Emerging Trends in Chitin-based Hydrogels: From Fundamental Properties to Advanced Applications

Chitin-based hydrogels have emerged as a versatile and sustainable material with significant potential in biomedical, environ- mental, and energy applications. Derived from the abundant biopolymer chitin, these hydrogels exhibit exceptional biocompat- ibility, biodegradability, and tunable physicochemical properties. This review highlights recent advancements in the synthesis and functional design of chitin-based hydrogels, focusing on solvent systems (e.g., alkali/urea solutions, ionic liquids, and deep eutectic solvents), crosslinking strategies, and structural modifications to enhance their mechanical strength, swelling capac- ity, and stimuli-responsiveness. Key applications include wound healing, drug delivery, tissue engineering, and environmental remediation, where their high-water retention, enzymatic degradability, and eco-friendly nature are particularly advantageous. Furthermore, innovations such as nanoparticle incorporation and chemical derivatization (e.g., carboxymethylation, hydrox- ypropylation) have expanded their utility in energy devices and smart sensors. Despite these advances, challenges remain in optimizing the energy efficiency of production methods for industrial scalability. This review provides a comprehensive overview of the current state of chitin-based hydrogels, offering insights into future directions for research and development in this promising field. Review Emerging Trends in Chitin-based Hydrogels: From Fundamental Properties to Advanced Applications Merreta Noorenza Biutty1, Ratri Puspita Wardani2, Zeno Rizqi Ramadhan3, Boram Yun4, Achmad Yanuar Maulana5*, Jongsik Kim4,5*, and Maulida Zakia6* Dr. M. N. Biutty Department of Chemical Engineering Universitas Pembangunan Nasional “Veteran” Yogyakarta Yogyakarta, 55283, Indonesia R. P. Wardani Department of Chemical Engineering Universitas Pembangunan Nasional “Veteran” Jawa Timur 1 Posted on 10 Dec 2025 | The copyright holder is the author/funder. All rights reserved. No reuse without permission. | https://doi.org/10.22541/au.176539179.91687105/v1 | This is a preprint and has not been peer-reviewed. Data may be preliminary. Surabaya, 60294, Indonesia Dr. Z. R. Ramadhan School of Chemistry The University of New South Wales Sydney, NSW 2052, Australia Dr. A. Y. Maulana, Prof. J. Kim Department of Chemistry Dong-A University Busan 49315, South Korea E-mail: [email protected] B. Yun, Prof. J. Kim Department of Chemical Engineering (BK21 FOUR Graduate Program) Dong-A University Busan 49315, South Korea E-mail: [email protected] Dr. M. Zakia Department of Chemical Engineering Universitas Negeri Semarang Semarang, 50229, Indonesia E-mail: [email protected]

chitin; hydrogels; stimuli-responsive materials; sustainable materials; biodegradable materials.

Chitin-based hydrogels have emerged as a versatile and sustainable material with significant poten- tial in biomedical, environmental, and energy applications. Derived from the abundant biopolymer chitin, these hydrogels exhibit exceptional biocompatibility, biodegradability, and tunable physicochemical prop- erties. This review highlights recent advancements in the synthesis and functional design of chitin-based hydrogels, focusing on solvent systems (e.g., alkali/urea solutions, ionic liquids, and deep eutectic solvents), crosslinking strategies, and structural modifications to enhance their mechanical strength, swelling capacity, andstimuli-responsiveness. Keyapplicationsincludewoundhealing, drugdelivery, tissueengineering, anden- vironmental remediation, where their high-water retention, enzymatic degradability, and eco-friendly nature are particularly advantageous. Furthermore, innovations such as nanoparticle incorporation and chemical derivatization (e.g., carboxymethylation, hydroxypropylation) have expanded their utility in energy devices and smart sensors. Despite these advances, challenges remain in optimizing the energy efficiency of produc- tion methods for industrial scalability. This review provides a comprehensive overview of the current state of chitin-based hydrogels, offering insights into future directions for research and development in this promising field. 1. Introduction Hydrogels are three-dimensional polymer networks characterized by their remarkable capacity to absorb and retain a significant fraction of water while remaining insoluble in aqueous environments.[1] Their high-water uptake capacity is attributed to the presence of hydrophilic functional groups along the polymer backbone. These moieties interact with water molecules through hydrogen bonding and osmotic pressure, facilitating extensive water uptake.[2] In addition, their insolubility is maintained due to covalent bonding from chemical crosslinking or physical forces such as hydrogen bonding, ionic interactions, and crystalline phase formation, which stabilize the three-dimensional matrix and prevent dissolution. 2 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. The classification of hydrogels is generally based on the source of the polymeric materials used in their synthesis.[3]Hydrogels may be derived from natural biopolymers or synthetic polymers, each offering distinct advantages depending on the intended application. Natural biopolymer-based hydrogels are generally favored for their excellent biocompatibility, biodegradability, and inherent bioactivity, making them particularly suit- able for biomedical applications such as wound healing[4-7], tissue engineering[8-12], and drug delivery[13-15]. These materials often exhibit minimal cytotoxicity and can mimic components of the extracellular matrix, enhancing cell attachment and proliferation. However, their mechanical properties and structural unifor- mity may be limited due to variability in natural sources and sensitivity to processing conditions[16]. In comparison, synthetic polymer-based hydrogels provide superior mechanical strength, structural stability, and reproducibility. They can be precisely engineered to achieve specific physical and chemical properties, such as tunable porosity, swelling behavior, and degradation rates. This level of control makes synthetic hydrogels highly suitable for applications requiring consistent performance and mechanical durability, in- cluding soft