Hydrogen-Bonding Assembly of Rigid-Rod Poly(p-sulfophenylene terephthalamide) and Flexible-Chain Poly(vinyl alcohol) for Transparent, Strong, and Tough Molecular Composites

A strong, excellent water resistance, and anti-ultraviolet poly(vinyl alcohol)/lignocellulose/poly(butylene adipate-co-terephthalate) composite with "sandwich" structure

Renewable materials derived from lignocellulose are promising materials for different practical applications, including biomedical, food, and agricultural fields. However, the poor mechanical performance and wet stability restrict their applications. Herein, we fabricated PVA/lignocellulose/PBAT (PVA/CLM/PBAT) composite with a "sandwich" structure. Cellulose-lignin mixture (CLM) was prepared by deep eutectic solvent (DES) pretreatment of corn straw. It served as a reinforcing scaffold of Poly (Vinyl Alcohol) (PVA) film for improving mechanical properties, UV blocking and decrease water permeability. Poly (butylene adipate-co-terephthalate) (PBAT) was used as "protector" with PVA/CLM composites by solvent coating method. The tensile strength of PVA/CLM/PBAT is 63.92 MPa. The results show that 27.93 MPa of tensile strength and 179.98 % of elongation at break of PVA/CLM/PBAT are achieved after immersing in water for 2 h, which was remarkable higher than that of PVA/CLM without PBAT coating (4.78 MPa and 111.43 %, respectively). The bonding strength of PVA/CLM/PBAT is 2.44 ± 0.06 N/15 mm. The hydrogen bonds, π-π interaction between PBAT, PVA, and CLM enhance the bonding strength of PVA/CLM/PBAT composites. The water absorption rate of PVA/CLM/PBAT is only 4.4 % after 20 h, which showed a remarkable water resistance. 25 wt% CLM content in PVA/CLM/PBAT composite can fully shield ultraviolet at 200-800 nm, indicating that the remarkable UV shielding capability.

Design and in vitro/in vivo evaluation of chitosan-polyvinyl alcohol copolymer material cross-linked by dynamic borate ester covalent for pregabalin film-forming delivery system

This research introduced a novel polymer synthesized by combining chitosan and modified polyvinyl alcohol, cross-linked with boric acid using dynamic covalent bonds. The polymer was developed to formulate a pregabalin Film-forming system (FFS) for treating postherpetic neuralgia via topical application, showcasing notable skin adhesion and drug delivery properites. The chitosan-boric acid-modified polyvinyl alcohol polymer was analyzed using NMR, FTIR. The exceptional features of the optimized FFS were evaluated through rheometer, Differential scanning calorimetry (Tg = 45.98 °C), contact angle (θ = 78.62°). The elongation (60.05 ± 3.67 %), cohesion (56.94 ± 4.65 MPa) and skin adhesion (58.12 ± 2.99 kPa) of chitosan-boric acid-modified polyvinyl alcohol were found to be 5.2, 6.8, and 8.3 times higher than those of the pure chitosan film, attributed to the double network structure formed by the cross-linked reversible dynamic covalent bond. The optimized pregabalin FFS exhibited increased in vitro (86.25 ± 1.87 μg/g) and in vivo (100.42 ± 7.44 μg/g) skin retention amounts compared to in vivo oral administration (28.43 ± 4.61 μg/g). In summary, the utilization of borate ester dynamic covalent bonds in developing chitosan-based film-forming polymer proved beneficial in improving skin adhesion and topical therapeutic effectiveness, thereby mitigating the risk of systemic side effects associated with oral administration.

High-strength fibrous sensors with an enhanced aggregate state for biomechanical monitoring of the Achilles tendon

Continuous monitoring of biomechanical signals generated from the injured Achilles tendon is essential for the deep understanding of the recovery or rehabilitation process, thus decreasing the risk of secondary injuries. With tissue-like components and adjustable properties, hydrogel-based biomechanical sensors are considered promising materials for human motion detection. However, existing hydrogels are characterized by inferior mechanical properties with strength and modulus typically lower than 1 MPa, as well as poor stability under physiological conditions, which hampers their applications in implantable devices. Moreover, acquiring the stress signal from collected electrical signals remains challenging. Herein, based on the regulation of polymer aggregation, a high-strength fibrous sensor composed of polyvinyl alcohol (PVA) and reduced graphene oxide (rGO) for in vivo monitoring is prepared through a two-step procedure, including freeze-thaw and freeze-soak. Benefiting from the synergy of crystallization, Hofmeister effect and nanocomposite, the hydrogel fibers feature high tensile strength (8.34 ± 0.66 MPa) and elastic modulus (1.15 ± 0.10 MPa). Meanwhile, the removal of salt ions during fabrication improves the water content (69.18 ± 1.47%) and anti-swelling performance of such fibers and minimizes side effects after implantation. It is demonstrated that the fibrous sensor could record the relative resistance changes upon stretching with ideal sensitivity (GF = 1.57) and convert them into bearing stress through formula derivation and calculations. In vitro and in vivo assays further confirm its feasibility for real-time monitoring of joint motion, providing important references for medical diagnosis and treatment.

