The use of cellulose as raw material is preferred in the manufacture of hydrogels based on natural polymers because of its inherent biocompatible and biodegradable properties, in addition to the excellent availability of various types of functional groups that can be used for modification, bio-adhesion, biocompatibility, accessibility, and affordability [ 15 , 47 ].
The OH group on C 1 is the OH found in aldehydes, referred to as reducing agents. This aldehyde group forms a pyranose ring through an intramolecular hemiacetal form. The OH groups on D-anhydroglucopyranose are one primary OH group and two secondary OH groups. In C 2 and C 6 –OH groups, intermolecular hydrogen bonds form. In the C 3 –OH group and oxygen on the pyranose ring, intramolecular hydrogen bonds form. Intramolecular and intermolecular hydrogen bonding occurs due to the large number of OH groups in cellulose [ 46 ].
The fundamental structure of plant cell walls is cellulose, and in certain woods, cellulose accounts for about 40–50% [ 43 ]. Cellulose is constructed from glucose chains linked via −1.4 glycosidic bonds formed between C 1 and C 4 of adjacent glucose groups. Each D-anhydroglucopyranose has three hydroxyl groups (OH) at positions C 2 , C 3 , and C 6 , as shown in [ 44 ].
Several cellulose derivatives that have been developed to synthesize hydrogels include methylcellulose (MC) [48], hydroxyethyl cellulose (HEC) [49], hydroxypropyl cellulose (HPC) [3], hydroxypropyl methylcellulose (HPMC) [50], and carboxymethyl cellulose sodium (CMCNa) [51]. These derivatives are known to be water-soluble cellulose derivatives. shows the molecular structure of HEC, HPC, HPMC, MC, and CMCNa. summarizes previous research related to the synthesis of cellulose-based hydrogels, the types of cellulose derivatives and crosslinking agents used, and the results.
The equilibrium swelling ratio of hydrogel was different from that of crosslinked MC hydrogel with control MCs, with average expansion values from 800% for MCs with 5% citric acid to 3000% MCs with 3% citric acid.
MC with 1% citric acid did not show a significant difference in the swelling equilibrium compared with MC control.
The addition of 0.25% plasticizer affected the barrier properties of hydrogel unlike hydrogel without plasticizer. Adding 5% citric acid to MC hydrogel plasticized with 0.25% sorbitol was possibly able to improve the barrier properties and decrease the affinity for water.
The gel fraction of hydrogel without WO3 was 59.7% and increased to 65.9% after the addition of 0.02% WO3.
The highest swelling of hydrogel was achieved for hydrogel without WO3 and with 0.02% WO3.
Unlike the MS-free sample, the addition of 10% medical stone increased the swelling capacity by 400%, and the initial expansion rate constant increased by 7.48 times.
Adding medical stone increased the initial swelling rate but decreased with increasing ion strength.
The incorporation of HAp and TCP nanoparticles on BCP in HPMC aqueous solution increased the viscosity of injection scaffold but decreased the gelation temperature.
With a higher pectin quantity, the swelling percentage increased from 79.58% to 92.62% at pH 7.4.
Increasing the amount of HPMC from 0.5 g to 1.5 g affected the percentage of swelling so that the swelling increases from 76.68% to 95.89% at pH 7.4.
When compared with HPC hydrogels at 25 °C, the MoS2-HPC/HPC hydrogels had a smaller expansion ratio as a result of the MoS2-HPC crosslinking action.
The cationic HPC hydrogel showed an excellent ability to adsorb anionic dye (dye orange (II)), and the maximum adsorption capacity at room temperature was 2478 (g/kg) at pH 3.96.
The highest WRV was 725 g distilled water/g gel and 118 g saline-water/g gel, with a composition of 3% of CMCNa and 4% of ECH.
As the temperature increased, the weight loss of CMCNa and crosslinked CMCNa/HEC hydrogel indicated the loss of moisture. The temperature of decomposition (TD) was 285.5 °C (weight loss: 68.2%) of CMCNa and 276.6 °C (weight loss: 56.8%) of crosslinked CMCNa/HEC (5/1).
