Cotton Cellulose-Derived Hydrogels with Tunable Absorbability: Research Advances and Prospects
Cotton is an important, worldwide cash crop and is considered as a ubiquitous resource offering the purest form of cellulose in nature. By far, the most industrially exploited natural resources containing cellulose are wood and cotton. Cellulose derived from either wood or cotton has the same chemical structure. Hydrogels are jellylike materials consisting of substantially hydrophilic cross-linked network filled with water. Upon replacing water with air, hydrogels are able to form aerogels. Cellulose and its derivatives can be used to prepare hydrogels with tailored absorbability and adsorbability. In the first section of this review, we discuss recent progress in the dissolution of high molecular weight cotton-derived cellulose as the dissolution of cellulose is an important step in preparing cellulose-based hydrogels. In the second section, we focus on the preparation of various cotton cellulose-based hydrogels and their derivatives by physical, chemical, and photocatalytic processes and their current applications. The third section includes the preparation and application of cellulose-based aerogels, which are a specific dry form of hydrogels. Overall, this review covers recent research developments in cotton cellulose-based hydrogels and their broad spectrum of applications in agriculture, environment, energy, health, and medicine.
KeywordsBiopolymer Cellulose Hydrogel Aerogels Cotton
Cotton is an agricultural crop that has fluffy staple fibers growing on a boll surrounding the seeds. The annual worldwide production of cotton is estimated to be 25 million tons, which makes cotton one of the leading cash crops in the world. The world’s largest producer of cotton is China, while the United States of America is regarded as the largest exporter of cotton in the world . Cotton fiber is almost pure cellulose, and it contains over 95% of cellulose . The biological growth of cotton fiber leads to the formation of cellulose starting with the primary cell wall development after the day post-anthesis (dpa) followed by the secondary cell wall development around 21 dpa [2, 3]. The conventional textile industry mostly uses cotton fiber to manufacture a variety of textile products including clothing, terry cloth, bed clothes, upholstery, and medical gauze. Despite the very large production, the current cotton industry is facing multiple challenges due to declining market price and considerable use of regenerated cellulose fiber, rayon, and synthetic fibers such as polypropylene and polyethylene terephthalate in the textile industry. As a result, there has been an increased demand for adding value to cotton fibers by preparing novel cotton-based materials which could potentially increase profitability and competitiveness of cotton.
Gels are a solid jellylike materials with a continuous, interconnected, and three-dimensional (3D) polymeric network . Hydrogels consist of hydrophilic polymeric chains and chemical groups that can hold and exchange a large amount of water with the environment . When water inside the hydrogels is replaced by a continuous gas phase, hydrogels become aerogels. These aerogels exhibit a wide range of applications in oil absorbability, metal recycling, probiotics, and environmental clarification [6, 7, 8]. The most intriguing feature of hydrogels is their capability of water retention and exchange with the environment. This property makes hydrogels adaptable to many areas including industrial, agricultural, environmental, and biomedical. Recent trends indicate that hydrogels synthesized from synthetic materials are gradually becoming popular in replacing the natural hydrogels generated from biosynthetic polymers due to their long lifetime and more tunable capacity of water absorption. However, hydrogels prepared from biopolymers still play a pivotal role in agricultural, environmental, and biomedical areas that especially require renewability, biodegradability, and biocompatibility.
This review discusses up-to-date research advances on the cotton cellulose-based hydrogels with tunable absorbability. Since the preparation of cellulose-based hydrogels involves cellulose dissolution, the first section of this review is focused on the dissolution of cotton cellulose and its derivatives. A diverse range of applications of cotton cellulose-based hydrogels is also discussed. The final section focuses on cotton cellulose-based aerogels and their current uses.