Robust, stretchable bioelectronic interfaces for cardiac pacing enabled by interfacial transfer of laser-induced graphene via water-response, nonswellable PVA gels

Implantable cardiac pacemakers are crucial therapeutic tools for managing various cardiac conditions. For effective pacing, electrodes should exhibit flexibility, deformability, biocompatibility, and high conductivity/capacitance. Laser-induced graphene (LIG) shows promise due to its exceptional electrical and electrochemical properties. However, the fragility of LIG and the non-stretchability of polyimide substrates pose challenges when interfacing with the beating heart. Here, we present a simple method for fabricating robust, flexible, and stretchable bioelectronic interfaces by transferring LIG via water-responsive, nonswellable polyvinyl alcohol (PVA) gels. PVA solution penetrates the porous structure of LIG and solidifies into PVA xerogel as the solvent evaporates. The robust PVA xerogel enables the smooth transfer of LIG and prevents stretching of the LIG network during this process, which helps maintain its conductivity. When hydrated, the xerogel becomes a stable, nonswellable hydrogel. This gives the LIG-PVA hydrogel (LIG-PVA-H) composites with excellent conductivity (119.7 ± 4.3Ω sq-1), high stretchability (up to 420%), reliability (cyclic stretch under 15% strain, with ∼ 1-time resistance increase), and good stability in phosphate buffered saline. The LIG-PVA-H composites were used as biointerfaces for electrocardiogram signal recording and electrical pacing on rat hearts ex vivo and in vivo, using commercial setups and a custom-built implantable wireless device. This work expands the application of LIG in bioelectronic interfaces and facilitates the development of electrotherapy for cardiac diseases.

Ultrastrong, Ductile, Tear- and Folding-Resistant Polyimide Film Doubly Reinforced by an Aminated Rigid-Rod Macromolecule and Graphene Oxide

As a high-performance polymer with exceptional mechanical, thermal, and insulating properties, polyimide (PI) has been widely used as flexible circuit substrates for microelectronics, portable electronics, and wearable devices. Due to the growing demand for further thinning and lightweighting of electronic products, PI films need to have further enhanced mechanical properties. Traditional nanofiller-reinforced PI films often exhibit reduced ductility and limited improvements in strength. Therefore, it remains a challenge to simultaneously improve the strength and toughness of PI films while preserving their ductility. In this study, we report an exceptionally strong and ductile PI doubly reinforced by one-dimensional rigid-rod para-aramid, poly(p-aminophenylene aminoterephthalamide ((NH2)2-PPTA), and two-dimensional graphene oxide (GO) nanosheets. The amino side groups of (NH2)2-PPTA react with the anhydride end groups of PI, forming covalent bonds. At a (NH2)2-PPTA content of only 0.4 wt %, the (NH2)2-PPTA/PI film displays significantly enhanced mechanical properties. When 0.4 wt % of GO is added together with (NH2)2-PPTA, the tensile strength, tensile toughness, and strain at break reach 284.8 ± 5.3 MPa, 277.9 ± 7.6 MJ/m3, and 132.6 ± 3.8%, which are ∼178, ∼312, and ∼51% higher, respectively, than those of pure PI. Moreover, the doubly reinforced PI film also exhibits a 206% increase in tear strength and significantly enhanced folding resistance. The dual reinforcement of PI with (NH2)2-PPTA and GO improves the mechanical properties more efficiently than any single reinforcing agents previously reported and overcomes the disadvantage of most inorganic nanofillers that reduce ductility.

Research on the Thermal Aging Mechanism of Polyvinyl Alcohol Hydrogel

Polyvinyl alcohol (PVA) hydrogels find applications in various fields, including machinery and tissue engineering, owing to their exceptional mechanical properties. However, the mechanical properties of PVA hydrogels are subject to alteration due to environmental factors such as temperature, affecting their prolonged utilization. To enhance their lifespan, it is crucial to investigate their aging mechanisms. Using physically cross-linked PVA hydrogels, this study involved high-temperature accelerated aging tests at 60 °C for 80 d and their performance was analyzed through macroscopic mechanics, microscopic morphology, and microanalysis tests. The findings revealed three aging stages, namely, a reduction in free water, a reduction in bound water, and the depletion of bound water, corresponding to volume shrinkage, decreased elongation, and a "tough-brittle" transition. The microscopic aging mechanism was influenced by intermolecular chain spacing, intermolecular hydrogen bonds, and the plasticizing effect of water. In particular, the loss of bound water predominantly affected the lifespan of PVA hydrogel structural components. These findings provide a reference for assessing and improving the lifespan of PVA hydrogels.

Self-healing, antioxidant, and antibacterial Bletilla striata polysaccharide-tannic acid dual dynamic crosslinked hydrogels for tissue adhesion and rapid hemostasis

Biomaterials capable of achieving effective sealing and hemostasis at moist wounds are in high demand in the clinical management of acute hemorrhage. Bletilla striata polysaccharide (BSP), a natural polysaccharide renowned for its hemostatic properties, holds promising applications in biomedical fields. In this study, a dual-dynamic-bonds crosslinked hydrogel was synthesized via a facile one-pot method utilizing poly(vinyl alcohol) (PVA)-borax as a matrix system, followed by the incorporation of BSP and tannic acid (TA). Chemical borate ester bonds formed around borax, coupled with multiple physical hydrogen bonds between BSP and other components, enhanced the mechanical properties and rapid self-healing capabilities. The catechol moieties in TA endowed the hydrogel with excellent adhesive strength of 30.2 kPa on the surface of wet tissues and facilitated easy removal without residue. Benefiting from the synergistic effect of TA and the preservation of the intrinsic properties of BSP, the hydrogel exhibited outstanding biocompatibility, antibacterial, and antioxidant properties. Moreover, it effectively halted acute bleeding within 31.3 s, resulting in blood loss of 15.6 % of that of the untreated group. As a superior hemostatic adhesive, the hydrogel in this study is poised to offer a novel solution for addressing future acute hemorrhage, wound healing, and other biomedical applications.

