Rapid shape memory TEMPO-oxidized cellulose nanofibers/polyacrylamide/gelatin hydrogels with enhanced mechanical strength

Nanocellulose/scleroglucan-enhanced robust, heat-resistant composite hydrogels for oilfield water plugging

In an oil exploitation process, hydrogel plugging agents can effectively reduce the water-oil intermixing, decrease water extraction volume, and enhance oil recovery rate. The practical applications of traditional polyacrylamide (PAM) hydrogel plugging agents in oilfield are limited by their non-biodegradability, poor mechanical performance, and inferior temperature-resistance. This work developed a mechanically stable and high-temperature-resistant composite hydrogel (STP) by incorporating biodegradable scleroglucan (Slg) and TEMPO-oxidized cellulose nanofibers (TOCN) in the PAM hydrogel. The addition of Slg conferred heat resistance to the PAM hydrogel, while TOCN reinforced the mechanical strength. Anti-aging analyses revealed that the STP endured for 108 h in a saline environment at 140 °C. In the water flooding characterization, the STP displayed a breakthrough pressure of 42.10 psi/ft. at a flow rate of 0.75 cm3/min. Under these extreme conditions, the plugging pressure reached 14.74 psi/ft., meeting the essential criteria for oilfield water plugging. This research demonstrates the potential of polysaccharides in the preparation of sustainable, tough, and heat-resistant water plugging materials.

Copper nanoparticles loaded gelatin/ polyvinyl alcohol/ guar gum-based 3D printable multimaterial hydrogel for tissue engineering applications

Hydrogels are becoming increasingly significant in tissue engineering because of their numerous benefits, including biocompatibility, biodegradability, and their ability to provide a supportive structure for cell proliferation. This study presents the synthesis and characterization of a new multimaterial hydrogel with 3D-printing capabilities composed of copper nanoparticle-reinforced gelatin, polyvinyl alcohol (PVA), and guar gum-based biomaterials intended for tissue engineering applications. Combining CuNPs aims to enhance the hydrogel's antibacterial properties, mechanical strength, and bioactivity, which are essential for successful tissue regeneration. Hydrogels are chemically cross-linked with glyoxal and analyzed through different assessments to examine the compressive behavior, surface morphology, sorbing capacity, biocompatibility, thermal stability, and degradation properties. The results demonstrated that including CuNPs significantly improved the hydrogel's compressive modulus (4.18 MPa) for the hydrogel with the CuNPs and provided better antibacterial activity against common pathogens with controlled degradation. All the hydrogels exhibited a lower coefficient of friction, which was below 0.1. In vitro cell culture studies using chondrocytes indicated that the CuNPs-loaded hydrogel supported cell proliferation and growth of chondrogenic genes such as collagen type II (COL2) and aggrecan (ACAN). The biocompatibility and enhanced mechanical properties of the multimaterial hydrogel make it a promising candidate for developing customized, patient-specific tissue engineering scaffolds.

Development of ZrO(OH)2@HCS co-modified molecularly imprinted gel-based electrochemical sensing platform for sensitive and selective detection of tert-butylhydroquinone in foods

Herein, a novel molecularly imprinted gel (MIG)-based electrochemical sensor equipped with hydrated zirconium oxide@hollow carbon spheres (ZrO(OH)2@HCS) was developed for highly sensitive and selective detection of tert-butylhydroquinone (TBHQ) in foods. The MIG was synthesized by using L-histidine to rapidly cross-link cationic guar gum, acrylamide and TBHQ through intermolecular hydrogen bonds and electrostatic interactions at room temperature, which offered outstanding specific recognition performance for TBHQ. ZrO(OH)2@HCS possessing excellent conductivity and water dispersibility was employed for signal amplification. Under optimal conditions, the MIG-ZrO(OH)2@HCS/GCE sensor showed a wide dynamic detection range (0.025-100 μM) with a low limit of detection (6.7 nM). TBHQ recovery experiments were conducted in spiked peanut oil and milk powder, yielding excellent recoveries. Moreover, the sensor was successfully utilized to detect TBHQ levels in snowflake chicken cutlets, crispy fried pork and boneless chicken fillets, and the results were in agreement with those obtained by the high performance liquid chromatography method.

Nanocellulose-mediated conductive hydrogels with NIR photoresponse and fatigue resistance for multifunctional wearable sensors

The rapid development of hydrogels has garnered significant attention in health monitoring and human motion sensing. However, the synthesis of multifunctional conductive hydrogels with excellent strain/pressure sensing and photoresponsiveness remains a challenge. Herein, the conductive hydrogels (BPTP) with excellent mechanical properties, fatigue resistance and photoresponsive behavior composed of polyacrylamide (PAM) matrix, 2,2,6,6-tetramethylpiperidin-1-yloxy-oxidized cellulose nanofibers (TOCNs) reinforcement and polydopamine-modified black phosphorus (BP@PDA) photosensitizer are prepared through a facile free-radical polymerization approach. The PDA adhered to the BP surface by π-π stacking promotes the optical properties of BP while also preventing BP oxidation from water. Through hydrogen bonding interactions, TOCNs improve the homogeneous dispersion of BP@PDA nanosheets and the mechanical toughness of BPTP. Benefiting from the synergistic effect of PDA and TOCNs, the conductive BPTP integrates superior mechanical performances, excellent photoelectric response and photothermal conversion capability. The BPTP-based sensor with high cycling stability demonstrates superior strain sensitivity (GF = 6.0) and pressure sensing capability (S = 0.13 kPa-1) to monitor various human activities. Therefore, this work delivers an alternative construction strategy for generating high-performance conductive hydrogels as multifunctional wearable sensors.