2 Dissolution of Cotton Cellulose and Its Derivatives
Most of the reported dissolution processes of cellulose carry limitations such as low cellulose solubility in terms of molecular weight and concentration, volatility, high cost, poor solvent recovery, high processing temperature, process instability, and environmental concerns . Two industrial processes, viscose and lyocell, use raw cellulose derived mostly from wood pulp to produce regenerated fibers . These industrial processes can also be applied to cotton cellulose, although cotton cellulose is not a popular feedstock to produce regenerated fibers . Both viscose and lyocell processes are associated with several issues, such as generation of highly toxic carbon disulfide (CS2), heavy metal compounds, hazardous by-products, unwanted side reactions, and harsh dissolution conditions [9, 25, 26].
2.1 Cotton Cellulose
Cellulose derived from wood remains the main source for the preparation of regenerated materials such as fibers and films in the industry . However, the dissolution of cellulose from other sources including cotton has been investigated in numerous solvents along with the preparation of regenerated materials such as fibers, hydrogels, and aerogels [27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41]. Unlike other cellulose sources, native cotton cellulose is obtained in the form of fibers, which can be directly processed into yarn. However, a significant amount of cotton fibers is discarded as waste during ginning and carding processes. Therefore, there is a need to utilize the waste cotton as a source of cellulose to prepare various regenerated materials.
Cellulose, hemicellulose, and lignin contents in wood and common agricultural residues 
Cotton seed hairs
Cost-effective and environmentally benign cellulose dissolution, in general, remains a challenge owing to its high molecular weight and high structural order. The problem becomes more complicated with cotton cellulose because it possesses high molecular weight and structural order (high amount of crystalline cellulose) among all plant cellulose macromolecules. Since the molecular weight of a polymer is a key parameter in determining its solubility and is inversely related to the entropic driving force which contributes to the dissolution, cotton cellulose of high molecular weight inherently presents a low solubility . Numerous studies have focused on both fundamental and applied aspects of cellulose dissolution. The majority of studies available in the literature use wood-based cellulose including microcrystalline cellulose (MCC) and wood pulps, which are different from cotton cellulose in terms of both molecular weight and crystallinity. These studies showed that different regenerated cellulose materials such as rayon and lyocell can be prepared from cellulose solution using different solvents [44, 45]. Since solvents effective for low molecular weight cellulose are not efficient on high molecular weight cellulose such as cotton under similar processing conditions, only few studies were focused on the dissolution of cotton cellulose in common cellulose solvents . Decreasing the molecular weight of cotton cellulose by means of chemical and enzymatic pretreatments could enhance the solubility of cotton cellulose. Other pretreatments which facilitate the dissolution of cotton cellulose include treatment with sodium hydroxide, solvent exchange with acetone, and freeze-drying processes [10, 46, 47].
2.2 Dissolution of Cotton Cellulose in Different Solvents
2.2.1 Viscose Process
Few studies were focused on the improvement of carbamate process using cotton linter cellulose. Yin et al. optimized the carbamation of cotton linter cellulose (DP ~520) with the aid of CO2 supercritical dehydration of cellulose . The study showed that CO2 supercritical dehydration could yield cellulose carbamate of high nitrogen content, which positively correlates with the solubility of cotton cellulose in aqueous NaOH. In addition, the pretreatment of cotton linter cellulose (DP ~550) by means of microwave heating at 255 W for 2–5 min under catalyst-free and solvent-free conditions was conducive to the enhancement of nitrogen content of cellulose carbamates from 0.651% to 2.427% . Fu et al. further optimized the carbamate process by improving the stability of the dissolved carbamate solution of cotton cellulose (DP ~790) by adding a small amount of ZnO in NaOH during the dissolution using conventional viscose method .
