1. Introduction

Considering the continuous demand for environmentally friendly materials in modern industrial and environmental fields, the paper industry, which is a leading sector in cellulose utilization, is focusing substantial efforts on the development of sustainable materials and expansion of its market presence. Increasing attention is being directed toward functional packaging materials that can replace plastics and a wide range of advanced functional materials derived from nanocellulose and cellulose derivatives. Cellulose can be processed into diverse material forms, including foams, sheets, hydrogels, aerogels, films, paper, and paperboard, broadening its applicability in multiple fields. In the case of sustainable packaging and biomedical fields, cellulose hydrogels are often compared with other green materials such as polylactic acid (PLA) and starch-based materials. While PLA offers good mechanical strength and processability, it lacks the hydrophilicity and swelling behavior essential for hydrogel functions. Starch-based hydrogels, on the other hand, show good biodegradability but often suffer from brittleness and limited swelling stability.

Cellulose hydrogels have garnered increasing attention owing to their unique physicochemical properties and biodegradability. As the most abundant natural polymer on Earth, cellulose is a renewable and inexpensive resource, and it is an ideal base material for environmentally friendly technologies. Hydrogels derived from cellulose exhibit high water absorption capacity, flexibility, biocompatibility, and potential for diverse functional modifications. These characteristics enable their application in diverse fields, including biomedical engineering, environmental remediation, packaging, and sensing technologies. Owing to their eco-friendly characteristics and high water content, cellulose hydrogels can provide environments similar to those of biological systems. Therefore, cellulose hydrogels hold considerable promise in biomedical applications, such as tissue engineering, drug delivery systems, and wound dressings. Furthermore, cellulose hydrogels exhibit responsive behaviors toward external stimuli, such as temperature, humidity, and pH, rendering them particularly suitable for smart packaging applications. Smart packaging aims to monitor food freshness and safety through its responses to environmental changes, and the responsive properties of cellulose hydrogels align well with these objectives. This paper aims to review the fabrication methods and properties of cellulose hydrogels and to analyze their potential and limitations across different application fields, particularly focusing on smart packaging. By presenting recent research trends, this study emphasizes the role of cellulose hydrogels and their environmentally friendly characteristics in emerging applications, offering insights into future development prospects.

2. Cellulose

Cellulose, which is the most abundant natural polymer, is a linear homopolysaccharide composed of D-glucose units linked by β-1,4-glycosidic bonds. Cellulose can be extracted from plants and certain bacterial species and has been widely utilized in the pulp and paper industry as well as in other fields owing to its numerous advantages, including biodegradability, excellent biocompatibility, nontoxicity, and high thermal and chemical stability.^1,2,3,4) Conventionally, wood-derived pulp has been used as the primary raw material for cellulose extraction.⁵⁾ However, recent research has increasingly focused on alternative sources, such as recycled pulp,⁶⁾ nonwood biomass, including seaweed and herbaceous plants, and agricultural by-products, such as corn husks and rice husks.^{7,8,9,10,11,12)} These sources have demonstrated potential for cellulose extraction and subsequent chemical modification for various applications. Structurally, cellulose is a homopolymer consisting of β-1,4-linked D-glucopyranose units. The β-glycosidic linkage induces a 180° rotation between adjacent glucose monomers, resulting in repeating disaccharide units known as cellobiose, each featuring a length of ~1.3 nm. The terminal ends of the cellulose chains are chemically distinct: one end is a reducing end possessing a hemiacetal form of D-glucopyranose, whereas the other end features a nonreducing terminus involved in the glycosidic linkage. Each glucose unit comprises three hydroxyl groups at the C2, C3, and C6 positions, which exhibit the typical reactivity of primary and secondary alcohols, facilitating diverse chemical modifications. These free hydroxyl groups enable the introduction of various functionalities via chemical modification.

3. Activation of Cellulose

The accessibility of cellulose is a measure of the degree to which enzymes, chemical reagents, or other agents interact with the surface or internal structure of cellulose molecules. The accessibility is primarily determined by the molecular arrangement and surface characteristics of cellulose. To enhance accessibility, the use of physicochemical treatments that disrupt or modify the crystalline regions and alter the particle size is often necessary. As illustrated in Fig. 1, the extensive hydrogen bonding network within cellulose considerably impedes the reactivity of cellulose, serving as a major barrier to chemical or enzymatic modification. To overcome this limitation, activation processes are employed to introduce reactive functional groups into cellulose or disrupt the hydrogen-bonded structure. These modifications facilitate the synthesis of various cellulose derivatives and the dissolution of cellulose in solvent systems for subsequent hydrogel fabrication.

