A strategy of tailoring polymorphs and nanostructures to construct self-reinforced nonswelling high-strength bacterial cellulose hydrogels

A review of advanced helical fibers: formation mechanism, preparation, properties, and applications

As a unique structural form, helical structures have a wide range of application prospects. In the field of biology, helical structures are essential for the function of biological macromolecules such as proteins, so the study of helical structures can help to deeply understand life phenomena and develop new biotechnology. In materials science, helical structures can give rise to special physical and chemical properties, such as in the case of spiral nanotubes, helical fibers, etc., which are expected to be used in energy, environment, medical and other fields. The helical structure also has unique charm and application value in the fields of aesthetics and architecture. In addition, helical fibers have attracted a lot of attention because of their tendrils' vascular geometry and indispensable structural properties. In this paper, the development of helical fibers is briefly reviewed from the aspects of mechanism, synthesis process and application. Due to their good chemical and physical properties, helical fibers have a good application prospect in many fields. Potential problems and future opportunities for helical fibers are also presented for future studies.

Balancing mechanical property and swelling behavior of bacterial cellulose film by in-situ adding chitosan oligosaccharide and covalent crosslinking with γ-PGA

Bacterial cellulose (BC) is an ideal candidate material for drug delivery, but the disbalance between the swelling behavior and mechanical properties limits its application. In this work, covalent crosslinking of γ-polyglutamic acid (γ-PGA) with the chitosan oligosaccharide (COS) embedded in BC was designed to remove the limitation. As a result, the dosage, time, and batch of COS addition significantly affected the mechanical properties and the yield of bacterial cellulose complex film (BCCF). The addition of 2.25 % COS at the incubation time of 0.5, 1.5, and 2 d increased the Young's modulus and the yield by 5.65 and 1.42 times, respectively, but decreased the swelling behavior to 1774 %, 46 % of that of native BC. Covalent γ-PGA transformed the dendritic structure of BCCF into a spider network, decreasing the porosity and increasing the swelling behavior by 3.46 times. The strategy balanced the swelling behavior and mechanical properties through tunning hydrogen bond, electrostatic interaction, and amido bond. The modified BCCF exhibited a desired behavior of benzalkonium chlorides transport, competent for drug delivery. Thereby, the strategy will be a competent candidate to modify BC for such potential applications as wound dressing, artificial skin, scar-inhibiting patch, and so on.

Compressible, anti-freezing, and ionic conductive cellulose/polyacrylic acid composite hydrogel prepared via AlCl3/ZnCl2 aqueous system as solvent and catalyst

From the perspective of environmental sustainability, introducing cellulose into ionic conductive hydrogel is an inevitable trend for the development of flexible conductive materials. We report a double-network cellulose/polyacrylic acid (Cel/PAA) composite hydrogel based on the dissolving of cellulose by AlCl3/ZnCl2 aqueous system. The Cel/PAA composite hydrogel consists of rigid cellulose chains and flexible polyacrylic acid, which synergistically realize the improvement of the mechanical properties. The AlCl3/ZnCl2 aqueous system not only serves as the green solvent for cellulose, but also the Al3+ and Zn2+ metal ions can be served as a catalyst to activate the initiator for polymerization of acrylic acid. Compared with pure cellulose hydrogel, the compression strain of the Cel/PAA composite hydrogel was significantly improved to 80 %, and its conductivity increased by 28.1 %. In addition, its compression stress was enhanced over 2 times than pure PAA hydrogel. The Cel/PAA composite hydrogel exhibits excellent anti-freezing (-45 °C), weight retention (90 %), and conductivity (2.70 S/m) properties, still maintaining transparency and storage stability in the extreme environment. This work presents a facile strategy to develop an ionic conductive cellulose-based composite hydrogel with good conductivity and mechanical properties, which shows potential for the application fields of flexible sensors and 3D-printing functional materials.