2.2.2 Aqueous Alkaline Systems
The potential of aqueous bases as cellulose solvents was reported as early as 1930, and it has been extensively studied since then [42, 47, 59, 60, 61, 62, 63]. Low molecular weight cellulose below 268,000 can be partially dissolved in aqueous NaOH of 7–10% concentration . The formation of NaOH hydrates in water is the major driving force to break down the intra- and intermolecular hydrogen bonding of cellulose . This system offers many advantages because of its low cost, minimal environmental issue, common chemicals, and facile dissolving process. However, NaOH/water system is unable to completely disrupt the semicrystalline region of cellulose, and the solubility is limited to cellulose of relatively low molecular weight. The improvement of cellulose dissolution in this system includes mechanical/physical, chemical, and enzymatic pretreatments of cellulose. Ball milling and steam explosion followed by grinding of wood cellulose pulp have been reported to enhance the solubility of cellulose in this system [47, 66]. Enzymatic pretreatments of cotton linter cellulose (DP ~850) could remarkably increase the solubility of cotton linter cellulose from 30% to 60% and significantly decrease the dissolution time . Additionally, the use of additives in NaOH/water solvent system, such as thiourea, urea, and polyethylene glycol, has been reported to enhance the dissolution ratio and the stability of the cellulose solution [28, 67, 68, 69, 70]. For example, NaOH/ urea system was used to dissolve relatively high MW cotton cellulose (DP ~620) within 30 min at a concentration of 4.5% urea in NaOH . Other bases, such as LiOH, KOH, and quaternary ammonium hydroxides, can also be used instead of NaOH to work with the additives to dissolve cellulose [48, 71, 72, 73]. One study reported that LiOH/urea/ H2O system was more effective than NaOH/urea/H2O system .
2.2.3 DMAc/LiCl System
2.2.4 Lyocell Process
Amine oxides constitute another important class of non-derivatizing cellulose solvents . Among the amine oxides, N-methylmorpholine-N-oxide monohydrate (NMMO) is considered the superior cellulose solvent. The lyocell process is based on the use of NMMO as a solvent for cellulose. NMMO monohydrate at 13% concentration in water can dissolve cellulose without derivatization to achieve a solubility of 23% cellulose concentration at 80–120 °C [9, 88]. The mechanism of NMMO to dissolve cellulose is due to the formation of one or two hydrogen bonds between the oxygen of NO group and hydroxyl groups of cellulose caused by the strong dipolar character of the NO group [74, 87]. Lyocell offers several advantages over industrially predominant viscose process. It can directly dissolve cellulose in NMMO without extra activation and pretreatment. It does not need an intermediate transfer to cellulose derivatization, and it significantly shortens the production route and also eliminates the release of highly toxic CS2. Additionally, the industrial recovery of NMMO can be as high as 99.7% . Lyocell fiber also has advantages over rayon in many respects such as its strength in both wet and dry states, modulus of elasticity, absorbable behavior, gloss, and touch. However, the lyocell process suffers from some drawbacks including unstable regenerated fiber, unwanted side reactions, and excessive degradation . Therefore, strategies have been applied to optimize the stabilization of the reaction and reduce the occurrence of unwanted side reactions .
2.2.5 Ionic Liquids
2.3 Other Methods for Preparing Cellulose-Based Hydrogels
Other methods for preparing cellulose-based hydrogels consist of synthesizing water-soluble cellulose derivatives and then forming hydrogels by means of physical, chemical, and radical cross-linking. Water-soluble cellulose derivatives contain methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), and hydroxypropyl methylcellulose (HPMC) . Generally, water-soluble cellulose derivatives have a low molecular weight (DP <100), and the substitution of hydroxyl groups on the cellulose macromolecular chain is more than 1.0 degree of substitution (DS) [9, 108]. These features make water-soluble cellulose derivatives quite easy to dissolve in water and other common solvents to prepare hydrogels. Most of water-soluble cellulose derivatives require that cellulose be well-dissolved or swollen prior to the modification, and they are produced from cellulose derived from wood because of the low molecular weight and abundant commodity of wood cellulose available. However, a few studies use cotton cellulose derived from waste cotton fabric, cotton by-products, and absorbent cotton that present low molecular weight to synthesize water-soluble cellulose derivatives [109, 110, 111].