robotics, sensors, and controlled-release systems. However, they are generally characterized by low biodegradability and limited biocompatibility. Therefore, the selection between natural and synthetic hydrogel materials is often dictated by the specific performance requirements of the target application, balancing the need for biocompatibility with the demand for mechanical integrity and processability. Among a wide range of natural biopolymers, chitin has emerged as a promising candidate for hydrogel development due to its unique structural and functional properties [17]. Structurally, chitin is a linear polysaccharide composed of β-(1-4)-linked N-acetyl-D-glucosamine units, and it is widely found in nature, particularly in the exoskeletons of crustaceans, insect cuticles, and fungal cell walls. Functionally, chitin- based hydrogels exhibit favorable properties such as high-water retention, biocompatibility, biodegradability, and hemostatic activity. These characteristics make them particularly suitable for biomedical applications. Their capacity to support cell adhesion and modulate drug release profiles further enhances their performance in therapeutic contexts. From a sustainability perspective, chitin offers the added advantage of being derived from renewable and underutilized seafood by-products, making it an eco-friendly and cost-effective option for the development of green hydrogel materials. Therefore, in this review, we primarily focus on recent advancements in the synthesis, crosslinking strategies, and functional design of chitin-based hydrogels, with an emphasis on their potential for multifunctional applications, including superabsorbent, controlled delivery systems, stimuli-sensitive systems, energy devices, and smart sensors. 2. Chitin Chitin is one of the most abundant natural polysaccharides and the second most prevalent biopolymer on Earth, following cellulose. It is a naturally occurring mucopolysaccharide that serves as the primary structural component in the exoskeletons of crustaceans, insects, and other arthropods. Although chitin is widely distributed in nature, its primary commercial sources to date have been limited to the exoskeletal waste of marine crustaceans, particularly crab and shrimp shells. Despite its abundance, only a small fraction of extracted chitin is utilized in value-added applications, while the majority remains underexploited. Due to its inherent biocompatibility, biodegradability, non-toxicity, and ability to be chemically modified, chitin has attracted considerable attention as a renewable feedstock for the development of advanced biomaterials in biomedical[18,19], environmental [20], and energy-related fields [21]. Structurally, chitin is composed of repeating units of 2-acetamido-2-deoxy- β-D-glucopyranose, connected via β-(1-4) glycosidic linkage (Fig. 1). The repeating monomer, 2-acetamido-2-deoxy- β-D-glucopyranose, forms the backbone of chitin, which shares structural similarity with cellulose but differs in the presence of ac- etamido groups that impart unique physicochemical characteristics. The degree of acetylation and molecular arrangement of chitin largely determine its crystallinity, solubility, and reactivity, which subsequently influ- ence its biological function and potential for modification [22]. Depending on the orientation of the polymer chains, chitin exists in three polymorphic forms ( α, β, and γ) with α-chitin being the most abundant and stable form found predominantly in crustacean shells. The polymer contains numerous functional groups, particularly hydroxyl and acetamido groups, which serve as reactive sites for chemical modifications such as deacetylation, carboxymethylation, or grafting, enabling the transformation of chitin into chitosan or 3 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. other derivatives with tailored properties. Biotechnological advances have facilitated the enzymatic or mi- crobial transformation of chitin into functional oligomers and nanostructures under mild and sustainable conditions[23-25]. These modified chitin-based materials exhibit enhanced bioactivity, mechanical properties, and processability, making them suitable for diverse applications, including drug delivery, tissue engineering, biosensing, and water treatment. Furthermore, amid escalating environmental and economic constraints linked to petroleum-derived polymers, chitin has emerged as a renewable, biodegradable biopolymer with significant potential for the development of high-performance bio functional materials. Consequently, chitin has garnered significant attention for its potential in replacing synthetic polymers across various sectors, offering eco-friendly pathways to produce advanced biomaterials, bio-plastics, membranes, and biomedical devices. Fig. 1 Chemical structure of chitin poly(N-acetyl- β-d-glucosamine). 3. Preparation of chitin-based hydrogels Due to its low solubility properties of natural chitin, chitin-based hydrogels can be prepared through several ways. It can be crosslinked from the native chitin dissolved in particular solvents, derivatives of chitin dissolved in water or acidic solution, and nano chitin dissolved in aqueous solution. 3.1. Preparation of hydrogels from native chitin The cross-linkage of chitin in solutions has limitations because of its high crystallinity. Its tightly packed structure is illustrated in Fig. 2a and 2b, corresponding to the ab- and bc- planes at 100 K, respectively. To date, various solvent systems have been explored for dissolving native chitin. The subsequent discussion highlights the dissolution behavior of native chitin in different solvent media. 4 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 2 Chitin packing at (a) ab- and (b) bc- planes. Reproduced with permission from [26]. 3.1.1. Alkali solutions Alkali/urea solutions are among the most widely employed systems for dissolving chitin. At low temper- ature, chitin can be effectively solubilized in such systems, particularly in NaOH-urea [27] and KOH-urea [28]solutions. Fang et al. reported that the dissolution mechanism involves the disruption of intermolecu- lar hydrogen bonds within the chitin chains and the formation of intramolecular hydrogen bonds between chitin and the solvent molecules, in which urea plays a crucial role in stabilizing the chitin solution, as illustrated in Fig. 3a [27]. Additionally, the process of chitin film formation using the KOH-urea system is schematically presented in Fig. 3b [28]. 