Cellulose Dissolution, Modification, and the Derived Hydrogel: A Review

The cellulose-based hydrogel has occupied a pivotal position in almost all walks of life. However, the native cellulose can not be directly used for preparing hydrogel due to the complex non-covalent interactions. Some literature has discussed the dissolution and modification of cellulose but has yet to address the influence of the pretreatment on the as-prepared hydrogels. Firstly, the "touching" of cellulose by derived and non-derived solvents was introduced, namely, the dissolution of cellulose. Secondly, the "conversion" of functional groups on the cellulose surface by special routes, which is the modification of cellulose. The above-mentioned two parts were intended to explain the changes in physicochemical properties of cellulose by these routes and their influences on the subsequent hydrogel preparation. Finally, the "reinforcement" of cellulose-based hydrogels by physical and chemical techniques was summarized, viz., improving the mechanical properties of cellulose-based hydrogels and the changes in the multi-level structure of the interior of cellulose-based hydrogels.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

Hydrogels with tailor-made swelling-shrinkable properties have aroused considerable interest in numerous biomedical domains. For example, as swelling is a key issue for blood and wound extrudates absorption, the transference of nutrients and metabolites, as well as drug diffusion and release, hydrogels with high swelling capacity have been widely applicated in full-thickness skin wound healing and tissue regeneration, and drug delivery. Nevertheless, in the fields of tissue adhesives and internal soft-tissue wound healing, and bioelectronics, non-swelling hydrogels play very important functions owing to their stable macroscopic dimension and physical performance in physiological environment. Moreover, the negative swelling behavior (i.e., shrinkage) of hydrogels can be exploited to drive noninvasive wound closure, and achieve resolution enhancement of hydrogel scaffolds. In addition, it can help push out the entrapped drugs, thus promote drug release. However, there still has not been a general review of the constructions and biomedical applications of hydrogels from the viewpoint of swelling-shrinkable properties. Therefore, this review summarizes the tactics employed so far in tailoring the swelling-shrinkable properties of hydrogels and their biomedical applications. And a relatively comprehensive understanding of the current progress and future challenge of the hydrogels with different swelling-shrinkable features is provided for potential clinical translations.

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

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

Self-Assembly of Nanocellulose Hydrogels Mimicking Bacterial Cellulose for Wound Dressing Applications

The self-assembly of nanocellulose in the form of cellulose nanofibers (CNFs) can be accomplished via hydrogen-bonding assistance into completely bio-based hydrogels. This study aimed to use the intrinsic properties of CNFs, such as their ability to form strong networks and high absorption capacity and exploit them in the sustainable development of effective wound dressing materials. First, TEMPO-oxidized CNFs were separated directly from wood (W-CNFs) and compared with CNFs separated from wood pulp (P-CNFs). Second, two approaches were evaluated for hydrogel self-assembly from W-CNFs, where water was removed from the suspensions via evaporation through suspension casting (SC) or vacuum-assisted filtration (VF). Third, the W-CNF-VF hydrogel was compared to commercial bacterial cellulose (BC). The study demonstrates that the self-assembly via VF of nanocellulose hydrogels from wood was the most promising material as wound dressing and displayed comparable properties to that of BC and strength to that of soft tissue.

Highly conductive and anti-freezing cellulose hydrogel for flexible sensors

Ionic conducting hydrogels (ICHs) are emerging materials for multi-functional sensors in the fields of healthcare monitoring and flexible electronics. However, there is a long-standing dilemma between ionic conductivity and mechanical properties of the ICHs. In this work, ionic conductive, flexible, transparent, and anti-freezing hydrogels are fabricated by dissolving cotton linter pulp in ZnCl2/CaCl2 solution and cross-linking with epichlorohydrin (ECH). The presence of inorganic salt imparts the hydrogel with high ionic conductivity and low-temperature tolerance. While the introduction of ECH as the second network gives the hydrogel with desirable mechanical performance. By tailoring the ECH addition, the tensile strength, compressive strength, elongation at break, and conductivity of the hydrogel could reach 0.82 MPa, 2.80 MPa, 260 %, and 5.48 S m-1, respectively. The prepared ICHs are fabricated into sensors for detecting full-range human body motions, and they demonstrate fast response and durable sensitivity to both tensile strain and compressive deformation. Moreover, flexible sensors can work at subzero temperatures. This work provides a new idea for the preparation of cellulose-based hydrogels with good ionic conductivity and mechanical properties under extreme conditions.

A UV-induced self-reinforced hydrogel based on in situ hydrophobic aggregation of strained 1,2-dithiolane rings

A novel responsive hydrogel exhibiting self-reinforcement and self-healing capacity was developed based on the hydrophobic aggregation of strained 1,2-dithiolane rings. Oligomerization of 1,2-dithiolane within hydrophobic domains under UV irradiation not only reinforced the hydrogel but also maintained its dynamic cross-linked nature by converting the intraring dynamic S-S bond to an outer one.