Ultra-stretchable and conductive polyacrylamide/carboxymethyl chitosan composite hydrogels with low modulus and fast self-recoverability as flexible strain sensors

There is a great demand for the fabrication of soft electronics using hydrogels due to their biomimetic structures and good flexibility. However, conventional hydrogels have poor mechanical properties, which restricts their applications as stretchable sensors. Herein, a facile one-step strategy is proposed to fabricate tough and conductive hydrogels by making use of the graftability of carboxymethyl chitosan without extra conductive matter and crosslinking agent. The obtained polyacrylamide/carboxymethyl chitosan composite hydrogels possess outstanding transmittance and excellent mechanical performances, with tensile breaking stress of 630 kPa, breaking strain of 4560 %, toughness of 8490 kJ/m3. These hydrogels have low modulus of 5-20 kPa, fast recoverability after unloading, high conductivity of ∼0.85 S/m without the addition of other conductive substances and good biocompatibility. The ionic conductivity of the gels originates from the counterions of carboxymethyl chitosan, affording the hydrogels as resistive-type sensors. The resultant hydrogel sensors demonstrate a broad strain window (0.12-1500 %), excellent linear response, high sensitivity with the gauge factor reaching 11.72, and great durability, capable of monitoring diverse human motions. This work provides a new strategy to develop stretchable conductive hydrogels with promising applications in the fields of artificial intelligence and flexible electronics.

Advancements in gelatin-based hydrogel systems for biomedical applications: A state-of-the-art review

A gelatin-based hydrogel system is a stimulus-responsive, biocompatible, and biodegradable polymeric system with solid-like rheology that entangles moisture in its porous network that gradually protrudes to assemble a hierarchical crosslinked arrangement. The hydrolysis of collagen directs gelatin construction, which retains arginyl glycyl aspartic acid and matrix metalloproteinase-sensitive degeneration sites, further confining access to chemicals entangled within the gel (e.g., cell encapsulation), modulating the release of encapsulated payloads and providing mechanical signals to the adjoining cells. The utilization of various types of functional tunable biopolymers as scaffold materials in hydrogels has become highly attractive due to their higher porosity and mechanical ability; thus, higher loading of proteins, peptides, therapeutic molecules, etc., can be further modulated. Furthermore, a stimulus-mediated gelatin-based hydrogel with an impaired concentration of gellan demonstrated great shear thinning and self-recovering characteristics in biomedical and tissue engineering applications. Therefore, this contemporary review presents a concise version of the gelatin-based hydrogel as a conceivable biomaterial for various biomedical applications. In addition, the article has recapped the multiple sources of gelatin and their structural characteristics concerning stimulating hydrogel development and delivery approaches of therapeutic molecules (e.g., proteins, peptides, genes, drugs, etc.), existing challenges, and overcoming designs, particularly from drug delivery perspectives.

A review of recent advances in biomedical applications of smart cellulose-based hydrogels

In biomedical engineering, smart materials act as media to communicate physiological signals inspired by environmentally responsive stimuli with outer indicators for timely scrutiny and precise therapy. Various physical and chemical processes are applied in the design of specific smart functions. Hydrogels are polymeric networks consisting of hydrophilic chains and chemical groups and they have contributed their unique features in biomedical application as one of the most used smart materials. Numerous raw materials can form hydrogels, in which cellulose and its derivatives have been extensively exploited in biomedicine due to their high hydrophilicity, availability, renewability, biodegradability, biocompatibility, and multifunctional reactivity. This review collates cellulose-based hydrogels and their extensive applications in the biomedical domain, specifically benefiting from the "SMART" concept in their design, synthesis and device assembly. The first section discusses the physical and chemical crosslinking and electrospinning techniques used in the fabrication of smart cellulose-based hydrogels. The second section describes the performance of these hydrogels, and the final section is a comprehensive discussion of their biomedical applications.

Recent Advances of Natural-Polymer-Based Hydrogels for Wound Antibacterial Therapeutics

Hydrogels have a three-dimensional network structure and high-water content, are similar in structure to the extracellular matrix, and are often used as wound dressings. Natural polymers have excellent biocompatibility and biodegradability and are commonly utilized to prepare hydrogels. Natural-polymer-based hydrogels can have excellent antibacterial and bioactive properties by loading antibacterial agents or being combined with therapeutics such as phototherapy, which has great advantages in the field of treatment of microbial infections. In the published reviews of hydrogels used in the treatment of infectious wounds, the common classification criteria of hydrogels include function, source of antibacterial properties, type of antibacterial agent, etc. However, there are few reviews on the classification of hydrogels based on raw materials, and the description of natural-polymer-based hydrogels is not comprehensive and detailed. In this paper, based on the principle of material classification, the characteristics of seven types of natural polymers that can be used to prepare hydrogels are discussed, respectively, and the application of natural-polymer-based hydrogels in the treatment of infectious wounds is described in detail. Finally, the research status, limitations, and prospects of natural-polymer-based hydrogels are briefly discussed.