5 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 3 (a) Chitin in NaOH/urea. Reproduced with permission from [27]; (b) Fabrication of chitin film in KOH/urea solution. Reproduced with permission from [28]. To enhance the dissolution process, the freeze–thaw technique is often employed. This method accelerates chitin dissolution in alkali/urea aqueous systems by facilitating structural disruption through thermal cycling [29]. Li et al. prepared a chitin hydrogel by dispersing chitin in 11 wt% NaOH-4 wt% urea aqueous solution using freeze-thaw process [30]. The resulting hydrogel demonstrated excellent performance, achieving up to 80% desalination efficiency in seawater treatment applications. Similarly, Zou et al dissolved chitin in a 20 wt% KOH–4 wt% urea solution using the same method [31]. The obtained hydrogel exhibited favorable properties, including biocompatibility, biodegradability, anti-cell adhesion, and mechanical strength, making it a promising material for post-operative adhesion prevention. The preparation of chitin hydrogels using alkali/urea systems has gained significant attention due to its simplicity, cost-effectiveness, and environmental friendliness. However, the process can be time-consuming, as multiple freeze–thaw cycles are typically required to achieve complete dissolution. 3.1.2. Ionic liquids Ionic liquids (ILs) are salts that remain in the liquid phase at room temperature and have been explored as effective solvents for chitin dissolution. The solubility of chitin in ILs is largely dependent on its degree of deacetylation (DA) and molecular weight [32]. Deng and Zhang successfully dissolved chitin in 1-butyl-3- methylimidazolium acetate ([BMIM]Ac) to form a chitin/[BMIM]Ac gel, as illustrated in Fig. 4 [33]. Subse- quently, the [BMIM]Ac was replaced by KOH solution to obtain chitin/KOH regenerated hydrogel, which is further used as the polymer electrolyte in supercapacitor application. Their study demonstrated that the regenerated chitin hydrogel exhibited superior electrochemical performance, including improved cyclic stability and higher capacitance, compared to conventional KOH aqueous solution. Despite its effectiveness, the dissolution of chitin in [BMIM]Ac requires continuous stirring at elevated temperatures (80 °C), leading to significant energy consumption. Consequently, there is a growing interest in identifying alternative solvent systems that enable efficient chitin dissolution under milder and more energy-efficient conditions. 6 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 4 Fabrication of chitin hydrogel in ([BMIM]Ac) solvent. Reproduced with permission from [33]. 3.1.3. Polar solvents Polar solvents are often combined with water to facilitate hydrolysis and deacetylation reactions in chitin processing. Zhang and Yan used D-Glucosamine (GlcN) with co-solvent of γ-valerolactone (GVL), tetrahy- drofuran (THF), 1,4-dioxane (DOX), dimethyl sulfoxide (DMSO), ethylene glycol dimethyl ether (EGDM), ethylene glycol diethyl ether (EGDE), and diethylene glycol diethyl ether (DGDE)[34]. Among these, etheric solvent exhibits superior performance compared to the other polar solvents evaluated. Notably, chitin was completely converted into a mixture of DGDE and water solution (4:1 V/V) with 80% GlcN in acidic con- dition from 100 mM sulfuric acid by heating at 175 °C for 1 h. The presence of an acid catalyst significantly enhanced reaction efficiency when combined with polar solvents, outperforming conventional hydrolysis con- ducted in water alone. This approach provides valuable insights into the importance of solvent selection and acid catalysis for effective chitin dissolution. In a separate study, Zhong et al. prepared a chitin propionate film by dissolving chitin powder in propionic anhydride/perchloride acid mixture [35]. The reaction was initiated at 0 degC for 30 min and continued at room temperature for 2.5 h with constant stirring. Following precipitation in water, washing, and multiple filtration steps, chitin propionate was successfully recovered, achieving a remarkably high yield of 132%. This high yield was attributed to the incorporation of propionyloxy groups into the chitin structure. The chitin propionate film was fabricated by casting in an ethanol/water binary solvent system with varying ethanol concentrations (90 wt%, 70 wt%, and 50 wt%). As shown in Fig. 5a, the optimal solubility (96%) was observed with 30 wt% water. The resulting films demonstrated excellent tensile strength and significantly enhanced water resistance, making them suitable for reinforcing paper substrates. Furthermore, as illustration in Fig. 5b, this work highlighted the improved solubility and performance of chitin propionate in ethanol/water systems, which are less toxic and offer promising potential for applications in coatings, films, and ink formulations. Fig. 5 (a) Chitin propionate solubility in ethanol/water solvent; (b) Hydrogen bonding interactions within molecules. Reproduced with permission from [35]. 7 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. 3.1.3. Deep eutectic solvents (DESs) Deep eutectic solvents (DES) are a mixture of two or more components that possess a lower melting point compared to their individual constituents [36]. Vicente et al. used four different hydrogen-bond acceptors (HBA), which are choline chloride ([Ch]Cl), choline dihydrogen citrate ([Ch]DHC), potassium bicarbonate, and potassium carbonate to six different hydrogen-bond donors (HBD) of ethylene glycol (EG), acetic acid glacial (AA), oxalic acid (OA), malic acid (MA), and citric acid (CA) monohydrate [37]. Each HBA–HBD combination was prepared by heating the components at 80 °C to form the DES. Chitin was then introduced into these DES systems under various temperature conditions and subjected to continuous stirring for 24 h. Among all the combinations, the [Ch]Cl:MA system at 120 degC yielded the highest degree of deacetylation (DDA) at 40%. A stepwise representation of the deacetylation reaction in the [Ch]Cl:AA system is provided in Fig. 6, illustrating the mechanism involved. This study demonstrates that DES can significantly enhance the chemical reactivity of chitin, enabling partial deacetylation and improved solubility. Although the maximum DDA achieved was below 50%, the findings highlight the potential of tailoring DES compositions to further increase deacetylation efficiency. This opens new avenues for developing more effective and environmentally friendly solvent systems for chitin processing. Fig. 6 Molecular structures of chitin deacetylation in [Ch]Cl:AA systems. Reproduced with permission from[37]. 