Lignosulfonate sodium and ionic liquid synergistically promote tough hydrogels for intelligent wearable human-machine interaction

Effect of sodium lignosulfonate as an additive in enhancing the corrosion and tribocorrosion behavior of micro-arc oxidation coatings on titanium alloy

A corrosion and wear resistant coating was developed on the surface of titanium alloy using micro-arc oxidation (MAO) technology with addition of lignin sulfonate (SLS) as an additive in electrolytes containing 15 g/L of Na2SiO3·9H2O and 10 g/L of Na3PO4·12H2O. The effects of concentration of SLS on the surface morphology, microstructure, and corrosion-wear performance of the MAO coatings were systematically investigated. Wetting properties and mechanical characteristics of MAO coatings were determined by contact angle measurements, microhardness testing, and bonding strength assessments. The corrosion resistance of the MAO coating in artificial saliva media was assessed through electrochemical testing, while its tribological and corrosion-wear performance was evaluated via dry friction tests and corrosion-wear tests, respectively. Experimental results indicate that the incorporation of lignin sulfonate as the electrolyte additive enhances pore filling within the coating matrix while improving both mechanical properties and corrosion resistance. Notably, MAO coatings with a concentration of 1 g/L exhibited superior mechanical properties; furthermore, their corrosion current density decreased by one to three orders of magnitude compared to that of the TC4 substrate.

Anti-freezing, long-term-usability conductive organo-hydrogels containing lignin for the manufacture of high-performance flexible strain and temperature sensors

The development of an all-in-one hydrogel that possesses the desired characteristics of multiple sensing modalities, exceptional electrical conductivity, and high strain sensing performance remains a significant challenge. Here, we employed calcium ions to crosslink and chelate sodium alginate and sodium lignosulfonate within a polyacrylic acid hydrogel network. Additionally, we introduced a conductive polyaniline network through in situ polymerization and performed a solvent exchange with glycerol to produce a multifunctional conductive organo-hydrogel. The resulting hydrogels exhibited remarkable mechanical properties, with a strength of 268 kPa at a tensile strain of 400 %. They also demonstrated resistance to drying and freezing, remaining unfrozen at low temperatures of -60 °C, as well as temperature sensitivity. The incorporation of polyaniline and Ca2+ ions contributed to enhanced electrical conductivity and sensing capabilities, evidenced by gauge factors of 1.39 and 2.13 in the strain ranges of 0-210 % and 210-400 %, respectively. Furthermore, the temperature sensing properties were characterized by a temperature coefficient of resistance of -4.49 % °C-1 and -0.92 % °C-1 in the temperature ranges of -25 to 25 °C and 25 to 60 °C, respectively. Leveraging these advantageous properties, this hydrogel is anticipated to serve as a real-time motion detection device for human activities in extreme conditions.

Facile fabrication of transparent and stretchable cellulose ionic gel paper for sustainable use in multifunctional sensors and optoelectronic devices

Although transparent paper derived from cellulose has been successfully demonstrated as an inexpensive, renewable and biodegradable substrate used for flexible electronics, the inherently stiff characteristic and intrinsic poor conductivity of the cellulose paper inevitably hinders its application in stretchable electronic devices. Herein, we report a new avenue for construction of highly stretchable, transparent, and ionic conductive cellulose gel paper via glycerol inducing plasticizing and CaCl2 initiating chelating, a facile casting and drying strategy. The renewable carboxymethyl cellulose is employed for its intrinsically abundant carboxyl groups for crosslinking with Ca2+ via ionic coordination bonds, benefiting the improvement of various performances. The resultant cellulose ionic gel paper (CIGP) displays high stretchability (tensile strain 320 % and strength 978 kPa at fracture), and transparency (over 90 % in 400 nm to 780 nm wavelength). In addition, the CIGP also has high ionic conductivity (82.78 mS/m), and displays highly reliable, sensitive and wide range strain sensing abilities to various stimuli. Significantly, the transparent CIGP with excellent sensing performances has been successfully integrated into multifunctional sensors and optoelectronic device, showing broad applications in flexible electronics.

