency, environmentally tolerance, and long-term stability. Sensors based on this hydrogel have a relatively high sensitivity, a fast response time, and a wide linear sensing range in monitoring human movements. It can also serve as electronic skin, like human skin, for touchscreen pen and writing. Thus, the all-in-one hydrogel was concluded to hold considerable promise for constructing the next generation of hydrogel platforms for human-motion sensors.

Development and Applications in Intelligent Sports of Hydrogel-Based Triboelectric Nanogenerators

As an emerging self-powered technology, triboelectric nanogenerators have the characteristics of a simple structure, high conversion efficiency, diverse material selection, and stable output. Hydrogels have the advantages of flexibility, extensibility, and shape adaptability, which means that hydrogel-based triboelectric nanogenerators (H-TENGs) have high flexibility, self-healing abilities, conductivity, and fatigue resistance. They can still operate normally in scenarios involving bending, pressing, stretching, and folding. H-TENGs offer a method of versatile and sustainable innovation in sports monitoring. This review elucidates the working principles and modes of H-TENGs, examines H-TENG characteristics that are relevant to intelligent sports, and summarizes their applications in this field. This paper concludes with a discussion on the development and applications of H-TENGs in intelligent sports.

Dynamic Double-Networked Hydrogels by Hybridizing PVA and Herbal Polysaccharides: Improved Mechanical Properties and Selective Antibacterial Activity

Chinese herbal medicine has offered an enormous source for developing novel bio-soft materials. In this research, the natural polysaccharide isolated from the Chinese herbal medicine Dendrobium was employed as the secondary building block to fabricate a "hybrid" hydrogel with synthetic poly (vinyl alcohol) (PVA) polymers. Thanks to the presence of mannose units that contain cis-diol motifs on the chain of the Dendrobium polysaccharides, efficient crosslinking with the borax is allowed and reversible covalent borate ester bonds are formed. Eventually, highly dynamic and double-networked hydrogels were successfully prepared by the integration of Dendrobium polysaccharides and PVA. Interestingly, the introduction of polysaccharides has given rise to more robust and dynamic hydrogel networks, leading to enhanced thermal stability, mechanical strength, and tensile capacity (>1000%) as well as the rapid self-healing ability (<5 s) of the "hybrid" hydrogels compared with the PVA/borax single networked hydrogel. Moreover, the polysaccharides/PVA double network hydrogel showed selective antibacterial activity towards S. aureus. The reported polysaccharides/PVA double networked hydrogel would provide a scaffold to hybridize bioactive natural polysaccharides and synthetic polymers for developing robust but dynamic multiple networked hydrogels that are tailorable for biomedical applications.

Hydrogel-based bimodal sensors for high-sensitivity independent detection of temperature and strain

Avoiding crosstalk between strain and temperature detection is crucial for bimodal hydrogel sensors, yet achieving high sensitivity for both parameters while maintaining signal decoupling remains a significant challenge. In this study, a bimodal sensor was developed by locally coating poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS) onto the hydrogel surface, creating distinct regions for strain and temperature detection. These regions form localized strain concentration zones and wrinkle structures, respectively. The localized strain concentration enhances the sensor's sensitivity from 8.5 to 18.5. Additionally, the sensor demonstrates a low detection limit (0.2 %), a wide detection range (up to 1356 %), a fast response time, and excellent cyclic stability for strain measurements. The temperature detection region, leveraging the thermoelectric effect, improves the Seebeck coefficient of the PEDOT: PSS coating from 20 to 122.86 μVK-1 through de-doping and energy band modulation. Moreover, the temperature sensing of the PEDOT: PSS coating features good cyclic stability, a rapid response time, and versatile testing capabilities. This innovative structural design effectively decouples strain and temperature signals across a broad strain range (0-600 %). These sensors hold potential applications in human health monitoring and as electronic skin for flexible robotics.