3.2. Preparation of hydrogels from chitin derivatives 3.2.1. Preparation of hydrogels from chitosan and its derivatives Chitosan is derived from chitin through a deacetylation process, in which the acetyl group at the C-2 position of the acetamide moiety is removed and replaced by an amino group. This structural transformation not only differentiates chitosan from chitin but also enhances its reactivity, enabling further functionalization 8 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. with various chemical groups that can modify its physical and chemical properties. Demirtas et al. pre- pared alginate, alginate-hydroxyapatite (alginate-HA), chitosan and chitosan-hydroxyapatite (chitosan-HA) hydrogels by thermal cross-linking reaction at 37 °C to investigate their suitability as bio-inks for tissue engineering [38]. The results indicated that chitosan-based hydrogels, particularly those incorporating HA, demonstrated significantly higher viscosity and elastic modulus compared to their alginate counterparts. Mo- reover, the addition of hydroxyapatite further enhanced the mechanical properties and overall performance of both alginate- and chitosan-based hydrogels. This study confirms the successful development of chito- san and chitosan-HA hydrogels as promising bio-ink materials, particularly for applications in bone tissue engineering, due to their improved rheological and mechanical characteristics. 3.2.2. Hydrogels from other chitin derivatives Liu et al. introduced carboxymethyl group to chitin to form carboxymethyl chitin (CMCH) hydrogel[39]. The modification was achieved via etherification, where the hydroxyl groups of chitins were reacted with sodium monochloroacetate in a NaOH/urea solvent system. Three different CMCH variants were prepared using chitin/sodium monochloroacetate ratios of 0.14, 0.21, and 0.31, respectively. Solubility analysis revealed that all CMCH samples were soluble in 0.1 M NaOH solution, a medium in which native chitin remains insoluble. It can be concluded that the introduction of carboxyl groups significantly enhances the aqueous solubility of chitin, offering improved processability for hydrogel applications. In another study, Xu et al. developed hydroxypropyl chitin (HPCH) hydrogel by grafting hydroxypropyl groups onto the chitin framework, as shown in Fig. 7a [40]. The synthesis was carried out in a NaOH/urea aqueous solution, followed by processing to obtain a moldable, thermosensitive hydrogel. The sol-gel transiti- on behavior and gelation mechanism are detailed in Fig. 7b and 7c. The resulting HPCH hydrogel exhibited excellent biocompatibility and low cytotoxicity, demonstrating strong potential for in vitro and in vivo app- lications, particularly in 3D cartilage tissue regeneration for clinical use. 9 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 7 (a) ¹H NMR spectrum; (b) Sol–gel transition; and (c) Gelation scheme of HPCH. Reproduced with permission from [40]. Additionally, Yang et al. introduced carboxyethyl groups to chitin to produce carboxyethyl chitin (CECT) [15]. The CECT was subsequently functionalized by reacting it with adipic dihydrazide (ADH) in a MES aqueous solution. Dialysis and freeze-drying were done afterwards to obtain adipic dihydrazide-grafted car- boxyethyl chitin (CECT-ADH) through synthetic route depicted in Fig. 8a. To form the hydrogel, a solution of CECT-ADH was mixed with dibenzaldehyde-terminated poly(ethylene glycol)/4-amino-DL-phenylalanine (PEG-DA/Phe-NH2), then cast into a mold and incubated. As shown in Fig. 8b, the resulting hydrogel exhibited favorable properties including self-healing, self-adaptation, and both in vitro and in vivo enzy- matic degradability. These characteristics make it a promising candidate for drug delivery applications in biomedical fields. Collectively, these studies underscore the value of chemical functionalization through carboxymethyl, hydroxypropyl, and carboxyethyl modifications in enhancing the solubility, tunability, and functionality of chitin-based hydrogels for diverse biomedical applications. 10 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 8 (a) Synthesis route of CECT-ADH; (b) CECT-ADH/PEG-DA hydrogel formation scheme. Repro- duced with permission from [15]. 3.3. Preparation of hydrogels from nano chitin The incorporation of nanoparticles into chitin-based hydrogels has emerged as a promising strategy for enhancing their functional properties. Wu et al. prepared chitin carbon nanotube (Ch/CNT), as illustrated in Fig. 9. Native chitin was first dissolved in a mixture of 70% sulfuric acid and 65% nitric acid at a volume ratio of 3:1, under heating at 60 °C for 2 h [41]. The resulting solution underwent multiple rounds of dialysis until a neutral pH was achieved, followed by freeze-drying to obtain purified chitin. Separately, functionalized CNTs were dispersed in an aqueous NaOH/urea solution and subsequently blended with native chitin at three distinct concentrations (1 wt%, 3 wt%, and 5 wt%). The resulting mixtures, labeled as Ch/CNT1, Ch/CNT2, and Ch/CNT3, respectively, were processed through a freeze-thaw method to form composite hydrogels. Mechanical testing revealed that the Ch/CNT hydrogels exhibited significantly improved mechanical strength and toughness compared to pure chitin hydrogels. This enhancement is attributed to the increased cross- linking density within the composite matrix. All Ch/CNT variants demonstrated notable tensile strength and elongation properties, indicating their strong potential for applications in peripheral nerve regeneration and other biomedical fields requiring mechanically robust, biocompatible scaffolds. 11 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 9 Scheme for chitin and chitin/carbon nanotube (Ch/CNT) composite hydrogels assembly. Reproduced with permission from [41]. 