Phytic acid/chitosan-assisted zwitterionic double-network hydrogels with enhanced mechanical properties, adhesion ability and ionic conductivity for wearable strain sensors

Ionic conductive hydrogels have recently attracted tremendous attention in flexible wearable strain sensors. However, achieving a combination of good mechanical properties, strong adhesion to various material surfaces, and remarkable ionic conductivity in a single ionic conductive hydrogel remains a challenge. Herein, new poly(acrylamide-co-sulfobetaine methacrylate)/chitosan/phytic acid (ASCP) ionic conductive hydrogels with double networks were prepared through free radical polymerization. The versatile functional groups from chitosan and phytic acid gave the hydrogels universal adhesion capabilities with a maximum adhesion strength of 18.7 kPa to paper. The obtained ASCP conductive hydrogels exhibited a large elongation of 675 % and a moderate tensile strength 52.8 kPa due to the synergy of chemical cross-linking and physical interactions. Phytic acid as the conductive component conferred the hydrogels with excellent ionic conductivity of 10.3 S m-1. Moreover, the incorporation of chitosan and phytic acid imparted the hydrogels with enhanced anti-drying capability, as evidenced by a residual mass ratio of 58.3 % after 10 days, and exhibited favorable anti-swelling behavior, with an equilibrium swelling ratio of 115 % in water after 4 days. The described ionic conductive hydrogels were assembled into wearable strain sensors to detect various human joint movements. This work offers a straightforward strategy to design multifunctional conductive hydrogels which envision prospective applications in wearable sensors and other flexible electronic devices.

Porous Carbon Nanoparticle Composite Paper Fiber with Laser-Induced Graphene Surface Microstructure for Pressure Sensing

In recent years, flexible pressure sensors have played an increasingly important role in human health monitoring. Inspired by traditional papermaking techniques, we have developed a highly flexible, low-cost, and ecofriendly flexible pressure sensor using shredded paper fibers as the substrate. By combining the properties of laser-induced graphene with the structure of paper fibers, we have improved the internal structure of pressure-sensitive paper and designed a conical surface microstructure, providing new insights into nanomaterial engineering. It features low resistance (424.44 Ω), low energy consumption of only 0.367 μW under a pressure of 1.96 kPa, high sensitivity (1.68 kPa-1), and a wide monitoring range (98 Pa-111.720 kPa). The pressure-sensitive paper with surface microstructure (MFTG) developed in this study has a total thickness comparable to A4 paper, is soft and bendable, can be cut into any shape like paper to fit the human body, and holds great potential for continuous monitoring of human activity status and physiological information.

Novel functional hydrogels based on lignin‑silver nanoparticles with adhesion, antimicrobial, antioxidant and anti-freezing properties for wound dressings and pressure strain sensors

As wound dressings and wearable electronics advance, it is critical to develop an efficacious strategy for integrating a variety of powerful functions into hydrogels. In this work, sodium lignosulfonate‑silver nanoparticles and the functional [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide structure (SBMA) are introduced into the multifunctional lignin-based hydrogel system. The sodium lignosulfonate‑silver nanoparticles, by catalyzing multiple redox reactions, facilitate the swift curing of hydrogels at room temperature. This process is advantageous for the structural refinement of hydrogel polymer segments and the integration of multiple functionalities. The synergistic effect of functional structure and nanoparticles bestows the hydrogel with superior adhesion, mechanical properties, antimicrobial properties and antioxidant properties. The introduction of a functional structure not only deferments the release of sodium lignosulfonate‑silver nanoparticles, but also imparts satisfactory conductivity and anti-freezing properties to the hydrogels. In applications related to wound dressings and pressure strain sensors, hydrogels demonstrate excellent potential. They effectively facilitate wound healing and enable the monitoring of limb movement. This work introduces a simple and effective approach to prepare lignin-based functional hydrogels, exhibiting significant potential for wound dressings and pressure strain sensors applications.