Ultrastretchable, Ultralow Hysteresis, High-Toughness Hydrogel Strain Sensor for Pressure Recognition with Deep Learning

Hydrogel, as a promising material for a wide range of applications, has demonstrated considerable potential for use in flexible wearable devices and engineering technologies. However, simultaneously realizing the ultrastretchability, low hysteresis, and high toughness of hydrogels is still a great challenge. Here, we present a dual physically cross-linked polyacrylamide (PAM)/sodium hyaluronate (HA)/montmorillonite (MMT) hydrogel. The introduction of HA increases the degree of chain entanglement, and the addition of MMT acts as a stress dissipation center and cross-linking agent, resulting in a hydrogel with high toughness and low hysteretic properties. This hydrogel synthesized by a simple strategy exhibited ultrahigh stretchability (3165%), high breaking stress (228 kPa), high toughness (4.149 MJ/m3), and ultralow hysteresis (≈2% at 100% of strain). The fabricated hydrogel flexible strain sensors possessed fast response and recovery times (62.5:75 ms), a wide strain detection range (2000%), a strain detection limit of 1%, and excellent cycling stability over 500 cycles. Furthermore, the hydrogel flexible strain sensor can be used for human motion monitoring, gesture recognition, and pressure recognition assisted by deep learning algorithms, showing enormous promise for applications.

Recent Advances in Natural-Polymer-Based Hydrogels for Body Movement and Biomedical Monitoring

In recent years, the interest in medical monitoring for human health has been rapidly increasing due to widespread concern. Hydrogels are widely used in medical monitoring and other fields due to their excellent mechanical properties, electrical conductivity and adhesion. However, some of the non-degradable materials in hydrogels may cause some environmental damage and resource waste. Therefore, organic renewable natural polymers with excellent properties of biocompatibility, biodegradability, low cost and non-toxicity are expected to serve as an alternative to those non-degradable materials, and also provide a broad application prospect for the development of natural-polymer-based hydrogels as flexible electronic devices. This paper reviews the progress of research on many different types of natural-polymer-based hydrogels such as proteins and polysaccharides. The applications of natural-polymer-based hydrogels in body movement detection and biomedical monitoring are then discussed. Finally, the present challenges and future prospects of natural polymer-based hydrogels are summarized.

Triple-Responsive, Multimodal, Visual Electronic Skin toward All-in-One Health Management for Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) is one of the most common metabolic disorders during pregnancy, leading to serious complications for pregnant women and a threat to life safety of infants. Therefore, it is particularly important to establish a multipurpose monitoring pathway to important physiological indicators of pregnant women. In this work, three kinds of double network hydrogels are prepared with poly(vinyl alcohol) (PVA), borax, and cellulose ethers with varying substituents of methyl (methyl cellulose, MC), hydroxypropyl (hydroxypropyl cellulose, HPC), or both (hydroxypropyl methyl cellulose, HPMC), respectively. The corresponding toughness (143.9, 102.3, and 135.9 kJ cm-3) and conductivity (0.69, 0.45, and 0.51 S m-1) of the hydrogels demonstrate that PB-MC was endowed with the prominent performance. Molecular dynamics simulations further revealed the essence that hydrogen bond interactions between PVA and cellulose ethers play a critical role in regulating the structure and properties of hydrogels. Thermochromic capsule powders (TCPs) were subsequently doped in to achieve a composite hydrogel (TCPs@PB-MC) to indicate the change in human body temperature. Furthermore, the process of the TCPs@PB-MC response to glucose, pH, and temperature was tracked in-depth through the electrochemical window. This work provides a novel strategy for all-in-one health management of GDM.