4. Physicochemical characteristics of chitin-based hydrogels 4.1. Mechanical Properties Naturally, chitin exists as part of a composite structure with proteins, minerals, and pigments. The choice of extraction method, whether mechanical, chemical, or enzymatic, not only removes these associated com- ponents but also affects the molecular structure of chitin, which in turn influences its mechanical properties [42]. In this context, key properties such as tensile strength, compressive modulus, shear rheology, elasticity, and structural integrity are influenced by factors including polymer concentration, molecular weight, degree of acetylation (DA), crosslinking method, and processing conditions (Fig. 10). 12 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 10 Schematic of factors enhancing chitin-based hydrogel strength. Chitin’s β-(1-4)-linked N-acetylglucosamine structure contributes to a semi-crystalline backbone, which im- parts inherent rigidity and strength. Increasing the polymer concentration and using high molecular weight chitin typically enhance strength of hydrogel by promoting greater chain entanglement and intermolecu- lar interactions [43-45]. While a lower degree of acetylation reduces hydrogen bonding, chain stability and crystallinity, resulting in a more flexible and tougher hydrogel network [46-48]. The mentioned inherent flexibility, however, must be supported by an appropriate crosslinking strategy to achieve the desired mechanical properties. Physically cross-linked chitin-based hydrogels are typically formed through hydrogen bonding or ionic interactions, often involving multivalent ions. These physical gels are generally soft, elastic, and exhibit reversible behavior, which can be advantageous for applications requiring conformability and responsiveness [49,50]. However, their reliance on weak intermolecular forces often results in poor mechanical strength and limited stability under physiological conditions. In contrast, recent studies report that chemically crosslinked chitin-based hydrogels synthesized using crosslinking agents such as 1,4-butanediol diglycidyl ether (BDDE) [31], genepin[51], and glutaraldehyde[52], form stable covalent networks that significantly enhance mechanical strength and durability. Consequently, it is more suitable for load-bearing biomedical applications, including drug release, cartilage regeneration and tissue scaffolds, where enhanced strength and durability are required [31,49-52]. In addition, processing condition parameters such as polarity [53], gelation temperature [53,54], and pH condition [55,56], may influence the internal mi- crostructure and crosslinking density of the chitin-based hydrogel, which affect its mechanical performance. Moreover, the incorporation of nanoscale reinforcements like carbon nanotube (CNT) [57] and poly(3,4- ethylenedioxythiophene) nanoparticles (PEDOT NPs) [58] enhances stress distribution within the polymer network, leading to improved mechanical strength and elastic flexibility. To quantitatively evaluate these mechanical attributes and understand how structural factors, crosslinking strategies, and processing conditions influence performance, rheological characterization is employed as a key analytical tool [33,59-62]. Chitin-based hydrogels demonstrate diverse rheological properties depending on the solvent system and formulation strategy [33,59-62]. Hydrogels prepared using calcium chloride–methanol exhibit elastic behavior with a storage modulus (G’) of 9.2 kPa, where G’ exceeds the loss modulus (G”), indicating solid-like characteristics. These gels are pseudoplastic, showing shear-thinning behavior with a phase angle below ten degrees, confirming high elasticity and good injectability [59]. In ionic liquid systems, the G’ value increases significantly up to 112 kPa, retaining shear-thinning and self-recovery properties after deformation [33]. NaOH/urea-based gels exhibit a moderate storage modulus (G’) of approximately 21.8 kPa, with G’ remaining higher than G” and showing shear-thinning behavior, although gelation is delayed in the 13 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. presence of water [60]. Deep eutectic solvent (DES)-derived hydrogels likewise demonstrate shear-thinning rheology, with mechanical strength tunable via processing conditions [61]. LiCl/DMAc-based hydrogels show pseudoplastic or Newtonian behavior, depending on polymer concentration, and exhibit temperature- sensitive viscosity [62]. Across systems, higher G’ values and G’ > G” consistently correlate with better mechanical strength and stability, making these hydrogels suitable for many applications requiring elasticity, injectability, and structural integrity[33,59-62]. 4.2. Microstructural Morphology The pore structure and surface features of chitin-based hydrogels critically influence their mechanical prop- erties, swelling behavior, and suitability for some applications. These characteristics are primarily dictated by the crosslinking mechanism, processing conditions, and drying techniques [45,63-69]. For instance, physical crosslinked chitin-based hydrogels coagulated in alkaline urea solutions exhibit enhanced structural unifor- mity, characterized by smoother surface topography, reduced pore diameter, and increased network density, compared to those precipitated in neutral or weakly polar solvents such as water or ethanol [63,64]. These microstructural refinements not only contribute to improved mechanical properties but also enhance optical transparency by minimizing light scattering through irregular interfaces and large pores [64]. In chemically crosslinked systems, the pore size generally falls within a few micrometers upon freeze-drying, with finer structures achievable through increased polymer concentration and crosslinking density [45,65]. The drying technique significantly influences the microstructure of materials [66]. Lyophilization (freeze- drying) typically promotes the formation of highly interconnected macroporous networks within chitin- based hydrogel through ice sublimation, which helps preserve the original three-dimensional structure [67]. In comparison, supercritical drying, which involves solvent exchange followed by depressurization beyond the critical point, enables the production of nanoporous with high surface area and minimal structural col- lapse [68]. Moreover, hydrogels derived from chitin nanowhiskers (ChNWs) exhibit uniformity and optical clarity, primarily due to physical entanglement and strong hydrogen bonding interactions within the poly- mer network [69]. The gelation process can be triggered by external stimuli including ultrasonication, pH modulation, or solvent displacement, leading to the development of robust three-dimensional networks even at relatively low concentrations. By systematically adjusting processing parameters, the pore structure of chitin-based hydrogels can be finely controlled to enhance their functionality for specific end-use applications. 