Advances in polysaccharide-based conductive hydrogel for flexible electronics

Polysaccharides, being the most abundant natural polymers, play a pivotal role in the development of hydrogel materials. Polysaccharide-based conductive hydrogels have found extensive applications in flexible electronics due to their excellent conductivity and biocompatibility. This review highlights recent advancements in this area, starting with an overview of polysaccharide materials such as chitosan, cellulose, starch, cyclodextrin, alginate, hyaluronic acid, and agarose. It then explores different classifications of conductive hydrogels: ionic conductive, electronic conductive, and ionic-electronic composite types. The review also covers key characteristics of these hydrogels, including mechanical properties, self-healing, adhesion, structural color, antibacterial, responsiveness, biocompatibility and anti-swelling. Representative applications, such as flexible sensors, triboelectric nanogenerators, supercapacitors, and flexible electronic wound dressings, are summarized. Finally, the review addresses current challenges and provides guidance for future research, aiming to advance the field of polysaccharide-based conductive hydrogels in flexible electronics.

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.

Fishing net-inspired PVA-chitosan-CNT hydrogels with high stretchability, sensitivity, and environmentally stability for textile strain sensors

Soft electronic products are being extensively investigated in diverse applications including sensors and devices, due to their superior softness, responsiveness, and biocompatibility. One-dimensional (1-D) fiber electronic devices are recognized for their lightweight, wearable, and stretchable qualities, thus emerging as critical constituents for seamless integration with the human body and attire, exhibiting great potential in wearable applications. However, wearable conductive hydrogel fibers usually face challenges in combining stretchability and excellent stability, notably in high-temperature environment. Herein, a novel stretchable conductive hydrogel fiber, namely PVA-CS-CNT (Polyvinyl Alcohol-Chitosan-Carbon Nanotube) hydrogel fiber, was successfully prepared through a straightforward low-temperature process. This hydrogel fiber not only maintains stable signal transmission at high temperatures but also exhibits significant mechanical and sensing capabilities, ensuring signal stability during repetitive cyclic stretching. Inspired by fishing net, textile sensors were fabricated by weaving PVA-CS-CNT hydrogel fibers, which offered breathability, high stability (withstanding over 500 stretch cycles), high sensitivity (detecting strains as low as 1 %), and exceptional mechanical strength (exceeding 17 MPa). The wearable sensor could not only accurately monitor human movements like stretching and bending, but also adeptly captured delicate signals such as pulses and sounds. These characteristics demonstrated the potential applications of the hydrogel fibers encompassing human motion tracking, intelligent textiles, and soft robotics.

Lignin sulfonate induced ultrafast fabrication of polypyrrole-based conductive organohydrogel for high performance flexible strain and temperature sensor

The ultrafast preparation of electrically conductive hydrogels to endow high sensing performance and temperature tolerance remains a critical challenge. Herein, lignosulfonate sodium-templated polypyrrole (LS-PPy) nanofillers were rapidly introduced into polyacrylic acid (PAA) hydrogel through ultrafast free radical polymerization in a glycerol/water binary solvent system. The resultant LS-PPy/PAA electrically conductive organohydrogel possesses satisfactory mechanical performance (strength of 56 kPa at a tensile strain of 800 %), strong adhesion, and a desirable low freezing point (-35 °C). Furthermore, this organohydrogel exhibits high strain sensitivity (gauge factor = 2.65), fast response time (~160 ms), low signal hysteresis, and excellent cyclic stability (over 1200 cycles). And the wearable LS-PPy/PAA organohydrogel sensor could accurately and real-time monitor various intense or subtle human movements, such as joint bending, facial expression and hand writing. Besides, the developed LS-PPy/PAA temperature sensor can respond to environmental temperature variations over a wide range of -20-100 °C. High resolution of 0.5 °C with remarkable sensitivity (-0.80 %/°C and linearity of R2 = 0.99) and repeatability were achieved within 36.5-40 °C, which makes it suitable for human body temperature monitoring. All these results demonstrate the substantial prospective value of the LS-PPy/PAA hydrogel in wearable sensors and other associated fields.

