Gradient Diffusion Anisotropic Carboxymethyl Cellulose Hydrogels for Strain Sensors

High-performance conductive double-network hydrogel base on sodium carboxymethyl cellulose for multifunctional wearable sensors

Sodium carboxymethyl cellulose showed great potential in wearable intelligent electronic devices due to its low price and good biocompatibility. This research aimed to develop a novel conductive hydrogel with stretchable, self-healing, self-adhesive, antibacterial, 3D printable properties, for the development of multifunctional flexible electronic materials based on sodium carboxymethyl cellulose. A multifunctional conductive hydrogel based on sodium carboxymethyl cellulose (SCMC) was synthesized by simple polymerization of SCMC, acrylic acid (AA) and alkaline calcium bentonite (AC-Bt). The multifunctional hydrogels (PAA-SCMC) possess excellent mechanical property (stress: 0.25 MPa; strain: 1675.0 %), Young's modulus (75.6 kPa), and conductivity (2.25 S/m). The multifunctional PAA-SCMC hydrogels serve as strain sensors (Gauge Factor (GF) = 12.68), temperature sensors (temperature coefficient of resistance (TCR) = -0.887 % °C at 20 °C-60 °C), sweat sensors, and pressure sensors. Importantly, the obtained hydrogels exhibited exceptional self-healing capability, self-adhesive properties, antimicrobial properties and 3D printability. The printed hydrogel has good mechanical properties, conductivity and antibacterial properties. Moreover, the hydrogel sensor possessed prominent sensitivity and cyclic stability to accurately monitor human motion, emotional changes, physiological signals in real time, and a hydrogel-based flexible touch keyboard was also fabricated to recognize writing trajectories. Overall, this study provided novel insights into the simple and efficient synthesis and sustainable manufacturing of environmentally friendly multifunctional flexible electronic skin sensors.

Fungal Chitin Nanofibrils Improve Mechanical Performance and UV-Light Resistance in Carboxymethylcellulose and Polyvinylpyrrolidone Films

Materials from renewable carbon feedstock can limit our dependence on fossil carbon and facilitate the transition from linear carbon-intensive economies to sustainable, circular economies. Chitin nanofibrils (ChNFs) isolated from white mushrooms offer remarkable environmental benefits over conventional crustacean-derived nanochitin. Herein, ChNFs are utilized to reinforce polymers of natural and fossil origin, carboxymethyl cellulose (CMC) and polyvinylpyrrolidone (PVP), respectively. Incorporation of 5 wt % ChNFs increases the Young's modulus from 1217 ± 11 to 1509 ± 22 MPa for PVP and from 1979 ± 48 to 2216 ± 102 MPa for CMC. ChNFs increase surface hydrophobicity and retard the scission of the C-H bond as a result of UV-light irradiation in both polymers under investigation. The yellowing from chain scission is reduced, while long-lasting retention of ductility is ensured. Given these results, we propose the utilization of ChNFs in sustainable polymeric materials from renewable carbon with competitive performance against fossil-based benchmark plastics.

Current status of research on polysaccharide-based functional gradient gel materials: A review

Functional gradient materials with material property anisotropy are one of the hotspots of current new material research. The gradient change of material properties comes from the change of the content of one or more components in the material, which is closely related to the preparation process of the material. Meanwhile, polysaccharide materials, as an environmentally friendly and green material, have attracted extensive attention from researchers. This paper focuses on the preparation process of functional gradient gel materials based on polysaccharides, analyzes the laws affecting the distribution of substances during the molding process from the basic principles of material molding, and clarifies the advantages and disadvantages of various methods, so as to promote the innovation of the theory of the preparation method of functional gradient gel materials. At the same time, the specific applications that can be realized by the gradient materials are introduced and compared with the traditional homogeneous materials to elucidate the enhancement of the usage properties brought by their unique gradient structure or properties, which will play a certain role as a reference for the direction of the application of the subsequent materials.