4.3. Swelling capacity and water retention Chitin-based hydrogels exhibit high swelling capacity and water retention due to the presence of hydrophilic functional groups such as hydroxyl and acetamido moieties, which facilitate hydrogen bonding with water[47]. The swelling behavior is primarily influenced by crosslinking density, pore architecture, and the method of drying [31,54,56,62]. Low crosslinking density and an open, interconnected pore structure allow more water to be absorbed, while highly crosslinked networks limit swelling due to their denser and less permeable structure [54,70]. In addition, freeze-dried and supercritically dried chitin hydrogels preserve porous structures, allowing rehydration and swelling ratios exceeding 106–107 g/g [56] and 60 g/g [71]. In contrast, air- or oven-dried samples tend to collapse structurally, reducing water retention. Molecular weight significantly influences performance, wherein chitin with higher molecular weight demon- strates increased water uptake as a result of enhanced polymer chain entanglement and elevated hydrophilic- ity [72]. Additionally, environmental conditions such as pH and ionic strength affect swelling, where alkaline or neutral pH favors higher absorption, particularly in chitosan-rich systems derived from partially deacety- lated chitin[56]. Tailoring swelling capacity or water uptake water uptake in chitin-based hydrogels enables their use in diverse applications. High swelling is ideal for wound dressings and absorbents, while con- trolled hydration suits drug delivery systems. By adjusting structure and processing, chitin hydrogels can be engineered for specific moisture-handling needs. 4.4. Degradation behavior Due to their polysaccharide backbone composed of β-(1-4)-linked N-acetylglucosamine units, chitin-based hy- 14 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. drogels exhibit inherent biodegradability. These bonds are susceptible to enzymatic hydrolysis by naturally occurring enzymes such as chitinase and lysozyme, allowing the hydrogel matrix to degrade gradually under physiological conditions [73]. This process produces non-toxic byproducts, making chitin hydrogels particu- larly suitable for biomedical applications where material resorption is desired, such as wound healing [74-78] or drug delivery [79,80]. The biodegradation rate of chitin hydrogels can be controlled by tuning key structural parameters. A lower degree of acetylation (DA) leads to increased enzymatic accessibility and faster degradation, as it reduces the crystalline regions that typically hinder enzyme penetration [48,81]. Similarly, hydrogels with lower crosslinking density and higher porosity allow easier water and enzyme diffusion into the network, accelerating breakdown. Physically crosslinked chitin hydrogels tend to degrade more quickly than chemically crosslinked ones due to the absence of strong covalent bonds within the network. Chemical modifications of chitin, including processes like carboxymethylation or hydroxypropylation, can fur- ther enhance biodegradability by increasing hydrophilicity and disrupting the crystalline structure [70,82,83]. These modifications not only improve solubility and gelation but also enable the design of hydrogels with adjustable degradation rates, depending on the application. For example, rapidly degrading hydrogels may be useful for short-term drug release [84-86], while slower-degrading systems are better suited for tissue scaffolding[87]. Overall, the biodegradability of chitin-based hydrogels is a highly tunable and application relevant property, offering a sustainable and safe alternative for transient biomedical and environmental uses. 5. Advanced Applications of Chitin-Based Hydrogels Owing to their intrinsic biocompatibility, enzymatic degradability, high moisture retention, and struc- tural adaptability, hydrogels derived from chitin have emerged as versatile materials across multiple disciplines[15,21,54,79,87-106] as shown in Table 1. Table 1. Multifunctional applications of chitin-based hydrogels. Application Area Mechanism Functionality Materials Key Features Example Use Superabsorbents Absorb large amounts of water or fluids via hydrogen bonding of hydrophilic groups Native chitin / modified derivatives (e.g. carboxymethyl chitin) High swelling ratio, porosity, moisture retention Wound dressing, wastewater treatment [87-92] Controlled delivery systems Encapsulation and timed release of drugs via swelling, diffusion, degradation CMCH, HPCH, chitosan, thermosensitive gels pH- or temperature-responsive release, biocompatibility, biodegradability Intestinal or injectable drug release [54,79,93-101] Stimuli-sensitive systems Respond to pH, temperature, magnetic/electric fields to modulate hydrogel properties CMCH, HPCH, AMC, Fe3O4-chitin composites On–off release, reversible swelling, electro-/magneto-responsive pH-triggered delivery, electric-actuated release [15,99,102-105] Energy devices Generate electricity from mechanical motion via triboelectric effect Chitin hydrogels (e.g. KOH/urea-prepared) High voltage output, flexibility, durability Triboelectric nanogenerator (TENG) [21,106] 5.1. Superabsorbent The absorption capacity of chitin-based hydrogels is one of their most important characteristics, driven by the presence of hydrophilic groups such as hydroxyl (-OH) and acetamido (-NHCOCH3) moieties in the chitin backbone. These groups facilitate strong hydrogen bonding with water molecules, resulting in high swelling ratios and water retention. In aqueous environments, chitin hydrogels act as superabsorbent materials [88], making them suitable for wound dressings, where they maintain a moist healing environment and absorb wound exudates [87,89]. Their three-dimensional (3D) porous structure provides