Fig. 1.

Hydrogen bonding of cellulose (a: intramolecular bonding, b: intermolecular bonding, c: crystalline and amorphous regions) (reproduced from Ref. 13, 14).^13,14)

3.1 Dissolution system of cellulose

For the fabrication of hydrogels using a dissolution system, an appropriate solvent capable of dissolving cellulose and a suitable counter solvent for inducing gelation are essential. The choice of the dissolution system considerably affects the properties of the resulting hydrogel. The complete dissolution of cellulose enhances the probability of the formation of a stable and uniform network structure during gelation. The current major cellulose dissolution systems include NaOH/urea,¹⁵⁾ N-methylmorpholine-N-oxide (NMMO),¹⁶⁾ ionic liquids (ILs), which maintain a liquid state over a wide temperature range and exhibit properties such as nontoxicity, nonflammability, and high ionic conductivity,¹⁷⁾ and deep eutectic solvents (DESs), which are the mixtures of two or more components that form a eutectic whose melting point is lower than that of individual components.^18,19) After the addition of a counter solvent to the cellulose solution, cellulose undergoes reprecipitation or aggregation, forming a cross-linked network structure and, ultimately, a hydrogel. The counter solvent and cellulose compete to interact with the solvent, inducing the separation of cellulose from the solution. For cellulose systems dissolved in ILs, the commonly used counter solvents include water, ethanol, and acetone. The macroscopic properties of cellulose hydrogels are influenced by factors such as the solvent, counter solvent, cellulose concentration, and colloidal environment.^20,21)

3.2 Cellulose derivatives

Cellulose derivatives are primarily synthesized via the chemical modification of dissolving-grade cellulose, which is characterized by a uniform molecular weight distribution and an α-cellulose content of >90%, resulting in high chemical reactivity. Representative methods for the preparation of cellulose derivatives include etherification, esterification, and oxidation, which yield derivatives possessing diverse physical properties. Cellulose ethers, such as methyl cellulose, carboxymethyl cellulose (CMC), and hydroxyethyl cellulose (HEC), are water soluble and possess a tunable viscosity. Cellulose esters, including cellulose acetate and nitrocellulose, are advantageous for improving the solvent solubility and mechanical strength. Additionally, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose, which introduces surface charges, can be applied in fields such as biomaterials and smart packaging. As depicted in Fig. 2, cellulose derivatives can be chemically modified via grafting with the various functional groups. Derivatization techniques, such as etherification, esterification, and oxidation, enhance the reactivity of cellulose, facilitating the fabrication of hydrogels. Etherification introduces ether groups into cellulose chains, improving the solubility and flexibility of the resulting material. The selective oxidation of cellulose introduces functional groups, such as aldehydes and carboxyls. Particularly, TEMPO-mediated oxidation converts primary hydroxyl groups into carboxyl groups, enhancing the hydrophilicity and ion-responsive behavior of the resulting hydrogels.

Fig. 2.

Classification of cellulose derivatives; a: chemical conversion procedures of cellulose to its derivatives, b: chemical structure of cellulose derivatives (reproduced from Ref. 4).⁴⁾

4. Fabrication Methods and Properties of Cellulose-Based

In contrast to other polymers, cellulose cannot be readily used in its native form because of its abundant hydroxyl groups. Cellulose must be subjected to various physicochemical modifications to form cellulose hydrogels. Cellulose hydrogels are three-dimensional (3D) hydrophilic polymer networks formed via cross-linking. Various crosslinking strategies have been developed for the fabrication of cellulose-based hydrogels, and the properties of the resulting materials considerably vary depending on the cross-linking method.