Surface Immobilization of Oxidized Carboxymethyl Cellulose on Polyurethane for Sustained Drug Delivery

Polyurethane (PU) has a diverse array of customized physical, chemical, mechanical, and structural characteristics, rendering it a superb option for biomedical applications. The current study involves modifying the polyurethane surface by the process of aminolysis (aminolyzed polyurethane; PU-A), followed by covalently immobilizing Carboxymethyl cellulose (CMC) polymer utilizing Schiff base chemistry. Oxidation of CMC periodically leads to the creation of dialdehyde groups along the CMC chain. When the aldehyde groups on the OCMC contact the amine group on a modified PU surface, they form an imine bond. Scanning electron microscopy (SEM), contact angle, and X-ray photoelectron spectroscopy (XPS) techniques are employed to analyze and confirm the immobilization of OCMC on aminolyzed PU film (PU-O). The OCMC gel incorporates Nitrofurantoin (NF) and immobilizes it on the PU surface (PU-ON), creating an antibacterial PU surface. The confirmation of medication incorporation is achieved using EDX analysis. The varying doses of NF have demonstrated concentration-dependent bacteriostatic and bactericidal effects on both Gram-positive and Gram-negative bacteria, in addition to sustained release. The proposed polyurethane (PU-ON) surface exhibited excellent infection resistance in in vivo testing. The material exhibited biocompatibility and is well-suited for biomedical applications.

Preparation of Gradient HEA-DAC/HPA Hydrogels by Limited Domain Swelling Method

Hydrogels are widely used in biological dressing, tissue scaffolding, drug delivery, sensors, and other promising applications owing to their water-rich soft structures, biocompatibility, and adjustable mechanical properties. However, most of the conventional hydrogels are isotropic. The anisotropic structures existed widely in the organizational structure of plants and animals, which played a crucial role in biological systems. In this work, a method of limited domain swelling to prepare anisotropic hydrogels is proposed. Through spatially controlled swelling, the extension direction of hydrogels can be limited by a tailored mold, further achieving anisotropic hydrogels with concentration gradients. The external solution serves as a swelling solution to promote swelling and extension of the hydrogel matrix in a mold which can control the extension direction. Due to the diversity of external solutions, the method can be applied to prepare a variety of stimulus-responsive polymers. The limited domain swelling method is promising for the construction of anisotropic hydrogels with different structures and properties.

Recyclable and mechanically tough nanocellulose reinforced natural rubber composite conductive elastomers for flexible multifunctional sensor

The development of flexible wearable multifunctional electronics has gained great attention in the field of human motion monitoring. However, developing mechanically tough, highly stretchable, and recyclable composite conductive materials for application in multifunctional sensors remained great challenges. In this work, a mechanically tough, highly stretchable, and recyclable composite conductive elastomer with the dynamic physical-chemical dual-crosslinking network was fabricated by the combination of multiple hydrogen bonds and dynamic ester bonds. To prepare the proposed composite elastomers, the polyaniline-modified carboxylate cellulose nanocrystals (C-CNC@PANI) were used as both conductive filler to yield high conductivity of 15.08 mS/m, and mechanical reinforcement to construct the dynamic dual-crosslinking network with epoxidized natural rubber latex to realize the high mechanical strength (8.65 MPa) and toughness (29.57 MJ/m3). Meanwhile, the construction of dynamic dual-crosslinking network endowed the elastomer with satisfactory recyclability. Based on these features, the composite conductive elastomers were used as strain sensors, and electrode material for assembling flexible and recyclable self-powered sensors for monitoring human motions. Importantly, the composite conductive elastomers maintained reliable sensing and energy harvesting performance even after multiple recycling process. This study provides a new strategy for the preparation of recyclable, mechanically tough composite conductive materials for wearable sensors.

Structural, rheological properties and antioxidant activities analysis of the exopolysaccharide produced by Rhizobium leguminosarum bv. viciae VF39