Dual Network Hydrogel with High Mechanical Properties, Electrical Conductivity, Water Retention and Frost Resistance, Suitable for Wearable Strain Sensors

With the progress of science and technology, intelligent wearable devices have become more and more popular in our daily life. Hydrogels are widely used in flexible sensors due to their good tensile and electrical conductivity. However, traditional water-based hydrogels are limited by shortcomings of water retention and frost resistance if they are used as the application materials of flexible sensors. In this study, the composite hydrogels formed by polyacrylamide (PAM) and TEMPO-Oxidized Cellulose Nanofibers (TOCNs) are immersed in LiCl/CaCl2/GI solvent to form double network (DN) hydrogel with better mechanical properties. The method of solvent replacement give the hydrogel good water retention and frost resistance, and the weight retention rate of the hydrogel was 80.5% after 15 days. The organic hydrogels still have good electrical and mechanical properties after 10 months, and can work normally at −20 °C, and has excellent transparency. The organic hydrogel show satisfactory sensitivity to tensile deformation, which has great potential in the field of strain sensors.

Hydroxypropyl methyl cellulose reinforced conducting polymer hydrogels with ultra-stretchability and low hysteresis as highly sensitive strain sensors for wearable health monitoring

Conducting polymer hydrogels have emerged as promising materials to fabricate highly sensitive strain sensors. However, due to weak bindings between conducting polymer and gel network, they usually suffer from limited stretchability and large hysteresis, failing to achieve wide-range strain sensing. Herein, we combine hydroxypropyl methyl cellulose (HPMC), poly (3,4-ethylenedioxythiophene):poly (styrene sulfonic acid) (PEDOT: PSS) with chemically cross-linked polyacrylamide (PAM) to prepare a conducting polymer hydrogel for strain sensors. Owing to abundant hydrogen bonds between HPMC, PEDOT:PSS and PAM chains, this conducting polymer hydrogel exhibits high tensile strength (166 kPa), ultra-stretchability (>1600 %) and low hysteresis (<10 % at 1000 % cyclic tensile strain). The resultant hydrogel strain sensor shows ultra-high sensitivity, wide strain sensing ranges of 2-1600 %, and excellent durability and reproducibility. Finally, this strain sensor can be used as wearable sensor to monitor vigorous human movement and fine physiological activity, and services as bioelectrodes for electrocardiograph and electromyography monitoring. This work provides new horizons to design conducting polymer hydrogels for advanced sensing devices.

Glucometer Readout for Portable Detection of Heterogeneous Circulating Tumor Cells in Lung Cancer Captured on a Dual Aptamer Functionalized Wrinkled Cellulose Hydrogel Interface

The rarity of circulating tumor cells (CTCs) poses a great challenge to their clinical application as reliable "liquid biopsy" markers for cancer diagnosis. Meanwhile, the epithelial-mesenchymal transition (EMT) led to a reduced efficiency in capturing cells with lost or downregulated epithelial cell adhesion molecule (EpCAM) expressions. In this study, we proposed an integrated, highly efficient strategy for heterogeneous CTC capture and portable detection from the blood of non-small-cell lung cancer (NSCLC) patients. First, the cellulose wrinkled hydrogel with excellent biocompatibility and high specific area was employed as the biointerface to capture heterogeneous CTCs with an improved capture efficiency in virtue of dual targeting against epithelial and mesenchymal ones. Meanwhile, the strategy of glucometer readout was introduced for the quantification of captured CTCs on the same hydrogel interface by a detection probe, Au-G-MSN-Apt, which was fabricated via entrapping glucose into the amino group functionalized mesoporous silica nanoparticle (MSN) framework sealed by l-cysteine modified gold nanoparticles (AuNPs) and then linked with dual aptamers of EpCAM and Vimentin. The number of captured CTCs on the hydrogel could be reflected according to the portable glucose meter (PGM) readings. Moreover, it was found that the captured cells maintained a higher viability on the hydrogel and could be in situ recultured without releasing from the substrate. Finally, this integrated strategy was successfully applied to inspect the correlations between the number of heterogeneous CTCs in the blood of NSCLC patients with disease stage and whether there was distant metastasis.

Preparation, properties, and applications of gelatin-based hydrogels (GHs) in the environmental, technological, and biomedical sectors

Gelatin's versatile functionalization offers prospects of facile and effective crosslinking as well as combining with other materials (e.g., metal nanoparticles, carbonaceous, minerals, and polymeric materials exhibiting desired functional properties) to form hybrid materials of improved thermo-mechanical, physio-chemical and biological characteristics. Gelatin-based hydrogels (GHs) and (nano)composite hydrogels possess unique functional features that make them appropriate for a wide range of environmental, technical, and biomedical applications. The properties of GHs could be balanced by optimizing the hydrogel design. The current review explores the various crosslinking techniques of GHs, their properties, composite types, and ultimately their end-use applications. GH's ability to absorb a large volume of water within the gel network via hydrogen bonding is frequently used for water retention (e.g., agricultural additives), and absorbency towards targeted chemicals from the environment (e.g., as wound dressings for absorbing exudates and in water treatment for absorbing pollutants). GH's controllable porosity makes its way to be used to restrict access to chemicals entrapped within the gel phase (e.g., cell encapsulation), regulate the release of encapsulated cargoes within the GH (e.g., drug delivery, agrochemicals release). GH's soft mechanics closely resembling biological tissues, make its use in tissue engineering to deliver suitable mechanical signals to neighboring cells. This review discussed the GHs as potential materials for the creation of biosensors, drug delivery systems, antimicrobials, modified electrodes, water adsorbents, fertilizers and packaging systems, among many others. The future research outlooks are also highlighted.