Bacterial cellulose reinforced chitosan-based hydrogel with highly efficient self-healing and enhanced antibacterial activity for wound healing

Biocompatible hydrogels with versatile functions are highly desired for demanding the complicated tissue issues, including irregular site and motional wound. Herein, a bio-based hydrogel with multifunctional properties is designed based on quaternized chitosan and dialdehyde bacterial cellulose. As a functional wound dressing, the hydrogel shows rapid self-healing performance and injectable behaviors due to dynamic Schiff-base interactions and presents superior antibacterial activity against E. coli (gram-negative) and S. aureus (gram-positive). The constructed 3D hydrogel also exhibits proper compressive property, desired water retention capacity. To be mentioned, the hydrogel could mimic the structure of natural extracellular matrix (ECM) in the presence of bacterial cellulose nanofibers. Thus, the biopolymer-based hydrogel shows good biocompatibility in terms of cell proliferation and cell spreading. The prepared chitosan-based hydrogel with self-healing, antibacterial, and low cost will become a promising biomaterial for wound healing.

Strong, tough, ionic conductive, and freezing-tolerant all-natural hydrogel enabled by cellulose-bentonite coordination interactions

Ionic conductive hydrogels prepared from naturally abundant cellulose are ideal candidates for constructing flexible electronics from the perspective of commercialization and environmental sustainability. However, cellulosic hydrogels featuring both high mechanical strength and ionic conductivity remain extremely challenging to achieve because the ionic charge carriers tend to destroy the hydrogen-bonding network among cellulose. Here we propose a supramolecular engineering strategy to boost the mechanical performance and ionic conductivity of cellulosic hydrogels by incorporating bentonite (BT) via the strong cellulose-BT coordination interaction and the ion regulation capability of the nanoconfined cellulose-BT intercalated nanostructure. A strong (compressive strength up to 3.2 MPa), tough (fracture energy up to 0.45 MJ m⁻³), yet highly ionic conductive and freezing tolerant (high ionic conductivities of 89.9 and 25.8 mS cm⁻¹ at 25 and −20 °C, respectively) all-natural cellulose-BT hydrogel is successfully realized. These findings open up new perspectives for the design of cellulosic hydrogels and beyond.

Cellulose based ion conductive hydrogels are emerging materials for application in flexible electronics but achieving simultaneously high conductivity and good mechanical properties remains challenging. Here, the authors propose a supramolecular engineering strategy to strengthen cellulosic hydrogel and to improve simultaneously its ionic conductivity and freezing tolerance.

Self-Stretchable Fiber Liquid Sensors Made with Bacterial Cellulose/Carbon Nanotubes for Smart Diapers

Liquid sensors for detecting water and body fluids are crucial in daily water usage and health monitoring, but it is challenging to combine sensing performance with high tensile deformation and multifunctional applications. Here, a substrate-free, self-stretchable bacterial cellulose (BC)/carbon nanotube (CNT) helical fiber liquid sensor was prepared by the solution spinning and coiling process using BC as the water-sensitive matrix and CNTs as the active sensing materials. The BC/CNT (BCT) fiber sensor has a high stretch ratio of more than 1000% and a rapid response for a current change rate of 10⁴% within 1 s, which is almost unaffected under washing and various stretching or knotting deformations. By combination of the BCT fiber, we can design smart diapers or water level detectors, which rapidly monitor the status of smart diapers or water level, and the monitoring result can be transferred on time through an alarm device or smartphone. In short, the scalable and continuous preparation of the self-stretchable BCT helical fiber will provide a capacious platform for the development of a wearable sensor applied in daily life (such as smart diapers, water level detection, etc.).