Microbial exopolysaccharide is an eco-friendly and non-toxic biopolymeric materials widely used in various industrial fields such as pharmaceutical, food and cosmetics based on its structural, rheological and physiochemical properties. A microbial exopolysaccharide (VF39-EPS) was directly isolated from Rhizobium leguminosarum bv. viciae VF39. Structural analysis using FTIR and 2D NMR spectroscopy confirmed the complete chemical structures of VF39-EPS as 3-hydroxybutanoylglycan with octasaccharide repeating units containing two pyruvyl, two acetyl, and one 3-hydroxybutanoyl group. VF39-EPS exhibited thermal stability up to 275 °C and showed characteristic rheological behaviors of structural fluid with weak gel-like properties above 4 % the aqueous solution, suggesting VF39-EPS as a potential effective thickener or hydrogel scaffolder. Flow behavior tests validated broad stability at a wide range of both pHs from 2 to 12 and temperatures from 25 to 75 °C, and even in the presence of various salts. Furthermore, VF39-EPS showed excellent antioxidant effects of 78.5 and 62.4 % (n = 3, p < 0.001) in DPPH scavenging activity and hydroxyl radical scavenging activity, respectively. Therefore, those structural, rheological and antioxidant properties suggest that VF39-EPS could be one of the excellent biomaterial candidates for cosmetic, food and pharmaceutical industries based on its characteristic rheological behaviors in various condition and excellent antioxidant activity.

Muscle-inspired anisotropic carboxymethyl cellulose-based double-network conductive hydrogels for flexible strain sensors

Conductive hydrogels are considered one of the most promising materials for preparing flexible sensors due to their flexible and extensible properties. However, conventional hydrogels' weak mechanical and isotropic properties are greatly limited in practical applications. Here, the internal structure of the hydrogel was regulated by pre-stretching synergistic ion crosslinking to construct a carboxymethyl cellulose-based double network-oriented hydrogel similar to muscle. The introduction of pre-stretching increased the tensile strength of the double-network hydrogel from 1.45 MPa to 4.32 MPa, and its light transmittance increased from 67.3 % to 84.5 %. In addition, the hydrogel's thermal stability and electrical conductivity were improved to a certain extent. Its good mechanical properties and conductive properties can be converted into stable electrical signal output during deformation. The carboxymethyl cellulose-based double network oriented hydrogels were further assembled as flexible substrates into flexible sensor devices. The hydrogel sensors can monitor simple joint movements as well as complex spatial movements, which makes them have potential application value in the research field of intelligent response electronic devices such as flexible wearables, intelligent strain sensing, and soft robots.

Tendon-Inspired Anisotropic Hydrogels with Excellent Mechanical Properties for Strain Sensors

Anisotropic conductive hydrogels mimicking the natural tissues with high mechanical properties and intelligent sensing have played an important role in the field of flexible electronic devices. Herein, tensile remodeling, drying, and subsequent ion cross-linking methods were used to construct anisotropic hydrogels, which were inspired by the orientation and functionality of tendons. Due to the anisotropic arrangement of the polymer network, the mechanical performance and electrical conductivity were greatly improved in specific directions. The tensile stress and elastic modulus of the hydrogel along the network orientation were 29.82 and 28.53 MPa, which were higher than those along the vertical orientation, 9.63 and 11.7 MPa, respectively. Moreover, the hydrogels exhibited structure-dependent anisotropic sensing. The gauge factors (GFs) parallel to the prestretching direction were greater than the GF along the vertical direction. Thus, the tendon-inspired conductive hydrogels with anisotropy could be used as flexible sensors for joint motion detection and voice recognition. The anisotropic hydrogel-based sensors are highly expected to promote the great development of emerging soft electronics and medical detection.

High-stretchable, self-healing, self-adhesive, self-extinguishing, low-temperature tolerant starch-based gel and its application in stimuli-responsiveness

Starch with active hydroxyl groups is one of the most attractive carbohydrates for the preparation of gels in recent years. However, the mechanical properties, self-healing properties, self-adhesion properties, especially low-temperature resistance are generally unsatisfactory for current starch-based gels. Based on that, a multiple network structure of amylopectin-carboxymethyl cellulose-polyacrylamide (ACP) gel was prepared by a "cooking" method. Tannic acid (TA) was used to construct multiple hydrogen bonds among molecular chains. ACP gel demonstrates high elongation at break (1090 %) and strength, self-healing performance and adhesion behavior, extraordinary low-temperature resistance (-80 °C) and self-extinguishing. As a sensor device, ACP gel can effectively monitor human movements and microscopic expression changes and achieve real-time monitoring under harsh conditions (After multiple cutting-healing steps, under low-temperature conditions, even a month later). Additionally, ACP gel could be served to detect temperature changes with a wide operating range and a high sensitivity of 33 %·°C^-1, which is promising to monitor the changes in temperature. More interestingly, ACP gel can even monitor the cooking process and breathing frequency with fast response, implying applications in food processing, disease diagnosis and medical treatment. This study provides new opportunities for the design and fabrication of carbohydrate-based gels with multiple performance and multifunctional electronic devices.