Highly stretchable, self-healing, antibacterial, conductive, and amylopectin-enhanced hydrogels with gallium droplets loading as strain sensors

In this study, we address the challenge of developing highly conductive hydrogels with enhanced stretchability for use in wearable sensors, which are critical for the precise detection of human motion and subtle physiological strains. Our novel approach utilizes amylopectin, a biopolymer, for the uniform integration of liquid metal gallium into the hydrogel matrix. This integration results in a conductive hydrogel characterized by remarkable elasticity (up to 7100 % extensibility) and superior electrical conductance (Gauge Factor = 31.4), coupled with a minimal detection limit of less than 0.1 % and exceptional durability over 5000 cycles. The hydrogel demonstrates significant antibacterial activity, inhibiting microbial growth in moist environments, thus enhancing its applicability in medical settings. Employing a synthesis process that involves ambient condition polymerization of acrylic acid, facilitated by a hydrophobic associative framework, this hydrogel stands out for its rapid gelation and robust mechanical properties. The potential applications of this hydrogel extend beyond wearable sensors, promising advancements in human-computer interaction through technologies like wireless actuation of robotic systems. This study not only introduces a viable material for current wearable technologies but also sets a foundation for future innovations in bio-compatible sensors and interactive devices.

Conductive, sensitivity, flexibility, anti-freezing and anti-drying silica/carbon nanotubes/sodium ions modified sodium alginate hydrogels for wearable strain sensing applications

The biocompatibility and salient gelling feature of alginate via forming the interpenetrating network structure has received extensive interests for different applications. Traditional alginate hydrogels freeze at low temperature and evaporate easily at room temperature, leading to reduced performance. Consequently, it is crucial to develop methods to prevent alginate hydrogel from freezing at subzero temperature and dehydration at normal temperature to maintain the performance stability. Utilizing polyacrylic acid, sodium alginate, and acrylamide-hydroxyethyl methacrylate copolymers as flexible matrix materials, this study develops a wearable silica (SiO2)/carbon nanotubes (CNT)/sodium ions (SiO2/CNT/Na+) modified sodium alginate hydrogel strain sensor characterized by high sensitivity, flexibility, and anti-freezing and anti-drying properties. The hydrogel doped with NaCl (50 mg), CNT (10 mg) and M-SiO2 (200 mg) shows excellent mechanical and electrical properties, the tensile strength is 436 KPa, the break elongation is 426 %, the elastic modulus is 99 KPa, and the toughness is 897 kJ/m3. The modified sodium alginate hydrogel used as strain sensor shows fast response time (∼100 ms), high sensitivity factor and excellent stability. The strain sensor exhibits excellent flexibility, ductility, self-adhesion, anti-freezing and anti-drying properties, significantly enhancing its strain sensing application field.

Tough, self-healing, adhesive double network conductive hydrogel based on gelatin-polyacrylamide covalently bridged by oxidized sodium alginate for durable wearable sensors

Pursuing high-performance conductive hydrogels is still hot topic in development of advanced flexible wearable devices. Herein, a tough, self-healing, adhesive double network (DN) conductive hydrogel (named as OSA-(Gelatin/PAM)-Ca, O-(G/P)-Ca) was prepared by bridging gelatin and polyacrylamide network with functionalized polysaccharide (oxidized sodium alginate, OSA) through Schiff base reaction. Thanks to the presence of multiple interactions (Schiff base bond, hydrogen bond, and metal coordination) within the network, the prepared hydrogel showed outstanding mechanical properties (tensile strain of 2800 % and stress of 630 kPa), high conductivity (0.72 S/m), repeatable adhesion performance and excellent self-healing ability (83.6 %/79.0 % of the original tensile strain/stress after self-healing). Moreover, the hydrogel-based sensor exhibited high strain sensitivity (GF = 3.66) and fast response time (<0.5 s), which can be used to monitor a wide range of human physiological signals. Based on this, excellent compression sensitivity (GF = 0.41 kPa-1 in the range of 90-120 kPa), a three-dimensional (3D) array of flexible sensor was designed to monitor the intensity of pressure and spatial force distribution. In addition, a gel-based wearable sensor was accurately classified and recognized ten types of gestures, achieving an accuracy rate of >96.33 % both before and after self-healing under three machine learning models (the decision tree, SVM, and KNN). This paper provides a simple method to prepare tough and self-healing conductive hydrogel as flexible multifunctional sensor devices for versatile applications in fields such as healthcare monitoring, human-computer interaction, and artificial intelligence.