Facile preparation of highly adhesive yet ultra-strong poly (vinyl alcohol)/cellulose nanocrystals composite hydrogel enabled by multiple networks structure

Poly (vinyl alcohol) (PVA) hydrogel showed potential applications in bioengineering and wearable sensors fields. It is still a huge challenge to prepare highly adhesive yet strong poly (vinyl alcohol) hydrogel with good biocompatibility. Herein, we prepared a highly self-adhesive and strong poly (vinyl alcohol)/tannic acid@cellulose nanocrystals (PVA/TA@CNCs) composite hydrogel using TA@CNCs as functional nanofiller via facile freezing-thawing method. Multiple networks consisting of hydrogen bonding and coordination interactions endowed the hydrogel with high mechanical strength, excellent flexibility and fracture toughness with adequate energy dissipation mechanism and relatively dense network structure. The tensile strength of PVA/TA@CNCs hydrogel reached the maximum of 463 kPa, increasing by 367 % in comparison with pure PVA hydrogel (99 kPa), demonstrating the synergistic reinforcing and toughening effect of TA@CNCs. The hydrogel exhibited extremely high adhesion not only for various dry and wet substrates such as plastic, metal, Teflon, rubber, glass, leaf, but also sweaty human skin, showing good adhesion durability. The highest adhesion strength to silicone rubber, steel plate and pigskin could reach 197 kPa, 100 kPa and 46.9 kPa, respectively. Meanwhile the hydrogel had negligible cytotoxicity to cells and showed good biocompatibility.

Designing a bi-layer multifunctional hydrogel patch based on polyvinyl alcohol, quaternized chitosan and gallic acid for abdominal wall defect repair

In abdominal wall defect repair, surgical site infection (SSI) remains the primary cause of failure, while complications like visceral adhesions present significant challenges following patch implantation. We designed a Janus multifunctional hydrogel patch (JMP) with antibacterial, anti-inflammatory, and anti-adhesive properties. The patch comprises two distinct layers: a pro-healing layer and an anti-adhesion layer. The pro-healing layer was created by a simple mixture of polyvinyl alcohol (PVA), quaternized chitosan (QCS), and gallic acid (GA), crosslinked to form PVA/QCS/GA (PQG) hydrogels through GA's self-assembly effect and hydrogen bonding. Additionally, the PVA anti-adhesive layer was constructed using a drying-assisted salting method, providing a smooth and dense physical barrier to prevent visceral adhesion while offering essential mechanical support to the abdominal wall. The hydrogel patch demonstrates widely adjustable mechanical properties, exceptional biocompatibility, and potent antimicrobial properties, along with a sustained and stable release of antioxidants. In rat models of skin and abdominal wall defects, the JMP effectively promoted tissue healing by controlling infection, inhibiting inflammation, stimulating neovascularization, and successfully preventing the formation of visceral adhesions. These compelling results highlight the JMP's potential to improve the success rate of abdominal wall defect repair and reduce surgical complications.

Multimodal and flexible hydrogel-based sensors for respiratory monitoring and posture recognition

The accurate monitoring of respiratory events and human motion states holds paramount importance in the realm of health surveillance and disease prognostication. An exquisitely precise, multifaceted, portable, and environmentally resilient sensor designed for health monitoring would undeniably be of utmost desirability, despite its persisting as a formidable challenge. Here, we propose a breath monitoring and posture recognition system that utilizes hydrogel electrolytes based on polyvinyl alcohol, sodium alginate, and starch, suitable for supercapacitors and multimodal wearable sensors. The multimodal smart sensors can independently detect mechanical and thermal changes through the output signals of capacitance and resistance, respectively. Moreover, we have cultivated an artificial neural network to achieve a finger-pressing posture recognition accuracy of up to 99.259%. Our hydrogel sensors have also been successfully employed in the diagnosis of obstructive sleep apnea syndrome. The flexible electronic device derived from this study exhibit a plethora of functionalities, thereby affording a novel perspective for the design and fabrication of advanced flexible electronic contrivances that find applications across diverse domains such as medicine and virtual reality.