Recent Development of Self-Powered Tactile Sensors Based on Ionic Hydrogels

Hydrogels are three-dimensional polymer networks with excellent flexibility. In recent years, ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications. In this paper, we introduce the basic properties of ionic hydrogels and highlight their application in self-powered sensors working in triboelectric, piezoionic, ionic diode, battery, and thermoelectric modes. We also summarize the current difficulty and prospect the future development of ionic hydrogel self-powered sensors.

Preparation of carbon dots-embedded fluorescent carboxymethyl cellulose hydrogel for anticounterfeiting applications

Fluorescent inks have been emerged as a desirable encoding technique to enhance anticounterfeiting printing of commercial goods. However, significant drawbacks with fluorescent inks, such as poor durability, low efficiency, and high cost. Herein, we describe the preparation of a self-healing authentication ink based on carboxymethyl cellulose (CMC) hydrogel immobilized with nitrogen-doped carbon dots (NCD) nanoparticles (NPs) for cutting-edge anticounterfeiting applications. Security inks that self-heal are very durable. Under ambient conditions, the prepared NCD@CMC hydrogel could self-heal with a high healing efficiency. It might stick to diverse surfaces such as plastic, glass and paper sheets. The self-healing composite ink demonstrated outstanding photostability under UV light. Straightforward and environmentally friendly method was applied on the agricultural waste of rice straw toward the production of NCD using hydrothermal carbonization in an aqueous medium, and in the presence of NH₄OH as an inexpensive passivating agent. The quantum yield (QY) for NCD reached 24.09 %. Various concentrations of NCD NPs were employed to produce self-healable nanocomposite inks with a variety of emission properties. Stamping homogeneous films onto paper surfaces produced a transparent layer. The CIE Lab and emission spectra of prints independently verified the capability of NCD nanocomposite inks to vary their color to blue under UV illumination. To measure the particle diameter of the prepared NCD, their morphological characteristics were examined by transmission electron microscopy (TEM) to indicate diameters of 10-25 nm. Utilizing various analytical techniques, the morphology and chemical composition of the fluorescent prints were examined. We examined the mechanical qualities of the stamped papers as well as the rheological characteristics of the ink hydrogel. Due to their colorless appearance, the excitation band of the printed films was peaked at 364 nm, while their emission was peaked at 465 nm. The current smart ink holds high potential for numerous applications like smart packaging and authentication, and shows great promise as a practical and mass production approach for easily creating anticounterfeiting stamps.

Construction of strong and tough carboxymethyl cellulose-based oriented hydrogels by phase separation

Anisotropic hydrogels have attracted extensive attention because they are similar to natural hydrogel-like materials and exhibit superiority and new functions that isotropic hydrogels cannot. Here, we fabricated strong and tough carboxymethyl cellulose-based conductive hydrogels with oriented hierarchical structures through pre-stretching, solvent displacement induced phase separation, and subsequent ionic crosslinking immobilization. Solvent displacement made the pre-stretched carboxymethyl cellulose-based polymer network more dense and linear, while the toughness of the hydrogel was further improved under the effect of phase separation. Strong and tough hydrogels were prepared by combining pre-stretching and phase separation; the variation range (tensile strength of 2.24-6.19 MPa and toughness of 19.41-22.92 MJ/m³) can be adjusted by the stretching ratio. Compared with traditional carboxymethyl cellulose-based hydrogels, the tensile strength and toughness were increased by 49 times and 15 times, respectively. In addition, the hydrogels had good underwater stability, ion cross-linking made the hydrogels have good conductivity, and the directional stratification structure gave the hydrogels conductive anisotropy. These characteristics give hydrogel sensors broad application prospects in flexible wearable devices, anisotropic sensors, and intelligent underwater devices.