Hofmeister effect in gelatin-based hydrogels with shape memory properties

The soaking strategy with the Hofmeister effect has been proposed to fabricate gelatin- based hydrogels with excellent properties. However, the modulation mechanism of hydrogels lacks in-depth study. In this work, we studied in detail the effects of Hofmeister ions on the structural, thermal, viscoelastic and mechanical properties of gelatin hydrogels. The results showed that kosmotropic anions (Cit^3-, SO₄^2-, H₂PO₄^- and S₂O₃^2-) enhanced hydrogen bonds and hydrophobic interactions between gelatin molecules, resulting in increases in the length and content of triple helices and thus improving the properties of gelatin hydrogels. In contrast, chaotropic anions (I^- and SCN^-) weakened the interactions between gelatin molecules, and thus attenuated the properties. Based on the Hofmeister effect, we successfully fabricated gelatin poly N-methylolacrylamide (PNMA) double network hydrogels with shape memory properties. The Hofmeister effect provides an excellent route for the rational design and fabrication of functional gelatin-based hydrogels.

Dual-Sensing, Stretchable, Fatigue-Resistant, Adhesive, and Conductive Hydrogels Used as Flexible Sensors for Human Motion Monitoring

Hydrogel-based sensors serve as an ideal platform for developing personalized wearable electronics due to their high flexibility and conformability. However, the weak stretchability and inferior conductivity of hydrogels have severely restricted their large-scale application. Herein, a natural polymer-based conductive hydrogel integrated with favorable mechanical properties, good adhesive performance, and excellent fatigue resistance was fabricated via interpenetrating tannic acid (TA) into a chitosan (CS) cross-linked network in an acidic aqueous solution. The hydrogel was composed of a regular hierarchical porous structure, which was built by the hydrogen bonding between TA and CS. In addition, the hydrogels exhibited adjustable mechanical properties (maximum yield stress of 7000 Pa) and good stretchability (strain up to 320%). Benefiting from the abundant catechol groups of TA, the proposed hydrogels could repeatedly adhere to various material surfaces and could be easily peeled off without residue. Moreover, the hydrogel exhibited stable conductivity, high stretching sensitivity (gauge factor of 2.956), rapid response time (930 ms), and excellent durability (>300 cycles), which can be assembled as a strain sensor to attach to the human body for precise monitoring of human exercise behavior, distinguishing physiological signals, and recognizing speech. Furthermore, the prepared hydrogels also exhibited stable sensing performance to temperature. As a result, the hydrogels exhibited dual sensory performance for both temperature and strain deformation. It is anticipated that the incorporation of strain sensors and thermal sensors will provide theoretical guidance for developing multifunctional conductive hydrogels and pave a way for the versatile application of hydrogel-based flexible sensors in wearable devices and soft actuators.

A self-healing, recyclable and conductive gelatin/nanofibrillated cellulose/Fe3+ hydrogel based on multi-dynamic interactions for a multifunctional strain sensor

Conductive hydrogels have emerged as promising material candidates for multifunctional strain sensors, attributed to their similarity to biological tissues, good wearability, and high accuracy of information acquisition. However, it is difficult to simultaneously manufacture conductive hydrogel-based multifunctional strain sensors with the synergistic properties of reliable healability for long-term usage and environmental degradability/recyclability for decreasing the electronic waste. This work reports a facile strategy to engineer a self-healing, recyclable and conductive strain sensor by virtue of molecular-level multi-dynamic interactions (MMDIs) including Schiff base complexes, hydrogen bonds, and coordination bonds, which were fabricated using a dialdehyde TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized nanofibrillated cellulose (DATNFC) pre-reinforced gelatin nanocomposite hydrogel (gelatin/DATNFC hydrogel, GDH) followed by dipping in an Fe³⁺ aqueous solution. The MMDI strategy allows synchronous regulation of both bulk and interfacial interactions to obtain exciting properties that outperform those of conventional hydrogels, including extraordinary compressive stress (1310 kPa), intriguing self-healing abilities, and remarkable electrical conductivity. With these outstanding merits, the as-prepared gelatin/DATNFC/Fe³⁺ hydrogel (GDIH) is developed to be a multifunctional strain sensor with appealing strain sensitivity (GF = 2.24 under 6% strain) and compressive sensitivity (S = 1.14 kPa^-1 under 15 kPa), which can be utilized to manufacture electronic skin and accurately discern subtle bodily motions, handwriting and personal signatures. Notably, this GDIH-based sensor also exhibited reliable self-healing properties for long-term usage, environmental degradability and complete recyclability for decreasing the electronic waste. In consideration of the extremely facile preparation process, biocompatibility, satisfactory functionalities, remarkable self-healing properties and recyclability, the emergence of the GDIH-based sensor is believed to propose a new strategy for the development of sustainable-multifunctional strain sensors and healthcare monitoring.

Nanocellulose-enhanced organohydrogel with high-strength, conductivity, and anti-freezing properties for wearable strain sensors

The use of ion-conductive hydrogels in strain sensors with high mechanical properties, conductivity, and anti-freezing properties is challenging. Here, high-strength, transparent, conductive, and anti-freezing organohydrogels were fabricated through the radical polymerization of polyacrylamide (PAM)/sodium alginate (SA)/TEMPO-oxidized cellulose nanofibrils (TOCNs) in a dimethyl sulfoxide (DMSO)/water solution, followed by soaking in a CaCl₂ solution. The resulting organohydrogels demonstrated a high strength (tensile strength of 1.04 MPa), stretchability (681%), transparency (>84% transmittance), and ionic conductivity (1.25 S m^-1). The organohydrogel-based strain sensor showed a high strain sensitivity (GF = 2.1). In addition, due to a synergistic effect between the DMSO/H₂O binary solvent and CaCl₂, the organohydrogel remained flexible (could bend 180°) and conductive (1.01 S m^-1) at -20 °C. Interestingly, the TOCNs exerted a reinforcing effect on both the mechanical properties and ionic conductivity. This research provides a novel strategy to prepare ion-conductive organohydrogels with good mechanical properties, conductivity, and anti-freezing properties for use as flexible electronic materials.