MC is a macromolecule of cellulose, with 27–32% of the hydroxyl group in the form of methyl ether. Various grades of MC with degrees of polymerization in the range of 50–1000, molecular weights in the range of 10,000–220,000 Da, and degree of substitution in the 1.64–1.92 range are commercially available [52]. This methyl derivative of cellulose has the special property of forming a thermally reversible hydrogel upon heating, thus being classified as a polymer with a lower critical solution temperature [53].
Bonetti et al. [48] developed MC-based hydrogels with citric acid as a crosslinking agent. In the first 24 h, all hydrogels showed an increase in weight due to water absorption. Swelling balance is reached in the next 24 h. Increasing the degree of crosslinking of the sample causes a significant decrease in the swelling ratio. The equilibrium swelling degree of hydrogels prepared with a constant amount of MC is dependent on the amount of critic acid, with the average swelling values ranging from 800% for MCs with 5% citric acid to 3000% for MCs with 3% citric acid. Conversely, MC with 1% citric acid did not show significant differences in terms of swelling at the equilibrium compared with MC control. This finding indicates that the specimen’s swelling behavior is slightly affected by low crosslinking. In fact, an increase in the crosslinking degree causes an increase in crosslinking points, preventing crosslinked MC network expansion in the water environment.
In line with previous research, Quiroz et al. [54] synthesized MC-based hydrogel with citric acid as a crosslinking agent. Citric acid functions as a crosslinking agent for MC hydrogels when used at low concentrations (5% w/w). The crosslinking decreased water vapor permeability and swelling, allowing good gas barrier properties to be obtained. The formulation of MC 1.5%, 0.25% sorbitol, and 5% citric acid (w/w MC) would allow reduced-affinity coating for water and oxygen to be obtained, which can be used to cover foods under low-humidity conditions and preserve nutrients susceptible to oxidation.
HEC is a partially substituted hydroxyethyl etherified cellulose. It is a hydrophilic polymer with a degree of substitution of at least 1.5. When the degree of substitution of HEC increases, the level of solubility in water will increase [53]. With its biocompatibility and non-immunogenicity, HEC is often used as stabilizer, thickener, film, hydrogel, nanofiber in tissue engineering applications, and it can improve the quality of the resulting hydrogel both mechanically and rheologically [55,56,57,58].
Fawal et al. [49] developed an HEC-based hydrogel with citric acid as a crosslinking agent and tungsten trioxide (WO3) as a support material for wound dressing applications. The FTIR analysis showed the presence of HEC and citric acid and that crosslinking had occurred. The gel fraction of hydrogel without WO3 and with 0.02% WO3 was 59.7% and 65.9%, respectively. Swelling or the highest water absorption was 300.1% without WO3 and 165.6% with 0.02% WO3, and decreased with increasing WO3. The percent of water absorption decreased with increasing concentration of WO3, because WO3 consumes some hydrogen bonds.
In line with previous research, Wang et al. [58] synthesized hydroxyethyl cellulose-g-poly(sodium acrylate)/medicinal stone (HEC-g-PNaA/medical stone)-based hydrogel with NMBA as a crosslinking agent. The addition of various amounts of medical stones can change the structure and composition of the hydrogel and affects the swelling capacity. With a medical stone of up to 10% by weight, the swelling capacity increased sharply by 400% and then decreased with further addition of medical stone. The addition of medical stone can decrease the degree of physical crosslinking and increase the swelling capacity because when NaA was grafted onto HEC and MS can participate in the polymerization reaction through its active silanol groups, contributing to the formation of ordinary polymer networks, preventing the intertwining of grafted polymer chains, and weakening hydrogen bonding interactions between groups. However, when the addition of medical stone exceeded 10% by weight, the swelling capacity decreased, because the tissue cavity for holding water was blocked and the hydrophilicity of the hydrogel decreased.
HPMC is a propyleneglycol ether of methylcellulose, described by the PhEur as a partly O-methylated and O-(2-hydroxypropylated) cellulose. HPMC is a water-soluble polymer that is available in several grades with different viscosities and substitution rates. HPMC hydrogel has high levels of transparency, stability, and viscosity because of its good biocompatibility and thermosensitive natural polymers [53,59,60].