Highly conductive, rapid self-healing, and anti-freezing poly(3,4-ethylenedioxythiophene)/lignosulfonate-cationic guar gum ionogels for multifunctional sensors

Soft ionic conductors exhibit immense potential for applications in soft ionotronics, including ionic skin, human-machine interface, and soft luminescent device. Nevertheless, the majority of ionogel-based soft ionic conductors are plagued by issues such as freezing, evaporation, liquid leakage, and inadequate self-healing capabilities, thereby constraining their usability in complex environments. In this study, we present a novel strategy for fabricating conductive ionogels through the proportionally mixing cationic guar gum (CGG), water, 1-butyl-3-methylimidazolium chloride (BmimCl)/glycerol eutectic-based ionic liquid, and poly(3,4-ethylenedioxythiophene)/lignosulfonate (PEDOT/LS). The resultant benefits from strong hydrogen bonding and electrostatic interactions among its constituents, endowing it with an ultrafast self-healing capability (merely 30 s) while sustaining high electrical conductivity (~16.5 mS cm-1). Moreover, it demonstrates exceptional water retention (62 % over 10 days), wide temperature tolerance (-20 to 60 °C), and injectability. A wearable sensor fabricated from this ionogel displayed remarkable sensitivity (gauge factor = 17.75) and a rapid response to variations in strain, pressure, and temperature, coupled with both long-term stability and wide working temperature range. These attributes underscore its potential for applications in healthcare devices and flexible electronics.

Multifunctional sodium lignosulfonate/xanthan gum/sodium alginate/polyacrylamide ionic hydrogels composite as a high-performance wearable strain sensor

Recently, conductive hydrogels have shown great promise in flexible electronics and are ideal materials for the preparation of wearable strain sensors. However, developing a simple method to produce conductive hydrogels with excellent mechanical properties, self-adhesion, transparency, anti-freezing, and UV resistance remains a significant challenge. A novel sodium lignosulfonate/xanthan gum/sodium alginate/polyacrylamide/Zn2+/DMSO (SLS/XG/SA/PAM/Zn2+/DMSO) ionic conductive hydrogel was developed using a one-pot method. The resulting ionic conductive hydrogels have excellent mechanical properties (stress: 0.13 MPa, strain: 1629 %), high anti-fatigue properties, self-adhesion properties (iron: 7.37 kPa, pigskin: 4.74 kPa), anti-freezing (freezing point: -33.49 °C) and UV resistance by constructing a chemical and physical hybrid cross-linking network. In particular, the conductivity of G hydrogel reached 6.02 S/m at room temperature and 5.52 S/m at -20 °C. Thus, the hydrogel was assembled into a flexible sensor that could distinguish a variety of large and small scales human movements, such as joint bending, swallowing and speaking in real time with high stability and sensitivity. Moreover, the hydrogel could be used as electronic skin just like human skin and touch screen pen to write.

Ionic conductive konjac glucomannan/liquid crystal cellulose composite hydrogels with dual sensing of photo- and electro-signals capacities as wearable strain sensors

The ionic conductive hydrogel-based sensor exhibits wide applications in wearable electronic devices. However, the strength and ductility trade-off, multimodal requirements, and water-soluble polymer alternatives are significant challenges for the hydrogel-based sensor. Herein, a stretchable and conductive hydrogel is developed with a double network formed by incorporating polyacrylamide and ionic liquid into the konjac glucomannan network. The hydrogel displays significantly enhanced mechanical properties, and good tear/puncture resistance owing to the existence of covalent and non-covalent interactions. In addition, by the introduction of nematic liquid crystal hydroxypropyl cellulose, the hydrogel/cellulose-based strain sensor demonstrates excellent sensing performance in monitoring human motions and writing recognition ability with optical and electrical bimodal sensing response. This work provides new insights to further expand the options of hydrogel-based sensor matrix and to construct bimodal sensors.