Monolith/Hydrogel composites as triamcinolone acetonide carriers for curing corneal neovascularization in mice by inhibiting the fibrinolytic system

Corneal neovascularization is a serious corneal pathological change caused by various factors. The drug delivery system is of great significance for the effective treatment of corneal neovascularization. Herein, we developed and characterized a monolith/hydrogel composite as the triamcinolone acetonide (TA) carrier for curing corneal neovascularization. The composite was prepared by photo-initiated free radical polymerization of multi-methacrylate substituted dodecamine organic molecular cage and post-modified by the sequential photo-initiated free radical polymerization of acrylated gelatin. The globular morphology and structural property of as-prepared composites were evaluated by scanning electron microscopy, Fourier-transform infrared spectroscopy and solid-state cross polarization magic angle spinning carbon-13 nuclear magnetic resonance. Then swelling ratio and the TA loading capacity were investigated then. Compared with gelatin hydrogel, the composites exhibited a decreased swelling ratio and an improved loading capacity. With good biocompatibility, the composite can sustainedly release TA for up to 28 days, and effectively inhibit corneal neovascularization with an alkali burn-induced corneal neovascularization model. Additionally, tandem mass tags-labeled quantitative proteomics were performed to identify differentially expressed proteins between vascularized and devascularized corneas. The Kyoto Encyclopedia of Genes and Genomes enrichment analysis revealed that the inhibition process could be primarily linked to the fibrinolytic system. These results demonstrated the potential of monolith/hydrogel composites as delivery systems in the therapy for biomedical diseases.

Recent Progress in High-Strength and Robust Regenerated Cellulose Materials

High-strength petroleum-based materials like plastics have been widely used in various fields, but their nonbiodegradability has caused serious pollution problems. Cellulose, as the most abundant sustainable polymer, has a great chance to act as the ideal substitute for plastics due to its low cost, wide availability, biodegradability, etc. Herein, the recent achievements for developing cellulose "green" solvents and regenerated cellulose materials with high strength via the "bottom-up" route are presented. Cellulose can be regenerated to produce films/membranes, hydrogels/aerogels, filaments/fibers, microspheres/beads, bioplastics, etc., which show potential applications in textiles, biomedicine, energy storage, packaging, etc. Importantly, these cellulose-based materials can be biodegraded in soil and oceans, reducing environmental pollution. The cellulose solvents, dissolving mechanism, and strategies for constructing the regenerated cellulose functional materials with high strength and performances, together with the current achievements and urgent challenges are summarized, and some perspectives are also proposed. The near future will be an exciting era for high-strength biodegradable and renewable materials. The hope is that many environmentally friendly materials with good properties and low cost will be produced for commercial use, which will be beneficial for sustainable development in the world.

Anisotropic bacterial cellulose hydrogels with tunable high mechanical performances, non-swelling and bionic nanofluidic ion transmission behavior

Water-rich hydrogels with tissue-like softness, especially ion conductive hydrogels with ion signal transfer systems similar to biological areas, are promising soft electrode materials, while too poor or unstable mechanical properties that come from uncontrollable swelling and biocompatibility issues caused by introducing high concentration ions are serious obstacles in practical applications. Herein, a simple method for fabricating strong, stable, ion-conductive, anisotropic bacterial cellulose hydrogels (ABCHs) is first reported. Relying on nanofibers with high aspect ratio in bacterial cellulose (BC), a tailor-made nanofiber-network-reinforced structure is constructed by controlled dissolution, followed by aligning them well via a simple fossilizing process under stretching. Therefore, tunable high mechanical performances can be achieved and the maximum tensile strength can reach 14.3 MPa with 70% water content. It is worth noting that ABCHs will not swell in water for 30 days and maintain 93% tensile strength. Most importantly, the unique nanofluid behaviors from nanochannels in nanofibers allow effective ion transport in ABCHs relying only on low concentrations of ions in body fluids (<300 mM), avoiding sacrificing biocompatibility to achieve useful conductivity. This facile strategy might be very scalable in fabricating high-strength, non-swelling, bio-ion conductive cellulose hydrogels for application in next-generation bio-interfacing and flexible implantable devices.

Vapor phase polymerization for electronically conductive nanopaper based on bacterial cellulose/poly(3,4-ethylenedioxythiophene)

Eco-friendly conductive polymer nanocomposites have garnered attention as an effective alternative for conventional conductive nanocomposites. Here, we report the fabrication and optimization of flexible, self-standing, and conductive bacterial cellulose/poly(3,4-ethylene dioxythiophene) (BC/PEDOT) nanocomposites using the vapor phase polymerization (VPP) method. Eco-friendly bacterial cellulose (BC) is used as a flexible matrix, and the highly conductive PEDOT polymer is introduced into the BC matrix to achieve electronic conductivity. We demonstrate that vapor phase polymerized BC/PEDOT composites exhibit more than 10 times lower sheet resistance (18 Ω/square) compared to solution polymerized BC/PEDOT (188 Ω/square). The resultant BC/PEDOT fabricated could be bent up to 100 times and completely rolled up without a notable decrease in electronic performance. Moreover, bent BC/PEDOT films enable operation of a green light-emitting diode (LED) light, indicating the flexibility and stability of conductive BC/PEDOT films. Overall, this study suggests a strategy for the development of eco-friendly, flexible, and conductive nanocomposite films.