Mussel-inspired adhesive gelatin-polyacrylamide hydrogel wound dressing loaded with tetracycline hydrochloride to enhance complete skin regeneration

Even though the global wound care market size was valued at USD 19.83 billion in 2020, it is still a challenge to develop a hydrogel-based wound dressing with a good mechanical property, adhesiveness and antibacterial property. This study established and validated a mussel-inspired adhesive hydrogel wound dressing with antibacterial activity by dispersing tetracycline hydrochloride into hydrogel based polydopamine, gelatin and polyacrylamide. A tough hydrogel with a fracture stress of 0.42 MPa was prepared by changing the contents of the gelatin and polyacrylamide. With the addition of polydopamine and tetracycline hydrochloride, the hydrogel was endowed with an adhesive property (with a tissue adhesive strength of 4.13 kPa) and antibacterial activity against both Escherichia coli and Staphylococcus aureus. Finally, a rat full-thickness skin defect wound model was used to evaluate the performance of the hydrogels in wound repair. The hydrogel group showed a significantly reduced wound area (95.72%) compared with the blank group (86.34%) on day 14. The hydrogel promoted the collagen deposition, weakened the inflammatory response and enhanced wound healing. Therefore, the hydrogel with multifunctional properties is a promising candidate for complete skin regeneration.

Cellulose nanocrystals enabled sustainable polycaprolactone based shape memory polyurethane bionanocomposites

In recent years, shape memory polyurethanes have gained substantial attention and are targeted for a range of smart and functional materials. In this work, the development of nanocrystalline celluloses (CNCs) enabled polycaprolactone-based shape memory polyurethane biocomposite using an in situ one-pot reactions is reported. The incorporation of up to 10 wt% CNCs resulted in a remarkable enhancement in the tensile strength at yield (from 0.2 MPa to 7.2 MPa), tensile strength at break (167% improvement), and modulus of elasticity (from 3.5 to 139.3 MPa) while maintaining the elongation at break. This was attributed to the simultaneous action of CNCs as a nucleating agent for crystallization and highly compatibilized reinforcing agent of the network. Moreover, the in situ incorporation of CNCs enhanced the shape memory capability of polyurethanes, which enables its employment in functional material applications, such as the biomedical sector. The intimate interfacial adhesion between the CNCs and the polymer matrix, which promoted shape fixating and recovery, was confirmed by fractured surface morphology studies. Rheology characterizations provided strong evidence that the addition of CNCs increased the shape fixity attributed to the stiffness of CNCs below the glass transition temperature (T_g) compared to the neat PU in conjuncture with the higher T_g of CNCs. Overall, the developed polymer nanocomposites are appealing materials for biomedical applications.

TEMPO-oxidised nanocellulose hydrogels and self-standing films derived from bacterial cellulose nanopaper

Hydrogels derived from TEMPO-oxidised cellulose nanofibrils (TOCNs) are not robust and inherently water unstable if the TOCNs are not crosslinked or coated with a water-swellable polymer. Furthermore, the manufacturing of self-standing TOCN films is still a challenge due to the small TOCN diameter and the viscosifying effect of TOCNs. Here, we report the TEMPO-mediated oxidation of bacterial cellulose (BC) nanopaper as a route to produce robust and water stable TOCN hydrogels without the need of additional additives or crosslinking steps. Pristine BC pellicle was first press-dried into a dried and well-consolidated BC nanopaper, followed by TEMPO-oxidation at various NaClO concentrations. The oxidation reaction introduced carboxylate moieties onto the exposed BC nanofibrils within the nanopaper network structure. This then led to the expansion and swelling of the nanopaper into a hydrogel. A swelling ratio of up to 100 times the original thickness of the BC nanopaper was observed upon TEMPO-oxidation. The water retention value of the TEMPO-oxidised BC hydrogels was also found to increase with increasing carboxylate content. These TEMPO-oxidised BC hydrogels were found to be robust and water-stable, even under prolonged (>1 month) magnetic stirring in water. We further showed that high grammage self-standing TOCN films (100 g m-2) can be fabricated as simple as press-drying these water stable TEMPO-oxidised BC hydrogels without the need of vacuum-assisted filtration or slow-drying, which is typically the rate-limiting step in the manufacturing of TOCN films.

Gelator-Enhanced Organohydrogels with Switchable Mechanics and High-Strain Shape-Memory Capacity

Hydrogels and organogels, as two crucial representatives of soft materials, have attracted immense interest. However, they develop independently along two parallel lines, and these gels with single networks have their inherent drawbacks. For example, hydrogels tend to freeze, and organogels are usually brittle. Herein, organogels were incorporated into a hydrogel matrix for the synthesis of organohydrogels GOHs through polymerization in Pickering emulsion. The rigid organogel domains contribute to enhancing the strength of organohydrogels. Besides this, the organogels derived from 12-HAS self-assembly behavior exhibit a gel-sol transition when the temperature reaches 70 °C, thus leading to a thermo-softening behavior in the GOHs. Due to the phase transition of organogel domains and the elastic hydrogel network, the resultant organohydrogels demonstrate high-strain shape-memory performance (over 1000%) which could help achieve full recovery in seconds. Consequently, GOHs are endowed with the potential of practical application in soft robots, wearable devices, and biological materials.