Seyedlar et al. [50] developed HPMC-based hydrogels with biphasic calcium phosphate (BCP) that were applied to tissue engineering. HPMC-based hydrogels can reduce the invasiveness of osteoplasty surgery, shorten the operating time, and cause homogeneous cell distribution. Incorporation of hydroxyapatite (HAp) and β-tricalcium phosphate (TCP) nanoparticles on BCP in an HPMC aqueous solution increased the viscosity of injection scaffold but decreased the gelation temperature.
In line with previous research, Bashir et al. [61] synthesized HPMC hydrogel with HPMC-pectin-co-acrylic acid as a polymer and NMBA as a crosslinking agent. PAA containing COOH group is the reason for the increase in the swelling pattern, which has a greater tendency to ionize as the high porosity of hydrogel increases at pH 7.4. The HPMC formulation gradually increased from 0.5 g to 1.5 g, causing the percentage of drug release to also increase simultaneously from 75.36% to 87.62% at pH 7.4, because HPMC has higher swellability and hydrophilic properties at pH 7.4.
HPC is a polymer in which some of the hydroxyl groups of cellulose have been hydroxypropylated, forming -OCH2CH(OH)CH3 groups. During the HPC manufacturing process, the added hydroxypropyl group can be esterified, having a mole substitution value (number of moles of hydroxypropyl groups per glucose ring) greater than 3. Therefore, HPC must have a degree of substitution (DS) value of 2.5 and a molarity of substitution (MS) of 4 to have good water solubility [52,62,63,64].
Chen et al. [3] developed HPC-based hydrogels made by modifying HPC to alkynyl-HPC as a polymer and molybdenum disulfide (MoS2) as a crosslinking agent. The hydrogels produced from this study had high water absorption capabilities and thicker pore walls. The addition of MoS2 with HPC can make the hydrogel to be effective in removing methylene blue dyes. The addition of MoS2 into HPC can induce a reduction in the swelling ratio of the hydrogel because the addition of MoS2 into HPC weakens the effect of the volume phase transition of hydroxypropyl cellulose, which causes an increase in crosslinking.
In line with previous research, Yan et al. [65] synthesized HPC hydrogel with ECH as a crosslinking agent, and ammonia as a co-crosslinking agent. It was found that the adsorption ability of the resin had a strong relationship with the pH value. The microporous structure and the chemical structure of the prepared crosslinked HPC resin are the key factors in producing hydrogels with high adsorption capacity of anionic dyes. The resin can also be used in neutral conditions with a high adsorption capacity for anionic dyes.
CMCNa is a hydrophilic polymer prepared by partial substitution of OH groups in the second, third, and sixth positions of cellulose by carboxymethyl groups. The DS value varies in the range of 0.6–1, affecting several physicochemical properties of the polymer. Therefore, due to the higher DS value, the water solubility and sodium content of CMCNa increase and the polymer tolerance for other components in the solution improves [53].
Alam et al. [51] developed a CMCNa-based hydrogel with ECH as a crosslinking agent. FTIR analysis showed the presence of CMCNa and ECH, as well as the fact that crosslinking had occurred. The hydrogel with the highest water absorption or water retention value (WRV) was obtained with a composition of 3% of CMCNa and 4% of ECH.
In line with previous research, Astrini et al. [66] synthesized CMCNa hydrogel with divinyl sulfone as a crosslinking agent. The weight loss of CMCNa and crosslinked CMCNa/HEC hydrogels indicated a loss of moisture in the samples when the temperature increased (100–170 °C). The TD was 285.5 °C (68.2% weight loss) for CMCNa and 276.6 °C (56.8% weight loss) for crosslinked CMCNa/HEC (5/1). The peak temperature of the main degradation step of CMCNa/HEC (5/1) shifted to a lower temperature compared with pure CMCNa. The crosslinked structure plays an important role in thermal decomposition and indicates that CMCNa is more stable than CMCNa/HEC. With increasing synthesis temperature and reaction time, water absorption capacity also increased.