Tannic acid and silicate-functionalized polyvinyl alcohol-hyaluronic acid hydrogel for infected diabetic wound healing

Healing of chronic diabetic wounds is challenging due to complications of severe inflammatory microenvironment, bacterial infection and poor vascular formation. Herein, a novel injectable polyvinyl alcohol-hyaluronic acid-based composite hydrogel was developed, with tannic acid (TA) and silicate functionalization to fabricate an 'all-in-one' hydrogel PTKH. On one hand, after being locally injected into the wound site, the hydrogel underwent a gradual sol-gel transition in situ, forming an adhesive and protective dressing for the wound. Manipulations of rheological characteristics, mechanical properties and swelling ability of PTKH could be performed via regulating TA and silicate content in hydrogel. On the other hand, PTKH was capable of eliminating reactive oxygen species overexpression, combating infection and generating a cell-favored microenvironment for wound healing acceleration in vitro. Subsequent animal studies demonstrated that PTKH could greatly stimulate angiogenesis and epithelization, accompanied with inflammation and infection risk reduction. Therefore, in consideration of its impressive in vitro and in vivo outcomes, this 'all-in-one' multifunctional hydrogel may hold promise for chronic diabetic wound treatment.

A nanocomposite hydrogel loaded with Ag nanoparticles reduced by aloe vera polysaccharides as an antimicrobial multifunctional sensor

Developing high-performance hydrogels with anti-freeze, and antimicrobial properties is crucial for the practical application of flexible sensors. In this study, we prepared silver nanoparticles (AgNPs) with aloe polysaccharide (AP) as a reducing agent. Then, the AP/AgNPs were added to a system of polyvinyl alcohol and borax crosslinked in water/glycerol to obtain a multifunctional conductive hydrogel. The incorporated AgNPs improved the conductivity (0.39 S/m) and mechanical properties (elongation at break: 732.9 %, fracture strength: 1267.6 kPa) of the hydrogel. In addition, resultant hydrogel exhibited potential for sensing strain, temperature, and humidity. When used as a strain sensor, the hydrogel system exhibited low detection limit (0.1 %), and fast response (0.08 s). The resistance of the hydrogel decreased with an increase in the absorbed moisture content, enabling humidity detection (25-95 %) to monitor breathing status. As a temperature sensor, the hydrogel supported a wide detection range (-50 to +90 °C) and sensitivity (-30-0 °C, temperature coefficient of resistance (TCR) = -5.64 %/°C) to detect changes in the ambient temperature. This study proposes a simple method for manufacturing multifunctional hydrogel sensors, which broadens their application prospects in wearable sensing and electronic products.

Injectable plant-derived polysaccharide hydrogels with intrinsic antioxidant bioactivity accelerate wound healing by promoting epithelialization and angiogenesis

Skin wound healing is a complex and dynamic process involving hemostasis, inflammatory response, cell proliferation and migration, and angiogenesis. Currently used wound dressings remain unsatisfactory in the clinic due to the lack of adjustable mechanical property for injection operation and bioactivity for accelerating wound healing. In this work, an "all-sugar" hydrogel dressing is developed based on dynamic borate bonding network between the hydroxyl groups of okra polysaccharide (OP) and xyloglucan (XG). Benefiting from the reversible crosslinking network, the resulting composite XG/OP hydrogels exhibited good shear-thinning and fast self-healing properties, which is suitable to be injected at wound beds and filled into irregular injured site. Besides, the proposed XG/OP hydrogels showed efficient antioxidant capacity by scavenging DPPH activity of 73.9 %. In vivo experiments demonstrated that XG/OP hydrogels performed hemostasis and accelerated wound healing with reduced inflammation, enhanced collagen deposition and angiogenesis. This plant-derived dynamic hydrogel offers a facile and effective approach for wound management and has great potential for clinical translation in feature.

Silver Nanoparticle-Assisted Electrochemically Exfoliated Graphene Inks Coated on PVA-Based Self-Healing Polymer Composites for Soft Electronics

Smart sensors with self-healing capabilities have recently aroused increasing interest in applications in soft electronics. However, challenges remain in balancing the sensors' self-healing and compatibility between their sensing and substrate layers. This study evaluated several self-healing polymer substrates and graphene ink-based strain-sensing coatings. The optimum electrochemically exfoliated graphene (e-graphene)/silver nanoparticle-coated tannic acid (TA)/superabsorbent polymer/graphene oxide (GO) blended poly(vinyl alcohol) polymer composites exhibited improvements of 47.1 and 39.2%, respectively, for the healing efficiency in a substrate crack area and in the graphene-based sensing layer due to conductive layer adhesion. While TA was found to improve healing efficiency on the coating surface by forming hydrogen bonds between the sensing and polymer layers, GO healed the polymer surface due to its ability to form bonds in the polymer matrix. The superabsorbent polymer was found to absorb excess water in e-graphene dispersion due to its host-guest interaction, while also reducing the coating thickness.