4.1 Fabrication methods and characteristics of cellulose-based macrogels

Hydrogels are physically or chemically cross-linked 3D hydrophilic polymer networks capable of absorbing large amounts of water or biological fluids, resulting in swelling.^22,23) These networks are composed of single polymers or copolymers and exhibit insolubility owing to chemical (e.g., bonding points and junctions) or physical (e.g., entanglements or crystalline domains) cross-links. Hydrogels swell in the presence of a solvent because of their thermodynamic interactions with water.²⁴⁾ The cross-linking process is essential for regulating the physicochemical properties of cellulose hydrogels. The cross-linking methods of cellulose hydrogels can be broadly classified into physical and chemical crosslinking. Physical crosslinking involves the formation of crosslinks via physical interactions. The freeze-thaw cycle method promotes hydrogen bonding and crystallization among cellulose chains, inducing physical cross-linking and producing hydrogels possessing excellent mechanical strength.²⁵⁾ Furthermore, the incorporation of multivalent cations (e.g., Ca²⁺ and Al³⁺) can induce ionic cross-linking in cellulose derivatives, such as alginate or CMC, enabling the adjustment of the mechanical properties and responsiveness of the resulting hydrogels.²⁶⁾ For example, the grafting of cellulose with poly(N-isopropylacrylamide) imparts thermal responsiveness to the resulting material, whereas poly(acrylic acid) grafting introduces pH sensitivity.²⁷⁾ Moreover, leveraging the inherent hydrogen bonding ability of cellulose, self-assembled hydrogels can be formed under mild conditions, and these hydrogels often exhibit reversible swelling and dehydration behavior in response to external stimuli. Moreover, cellulose nanomaterials can be incorporated into hydrogels to enhance the mechanical strength, structural integrity, and thermal stability. Hydrogels reinforced with cellulose nanomaterials form microporous networks that improve the water retention and transport properties of the material. Furthermore, the incorporation of nanoparticles, such as silica, into cellulose enhances the mechanical performance, rendering the as-formed materials suitable for applications such as wound healing.²⁸⁾Chemical crosslinking forms 3D network structures through covalent bonding between polymer chains. Chemical cross-linking methods involves the addition of cross-linking agents capable of forming covalent bonds with cellulose, such as epichlorohydrin (ECH),²⁹⁾ diisocyanates,³⁰⁾ aldehydes,³¹⁾ and acrylic acid.³²⁾ Furthermore, dual cross-linking strategies that combine both physical and chemical crosslinking have been developed to optimize the mechanical properties of hydrogels.^33,34) Chemically or dually crosslinked hydrogels offer superior structural stability and multifunctionality compared to the hydrogels featuring exclusively physical crosslinks; however, they present challenges such as the potential toxicity of crosslinking agents and high manufacturing costs.

4.2 Properties and fabrication methods of cellulose-based microgels

Microgels are soft, swellable colloidal materials formed from crosslinked polymers through covalent bonding or other strong interactions and are characterized by microsized gel particles containing an internal gel-like network structure. Specifically, they are particulate gel materials and are frequently referred to as hydrogel particles, nanogels, microspheres, or microbeads.³⁵⁾ These microgels comprise crosslinked polymer networks, and their degree of swelling is determined by the solvent properties and crosslinking density.³⁶⁾ Owing to the aforementioned unique characteristics, microgels serve as versatile platform materials in fields such as food, biotechnology, environmental science, and energy, functioning as emulsion stabilizers, delivery systems, building blocks for cell culture and tissue engineering, catalytic platforms, sensors, and purifier systems. In principle, all gelling materials can be fabricated into microgel particles; however, the majority of systems studied thus far are composed of synthetic or natural polymers. Microgels can be obtained via the controlled growth of polymer networks, which is known as the “bottom-up” approach, or by the breaking down of bulk hydrogels into fine particles, which is referred to as the “top-down” method.³⁷⁾ The fabrication of cellulose-based microgels requires a combination of cellulose material pretreatment and advanced production techniques.