Physical cross-linking is not involved in the covalent bonding. Cellulose derivatives are simply dissolved in water at a relatively high concentration under different conditions (heat or stir) to form hydrogels by means of ionic bonding, hydrogen bonding, and molecular interaction between polymeric chains [112, 113, 114]. Chemical and radical cross-linking of cellulose derivatives to form hydrogels are achieved through covalent bonding by functionalized cross-linker or self-induced bonding by light, heat, and pressure [115, 116]. For example, cellulose from cotton waste textiles was carboxymethylated and cross-linked by divinyl sulfone to form superabsorbent hydrogel using NMMO to dissolve cellulose. The resulting CMC products exhibit a high binding capacity of 541 g/g and a rapid absorption rate within 60 min when the sample is immersed in water .
3 Applications of Cotton Cellulose-Based Hydrogels
Most of cellulose-based hydrogels have been used in agricultural, environmental, health, and medical areas depending on their super-absorbability and tunable absorbency. Cotton cellulose is not the major feedstock for making hydrogels, and only cotton cellulose products, including linter, linter pulp, and waste with low molecular weight, have been explored to prepare various hydrogels for diverse applications. This review discusses only the applications of hydrogels derived from cotton cellulose.
3.1 Applications in Agriculture
There are only a few examples using cotton cellulose to prepare hydrogels as soil additives for adjusting water requirement. Kono and coworker reported a superabsorbent hydrogel prepared from cotton cellulose with a high DP of 2400 . The authors used LiCl/N-methyl-2-pyrrolidone (LiCl/NMP) to dissolve cotton cellulose followed by esterification of the dissolved cotton cellulose using 1,2,3,4-butanetetracarboxylic dianhydride (BTCA). Then the esterified product was converted to sodium carboxylates in aqueous NaOH. The final hydrogel product demonstrates its potential in water retention and discharges in the soil with an excellent absorbency of 720 times of its dry weight and a good biodegradability. Cross-linked CMC prepared from cotton linter pulp and cross-linker (epichlorohydrin) was also reported . Cotton linter was first swollen and then carboxymethylated followed by cross-linking. The resulting cross-linked CMC hydrogel exhibited an improved swelling behavior, which is sensitive to a change in the pH variation. This hydrogel may satisfy different soil pH requirements under different environments.
3.2 Environmental Applications
Cellulose-based hydrogels as pollutant adsorbents can remove heavy metal ions, dyes, and pesticides from water . The absorbency of cellulose-based hydrogels can be easily tailored by using different cellulose derivatives. For example, cellulose grafted with acrylonitrile is a good adsorbent of Cr5+, and oxidized cellulose-based hydrogel is considered a good absorbent for Hg2+ with an adsorption capacity of 258.75 mg/g at 30 °C and pH 4.0 . The preparation of hydrogels from these cellulose derivatives requires the dissolution of cotton or wood cellulose, modification of cellulose by specific functional groups, and then formation of hydrogels by means of physical or chemical cross-linking. In addition, hydrogel composites prepared from cotton cellulose and functional additives also play a role in the removal of heavy metal ions. Zhou and coworkers developed a fixed-bed column filled with cellulose/chitin hydrogel beads for the removal of a number of heavy metal ions in a broad range . A mixture of 4% cellulose solution prepared in DMAc/LiCl and 2% chitin solution prepared in NaOH/thiourea was used to coagulate and prepare composite hydrogel beads. The assembled bed with hydrogel beads could perform four cycles of adsorption/desorption to reach an adsorption efficiency of Pb2+ more than 70%. The cellulose/chitin beads also exhibited an excellent recycling capability. Beads treated with 0.1 M HCl can restore the regeneration efficiency up to 99%.