Drug cross-linking electrospun fiber for effective infected wound healing

The management of infected wounds is still an intractable challenge in clinic. Development of antibacterial wound dressing is of great practical significance for wound management. Herein, a natural-derived antibacterial drug, tannic acid (TA), was incorporated into the electrospun polyvinyl alcohol (PVA) fiber (TA/PVA fiber, 952 ± 40 nm in diameter). TA worked as a cross-linker via hydrogen bonding with PVA to improve the physicochemical properties of the fiber and to reach a sustained drug release (88% release of drug at 48 h). Improved mechanical property (0.8-1.2 MPa) and computational simulation validated the formation of the hydrogen bonds between TA and PVA. Moreover, the antibacterial and anti-inflammatory characteristics of TA laid the foundation for the application of TA/PVA fiber in repairing infected wounds. Meanwhile, in vitro studies proved the high hemocompatibility and cytocompatibility of TA/PVA fiber. Further in vivo animal investigation showed that the TA/PVA fiber promoted the repair of infected wound by inhibiting the bacterial growth, promoting granulation formation, and collagen matrix deposition, accelerating angiogenesis, and inducing M2 macrophage polarization within 14 days. All the data demonstrated that the TA cross-linked fiber would be a potent dressing for bacteria-infected wound healing.

Facile Solvent Regulation for Highly Strong and Tough Physical Eutectogels with Remarkable Strain Sensitivity

Physical eutectogels have great potential for applications in many fields due to their electrical conductivity, broad temperature stability, and biocompatibility. However, the preparation of high-performance physical eutectogels in a simple, efficient, and cost-effective way remains a challenge. In this study, a facile but efficient solvent regulation strategy was proposed to construct a highly robust poly(vinyl alcohol) (PVA) physical eutectogel. Hydrogen bonds within the polymer-containing deep eutectic solvent system were dynamically regulated by the introduction-removal of water to induce the formation of a uniform and dense polymer cross-linked network, which imparted excellent mechanical properties to the resulting eutectogel. For the eutectogel with 15 wt % PVA, the tensile strength and toughness were 1.67 MPa and 6.81 MJ m-3, respectively, which were at a high level among existing physical eutectogels. This high-performance eutectogel was available as a strain sensor and exhibited high sensitivity. In addition, this eutectogel can be endowed with a directional muscle-like stretching performance through convenient mechanical training. The easy scalability and low cost made our method an effective strategy for developing high-performance physical eutectogels, which would further promote the application of such materials in areas such as wearable electronics and soft robotics.

Water-Induced Cellulose Nanofibers/Poly(vinyl alcohol) Hydrogels Regulated by Hydrogen Bonding for In Situ Water Shutoff

Hydrogels have been widely applied to the water shutoff in oilfields due to their excellent three-dimensional network and thermal and physicochemical stability, and it is still a huge challenge to develop new hydrogels with simple preparation, low cost, and high mechanical performance that can meet the requirements of practical applications. Herein, we devised a simple and universal manufacturing method for regulating the hydrogen bonds between poly(vinyl alcohol) (PVA) and cellulose nanofibers (CNF) via the water-diffusion action, thus fabricating a physically tough PVA-CNF hydrogel for the in situ water shutoff. This method allowed the polymer chains to strengthen the molecular interactions between polymers upon replacing with water (a poor solvent) to regulate the cross-linking structure, characterizing by the nano-crystallinity domains and fibrillar segments, which also accounted for the thermal stability, extraordinary elasticity, high stretchability, and toughness of PVA-CNF hydrogel. Further, the obtained PVA-CNF hydrogel exhibited superb plugging performance, that is, the breakthrough pressure gradient could reach 71.56 MPa·m-1, surpassing all currently reported gelling water shutoff agents. This water-induced in situ hydrogelation made it well suited as a water shutoff agent in oilfields and may provide a promising strategy to fabricate mechanically robust smart materials for the water shutoff projects with low cost, simple processing, and high efficiency.

Ultrastrong and Tough Urushiol-Based Ionic Conductive Double Network Hydrogels as Flexible Strain Sensors

Ionic conductive hydrogels have attracted increasing research interest in flexible electronics. However, the limited resilience and poor fatigue resistance of current ionic hydrogels significantly restrict their practical application. Herein, an urushiol-based ionic conductive double network hydrogel (PU/PVA-Li) was developed by one-pot thermal initiation polymerization assisted with freeze-thaw cycling and subsequent LiCl soaking. Such a PU/PVA-Li hydrogel comprises a primary network of covalently crosslinked polyurushiol (PU) and a secondary network formed by physically crosslinked poly(vinyl alcohol) (PVA) through crystalline regions. The obtained PU/PVA-Li hydrogel demonstrates exceptional mechanical properties, including ultrahigh strength (up to 3.4 MPa), remarkable toughness (up to 1868.6 kJ/m3), and outstanding fatigue resistance, which can be attributed to the synergistic effect of the interpenetrating network structure and dynamic physical interactions between PU and PVA chains. Moreover, the incorporation of LiCl into the hydrogels induces polymer chain contraction via ionic coordination, further enhancing their mechanical strength and resilience, which also impart exceptional ionic conductivity (2.62 mS/m) to the hydrogels. Based on these excellent characteristics of PU/PVA-Li hydrogel, a high-performance flexible strain sensor is developed, which exhibits high sensitivity, excellent stability, and reliability. This PU/PVA-Li hydrogel sensor can be effectively utilized as a wearable electronic device for monitoring various human joint movements. This PU/PVA-Li hydrogel sensor could also demonstrate its great potential in information encryption and decryption through Morse code. This work provides a facile strategy for designing versatile, ultrastrong, and tough ionic conductive hydrogels using sustainable natural extracts and biocompatible polymers. The developed hydrogels hold great potential as promising candidate materials for future flexible intelligent electronics.