a large surface area for fluid uptake, while also allowing gas exchange and nutrient transport. In environmental applications, derivate chitin-based hydrogels serve as biosorbents capable of removing pollutants such as heavy metals (e.g., Cu²+, Pb²+) and dyes from wastewater [90-92]. For example, Fig. 11a outlines the conceptual mechanism by which the chitin-based hydrogel interacts with contaminants [90]. The adsorption process is governed by the synergistic action of hydroxyl, carboxyl, and amino functional groups embedded within the hydrogel matrix. These groups facilitate strong interactions with metal ions (Pb ²+, Cu²+) and dyes (MB) through mechanisms such as ion exchange, electrostatic attraction, and chelation. Notably, chemical modification of chitin or its derivatives through carboxymethylation significantly enhances interaction efficiency, thereby greatly improving the hydrogel’s capacity for contaminant adsorption and retention. As illustrated in Fig. 11b, under competitive adsorption conditions with multiple coexisting pollutants, the hydrogel exhibits a 15 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. clear preference for capturing Pb ²+ over Cu²+ and MB. This selective behavior is largely influenced by the higher electronegativity and stronger binding affinity of Pb²+, which facilitates its more effective interaction with the functionalized hydrogel surface [90]. In this regard, chitin-based hydrogels show excellent reusability and regeneration potential, making them sustainable and cost-effective for repeated cycles of adsorption– desorption in water treatment systems. Fig. 11 (a) Adsorption pathways and (b) Selective adsorption of pollutant system. Reproduced with permission from [90]. 5.2. Controlled delivery systems With their biodegradable nature, non-cytotoxicity, and adjustable physical properties, chitin-based hydrogels are well-suited for use in controlled drug delivery systems [94,95]. The porous matrix of chitin-based hydrogels allows for effective encapsulation of both hydrophilic and hydrophobic drug molecules due to their tunable physicochemical properties, such as degree of deacetylation and crosslinking density. The drug release kinetics is primarily governed by diffusion through the hydrated gel layer, polymer degradation via enzymatic action (e.g., lysozyme in biological environments), and swelling-controlled mechanisms, which are influenced by pH-dependent solubility and gel-layer dynamics [94,96]. For instance, chitosan (derived from chitin) forms a mucoadhesive gel layer in acidic conditions, enabling sustained release, while its erosion and degradation further modulate release rates in physiological environments [97,98]. Stimuli-responsive chitin derivatives, such as carboxymethyl chitin (CMHC) and hydroxypropyl chitin (HPCH), offer advanced control over drug [79,99]. Zheng et al. demonstrated that CMCH forms pH-sensitive hydrogels where ionization of carboxyl groups induces swelling at neutral or alkaline pH, suitable for tar- geted delivery in intestinal environments. On the other hand, HPCH hydrogels exhibit thermoreversible gelation, forming a gel at physiological temperature, ideal for injectable in situ gel systems. This property facilitates site-specific drug delivery with minimal procedural invasiveness [79]. Furthermore, by blending chitin with other polymers [54,93] or incorporating nanoparticles [100,101], multifunctional hydrogel systems can be created with enhanced mechanical stability, drug loading capacity, and responsive behavior to pH, temperature, or ionic strength. For example, chitin-based hydrogel composites formed by blending chitin nanowhiskers (ChWs) with poly(vinyl alcohol) (PVA) exhibit an interconnected porous network capable of efficiently encapsulating and gradually releasing biomolecules such as BSA, as illustrated in Fig. 12a [93]. As illustrated at Fig. 12b, the corresponding release profile reveals a two-phase mechanism: an initial burst release driven by rapid swelling and diffusion, followed by a more gradual and sustained release as equilib- rium is approached. Among the tested formulations, the hydrogel containing 40% ChWs demonstrated the most effective release performance, likely due to its well-balanced combination of porosity and mechanical strength, which highlights their strong potential for use in implantable drug delivery platforms. 16 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 12 (a) Schematic of preparation PVA/ChWs hysrogels and (b) the BSA release profile (25 °C). Reproduced with permission from [93]. 5.3. Stimuli-sensitive systems Chitin-based hydrogels can be engineered to exhibit stimuli-sensitive behavior, enabling them to undergo physical or chemical changes in response to external stimuli such as pH, temperature, magnetic field or electric field at show in Table 2 [15,99,102-105]. Table 2. Stimuli-sensitive chitin-based hydrogels. Stimulus Modification/ Composite Response Mechanism References pH Carboxymethyl chitin (CMCH) Ionization of carboxyl groups in increases swelling in neutral/alkaline pH [15,99] Temperature Poly(diethylene glycol methyl ether methacrylate) (PMEO2MA)-or PNIPAm-grafted chitin Thermoreversible gelation at physiological temperature (in situ gel formation) [102,103] Magnetic Field Fe3O4 nanoparticles embedded in chitin hydrogel Magnetic actuation and spatially-controlled release [99] Electric Field Hericium erinaceus-derived chitin hydrogel Ion migration causes reversible bending/swelling [104] Dual (pH/Temp) Acrylamide-modified chitin (AMC) Sol-gel transitions triggered by electrochemical pH changes or redox ions (e.g., Fe ³+/Fe²+) [105] Pure chitin lacks inherent stimuli-responsiveness; however, chemical modifications or blending with respon- sive polymers can impart these functionalities. For instance, the pH-responsiveness is commonly introduced through carboxymethylation of chitin, producing carboxymethyl chitin (CMCH). The carboxyl groups in CMCH ionize in neutral or alkaline media, leading to electrostatic repulsion within the network and en- hanced swelling [79,99]. This enables pH-triggered drug release, ideal for targeting specific regions of the gastrointestinal tract. Building upon this concept of environmental responsiveness, temperature sensitiv- 17 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. ity can be