The removal of dye and pesticides can be achieved using cellulose-based hydrogel prepared from cotton cellulose modified with functional groups to synthesize super-adsorbent hydrogels or cellulose nanocrystals (CNCs) hydrogel composites. Zhou and coworker synthesized a super-adsorbent cellulose-graft-acrylic acid hydrogel using commercial cotton linter powder for the removal of methylene blue (MB) dyes from wastewater . The resulting hydrogels exhibited excellent MB adsorption capacity of 2197 mg/g, and nearly 70% MB in weak acid solution could be removed during the desorption process. In addition, CNCs/alginate hydrogels beads were used for MB removal up to 97% efficiency , as well as CNCs-reinforced polyacrylamide-methylcellulose hydrogels were found highly effective for agricultural applications as an adsorbent for insecticides or as a carrier for agrochemical controlled release .
3.3 Applications in Health and Medicals
Cotton cellulose has a long history of application in health and medical areas. A great amount of traditional medical gauze tape and bandage is employed every day in hospitals, although modern gauze tape made of synthetic fibers gradually becomes popular due to its versatile properties such as self-adhesion and antibacterial. Therefore, it is not necessary to dissolve cotton cellulose to prepare cellulose-based hydrogels for this type of applications. However, when advanced applications are required, such as diapers and wound healing contact films, cellulose-based hydrogels are required. Two kinds of cellulose samples including cotton linter pulps and wood pulps are the major feedstock needed for the preparation of cellulose-based hydrogels  for health and medical uses. Diapers were the first cellulose-based super-absorbent, initially used in the healthcare industry in 1982 in Japan . They were made from cellulose acrylate which is mostly used as a major functional compound in disposable diapers . In recent years, a new generation of diapers with advanced functions has emerged. Peng and coworker synthesized multifunctional cellulose-based superabsorbent hydrogels by means of cross-linking and quaternization of cellulose in NaOH/urea solution . The resulting hydrogel exhibited a high swelling ratio of 984 g/g and a good antimicrobial property against Saccharomyces cerevisiae.
Regarding wound healing and drug delivery, cellulose-based hydrogels play a key role as well. In addition to the traditional use as wound dressing materials, regenerated fibers made of cotton cellulose solution can be obtained by means of electrospinning or wet-spinning approaches. Various modifiers and chemicals can be added during the regeneration of dissolved cotton cellulose to exhibit versatile features of liquid exchange and drug delivery . Further research reported that functionalized cellulose-based hydrogels may work as a temporary skin substitute to compensate the dysfunction of wound tissue, such as providing moisture to wound bed, as well as absorbing wound exudates [132, 133]. However, due to its non-biodegradation in the body, cellulose-based hydrogels are constrained to the long-term use as skin substitutes for chronic wound healing (e.g., second-degree burn and above). Hu and coworkers performed considerable work to prepare cellulose-based hydrogel bioabsorbable by means of enzymatic incorporation to cellulose-based hydrogels [12, 134]. For drug delivery, a superabsorbent hydrogel synthesized from cotton cellulose and succinic anhydride exhibited an excellent water retention capacity of 400 times its dry weight . This hydrogel can be degraded completely after 25 days, which is a good candidate for a drug carrier vehicle to transfer and release effective ingredients in the body.
4 Cotton Cellulose-Based Aerogels
Aerogels are organic, coherent, highly porous, spongelike, or skeleton-like solid materials formed by the solidification of a colloidal gel network followed by replacing the liquid in the gel with one continuous gas or air [135, 136]. Generally, the preparation of aerogels is completed by removing the liquid from a hydrated interconnected gel network to leave a gaseous phase via freeze-drying or supercritical drying operations, and inversely the process of reabsorbing the liquid makes the aerogels become hydrogels again [135, 136]. When the liquid in the gel is water, aerogels may be deemed as pre-hydrogels. Most of the hydrogels require dehydration or remain a low hydrous level materials to ensure the best absorbability of liquids. Of course, some hydrogels need to maintain a high hydrous level to discharge the desired liquids to the environment, such as hydrogels used for wound healing that require keeping the wound moisture for tissue cell growth [12, 133]. Therefore, this section describes aerogels with super-absorbability and tunable absorbency.