Preparation of an Aminated Lignin/Fe(III)/Polyvinyl Alcohol Film: A Packaging Material with UV Resistance and Slow-Release Function

To reduce the usage of petroleum-based plastic products, a lignin-based film material named aminated lignin/Fe(III)/PVA was developed. The mixture of 8 g lignin, 12 mL diethylenetriamine, 200 mL NaOH solution (0.4 mol·L^-1), and 8 mL formaldehyde was heated at 85 °C for 4 h; after the aminated lignin was impregnated in the Fe(NO₃)₃ solution, a mixture of 3 g aminated lignin/Fe(III), 7 g PVA, and 200 mL NaOH solution (pH 8) was heated at 85 °C for 60 min; after 2 mL of glycerin was added, the mixture was spread on a glass plate to obtain the aminated lignin/Fe(III)/PVA film. This film demonstrated hydrophobicity, an UV-blocking function, and a good slow-release performance. Due to the formation of hydrogen bonds between the hydroxyl groups of lignin and PVA, the tensile strength, the elongation at break, and the fracture resistance of the film were 9.1%, 107.8%, and 21.9% higher than that of pure PVA film, respectively. The iron content of aminated lignin/Fe(III)/PVA was 1.06 wt%, which mainly existed in a trivalent form. The aminated lignin/Fe(III)/PVA film has the potential to be used as a food packaging material with anti-ultraviolet light function and can also be developed as other packaging materials, such as seedling bowls, pots for transplanting, and coating films during transport.

Tough, Transparent, and Slippery PVA Hydrogel Led by Syneresis

Slippery and transparent polyvinyl alcohol (PVA) hydrogels with mechanical robustness exhibit broad applications in artificial biological soft tissues, flexible wearable electronics, and implantable biomedical devices. Most of the current PVA hydrogels, however, are unable to integrate these features, which compromises its performance in biological and engineering applications. To achieve such purpose, herein, a novel tactic is proposed, salting-out-after-syneresis of PVA, to realize a mechanically robust and highly transparent slippery PVA hydrogel. The syneresis of PVA sol is first conducted to form highly dense and transparent PVA polymer networks, then the salting-out effect tunes the aggregation of the polymer chains to rapidly induce the phase separation and crystallization. The resultant hydrogels show the transparency up to 98% in the visible region, the tribological coefficient down to 0.0081, and the excellent mechanical properties with strength, modulus, and toughness of 26.72 ± 1.05, 6.66 ± 0.29 MPa, and 55.21 ± 1.62 MJ m^-3 , respectively. To reveal the potentials, PVA contact lens that combine remarkable lubrication, anti-protein adhesion, biocompatibility, and drug-loading functions are demonstrated. This strategy provides a simple and new avenue for developing the mechanically robust, transparent, and hydrated hydrogels, showing the potential in biomedicine and wearable 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.

An Ultra-Stretchable Polyvinyl Alcohol Hydrogel Based on Tannic Acid Modified Aramid Nanofibers for Use as a Strain Sensor

The mechanical performance is critical for hydrogels that are used as strain sensors. p-Aramid nanofiber (ANF) is preferable as an additive to the reinforce the mechanical performance of a poly(vinyl alcohol) (PVA). However, due to the limited hydrogen bond sites, the preparation of ultra-stretchable, ANF-based hydrogel strain sensor is still a challenge. Herein, we reported an ultra-stretchable PVA hydrogel sensor based on tea stain-inspired ANFs. Due to the presence of numerous phenol groups in the tannic acid (TA) layer, the interaction between PVA and the ANFs was significantly enhanced even though the mass ratio of TA@ANF in the hydrogel was 2.8 wt‰. The tensile breaking modulus of the PVA/TA@ANF/Ag hydrogel sensor was increased from 86 kPa to 326 kPa, and the tensile breaking elongation was increased from 356% to 602%. Meanwhile, the hydrogel became much softer, and no obvious deterioration of the flexibility was observed after repeated use. Moreover, Ag NPs were formed in situ on the surfaces of the ANFs, which imparted the sensor with electrical conductivity. The hydrogel-based strain sensor could be used to detect the joint movements of a finger, an elbow, a wrist, and a knee, respectively. This ultra-stretchable hydrogel described herein was a promising candidate for detecting large-scale motions.

Frost-resistant and ultrasensitive strain sensor based on a tannic acid-nanocellulose/sulfonated carbon nanotube-reinforced polyvinyl alcohol hydrogel

The operating temperature of hydrogels, especially at low temperatures, is crucial due to their wide applicability in soft robots, sensors, and electronic skin. Hydrogels are often used at room temperature, but their performance may deteriorate at low temperatures. Therefore, it is crucial to develop hydrogels that can be used at low temperatures to expand their range of use. Herein, we have proposed a simple one-pot method to prepare a frost-resistant (-70 °C) and conductive hydrogel consisting of a glycerol (Gly)-water binary solvent. We have added tannic acid (TA)-coated carboxymethylated cellulose nanofibrils (CMCNFs) to poly (vinyl alcohol) (PVA) as a functional filler to improve the hydrogel's mechanical properties. The introduction of sulfonated carbon nanotubes (SCNT) has provided the hydrogel with high conductivity (0.1 S/m), strain sensitivity (gauge factor of 3.76), and cyclic stability (1600 cycles). Due to the strong hydrogen bonding and physical entanglement effects between the components, the hydrogel exhibied excellent tensile properties (297 %), high toughness (0.44 MJ/m³), and a high Young's modulus (1.25 MPa). These characteristics ensure that the hydrogel is well suited for low-temperature environments, health monitoring, and wearable devices.