Direct ink writing of aloe vera/cellulose nanofibrils bio-hydrogels

Direct-ink-writing (DIW) of hydrogels has become an attractive research area due to its capability to fabricate intricate, complex, and highly customizable structures at ambient conditions for various applications, including biomedical purposes. In the current study, cellulose nanofibrils reinforced aloe vera bio-hydrogels were utilized to develop 3D geometries through the DIW technique. The hydrogels revealed excellent viscoelastic properties enabled extruding thin filaments through a nozzle with a diameter of 630 μm. Accordingly, the lattice structures were printed precisely with a suitable resolution. The 3D-printed structures demonstrated significant wet stability due to the high aspect ratio of the nano- and microfibrils cellulose, reinforced the hydrogels, and protected the shape from extensive shrinkage upon drying. Furthermore, all printed samples had a porosity higher than 80% and a high-water uptake capacity of up to 46 g/g. Altogether, these fully bio-based, porous, and wet stable 3D structures might have an opportunity in biomedical fields.

Nanocellulose: Recent Fundamental Advances and Emerging Biological and Biomimicking Applications

In the effort toward sustainable advanced functional materials, nanocelluloses have attracted extensive recent attention. Nanocelluloses range from rod-like highly crystalline cellulose nanocrystals to longer and more entangled cellulose nanofibers, earlier denoted also as microfibrillated celluloses and bacterial cellulose. In recent years, they have spurred research toward a wide range of applications, ranging from nanocomposites, viscosity modifiers, films, barrier layers, fibers, structural color, gels, aerogels and foams, and energy applications, until filtering membranes, to name a few. Still, nanocelluloses continue to show surprisingly high challenges to master their interactions and tailorability to allow well-controlled assemblies for functional materials. Rather than trying to review the already extensive nanocellulose literature at large, here selected aspects of the recent progress are the focus. Water interactions, which are central for processing for the functional properties, are discussed first. Then advanced hybrid gels toward (multi)stimuli responses, shape-memory materials, self-healing, adhesion and gluing, biological scaffolding, and forensic applications are discussed. Finally, composite fibers are discussed, as well as nanocellulose as a strategy for improvement of photosynthesis-based chemicals production. In summary, selected perspectives toward new directions for sustainable high-tech functional materials science based on nanocelluloses are described.

Preparations of Tough and Conductive PAMPS/PAA Double Network Hydrogels Containing Cellulose Nanofibers and Polypyrroles

To afford an intact double network (sample abbr.: DN) hydrogel, two-step crosslinking reactions of poly(2-acrylamido-2-methylpropanesulfonic acid) (i.e., PAMPS first network) and then poly(acrylic acid) (i.e., PAA second network) were conducted both in the presence of crosslinker (N,N'-methylenebisacrylamide (MBAA)). Similar to the two-step processes, different contents of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) oxidized cellulose nanofibers (TOCN: 1, 2, and 3 wt.%) were initially dispersed in the first network solutions and then crosslinked. The TOCN-containing PAMPS first networks subsequently soaked in AA and crosslinker and conducted the second network crosslinking reactions (TOCN was then abbreviated as T for DN samples). As the third step, various (T-)DN hydrogels were then treated with different concentrations of FeCl3(aq) solutions (5, 50, 100, and 200 mM). Through incorporations of ferric ions into (T-)DN hydrogels, notably, three purposes are targeted: (i) strengthen the (T-)DN hydrogels through ionic bonding, (ii) significantly render ionic conductivity of hydrogels, and (iii) serve as a catalyst for the forth step to proceed with in situ chemical oxidative polymerizations of pyrroles to afford polypyrrole-containing (sample abbr.: Py) hydrogels [i.e., (T-)Py-DN samples]. The characteristic functional groups of PAMPS, PAA, and Py were confirmed by FT-IR. Uniform microstructures were observed by cryo scanning electron microscopy (cryo-SEM). These results indicated that homogeneous composites of T-Py-DN hydrogels were obtained through the four-step process. All dry samples showed similar thermal degradation behaviors from the thermogravimetric analysis (TGA). The T2-Py5-DN sample (i.e., containing 2 wt.% TOCN with 5 mM FeCl3(aq) treatment) showed the best tensile strength and strain at breaking properties (i.e., σTb = 450 kPa and εTb = 106%). With the same compositions, a high conductivity of 3.34 × 10-3 S/cm was acquired. The tough T2-Py5-DN hydrogel displayed good conductive reversibility during several "stretching-and-releasing" cycles of 50-100-0%, demonstrating a promising candidate for bioelectronic or biomaterial applications.

Facile fabrication of pH-sensitive nanoparticles based on nanocellulose for fast and efficient As(V) removal

This study reported a facile method to synthesize novel pH-sensitive nanoparticle based on nanocellulose, involving cross-linking polyethyleneimine and glutaraldehyde. The adsorbent was characterized and found to be sensitive to the solution pH, especially at pH 3. Additionally, the biosorbent exhibited rapid adsorption during the initial 10 min and the As(V) adsorption capacity of the nanoparticles reached approximately 255.19 mg g^-1 at pH 3, which was five times greater than that achieved with the As(V) solution at its initial pH (44.33 mg g^-1). To reflect its performance in actual acidic wastewater, the effects of coexisting anions were also investigated, showing that these anions had little influence on As(V) adsorption. Meanwhile, the adsorbent displayed excellent performance even after eight regeneration cycles. This novel material demonstrates enormous potential for the removal of arsenic contaminants and for the development of pH-sensitive materials.