4.2.1 Bottom-up fabrication of cellulose-based microgels

Cellulose-based microgels are generally fabricated through methods such as the dissolution and regeneration of cellulose, the self-assembly of nanocellulose, and the combination of cellulose nanomaterials with other polymers. The dissolution and regeneration method requires a suitable solvent that dissolves cellulose and an antisolvent that induces gelation. Currently, the major solvents employed for cellulose dissolution include aqueous alkali/urea solvent systems,³⁸⁾ NMMO,³⁹⁾ ILs,⁴⁰⁾ and DESs.⁴¹⁾ Upon the addition of an antisolvent possessing contrasting properties, such as water, ethanol, and acetone, the cellulose solution undergoes regeneration, forming a cross-linked network and precipitating or coagulating cellulose. The physicochemical properties of regenerated cellulose microgels are mainly influenced by the solvent, antisolvent, cellulose concentration, and physical aggregation conditions.³⁷⁾Emulsification is among the most widely used approaches for fabricating microgels. An emulsion is a thermodynamically unstable colloidal system in which the droplets of one phase (ranging from ~10 nm to 100 µm) are dispersed within another continuous phase. In the emulsion-templated method, a preprepared water-in-oil emulsion serves as a template for gelation. The first step of this method involves the formation of oil-water emulsions. In this method, during high-shear mixing processes, such as high-pressure homogenization and colloid milling, aqueous droplets are dispersed and adsorbed on the oil-droplet surface owing to their surface-active properties. This adsorption reduces the interfacial tension and stabilizes the oil droplets via electrostatic mechanisms.⁴²⁾ After gelation, the microgels are typically separated from the oil phase via centrifugation or filtration. The size of the fabricated microgels is influenced by factors such as the oil viscosity, stirring strength, oil-to-water ratio, and surfactant properties. Surfactants are commonly utilized as stabilizers, which lower the surface tension and prevent phenomena such as droplet coagulation, aggregation, and Ostwald ripening.⁴³⁾ Typically, amphiphilic organic compounds serve the aforementioned purposes by diffusing into the aqueous phase and adsorbing at the interfaces to impart colloidal stability. When solid particles are used instead of molecular surfactants, the resulting emulsions are known as Pickering or Ramsden emulsions.⁴⁴⁾ Pickering stabilization is achieved by the solid particles adsorbed at the interface, such as fat crystals,⁴⁵⁾ starch,⁴⁶⁾ chitin nanomaterials,⁴⁷⁾ and cellulose nanoparticles.⁴⁸⁾ Cai et al.⁴⁹⁾ prepared microgels using cellulose nanofibers to stabilize a Pickering emulsion containing lemon essential oil, achieving excellent interfacial wettability and antibacterial activity. They engineered cellulose nanomaterials as emulsifier stabilizers to successfully endow the microgels with the targeted characteristics. Conventional emulsification methods, such as high-shear homogenization, ultrasonic emulsification, and high-speed mixing, achieve emulsion formation via the physical dispersion of the oil and water phases. Although they demonstrate simple operation, these methods have limitations in the precise control of the droplet size and distribution, and they are energy intensive. Conversely, microfluidic techniques provide superior control over the fluid behavior, enabling the formation of monodisperse droplets through the precise manipulation of the interfacial tension and shear forces in the microchannels.⁴⁹⁾ Microfluidic methods offer the fine control of the droplet size and distribution via the adjustment of the flow rates, oil-to-water volume ratios, and emulsifier types and concentrations, minimizing the instability and enabling microgel fabrication in small reaction volumes with high automation and precision.⁵⁰⁾

4.2.2 Top-down fabrication of cellulose-based microgels

Microgels can be obtained via the mechanical disintegration of macrogels into particles possessing microscale dimensions. The particle size of the resulting microgels can be controlled by adjusting the shear rate of the mechanical device. Li et al.⁴²⁾ applied high shearing to chemically cross-linked regenerated cellulose macrogels to prepare cellulose microhydrogels featuring a narrow particle size distribution and well-defined microstructures. These microgels could be uniformly dispersed in water and formed interpenetrating networks with polymerized polyacrylamide. The microstructure of the microgels could be fine-tuned by controlling the cross-linking density, resulting in enhanced mechanical properties.⁵¹⁾ Zhang et al.⁵²⁾ formed macrogels by mixing CMC with a cross-linking agent and subsequently disrupted these macrogels via physical shearing to fabricate lysozyme-microgel composites. Furthermore, nanocellulose can be used as an additive in the preparation of composite macrogels. Lu et al.⁵³⁾ demonstrated the fabrication of antimicrobial TOCNF/nisin hydrogel microparticles (microgels) through the mechanical shearing of cellulose-derived macrogels. The TOCNF/nisin hydrogel was structured via electrostatic interactions between the peptide nisin and TEMPO-oxidized cellulose nanofibers derived from sugarcane, and it was applied as a coating material for antimicrobial paper packaging. Although this top-down approach is simpler than the bottom-up strategies, it requires considerable time and energy. Moreover, the control of the particle size in this method is challenging, frequently leading to the formation of polydisperse microgels.