Pullulan-1-Ethyl-3-Methylimidazolium Tetrafluoroborate Composite as a Water-Soluble Active Component of a Vibration Sensor

In recent years, the issue of electronic waste production has gained significant attention. To mitigate the environmental impact of e-waste, one approach under consideration involves the development of biodegradable electronic devices or devices that dissolve in the environment at the end of their life cycle. This study presents results related to the creation of a sensor that effectively addresses both criteria. The device was constructed using a composite material formed by impregnating a pullulan membrane (a biodegradable water-soluble biopolymer) with 1-Ethyl-3-Methylimidazolium tetrafluoroborate (a water-soluble ionic liquid) and coating the product with a conductive silver-based varnish. Capitalizing on the piezoionic effect, the device has demonstrated functionality as a vibration sensor with a sensitivity of approximately 5.5 × 10-5 V/mm and a resolution of about 1 mm. The novelty of this study lies in the unique combination of materials. Unlike the use of piezoelectric materials, this combination allows for the production of a device that does not require an external potential difference generator to function properly as a sensor. Furthermore, the combination of a biopolymer, such as pullulan, and an ionic liquid, both readily soluble in water, in creating an active electronic component represents an innovation in the field of vibration sensors.

In situ reduction of Ag nanoparticles using okra polysaccharides for the preparation of flexible multifunctional sensors

This paper reports the fabrication of flexible films loaded with Ag nanoparticles (Ag NPs) and annotated as POPA films from polyvinyl alcohol, okra polysaccharides, phytic acid, and AgNO3 via an in situ reduction and solution-casting method. The prepared films exhibit strain, temperature, and humidity sensing. As a flexible strain sensor, the POPA sensor has a wide strain sensing range (1-250 %), and fast response/recovery (0.22/0.28 s), while as a temperature sensor, it senses the human body temperature and exhibits excellent temperature sensitivity (TCR = -1.401 % °C-1) and good linearity (R2 = 0.994) in the temperature range of 30-55 °C. Additionally, in the relative humidity (RH) of range 35-95 %, the POPA humidity sensor outputs stable electrical signals during adsorption and desorption. Moreover, it exhibits low hysteresis values (3.19 % RH) and good linearity (R2 = 0.989) for the detection of breathing rates during different human body states. Consequently, the POPA sensor exhibits good stability, repeatability, and reversibility for strain, temperature, and humidity sensing. The designed multifunctional POPA sensor thus holds great potential for its application in flexible wearable devices and electronics.

Highly stretchable anti-freeze hydrogel based on aloe polysaccharides with high ionic conductivity for multifunctional wearable sensors

Conductive hydrogels have limitations such as non-degradability, loss of electrical conductivity at sub-zero temperatures, and single functionality, which limit their applicability as materials for wearable sensors. To overcome these limitations, this study proposes a bio-based hydrogel using aloe polysaccharides as the matrix and degradable polyvinyl alcohol as a reinforcing material. The hydrogel was crosslinked with borax in a glycerol-water binary solvent system, producing good toughness and compressive strength. Furthermore, the hydrogel was developed as a sensor that could detect both small and large deformations with a low detection limit of 1 % and high stretchability of up to 300 %. Moreover, the sensor exhibited excellent frost resistance at temperatures above -50 °C, and the gauge factor of the hydrogel was 2.86 at 20 °C and 2.12 at -20 °C. The Aloe-polysaccharide-based conductive hydrogels also functioned effectively as a wearable sensor; it detected a wide range of humidities (0-98 % relative humidity) and exhibited fast response and recovery times (1.1 and 0.9 s) while detecting normal human breathing. The polysaccharide hydrogel was also temperature sensitive (1.737 % °C-1) and allowed for information sensing during handwriting.