As a polyelectrolyte, CMCNa is sensitive to pH and ionic strength. Therefore, the compatibility of CMCNa in a solution with other components is an important characteristic. CMCNa is highly compatible with most 10% and 50% monovalent inorganic salt solutions of the cations that form CMCNa soluble salts. Crosslinked CMCNa is capable of absorbing large amounts of water and swells to form superabsorbent hydrogels that exhibit superior mechanical and viscoelastic properties compared with other crosslinked cellulose derivatives hydrogels [1].
CMCNa-based hydrogels can be used in enzyme immobilization, wound healing, drug delivery, and adsorbents. They can be made into materials for applications involving anti-bacterial activity, drug delivery, wound healing, and tissue engineering [67,68,69]. CMCNa is easily synthesized from cellulose derived from waste biomass extraction, such as oil palm empty fruit bunches and bagasse because it provides unique CMCNa properties, such as good adsorption, high swelling capacity, and good optical properties (i.e., how it interacts with light, focusing on biomedical applications). The high methylation group in the biomass waste is also an advantage for the production of CMCNa-based hydrogels.
Among the five cellulose derivatives mentioned above, CMCNa remains a favorite raw material for developing hydrogel materials. This is supported by the statistics shown in , obtained from Scopus and ScienceDirect.
The statistical data in were collected by searching for related articles using several keywords, such as “MC hydrogel,” “HPMC hydrogel,” “HEC hydrogels,” “HPC hydrogel,” and “CMCNa hydrogel,” in the years ranging from 2011–2021. The data indicate that Scopus and ScienceDirect had 260 and 161 scientific articles on the topic of CMCNa-based hydrogels, respectively. This result may be due to the nature of CMCNa itself; CMCNa exhibits a relatively constant level of viscosity over a wide temperature range. The carboxyl group present in CMCNa is the reason for this advantage, because the addition of the carboxyl group to cellulose can adjust the properties and allow the end user to obtain a certain texture beyond the thickness. CMCNa also has high water absorption [51] and swelling ratio [30] when used for hydrogel materials.
Most CMCNa that is used as a raw material in hydrogel synthesis is made from natural materials. Research on manufacturing CMCNa with natural ingredients has been conducted in the past. Rachtanapun et al. [70] reported cellulose from durian rind isolated with NaOH and bleached with hydrogen peroxide. The cellulose was converted to CMCNa using various NaOH concentrations for carboxymethylation. The best results showed that the DS values increased with increasing NaOH concentrations.
Recently, Phan and Thi [71] synthesized CMCNa from another natural material, namely, passion fruit peel cellulose. Passion fruit peel has excellent potential with a dry weight of cellulose of about 42% [71] and high cellulose content of about 86.2 g/kg [72]. They conducted an experiment to extract the cellulose from passion fruit peel, which was then synthesized into CMCNa. The highest cellulose extraction yield was 32.13% at 1 M NaOH and 1.25 M HNO3. The obtained cellulose was then characterized using FTIR; several peaks were observed, indicating that the cellulose produced was pure cellulose and showing the presence of β-(4, 17)-glycosidic linkages between the glucose units in cellulose. This cellulose was synthesized into CMCNa, with a maximum CMCNa yield of 79.5% and a degree of substitution of 0.78, which were achieved at 20% NaOH concentration and 2 g monochloroacetic acid (MCA). The functional groups of CMCNa were analyzed using FTIR. The presence of –COO and –COONa groups was observed, indicating that cellulose etherification was successful.
Many studies have been conducted on the manufacture of CMCNa from various natural materials, with good and high yields; therefore, CMCNa from natural materials has the potential to be used as a raw material in the manufacture of hydrogels. In particular, passion fruit peel has been used only as a feed mixture [73] and in the manufacture of pectin extracts [64]. In the material sector, passion fruit peel is only used as a film [74], activated carbon [75], and microcrystalline cellulose [76].
In recent decades, crosslinked CMCNa networks have been obtained by applying crosslinking technology chemically and physically. Chemical crosslinking involves the use of bifunctional crosslinkers such as ECH, multifunctional carboxylic acid, and PEGDE. However, some diglycidyl ethers produce large amounts of toxic by-products under crosslinking conditions that require elimination by extensive washing, thereby affecting the hydrogel biocompatibility and environmental safety of the production process.