Direct Ink Writing Based 4D Printing of Materials and Their Applications

4D printing has attracted academic interest in the recent years because it endows static printed structures with dynamic properties with the change of time. The shapes, functionalities, or properties of the 4D printed objects could alter under various stimuli such as heat, light, electric, and magnetic field. Briefly, 4D printing is the development of 3D printing with the fourth dimension of time. Among the fabrication techniques that have been employed for 4D printing, the direct ink writing technique shows superiority due to its open source for various types of materials. Herein, the state-of-the-art achievements about the topic of 4D printing through direct ink writing are summarized. The types of materials, printing strategies, actuated methods, and their potential applications are discussed in detail. To date, most efforts have been devoted to shape-shifting materials, including shape memory polymers, hydrogels, and liquid crystal elastomers, showing great prospects in areas ranging from the biomedical field to robotics. Finally, the current challenges and outlook toward 4D printing based on direct ink writing are also pointed out to leave open a significant space for future innovation.

Salt-Induced Shape-Memory Effect in Gelatin-Based Hydrogels

Hydrophilic biopolymers display a strong tendency for self-organization into stable secondary, tertiary, and quaternary structures in aqueous environments. These structures are sensitive to changes in external conditions, such as temperature, pH or ions/salts, which may lead to molecular and/or macroscopic transitions. Here, we report on biopolymer-based stimuli-sensitive switchable matrices showing a shape-memory function as an output being alternatively switched by two different input signals, such as environmental changes in salt concentration or temperature. This was realized by implementing a shape-memory function in hydrogels based on the coil-to-helix transition of protein chains in gelatin-based networks. The hydrogels exhibited mechanical properties similar to that of soft tissue (storage modulus G' = 1-100 kPa) and high swelling capabilities (Q = 1000-3000 vol %). In these gelatin-based networks, the covalent netpoints defined the permanent shape while after deformation helicalization of the gelatin acted as reversible stimuli-sensitive switches providing additional crosslinks capable of fixing the deformed temporary shape. By using either chaotropic salts to suppress gelatin helicalization or kosmotropic salts to support conformational changes of gelatin toward a helical orientation, these additional crosslinks could be cleaved or formed. In bending experiments, the strain fixity (R_f) and strain recovery ratios (R_r) were determined. While R_f ranged from 65 to 95% and was depending on the network composition, R_r were independent of the hydrogel composition with values about 100%. In addition, R_f and R_r were independent of the type of chaotropic salt that was used in this study, showing equal R_f and R_r values for MgCl₂, NaSCN, and Mg(SCN)₂.

Xanthan gum derivatives: review of synthesis, properties and diverse applications

Natural polysaccharides are well known for their biocompatibility, non-toxicity and biodegradability. These properties are also inherent to xanthan gum (XG), a microbial polysaccharide. This biomaterial has been extensively investigated as matrices for tablets, nanoparticles, microparticles, hydrogels, buccal/transdermal patches, tissue engineering scaffolds with different degrees of success. However, the native XG has its own limitations with regards to its susceptibility to microbial contamination, unusable viscosity, poor thermal and mechanical stability, and inadequate water solubility. Chemical modification can circumvent these limitations and tailor the properties of virgin XG to fulfill the unmet needs of drug delivery, tissue engineering, oil drilling and other applications. This review illustrates the process of chemical modification and/crosslinking of XG via etherification, esterification, acetalation, amidation, and oxidation. This review further describes the tailor-made properties of novel XG derivatives and their potential application in diverse fields. The physicomechanical modification and its impact on the properties of XG are also discussed. Overall, the recent developments on XG derivatives are very promising to progress further with polysaccharide research.

Due to presence of hydroxy and carboxy functional groups, xanthan gum is amenable to various chemical modification for producing derivatives such as carboxymethyl xanthan and carboxymethyl hydroxypropyl xanthan with desirable properties for end use.

Synthesis of a TEMPO-Substituted Polyacrylamide Bearing a Sulfonate Sodium Pendant and Its Properties in an Organic Radical Battery

A novel nitroxyl radical polymer poly(TEMPO-acrylamide-co-sodium styrene sulfonate) (abbreviated as poly(TAm-co-SSS)) was synthesized using 4-acrylamido-2,2,6,6- tetramethylpiperidine (AATP) copolymerized with styrene sulfonate sodium (SSS). AATP was synthesized through a substitution reaction of acryloyl chloride. Meanwhile, poly(4-acrylamido-2,2,6,6-tetramethylpiperidine-1-nitroxyl radical) (PTAm) was prepared as a control sample. Then, the structures of products were characterized by nuclear magnetic resonance spectroscopy (1H-NMR), Fourier transform infrared spectroscopy (FT-IR), high performance liquid chromatography-mass spectrometry (HPLC-MS), differential scanning calorimetry (DSC), X-Ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR), respectively. Additionally, the electrochemical impedance spectra (EIS) and the charge-discharge cycling properties were studied. The results demonstrated that the poly(TAm-co-SSS) with the side group of sodium sulfonate adjacent to TEMPO group exhibits a better charge-discharge cycling stability than that of the PTAm. Moreover, the charge specific capacity of the poly(TAm-co-SSS) is larger than that of the PTAm. Besides, the first coulombic efficiency of poly(TAm-co-SSS) is higher in comparison with that of PTAm. These superior electrochemical performances were ascribed to the synergistic effect of sulfonate ions group and nitroxyl radical structure, which benefits the improvement of charge carrier transportation of the nitroxyl radical polymers. Consequently, the nitroxyl radical poly(TAm-co-SSS) is promising for use in organic radical battery materials, based on the good electrochemical properties.