Crosslinking of Epoxidized Natural Rubber with Borax for Self-Healing and Self-Repairing Properties: pH Dependence

Epoxidized natural rubber (ENR) crosslinked using borax, which exhibits self-healing and self-repairing properties, is successfully developed. The crosslink formation of ENR by using borax under neutral and alkaline conditions is investigated. Fourier transform infrared spectroscopy (FTIR) shows that the borate-ester bond is formed in ENR prepared under both neutral and alkaline conditions, whereas boron nuclear magnetic resonance (11 B-NMR) results exhibit that the ENR prepared under alkaline conditions more actively forms crosslink networks with borax. Moreover, the crosslink density and gel content increase significantly with the presence of borax in alkaline conditions. The crosslink density and gel content of ENR with 10 phr borax are higher by 155% and 36%, respectively, than those of neat ENR. Furthermore, the formation of the crosslinking ENR by borax enhances self-healing and self-repairing properties. The healing efficiency significantly increases from 1.09% to 85.90%, when ENR is developed under alkaline conditions with 30 phr borax. These results represent the first successful demonstration of the efficient use of borax as a crosslinker in ENR, which exhibits its promising self-healing and self-repairing properties under atmospheric conditions without the need for external stimuli. The ENR prepared in this work holds great promise for various self-healing rubber applications.

Biodegradable ion-conductive polyvinyl alcohol/okra polysaccharide composite films for fast-response respiratory monitoring sensors

Polyvinyl alcohol (PVA) and okra polysaccharide (OP) are biodegradable polymers with high hydrophilicity and good biocompatibility with potential for use as flexible humidity-sensitive materials. Herein, biodegradable flexible composite films (named POP films) were prepared from PVA, OP, and phytic acid using a solution-casting method based on. POP films exhibited excellent mechanical strength, flexibility, flame retardancy, water resistance, humidity response, and humidity-sensing characteristics. Notably, the POP humidity sensors exhibited a hysteresis value of 1.88 % relative humidity for the adsorption and desorption processes and good sensitivity over a wide humidity range of 35-95 %. In addition, the humidity sensor distinguished the frequency of nose breathing, and its response and recovery times were 0.9 and 1.98 s, respectively. The excellent performance of POP sensors in monitoring humidity and human respiratory rates demonstrates the sensor's potential for wearable smart devices.

Tremella polysaccharide-based conductive hydrogel with anti-freezing and self-healing ability for motion monitoring and intelligent interaction

The application of conductive hydrogels in flexible wearable devices has garnered significant attention. In this study, a self-healing, anti-freezing, and fire-resistant hydrogel strain sensor is successfully synthesized by incorporating sustainable natural biological materials, viz. Tremella polysaccharide and silk fiber, into a polyvinyl alcohol matrix with borax cross-linking. The resulting hydrogel exhibits excellent transparency, thermoplasticity, and remarkable mechanical properties, including a notable elongation (1107.3 %) and high self-healing rate (91.11 %) within 5 min, attributed to the dynamic cross-linking effect of hydrogen bonds and borax. A strain sensor based on the prepared hydrogel sensor can be used to accurately monitor diverse human movements, while maintaining exceptional sensing stability and durability under repeated strain cycles. Additionally, a hydrogel touch component is designed that can successfully interact with intelligent electronic devices, encompassing functions like clicking, writing, and drawing. These inherent advantages make the prepared hydrogel a promising candidate for applications in human health monitoring and intelligent electronic device interaction.

A robust and sensitive flexible strain sensor based on polyurethane cross-linked composite hydrogels for the detection of human motion

Polyurethane cross-linked composite hydrogel and wireless Bluetooth module were assembled for the detection of human motion.