5. Applications of Cellulose-Based Hydrogels

Cellulose hydrogels possess nontoxicity, biodegradability, and excellent biocompatibility, together with easily tunable microstructures and porosity. Owing to these favorable characteristics, extensive research has been conducted to explore the application of cellulose hydrogels as active substance carriers, emulsion stabilizers, intelligent responsive materials, etc. Such studies explore the potential of cellulose hydrogels in various fields, such as biomedical materials, smart materials, and food science.

5.1 Biomedical applications

Owing to the increasing concerns regarding environmental issues and the growing demand for eco-friendly materials, the application of cellulose as a biomedical material has garnered considerable attention. Cellulose exhibits excellent biocompatibility, favorable physical and mechanical properties, and long-term physiological stability. Consequently, cellulose hydrogels have emerged as highly promising materials in the field of biomaterials, particularly for applications in drug delivery, tissue engineering, and wound healing. Because of its intrinsic properties, such as biocompatibility and biodegradability, cellulose is considered an ideal candidate for hydrogel fabrication. The introduction of various functional groups into cellulose derivatives can enhance the reactivity of these materials and facilitate their diverse modifications, expanding their applicability to multiple domains.⁵⁴⁾ Recently, cellulose hydrogels have received increasing attention in the biomedical sector, particularly in drug delivery, wound healing, and tissue engineering, owing to their excellent biocompatibility, biodegradability, and chemical stability.^55,56)

Conventional dressings used for wound healing are limited by weak bioactivity and low functionality, resulting in poor therapeutic outcomes and prolonged healing times. In this context, Zhang et al.⁵⁷⁾ developed a hybrid hydrogel matrix composed of cellulose nanofibers and poly(vinyl alcohol) (PVA), and incorporated curcumin and Ag nanoparticles into it. This hydrogel exhibited high mechanical strength, self-healing properties, and antimicrobial activity against Staphylococcus aureus and Escherichia coli, in addition to antioxidant capacity and sustained drug release behavior, rendering it effective for the treatment of chronic wounds. Jeong et al.⁵⁸⁾ developed a gelatin/oxidized CMC-based hydrogel loaded with fucoidan extracted from Undaria pinnatifida for wound healing. Owing to its modulated drug release, this hydrogel effectively promoted reepithelialization and collagen deposition, accelerating wound recovery. Foam-based wound dressings, which were developed by dispersing gases in polymeric materials, offer the advantage of the efficient absorption of wound exudates owing to their porous structure. Consequently, research on the replacement of petroleum-based foam dressings with cellulose hydrogel-based alternatives is ongoing. Orhan et al.⁵⁹⁾ fabricated a hydrogel dressing by foaming a mixture of CMC, PVA, and cerium oxide nanoparticles, followed by cross-linking with a sodium tetraborate solution and a freeze-thaw process. The as-formed hydrogel exhibited antimicrobial activity, excellent swelling behavior, and effective drug release performance. Despite their biocompatibility, cellulose-based biomaterials used in wound dressings or drug delivery must meet strict regulatory guidelines. Adverse effects such as the modulation of immune cell function and a reduction in lysosomal stability, and long-term degradation behavior are critical for clinical approval by agencies.

5.2 Packaging applications

Owing to the increasing environmental concerns associated with plastic pollution, the development of sustainable packaging materials has garnered growing research attention. The key requirements for such materials are reusability, recyclability, and biodegradability after disposal, which minimize their ecological impact. From this perspective, cellulose is a highly promising material for application in sustainable packaging. However, its practical application is hindered by certain limitations, such as low elongation at break and hydrophilicity, which result in their inferior barrier properties against moisture, oxygen, and microorganisms compared to those of conventional plastics. As depicted in Fig. 3, cellulose hydrogels exhibit potential as efficient packaging materials owing to their durability, biodegradability, and mechanical strength. Seo et al.⁶⁰⁾ developed a transparent film, possessing excellent mechanical properties, composed of polyvinylpyrrolidone and CMC, which could biodegrade within 8 weeks. This hydrogel-based packaging material was suitable for the packaging of moisture-sensitive vegetables and fruits. Ebranimi et al.⁶¹⁾ demonstrated that CMC films incorporated with Ag, ZnO, and CuO nanoparticles could effectively inhibit the growth of Escherichia coli and Staphylococcus aureus, and Ag nanoparticles exhibited the highest antibacterial activity.

Fig. 3.