Mussel-inspired agarose hydrogel scaffolds for skin tissue engineering

Polysaccharide hydrogels are widely used in tissue engineering because of their superior biocompatibility and low immunogenicity. However, many of these hydrogels are unrealistic for practical applications as the cost of raw materials is high, and the fabrication steps are tedious. This study focuses on the facile fabrication and optimization of agarose-polydopamine hydrogel (APG) scaffolds for skin wound healing. The first study objective was to evaluate the effects of polydopamine (PDA) on the mechanical properties, water holding capacity and cell adhesiveness of APG. We observed that APG showed decreased rigidity and increased water content with the addition of PDA. Most importantly, decreased rigidity translated into significant increase in cell adhesiveness. Next, the slow biodegradability and high biocompatibility of APG with the highest PDA content (APG3) was confirmed. In addition, APG3 promoted full-thickness skin defect healing by accelerating collagen deposition and promoting angiogenesis. Altogether, we have developed a straightforward and efficient strategy to construct functional APG scaffold for skin tissue engineering, which has translation potentials in clinical practice.

High-Strength Superstretchable Helical Bacterial Cellulose Fibers with a "Self-Fiber-Reinforced Structure"

As a hydrogel membrane grown on the gas-liquid interface by bacterial culture that can be industrialized, bacterial cellulose (BC) cannot give full play to the advantages of its natural nanofibers. Conversion to the properties of nanofibers from high-performance to macrofibers represents a difficult material engineering challenge. Herein, we construct high-strength BC macrofibers with a "self-fiber-reinforced structure" using a dry-wet spinning method by adjusting the BC dissolution and concentration. The macrofiber with a tensile strength of 649 MPa and a strain of 17.2% can be obtained, which is one of the strongest and toughest cellulose fibers. In addition, the macrofiber can be fabricated to a superstretchable helical fiber without adding other elastomers or auxiliary materials. When the helical diameter is 1.6 mm, the ultimate stretch reaches 1240%. Meanwhile, cyclic tests show that the mechanical properties and morphology of the fiber remained stable after 100 times of 100% cyclic stretching. It is exciting that the helical fiber also owns outstanding knittability, washability, scalability, and dyeability. Furthermore, superstretchable functional helical BC fibers can be fabricated by embedding functional materials (carbon materials, conductive polymers, etc.) on BC or in the spinning dope, which can be made to wearable devices such as fiber solid-state supercapacitors. This work provides a scalable way for high-strength superstretchable and multifunctional fibers applied in wearable devices.

A new type of bilayer dural substitute candidate made up of modified chitin and bacterial cellulose

In the field of neurosurgery, timely and effective repair of dura mater plays an important role in stabilizing the physiological functions of the human body. Therefore, the aim of this study is to develop a new type of bilayer membrane as a dural substitute candidate. It consists of a dense layer that prevents cerebrospinal fluid leakage and a porous layer that promotes tissue regeneration. The dense layer, a composite polysaccharid film, was composed of high molecular weight chitosan (CS) and bacterial cellulose (BC). The porous layer, a composite polysaccharid scaffold cross-linked by glutaraldehyde (GA) or citric acid (CA) respectively, was composed of O-carboxymethyl chitin (O-CMCH) and BC. The bilayer dural substitutes were characterized in terms of SEM, mechanical behavior, swelling rate, anti-leakage test, in vitro cytotoxicity, proliferation, and animal experiment. Results indicated that all prepared dural substitutes were tightly bound between layers without excessively large cavities. The porous layer showed appropriate pore size (90～200 μm) with high porous connectivity. The optimized bilayer dural substitutes showed suitable swelling rate and mechanical behavior. Furthermore, no leakage was observed during testing, no cytotoxicity effect on NIH/3T3 cells, and exhibited excellent cell proliferation promoting properties. Also, it was observed that it did not deform in the peritoneal environment of mice, and tissue inflammation was mild.