Fabrication and Characterization of Buccal Film Loaded Self-emulsifying Drug Delivery System containing Lysiphyllum strychnifolium Stem Extracts

Lysiphyllum strychnifolium has long been used as a popular herbal medicinal plant for treating fever and alcohol intoxication. This study aimed to prepare buccal film for L. strychnifolium stem extracts. These extracts were less soluble in water and were therefore loaded in self-emulsifying systems before being mixed into the film. Astilbin was selected as a chemical marker in L. strychnifolium stem extracts. Firstly, the L. strychnifolium stem extracts were entrapped in the self-emulsifying systems which were designed and optimized based on 3² factorial design. The optimal formulation was 0.60 g of surfactant-co-surfactant mixture (Tween^® 80 and polyethylene glycol 400 in the ratio of 7.5:1) and 0.40 g of caprylic/capric triglyceride. Secondly, the optimal self-emulsifying system was loaded in the polymeric film which consisted of polyvinyl alcohol blended with poloxamer 407 using glycerin as a plasticizer. The properties of the prepared buccal film were unchanged, and the film showed an amorphous state, indicating all ingredients might be completely dissolved in the film. The buccal film could be placed in direct contact with the mouth without oral mucosal irritation, and showed a smooth and homogeneous surface with a rough and compact cross-sectional morphology. Astilbin content in the buccal film was 61.39 ± 11.45 µg/cm². Astilbin was released from the buccal film while the permeation rate was low. The release mechanism was both swelling and diffusion, and followed anomalous or non-Fickian transfer. The permeability coefficient of the cumulative amount of astilbin permeated from buccal film was 1.0192 ± 0.1395 ×10^-3 cm/h. Thus, the buccal film can be prepared by using a self-emulsifying system for herbal applications and shows potential as a safe and convenient form of oral drug administration.

Transparent and ultra-tough PVA/alkaline lignin films with UV shielding and antibacterial functions

Lignin and its derivatives can be used to make membranes with natural polymer materials for its properties including ultraviolet adsorption, biodegradable, antibacterial, and antioxidant. However, the lignin film has poor transparency due to the dark color, and how to control the proportion of each component to enhance properties is the main research topic. In this study, a polyvinyl alcohol /alkaline lignin (PVA/AL) composite film with excellent UV-shielding and visible-transparent performance successfully prepared by solution casting. By mixing with the N-halamine precursor 3-(2,3-dihydroxypropyl)-5,5-dimethylhydantoin (DPDMH), the composite film after chlorination showed superb antibacterial efficacy and could inactivate 6.85 log Escherichia coli (E. coli) and 6.57 log Staphylococcus aureus (S. aureus) respectively within 5 min of contact. Moreover, the composite film with 5 wt% AL exhibited ultra-high elongation of 449 % and toughness of 92 MJ/m³, and the toughness and malleability was greatly improved. In addition, with the introduction of AL, the composite film could shield 100 % of the UVB (320-275 nm) and UVC (275-200 nm) spectra and most of the UVA (400-320 nm) spectrum. The films prepared in this work are expected to find applications in promising fields such as in packaging materials and ultraviolet shielding.

A Chewing Gum Residue-Based Gel with Superior Mechanical Properties and Self-Healability for Flexible Wearable Sensor

Chewing gum residue is hard to decompose and easy to cause pollution, which is highly desirable to realize the recycling. In this paper, a chewing gum gel with enhanced mechanical properties and self-healing properties is prepared by using polyvinyl alcohol (PVA) as the backbone in chewing gum residue. The hydrogen bond and the borax ester bond are employed to construct reversible interaction to enhance the self-healing ability. The physical crosslinking is realized by further freeze-thaw treatment to improve its mechanical properties. The gel demonstrates high elongation at break of 610% and strength of 0.11 MPa, as well as excellent self-healing performance and recyclable property. In particular, the gel with a fast signal response is successfully applied as a wearable strain sensor to monitor different types of human motion. The gel as a sensor exhibits self-healing properties suggesting superior safety and stability, and displays wide linear sensitivity (the gauge factor is 0.417 and 0.170). The gel can be further served to explore temperature changes, implying the application in temperature monitoring. This study develops a novel approach for the recycle and reuse of chewing gum residue. The obtained gel may be a promising candidate for the fabrication of flexible wearable sensor. This article is protected by copyright. All rights reserved.