Application of hydrogels in food packaging (reprinted from Ref. 66).⁶⁶⁾

Hydrogel films fabricated using cellulose microgels exhibit high mechanical toughness and enable the integration of various functional microgels for constructing customized functional matrices.⁶²⁾ For instance, a composite hydrogel film was prepared by embedding microgels, which were obtained via cross-linking from dialdehyde nanocellulose and tannin, into a gelatin network. The resulting composite, which was formed via Schiff base linkages, hydrogen bonding between nanocellulose and gelatin, and interactions between tannin and gelatin, exhibited excellent mechanical properties, ultraviolet-blocking ability, and antioxidant capacity.⁶³⁾ Liu et al.⁶⁴⁾ developed a method to produce an eco-friendly, biodegradable packaging material for preserving refrigerated tilapia fillets. A composite microgel film possessing antimicrobial and biodegradable properties was prepared by cross-linking a matrix comprising cellulose and chitosan. The resulting film displayed enhanced tensile strength and elongation at break because of the formation of a cross-linked structure and strong hydrogen bonding within the microgel. Moreover, it demonstrated a moisture barrier and antibacterial performance, consequently extending the shelf life of refrigerated tilapia. Cai et al.⁶⁵⁾ used nanocellulose microgels as stabilizers in the emulsification of lemon essential oil for preparing Pickering emulsions. A transparent and flexible film was fabricated via chemical bonding with polyethyleneimine and nanocelluloses, which exhibited excellent interfacial wettability and antimicrobial properties.

5.3 Environmental applications

Cellulose hydrogels have considerable potential for the effective adsorption of heavy metals, dyes, and organic pollutants from wastewater. Their ability to respond to environmental stimuli, such as pH and temperature, enables the controlled adsorption and desorption of pollutants, rendering them as reusable and efficient materials. Moreover, the inherent porous structure of cellulose hydrogels enhances their contact area with contaminants, improving the purification performance. Al-Hazmi et al.⁶⁷⁾ developed hydrogel beads by cross-linking chitosan and CMC with ECH and incorporating a cerium-based metal-organic framework into the matrix. As shown in Fig. 4, These bead-shaped hydrogels exhibited high efficacy in the removal of Ni²⁺ ions from aqueous media and maintained excellent removal efficiency even after six reuse cycles. Yi et al.⁶⁸⁾ synthesized a hydrogel via free-radical polymerization using cellulose, acrylic acid, and acrylamide, which selectively adsorbed heavy metal cations, such as Pb²⁺, Cd²⁺, and Cu²⁺, in multiion aqueous environments. Wang et al.⁶⁹⁾ prepared a hydrogel by combining cellulose nanofibers with poly(acrylic acid) and introducing dansyl chloride derivatives possessing fluorescent properties. The chelation interactions between the dansyl chloride groups and metal ions enabled the efficient adsorption and removal of Cu²⁺ and Cr⁶⁺ from water. Modified cellulose hydrogels possessing hydrophobic or oleophilic characteristics are biodegradable and can selectively absorb oils and organic solvents from water, offering a sustainable method for oil spill remediation. The tuning of the surface properties of the porous structure of these hydrogels enables effective oil-water separation. Liu et al.⁷⁰⁾ fabricated a superhydrophilic hydrogel using PVA and cellulose nanocrystals (CNCs). The introduction of CNCs into the PVA network prevented the collapse of internal microporous structures during adsorption and inhibited oil infiltration, resulting in efficient oil-water separation. Zhao et al.⁷¹⁾ developed a hydrophobic gel via the dispersion of electrospun cellulose fibers in water, inducing covalent bonding, and subsequent silanization to enhance the hydrophobicity of the fibrous matrix. The resulting hydrogel, featuring a 3D microporous structure, demonstrated the ability to absorb various oils and organic solvents.

Fig. 4.