A Polyvinyl Alcohol-Tannic Acid Gel with Exceptional Mechanical Properties and Ultraviolet Resistance

Design and preparation of gels with excellent mechanical properties has garnered wide interest at present. In this paper, preparation of polyvinyl alcohol (PVA)-tannic acid (TA) gels with exceptional properties is documented. The crystallization zone and hydrogen bonding acted as physical crosslinkages fabricated by a combination of freeze-thaw treatment and a tannic acid compound. The effect of tannic acid on mechanical properties of prepared PVA-TA gels was investigated and analyzed. When the mass fraction of PVA was 20.0 wt% and soaking time was 12 h in tannic acid aqueous solution, tensile strength and the elongation at break of PVA-TA gel reached 5.97 MPa and 1450%, respectively. This PVA-TA gel was far superior to a pure 20.0 wt% PVA hydrogel treated only with the freeze-thaw process, as well as most previously reported PVA-TA gels. The toughness of a PVA-TA gel is about 14 times that of a pure PVA gel. In addition, transparent PVA-TA gels can effectively prevent ultraviolet-light-induced degradation. This study provides a novel strategy and reference for design and preparation of high-performance gels that are promising for practical application.

Ultrastretchable, Antifreezing, and High-Performance Strain Sensor Based on a Muscle-Inspired Anisotropic Conductive Hydrogel for Human Motion Monitoring and Wireless Transmission

Integrating structural anisotropy, excellent mechanical properties, and superior sensing capability into conductive hydrogels is of great importance to wearable flexible electronics yet challenging. Herein, inspired from the aligned structure of human muscle, we proposed a facile and universal method to construct an anisotropic hydrogel composed of polyacrylamide and sodium alginate by pre-stretching in a confined geometry and subsequent ionic cross-linking. The designed hydrogels showed extraordinary mechanical performances, such as ultrahigh stretchability, a comparable modulus to that of human tissues, and good toughness, ascribed to their anisotropically aligned polymer networks. Additionally, the hydrogel possessed anisotropic conductivity due to the anisotropy in ion transport channels. The hydrogel along the vertical direction was further cut and assembled into a flexible strain sensor, exhibiting a low detection limit (0.1%), wide strain range (1585%), rapid response (123 ms), distinct resilience, good stability, and repeatability, thereby being capable of monitoring and discriminating different human movements. In addition, the relatively high ionic conductivity and superior sensitivity enabled the anisotropic hydrogel sensor to be used for wireless human-machine interaction. More interestingly, the Ca²⁺-cross-linking strategy also endowed the hydrogel sensor with antifreezing ability, further broadening their working temperature. This work is expected to speed up the development of hydrogel sensors in the emerging wearable soft electronics.

Synthesis and Applications of Carboxymethyl Cellulose Hydrogels

Hydrogels are basic materials widely used in various fields, especially in biological engineering and medical imaging. Hydrogels consist of a hydrophilic three-dimensional polymer network that rapidly expands in water and can hold a large volume of water in its swelling state without dissolving. These characteristics have rendered hydrogels the material of choice in drug delivery applications. In particular, carboxymethyl cellulose (CMC) hydrogels have attracted considerable research attention for the development of safe drug delivery carriers because of their non-toxicity, good biodegradability, good biocompatibility and low immunogenicity. Aiming to inspire future research in this field, this review focuses on the current preparation methods and applications of CMC gels and highlights future lines of research for the further development of diverse applications.

Advances in Cellulose-Based Hydrogels for Biomedical Engineering: A Review Summary

In recent years, hydrogel-based research in biomedical engineering has attracted more attention. Cellulose-based hydrogels have become a research hotspot in the field of functional materials because of their outstanding characteristics such as excellent flexibility, stimulus-response, biocompatibility, and degradability. In addition, cellulose-based hydrogel materials exhibit excellent mechanical properties and designable functions through different preparation methods and structure designs, demonstrating huge development potential. In this review, we have systematically summarized sources and types of cellulose and the formation mechanism of the hydrogel. We have reviewed and discussed the recent progress in the development of cellulose-based hydrogels and introduced their applications such as ionic conduction, thermal insulation, and drug delivery. Also, we analyzed and highlighted the trends and opportunities for the further development of cellulose-based hydrogels as emerging materials in the future.