Hydrogen-bond-dominated mechanical stretchability in PVA films: from phenomenological to numerical insights

Hydrogen bonds (H-bonds) in poly(vinyl alcohol) (PVA) play a crucial role in macroscopic mechanical properties, particularly for stretchability. However, there is still some ambiguity about the quantitative dependence of H-bond interactions on the mechanical performance, mainly attributed to the difficulty in the discrimination of various H-bond types. Herein, small molecular chemicals as plasticizers were incorporated into the PVA matrix to tailor the H-bonding interactions. By altering the PVA molecular weight, plasticizer type and loading, both the stretchability and H-bond content were regulated on a large scale. By a combination of DMA, IR spectroscopy, MD simulation and solid-state ¹³C-NMR, every sort of H-bond in PVA was assigned, and their relative fractions were ascertained quantitatively. After correlating the elongation ratio with the relative fraction of the different types of H-bonding interaction, it was found that all the pairs of elongation vs. intermolecular H-bond content derived from different series of PVA/plasticizer films could be plotted into a master curve and exhibited good linearity, indicating that intermolecular H-bonds dominate the mechanical stretchability in PVA films. Our efforts contribute towards an in-depth understanding of performance optimization induced by H-bond manipulation from empirical, phenomenological aspects to intrinsic, numerical insights.

One-pot freezing-thawing preparation of cellulose nanofibrils reinforced polyvinyl alcohol based ionic hydrogel strain sensor for human motion monitoring

Ionic conductive hydrogels have been widely applied in sensors, energy storage and soft electronics recently. However, most of the polyvinyl alcohol (PVA) based ionic hydrogels are mainly fabricated by soaking the hydrogels in high concentration electrolyte solution which can induce the waste of electrolyte and solvent. Herein, we have designed cellulose nanofibrils (CNF) and ZnSO₄ reinforced PVA based hydrogels through a one-pot simple freezing-thawing method at low ZnSO₄ concentration without any soaking process. Furthermore, the hydrogel with 0.4% CNF exhibited stress up to 0.79 MPa (242% strain) and high ionic conductivity of 0.32 S m^-1 (0.07 M ZnSO₄). Moreover, hydrogel sensor displayed high linear gauge factor 1.70 (0-200% strain), excellent stability, durability and reliability. The integrated hydrogel sensor also showed excellent sensor performance for human motion monitoring. This work provides a new prospect for the design of cellulose reinforced conductive hydrogels via a facile method.

Dynamic Nanoconfinement Enabled Highly Stretchable and Supratough Polymeric Materials with Desirable Healability and Biocompatibility

Lightweight polymeric materials are highly attractive platforms for many potential industrial applications in aerospace, soft robots, and biological engineering fields. For these real-world applications, it is vital for them to exhibit a desirable combination of great toughness, large ductility, and high strength together with desired healability and biocompatibility. However, existing material design strategies usually fail to achieve such a performance portfolio owing to their different and even mutually exclusive governing mechanisms. To overcome these hurdles, herein, for the first time a dynamic hydrogen-bonded nanoconfinement concept is proposed, and the design of highly stretchable and supratough biocompatible poly(vinyl alcohol) (PVA) with well-dispersed dynamic nanoconfinement phases induced by hydrogen-bond (H-bond) crosslinking is demonstrated. Because of H-bond crosslinking and dynamic nanoconfinement, the as-prepared PVA nanocomposite film exhibits a world-record toughness of 425 ± 31 MJ m-3 in combination with a tensile strength of 98 MPa and a large break strain of 550%, representing the best of its kind and even outperforming most natural and artificial materials. In addition, the final polymer exhibits a good self-healing ability and biocompatibility. This work affords new opportunities for creating mechanically robust, healable, and biocompatible polymeric materials, which hold great promise for applications, such as soft robots and artificial ligaments.

Aramid nanofibers reinforced polyvinyl alcohol/tannic acid hydrogel with improved mechanical and antibacterial properties for potential application as wound dressing

The poor mechanical properties and the lack of antibacterial ability of hydrogels limit their applications as wound dressing. In this work, a novel and high strength polyvinyl alcohol (PVA)/tannic acid (TA) hydrogel with aramid nanofibers (ANFs) as the reinforcement was successfully fabricated. The surface composition and microstructure of the hydrogel were characterized by fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties, water content and swelling behaviors, as well as the antibacterial abilities and biocompatibility of the prepared hydrogel were systematically analyzed as well. The results indicated that the prepared hydrogel showed excellent mechanical properties. The tensile strength and elongation of the prepared hydrogel can respectively reach 2.06 MPa and 950% owing to the formation of the multiple H bonds among PVA, ANFs and TA. What's more, PVA/ANFs/TA (PAT) hydrogel possessed shape memory and broad-spectrum antibacterial properties against S. aureus, E. coli and P. aeruginosa (100% antibacterial rate) at the concentration of 12 mg/mL. PAT hydrogels also had low cytotoxicity, affirming its potential application as wound dressing.