Although cotton contains over 90% of cellulose, there is a scant information in the literature about the uses of cotton cellulose-based aerogels. In this section, we report on the preparation of cotton cellulose-based aerogels and their current applications.
4.1 Preparation of Cotton Cellulose-Based Aerogels
4.2 Applications of Cotton Cellulose-Based Aerogels with a Good Absorbency
4.2.1 Oil and Solvent Spillage Cleanup
Oil spillage has become a serious environmental concern worldwide due to the rapidly increasing use and transportation of oils and accidental oil leakage and spillage. Those spillages can cause severe pollution of natural water resources including underground water streams and coastal waters and consequently affecting both human and animal health. Currently used technologies for the removal and separation of oil spillage include chemical treatments , bioremediation , and physical methods . Among these, adsorbents used in physical methods are considered as one of the most efficient and effective approaches. Recently, cellulose-based aerogels have been investigated as suitable sorbents for removal of oil due to their natural abundance, sustainability, low cost, and environmental benignity. Cheng and coworkers reported on the utilization of pure cotton aerogels and cotton cellulose aerogels derived from pure cotton and cellulose fiber from paper waste for the adsorption of oil and organic solvents . Cotton cellulose-based aerogels showed a better performance over pure cotton aerogels because of the synergistic effects of two different cellulose fiber sources. The adsorption capacity of the composite aerogel is 72.3 and 94.3 g/g for machine oil and dichloromethane, respectively. Wan et al. investigated the waste motor oil adsorption capacities of cotton cellulose-based aerogels modified by methyltrichlorosilane (MTCS) . These MTCS-modified cotton cellulose aerogels exhibited an adsorption of approximately 14 times of its dry weight. Wang and coworkers also demonstrated the adsorption of diesel oil onto cotton-derived carbon fiber aerogels grafted with nitrogen-doped graphene . Bi et al. investigated the adsorption of oil and organic solvents onto aerogels prepared from twisted carbon fibers (TCF) derived from cotton fibers . They reported adsorption capacity in the range of 50–190 times the initial weight of TCF aerogels with excellent reusability.
4.2.2 Removal of Organic Pollutants and Heavy Metals
With the rapid industrialization and urbanization, a variety of industrial effluents containing organic and inorganic pollutants are discharged into natural water supply. Among those, pollutants such as organic dyes and heavy metal ions are extremely stable and persistent environmental contaminants and could not be eliminated naturally. Therefore, these pollutants pose a major threat to human health as well as aquatic ecosystems. Different approaches including chemical precipitation and adsorption, advanced oxidation and reduction, biological degradation, coagulation and flocculation, membrane separation, ultrafiltration, and photocatalytic and sonochemical degradation have been employed for the removal of organic dyes and heavy metals . The adsorption technique is considered to be the most efficient physiochemical approach due to its low operation cost and simplicity. Cellulose-based aerogels have been extensively investigated as adsorbents for the removal of organic contaminants and heavy metals. Chen and coworkers studied the removal of organic pollutants (phenol (PhOH), aniline (PhNH2), methyl orange (MO), methylene blue (MB), rhodamine B (RhB), and Victoria blue (VB)) and heavy metal ions (Pb2+, Co2+, Cd2+, and Sr2+) using cotton-derived porous carbon (CDPC) and cotton-derived porous carbon oxide (CDPCO) aerogels . They reported maximum adsorption capacity of 1519 and 1020 mg/g of MB for CDPC and CDPCO. The maximum Pb2+ adsorption of 111.1 and 21.2 mg/g was achieved for CDPC and CDPCO, respectively. Li et al. showed the maximum monolayer methylene blue (MB) adsorption of 101.23 mg/g for carbon fiber aerogels prepared from waste cotton . Zhou et al. reported maximum MB adsorption capacity of 2197 mg/g for cellulose-graft-acrylic acid (C-g-AA) hydrogels derived from cotton linters . Melone and coworkers reported on the preparation of ceramic aerogels via one-pot method by mixing aqueous hydrogels of cotton cellulose with titanium dioxide (TiO2) or TiO2/silicon dioxide (SiO2) solutions and subsequent freeze-drying . The calcined ceramic aerogels showed pronounced adsorption and photodegradation efficiency toward MB and RhB dyes.