Cellulose hydrogels for the removal of heavy metal ions in aqueous media (reproduced from Ref. 68).⁶⁸⁾

5.4 Sensing applications

The integration of sensing elements into cellulose hydrogels enables the detection of changes in environmental parameters, such as pH, temperature, and specific pollutants. When ambient humidity rises, the hydrogel absorbs moisture and swells, altering its internal structure. This swelling causes variations in conductivity and capacitance, allowing it to function as a humidity sensor. Under mechanical stress (e.g., stretching or compression), the molecular chains within the cellulose-based hydrogel slide and rearrange, leading to changes in its shape and conductivity. Zou et al.⁷²⁾ developed a highly sensitive hydrogel capable of detecting human motion under extreme conditions (–80°C) via the polymerization of acrylamide monomers around cellulose nanofiber-stabilized liquid metal droplets and the subsequent incorporation of reduced graphene oxide and additional cross-linking with glycerol (Fig. 5). Zhang et al.⁷³⁾ developed a smart conductive hydrogel based on cellulose and MXene. The as-formed hydrogel was capable of sensing various strain signals generated by human activity. Additionally, core-shell-structured hydrogel fibers were fabricated using CMC-alginate and blueberry anthocyanins, and they exhibited distinct colorimetric responses under different pH conditions. This study demonstrated the potential for developing eco-friendly and renewable fiber-based color sensors. Such cellulose hydrogel-based sensors can provide real-time data for air and water quality monitoring, contributing to environmental protection efforts. In cellulose-based hydrogel sensors, cellulose contributes flexibility due to its fibrous, networked structure and acts as an effective dispersion matrix for conductive fillers such as graphene or MXenes.⁷⁴⁾

Fig. 5.

Highly stable and skin-compatible flexible strain sensor based on a hydrogel for human activity monitoring. Detection of (a) finger deformation at different bending angles, (b) wrist bending at different directions, (c) tricep muscle contractions and relaxations, and (d) frowning and (e) swallowing activities (reprinted from Ref. 73).⁷³⁾

5.5 Agricultural applications

As depicted in Fig. 6, cellulose hydrogels employed in the agricultural sector can absorb and gradually release water in response to soil drying, regulating the soil moisture and porosity levels. This promotes sustainable water usage and enhances crop yields. Qin et al.⁷⁵⁾ demonstrated that when cellulose hydrogels possessing a high water retention capacity were added as soil conditioners, they exerted a positive effect on soil improvement and contributed to the germination and growth of wheat and lettuce seeds, sustainably increasing agricultural productivity. Similarly, Lee et al.⁷⁶⁾ reported that when the hydrogels synthesized from CMC were applied to soil, they improved the water retention capacity and supported crop growth, which can be utilized for the development of eco-friendly fertilizers. Currently, most fertilizer coatings rely on petrochemical-based materials; however, biodegradable cellulose or cellulose derivatives can be employed for the production of environmentally safe fertilizers without contributing to soil pollution. Kassem et al.⁷⁷⁾ coated monoammonium phosphate fertilizer particles with CMC and HEC, achieving improved soil water retention and a prolonged release time of phosphorus and nitrogen. Ahmad et al.⁷⁸⁾ extracted cellulose from paper waste and synthesized cellulose-based hydrogels using ECH and CMC as the cross-linking and gelling agents, respectively. When applied to soil, the porous cellulose hydrogel formulation was effective in regulating the soil moisture content and the release characteristics of urea.

Fig. 6.

Effects of cellulose hydrogels on soil properties and urea release (reprinted from Ref. 78).⁷⁸⁾

6. Conclusion

Cellulose has garnered considerable attention as a sustainable material owing to its abundance, biodegradability, and excellent mechanical and chemical properties. In particular, cellulose-based hydrogels are environmentally friendly and possess high water retention capacity as well as tunable physical and chemical characteristics, rendering them suitable for a wide range of applications. This review discusses the various applications of cellulose hydrogels in fields such as smart packaging, biomedical engineering, environmental remediation, sensors, and electronic materials. In smart packaging, cellulose hydrogels play a critical role in maintaining the freshness and quality of food and pharmaceutical products. In the field of biomedicine, they exhibit promising applications in tissue engineering, drug delivery systems, and wound healing. For environmental remediation, cellulose hydrogels are applied in pollutant adsorption and water treatment technologies. Furthermore, the application of cellulose hydrogels as sensors and electronic materials is actively being explored, increasing their value as next-generation functional materials. However, several challenges remain in maximizing the industrial utilization of cellulose hydrogels. First, the establishment of efficient and cost-effective large-scale production processes is essential. Second, the development of customized functionalization technologies tailored for specific application requirements is crucial. Third, further research is required for enhancing the mechanical strength and durability of cellulose hydrogels while maintaining biodegradability and environmental safety. Owing to their substantial potential as sustainable materials, cellulose hydrogels are expected to considerably contribute to the realization of an environmentally friendly society in future through continued technological development and commercialization.