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

The development of sustainable and high-performance flexible electronics is a rapidly growing field. Ionic conductive hydrogels, particularly those derived from renewable resources like cellulose, offer significant advantages in terms of biocompatibility, flexibility, and cost-effectiveness. Cellulose, the most abundant biopolymer on Earth, possesses inherent properties that make it attractive for hydrogel synthesis, including its biodegradability, renewability, and ability to form strong hydrogen bonds. However, a major challenge lies in creating cellulosic hydrogels that simultaneously exhibit high mechanical strength and high ionic conductivity. The introduction of ionic charge carriers, essential for conductivity, often disrupts the crucial hydrogen bonding networks that contribute to the hydrogel's structural integrity, leading to weak and brittle materials. Existing cellulosic hydrogels often compromise on either mechanical strength or conductivity. This research aims to address this challenge by developing a novel supramolecular engineering strategy to create a cellulosic hydrogel with both superior mechanical properties and high ionic conductivity. The successful creation of such a material will open up new possibilities for flexible electronics, sensors, actuators, and biomedical applications requiring robust, conductive, and sustainable materials. The incorporation of bentonite, a naturally occurring clay mineral, is proposed as a key component in overcoming this limitation. Bentonite possesses unique properties including its layered structure, high cation exchange capacity, and ability to form strong coordination bonds with cellulose, making it an ideal candidate for enhancing both the mechanical strength and ionic conductivity of the resulting hydrogel.

Numerous studies have investigated the development of ionic conductive hydrogels, with a particular focus on achieving high mechanical strength and flexibility. Many approaches have explored the use of synthetic polymers, such as polyacrylamide (PAAm) and poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS), often incorporating inorganic salts or conductive fillers to enhance ionic conductivity. However, these synthetic approaches often involve complex synthesis methods, potentially harmful chemicals, and lack the biodegradability and sustainability of natural alternatives. Other research has focused on using cellulose-based hydrogels, but these have faced the challenge of balancing mechanical strength with ionic conductivity. Previous studies have shown that incorporating various additives into cellulose-based hydrogels can improve their properties, such as the addition of ionic liquids or conductive polymers. However, these modifications often compromise the overall sustainability and biocompatibility of the resulting hydrogel. This paper aims to address these limitations by proposing a novel approach that utilizes the synergistic effects of cellulose and bentonite to create a high-performance, all-natural hydrogel. The use of bentonite in polymer composites has been previously reported to enhance mechanical properties, but its application in generating highly ionic conductive hydrogels with cellulose remains unexplored.

The research employed a supramolecular engineering strategy to synthesize the cellulose-bentonite (cellulose/BT) hydrogel. The process began with the extraction of high-purity cellulose from poplar wood powder through a series of chemical treatments, including alkaline extraction and sodium chlorite bleaching. This ensured the removal of lignin and hemicellulose, leaving behind purified cellulose fibers. Bentonite nanoplatelets were prepared via mechanical exfoliation, a method that disperses the clay into individual nanosheets. A 3 wt% cellulose solution was prepared by dissolving the cellulose pulp in an aqueous solution containing 7 wt% NaOH and 12 wt% urea at -12 °C. A specific amount of bentonite nanoplatelets (0.3g) and a chemical cross-linker, 1,4-Butanediol diglycidyl ether (BDE, 0.75g), were added to the cellulose solution (100g). The mixture was degassed and transferred into molds to form the gel. The gels were then immersed in deionized water for three days. Finally, to introduce ionic conductivity, the cellulose/BT hydrogels were soaked in 2 M LiCl solutions for 24 hours. This resulted in the ionic conductive cellulose/BT hydrogel (Ion-CB hydrogel). Various characterization techniques were used to thoroughly investigate the structure, morphology, and properties of the resulting hydrogel. Confocal laser scanning microscopy (CLSM), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) were employed to analyze the microstructure and elemental composition. Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and solid-state NMR spectroscopy were used to investigate the molecular interactions and structural changes within the hydrogel. Density functional theory (DFT) calculations were performed to investigate the interaction energies between cellulose and bentonite and evaluate the impact of LiCl on the system. Mechanical properties were evaluated through compressive and tensile tests, including cyclic loading to assess the hydrogel's resilience. Electrochemical impedance spectroscopy (EIS) was used to measure ionic conductivity at various temperatures, including subzero conditions. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to analyze the phase transition behavior of the hydrogel and understand its freezing tolerance. Finally, proof-of-concept demonstrations were performed to showcase the hydrogel's applications in flexible electronics and sensing.