4.2.3 Energy Storage and Carbon Dioxide Adsorption
The development of energy storage systems such as batteries and supercapacitors has continued to evolve over the last few years due to the variable production of energy from some renewable energy technologies. The energy storage systems have great potential for providing a stable energy supply and ensuring that the supply of energy matches the demand. Recently, supercapacitors have been developed as one of the most promising energy storage devices for their faster charge-discharge rates, excellent stability and cyclic retention, higher power, and minimal charge separation. Carbon materials such as graphene, activated carbon, carbon nanotubes, and carbon aerogels have been broadly used in supercapacitors [169, 170]. Carbon aerogels have advantages over other carbonaceous materials, including high surface area and porosity, excellent electrical conductivity, and high chemical and mechanical strength. Carbon aerogels derived from cotton cellulose have also been investigated for high-performance supercapacitor applications. Hu et al. reported the supercapacitance of 225 F/g and excellent cycle life with 94% of capacitance retention after 5000 charge/discharge cycles for nitrogen-doped carbon aerogels prepared from cotton linters and ammonia as a nitrogen source . They also reported CO2 gas adsorption of 4.99 mmol/ g for their material. Nitrogen-doped graphene aerogels prepared from raw cotton and urea showed a specific supercapacitance of 107.5 F/g . Tian and coworkers investigated the super capacitance properties of silver (Ag)/polyaniline (PANI)/cellulose aerogel nanocomposite prepared from cotton linter pulp . They reported a specific capacitance of 217 F/g with capacitance retention of 83% after 1000 cycles.
Owing to their high surface area and porosity, interconnected 3D network, biocompatibility, biodegradability, relative abundance, and cost-effectiveness, cellulose aerogels have attracted tremendous attention in a variety of biomedical applications. The applications of cellulose aerogels include tissue engineering, biosensors, antimicrobial agents, ultrasound contrast agents, drug delivery, and disease diagnosis . However, the use of cotton-derived cellulose aerogels in biomedical field has not been fully explored. Wansapura et al. reported on the preparation of cotton cellulose-cadmium-tellurium quantum dot aerogels and its antibacterial activity against gram-positive bacteria, Streptococcus aureus . Edwards and coworkers reported on the utilization of nanocellulose aerogels in biosensing of proteases [175, 176]. They prepared peptide-nanocellulose aerogels (PepNA) derived from cotton as biosensors for detecting proteases and reported the detection sensitivity of 0.13 units/mL for human neutrophil elastase.
5 Conclusion and Prospects
Cotton is a worldwide industrial crop which inherits the purest form of cellulose in nature. Most of harvested cotton goes to textile industry to produce fabrics for clothing, tablecloth, medical gauze, and home decoration. Compared to wood cellulose, it is uncommon to use cotton cellulose as a major feedstock to produce hydrogels. Because cellulose dissolution is a prerequisite to prepare regenerated cellulose materials, the high molecular weight and high crystallinity combined with extensive hydrogen bonding of cotton cellulose are the major obstacles for converting cotton cellulose to hydrogel materials. New emerging solvents can effectively dissolve cotton cellulose which can subsequently be converted to various materials including cellulose hydrogels. This review summarizes the current research trends in cotton cellulose-based hydrogels with tunable absorbability and cotton cellulose-based aerogels and their diverse range of applications in agriculture, environment, energy, health, and medical fields.
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