imparted by grafting chitin or its derivatives with thermoresponsive polymers such as poly(N- isopropylacrylamide) (PNIPAm) [102] and poly (di(ethylene glycol) methyl ether methacrylate) (PMEO2MA) [103]. These hydrogels swell below their lower critical solution temperature (LCST) and shrink above it, al- lowing on–off switching of drug release at body temperature. Further expanding their functionality, magnetic responsiveness can be achieved by embedding magnetic nanoparticles (e.g., Fe3O4) into the hydrogel struc- ture [99]. These composite hydrogels respond to external magnetic fields, enabling remote actuation and spatially controlled release, which are especially beneficial for non-invasive targeting in biomedical systems. In parallel, electric field-sensitive chitin-based hydrogels have also been developed to respond to electrical stimuli, offering additional potential for on-demand therapeutic control. For instance, chitin derived hydrogel from Hericium erinaceus demonstrates reversible bending and swelling under electric fields, driven by ion migration and osmotic gradients [104]. This behavior enabled controlled curcumin release and showcased promising electromechanical responsiveness for biomedical applications such as soft actuators and electrically triggered drug delivery. In another study, acrylamide-modified chitin (AMC) exhibited dual responsiveness to both pH and electric signals. This water-soluble chitin derivative could undergo sol-gel transitions in response to electrochemically induced pH changes or redox-active ions (e.g., Fe ³+/Fe²+), enabling programmable protein entrapment and release directly on electrode surfaces [105]. 5.4. Energy devices and smart sensors Beyond biomedical use, chitin-based hydrogels and their derivatives have gained increasing attention in the development of energy devices and smart sensor platforms. Their intrinsic biocompatibility, biodegradabil- ity, and mechanical flexibility make them ideal for wearable electronics, energy harvesting systems, and environmental sensing applications. For instance, chitin hydrogel fabricated via non-freezing dissolution in KOH/urea systems have been employed as a tribopositive layers in triboelectric nano-generators (TENGs), achieving high output voltages (up to 182.4 V) and power densities over 1.25 W/m ², thus enabling self- powered sensing systems for tactile recognition and physiological monitoring [21]. In addition, carboxyethyl chitin/polyacrylamide hydrogels have been developed with ultra-stretchability (>1500%), high transparency, and strong adhesion, making them well-suited as electronic skins capable of detecting fine strains and pressu- res as described in Fig. 13a, which illustrates their molecular interaction network and structural design [106]. These hydrogels demonstrate high gauge factors (˜18.5) and broad sensing ranges, supporting applications in human–machine interfaces (Fig. 13b), including Morse code recognition via finger gestures, and spatially resolved pressure sensing via integrated electronic skin (e-skin) arrays for wearable electronics (Fig.13c) [106]. 18 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. Fig. 13 (a) Interaction schematic; (b) A human–machine interface and (c) E-skin array of carboxyethyl chitin/polyacrylamide hydrogels. Reproduced with permission from [106]. Further advancement includes the integration of chitin nanoparticle into deep eutectic eutectogels, resulting in self-healing, temperature-tolerant, and ultra stretchable sensors that can conform to dynamic surfaces like human joints or internal organs [107,108]. These multisignal wearable sensors exhibit high mechanical durability and maintain sensing function under extreme environments, expanding their use for long-term health monitoring. Moreover, amide-modified chitin-based hydrogels loaded with ε-polylysine and Al ³+ ions provide antibacterial, adhesive, and electrically conductive properties [109]. These hydrogels function effectively as real-time motion sensors, capable of detecting both subtle and gross human movements while preserving long-term performance through self-healing mechanisms. Collectively, these innovations highlight the vast potential of chitin-derived materials in building eco-friendly, biocompatible, and high-performance components for self-powered wearable electronics and soft intelligent devices. 6. Conclusion and Prospect The number of research on chitin hydrogels has expanded significantly due to its remarkable advantages of abundant biopolymer, biocompatibility, degradability, non-toxicity, and ability to be chemically modified. The limitation of poor solubility in native chitin has attracted many researchers to investigate hydrogels 19 Posted on 10 Dec 2025 — The copyright holder is the author/funder. All rights reserved. No reuse without permission. — https://doi.org/10.22541/au.176539179.91687105/v1 — This is a preprint and has not been peer-reviewed. Data may be preliminary. preparation in various ways, through solvent modification, chitin structural change to generate derivatives, and incorporating nanotubes. Each method has benefits and drawbacks. Future development should focus on less energy consumption in chitin hydrogels preparation for further industrial application. Chitin hydrogels exhibit remarkable mechanical properties and high swelling capacity and retention which make it versatile in various applications in drug delivery systems, stimuli responsive capability, biosensors and electrodes. We believe that this review presents a compact understanding that might provide insight for future chitin-based hydrogels processing to improve its chemical and physical properties.

I wish to state that no individuals or organizations require acknowledgment for their contributions to this study. This research was supported by the Korea Institute of Energy Technology Evaluation and Plan- ning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20224000000400) and Global - Learning & Academic research institution for Master’s ·PhD students, and Postdocs (G-LAMP) Program of NRF grant funded by the Ministry of Education (RS-2025-25440216). Received: (will be filled in by the editorial staff) Revised: (will be filled in by the editorial staff) Published online: (will be filled in by the editorial staff)

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