The study successfully synthesized a novel all-natural cellulose-bentonite hydrogel with remarkable properties. The hydrogel demonstrated exceptional mechanical strength, with a compressive strength of up to 3.2 MPa and a fracture energy of up to 0.45 MJ m⁻³. This represents a significant improvement compared to previously reported cellulose-based hydrogels. Importantly, the hydrogel also exhibited high ionic conductivity, reaching 89.9 mS cm⁻¹ at 25 °C and maintaining a conductivity of 25.8 mS cm⁻¹ even at −20 °C. This freezing tolerance is superior to most reported conductive hydrogels. Detailed characterization techniques revealed the mechanism underlying these properties. FTIR, Raman, XRD, XPS, and solid-state NMR analyses confirmed the formation of strong Al-O-C bonds between the cellulose and bentonite, creating a robust cross-linked network responsible for the enhanced mechanical strength. DFT calculations further supported this finding, showing a high binding energy between cellulose and bentonite (-5.435 eV), even in the presence of LiCl. The addition of LiCl was found to significantly lower the freezing point of the hydrogel, leading to its remarkable freezing tolerance. The layered structure of the bentonite, interspersed with cellulose fibers, facilitated ion transport, contributing to the high ionic conductivity. The hydrogel's ability to maintain both its mechanical properties and high ionic conductivity at subzero temperatures was demonstrated through the successful operation of an LED at −20 °C while the hydrogel was subjected to bending and twisting. Finally, proof-of-concept demonstrations showed the hydrogel's potential as a sensor for detecting body movements and physiological signs.

This research successfully addressed the longstanding challenge of creating a cellulosic hydrogel with both high mechanical strength and high ionic conductivity. The incorporation of bentonite nanoplatelets, facilitated by strong Al-O-C cross-linking and hydrogen bonding, effectively countered the detrimental effects of ionic salts on the hydrogel's mechanical integrity. The synergistic combination of cellulose and bentonite, coupled with the addition of LiCl, led to a material exhibiting superior properties compared to most existing conductive hydrogels. The high ionic conductivity at subzero temperatures is particularly significant, opening possibilities for applications in extreme environments. The all-natural composition, combined with the straightforward and scalable synthesis method, makes this hydrogel a highly attractive candidate for sustainable and biocompatible applications in flexible electronics and sensors. The findings demonstrate the potential of supramolecular engineering strategies for designing and constructing high-performance hydrogels with precisely tailored properties. Future research should explore the long-term stability of the hydrogel under various conditions and its compatibility with different electronic devices. Optimizing the cellulose-bentonite ratio and exploring other naturally occurring clay minerals could further enhance the hydrogel's properties.

This study presents a novel all-natural, strong, tough, ionic conductive, and freezing-tolerant hydrogel based on cellulose and bentonite coordination interactions. The unique combination of exceptional mechanical strength, high ionic conductivity, and excellent freezing tolerance makes this material highly promising for various applications, particularly in flexible electronics, sensors, and biomedical devices. The sustainable nature of its components and the simple, scalable fabrication process further enhance its appeal. Future studies should focus on exploring broader applications and long-term stability assessments to further realize the full potential of this innovative material.

While the study demonstrates the exceptional properties of the developed hydrogel, some limitations need to be acknowledged. The long-term stability of the hydrogel under various environmental conditions (e.g., high humidity, prolonged exposure to extreme temperatures) requires further investigation. The effects of repeated bending and flexing on the long-term performance and durability of the hydrogel also need more extensive study. Although the proof-of-concept demonstrations showcase the potential of the hydrogel in sensing applications, a comprehensive evaluation of its performance in different types of sensors and its comparison with existing technologies would strengthen the findings. Finally, a more detailed cost analysis comparing the production of this hydrogel with existing materials would be beneficial for future commercialization.