Highly Stretchable, Strain-Sensitive, and Ionic-Conductive Cellulose-Based Hydrogels for Wearable Sensors

To extend the applications of natural polymer-based hydrogels to wearable sensors, it is both important and a great challenge to improve their mechanical and electrical performance. In this work, highly stretchable, strain-sensitive, and ionic-conductive cellulose-based hydrogels (CHs) were prepared by random copolymerization of allyl cellulose and acrylic acid. The acquired hydrogels exhibit high stretchability (~142% of tensile strain) and good transparency (~86% at 550 nm). In addition, the hydrogels not only demonstrate better sensitivity in a wide linear range (0–100%) but also exhibit excellent repeatable and stable signals even after 1000 cycles. Notably, hydrogel-based wearable sensors were successfully constructed to detect human movements. Their reliability, sensitivity, and wide-range properties endow the CHs with great potential for application in various wearable sensors.

Physicochemical and bioactive properties of edible methylcellulose films containing Rheum ribes L. extract

Methylcellulose (MC) polymer was used to prepare the edible films. Then, the Rheum ribes L. ethanol extract (RE) was added to the films in order to act as a natural antimicrobial agent. All the methylcellulose films were characterized for bioactivity and the physicochemical analysis were carried out to define functional group interactions between the polymer and RE. The strongest antimicrobial effect was obtained with MC films enriched with 2% (w/w) RE against to Listeria monocytogenes with 20.3 ± 2.5 mm zone diameter. In the presence of RE, the tensile strength of film decreased, whereas the extract concentration significantly increased. The elongation at break and the water vapor permeability (WVP) values of the films were found to have decreased significantly in parallel with the increase in RE content. Moreover, the increase in the concentration of RE caused an increment in the hydrophilic properties, especially contact angle values of the films.

Advances in tissue engineering of nanocellulose-based scaffolds: A review

Scaffolds based on nanocellulose (NC) have crucial applications in tissue engineering (TE) owing to the biocompatibility, water absorption, water retention, optical transparency, and chemo-mechanical properties. In this review, we summarize the scaffolds based on nanocellulose, including nanocrystalline cellulose and nanofibrillated cellulose. We compare four representative methods to prepare NC-based scaffolds, containing electrospinning, freeze-drying, 3D printing, and solvent casting. We outline the characteristics of scaffolds obtained by different methods. Our focus is on the applications of NC-based scaffolds to repair, improve or replace damaged tissues and organs, including skin, blood vessel, nerve, skeletal muscle, heart, liver, and ophthalmology. NC-based scaffolds are attractive materials for regeneration of different tissues and organs due to the remarkable features. Finally, we propose the challenges and potentials of NC-based TE scaffolds.

High strength, anti-freezing and strain sensing carboxymethyl cellulose-based organohydrogel

The gel-based sensor was extensively investigated due to the flexible and extensible properties. Here, a flexible, excellent mechanical (fracture energy of 5238 kJ/m³) and anti-freezing ionic conductive carboxymethyl cellulose-based organohydrogel sensor was prepared via Fe³⁺ cross-linked sodium carboxymethyl cellulose (CMC) as the first network and covalently cross-linked polyacrylamide as the second network in the co-solvents of water and ethylene glycol. Owing to the clipping transportation of Fe³⁺ in the water channels, the gel sensor had good sensitivity (GF = 1.4, 0˜30% strain) and fast strain-responsiveness (0.98 s) to monitor the subtle motions of human body. The CMC-based organohydrogel with high strain-sensitivity exhibited more potential applications of next-generation bioelectronic materials and devices.

Thermal- and salt-activated shape memory hydrogels based on a gelatin/polyacrylamide double network

Shape memory hydrogels have been extensively studied in the past decades owing to their exceptionally promising potential in a wide range of applications. Here, we present a gelatin/polyacrylamide double network hydrogel with thermal- and salt-activated shape memory effect. The thermally activated behavior is attributed to the reversible triple helix transformation of gelatin, and the salt-activated performance can be ascribed to the formation of hydrophobic interaction domains under the Hofmeister effect. The hydrogel can memorize a temporary shape successfully through soaking with (NH4)2SO4 solution or decreasing temperature, and recovers its permanent shape by extracting ions with deionized water or increasing temperature. In particular, the hydrogel exhibits excellent shape fixity and recovery ratio. The presented strategy may enrich the construction as well as application of biopolymer based shape memory hydrogels.

Highly Stretchable and Compressible Cellulose Ionic Hydrogels for Flexible Strain Sensors

Stretchable and compressible hydrogels based on natural polymers have received immense considerations for electronics. The feasibility of using pure natural polymer-based hydrogels could be improved if their mechanical behaviors satisfy the requirements of practical applications. Herein, we report highly stretchable (tensile strain ∼126%) and compressible (compression strain ∼80%) cellulose ionic hydrogels (CIHs) among pure natural polymer-based hydrogels including cellulose, chitin, and chitosan via chemical cross-linking based on free radical polymerization of allyl cellulose in NaOH/urea aqueous solution. In addition, the hydrogels have good transparency (transmittance of ∼89% at 550 nm) and ionic conductivity (∼0.16 mS cm^-1) and can be worked at -20 °C without freezing and visual loss of transparency. Moreover, the CIHs can serve as reliable and stable strain sensors and have been successfully used to monitor human activities. Significantly, the various properties of hydrogel can be controlled through rationally adjusting the chemically cross-linked density. Our methodology will prove useful in developing the satisfied mechanical and transparent CIHs for a myriad of applications in flexible electronics.