Remalleable, Healable, and Highly Sustainable Supramolecular Polymeric Materials Combining Superhigh Strength and Ultrahigh Toughness

To build a sustainable society, it is of significant importance but highly challenging to develop remalleable, healable, and biodegradable polymeric materials with integrated high strength and high toughness. Here, we report a superstrong and ultratough sustainable supramolecular polymeric material with a toughness of ca. 282.3 J g^-1 (395.2 MJ m^-3) in combination with a tensile strength as high as ca. 104.2 MPa and a Young's modulus of ca. 3.53 GPa. The toughness is even higher than that of the toughest spider silk (ca. 354 MJ m^-3) ever found in the world, while the material also exhibits a superior tensile strength over most engineering plastics. This material is fabricated by topological confinement of the biodegradable linear polymer of poly(vinyl alcohol) (PVA) via the naturally occurring dendritic molecules of tannic acid (TA) based on high-density hydrogen bonds. Simply blending TA and PVA in aqueous solutions at acidic conditions leads to the formation of TA-PVA complexes as precipitates, which can be processed into dry TA-PVA composite products with desired shapes via the compression molding method. Compared to the conventional solution casting method for the fabrication of PVA-based thin films, the as-developed strategy allows large-scale production of bulk TA-PVA composites. The TA-PVA composites consist of interpenetrating three-dimensional supramolecular TA-PVA clusters. Such a structural feature, revealed by computational simulations, is crucial for the integrated superhigh strength and ultrahigh toughness of the material. The biodegradable TA-PVA composites are remalleable for multiple generations of recycling and healable after break, at room temperature, by the assistance of water to activate the reversibility of the hydrogen bonds. The TA-PVA composites show high promise as sustainable substitutes for conventional plastics because of their remalleability, healability, and biodegradability. The integrated superhigh strength and ultrahigh toughness of the TA-PVA composites ensure their high reliability and broad applicability.

High-Performance Polymeric Materials through Hydrogen-Bond Cross-Linking

It has always been critical to develop high-performance polymeric materials with exceptional mechanical strength and toughness, thermal stability, and even healable properties for meeting performance requirements in industry. Conventional chemical cross-linking leads to enhanced mechanical strength and thermostability at the expense of extensibility due to mutually exclusive mechanisms. Such major challenges have recently been addressed by using noncovalent cross-linking of reversible multiple hydrogen-bonds (H-bonds) that widely exist in biological materials, such as silk and muscle. Recent decades have witnessed the development of many tailor-made high-performance H-bond cross-linked polymeric materials. Here, recent advances in H-bond cross-linking strategies are reviewed for creating high-performance polymeric materials. H-bond cross-linking of polymers can be realized via i) self-association of interchain multiple H-bonding interactions or specific H-bond cross-linking motifs, such as 2-ureido-4-pyrimidone units with self-complementary quadruple H-bonds and ii) addition of external cross-linkers, including small molecules, nanoparticles, and polymer aggregates. The resultant cross-linked polymers normally exhibit tunable high strength, large extensibility, improved thermostability, and healable capability. Such performance portfolios enable these advanced polymers to find many significant cutting-edge applications. Major challenges facing existing H-bond cross-linking strategies are discussed, and some promising approaches for designing H-bond cross-linked polymeric materials in the future are also proposed.

Gallic Acid and Quercetin as Intelligent and Active Ingredients in Poly(vinyl alcohol) Films for Food Packaging

Gallic acid (GA) and quercetin (QC) were used as active ingredients in poly(vinyl alcohol) (PVA) film formulations obtained by solvent casting process. The effect of two different percentages (5 and 10 % wt.) on morphological behavior, thermal stability, optical, mechanical, and release properties of PVA were investigated, while migration with food stimulants and antioxidant properties were tested taking into account the final application as food packaging systems. The results showed how different dispersability in PVA water solutions gave different results in term of deformability (mean value of ε PVA/5GA = 280% and ε PVA/5QC = 255%, with 190% for neat PVA), comparable values for antioxidant activity at the high contents (Radical Scavenging Activity, RSA(%) PVA/10GA = 95 and RSA(%) PVA/10QC = 91) and different coloring attitude of the polymeric films. It was proved that GA, even if it represents the best antioxidant ingredient to be used with PVA and can be easily dispersed in water, it gives more rigid films in comparison to QC, that indeed was more efficient in tuning the deformability of the PVA films, due the presence of sole hydroxyl groups carrying agent. The deviation of the film coloring towards greenish tones for GA films and redness for QC films after 7 and within 21 days in the simulated conditions confirmed the possibility of using easy processable PVA films as active and intelligent films in food packaging.

A Dual Cross-Linked Strategy to Construct Moldable Hydrogels with High Stretchability, Good Self-Recovery, and Self-Healing Capability

Most conventional synthetic hydrogels suffer from poor mechanical properties; despite recent significant progress in fabricating tough hydrogels, it is still a challenge to simultaneously realize high stretchability, self-recovery, and self-healing capability in a hydrogel. In this work, a new type of starch/PVA/borax hybrid dual cross-linked (DC) hydrogel was synthesized by a one-pot method. The as-prepared DC hydrogels exhibited mechanical properties of remarkable extensibility (ca. 2485%), excellent toughness (ca. 290.5 kJ m^-3), high compression strength (ca. 547.8 kPa), rapid recoverability (81.9% energy recovery after 30 min), and free-shapeable behavior. More impressively, the DC gels sustained approximately 300 times their own weight and exhibited an outstanding self-healing capability at room temperature both in air and underwater. Furthermore, the adsorption amount of methylene blue onto the anionic DC gel (144.68 mg/g) was much higher than that of corn starch gel. Consequently, the eco-friendly, stable, and biodegradable hydrogels will have a great potential application in removing anionic dyes from the wastewater produced by agriculture and industry.