Chitosan as a Tool for Sustainable Development: A Mini Review

New developments require innovative ecofriendly materials defined by their biocompatibility, biodegradability, and versatility. For that reason, the scientific society is focused on biopolymers such as chitosan, which is the second most abundant in the world after cellulose. These new materials should show good properties in terms of sustainability, circularity, and energy consumption during industrial applications. The idea is to replace traditional raw materials with new ecofriendly materials which contribute to keeping a high production rate but also reducing its environmental impact and the costs. The chitosan shows interesting and unique properties, thus it can be used for different purposes which contributes to the design and development of sustainable novel materials. This helps in promoting sustainability through the use of chitosan and diverse materials based on it. For example, it is a good sustainable alternative for food packaging or it can be used for sustainable agriculture. The chitosan can also reduce the pollution of other industrial processes such as paper production. This mini review collects some of the most important advances for the sustainable use of chitosan for promoting circular economy. Hence, the present review focuses on different aspects of chitosan from its synthesis to multiple applications.


Introduction: Necessity of Alternative Materials for a Circular Economy
The new regulations promoted by numerous governments are trying to take care of the environment by protecting actions and behaviors to develop a new sustainable economy. Some of the most important goals of these laws are aimed at the reduction of the excessive consumption of non-renewable raw materials, especially those derived from natural sources. The extraction and cleaning of raw materials are responsible for soil degradation, biodiversity loss, water shortages, and global warming. The use of residues as raw materials is a new concept derived from the circular economy which could definitely contribute to the reduction of the huge amounts of trash accumulated in landfills. The concept of a circular material means that a new product can be obtained from the old one which is acting as a raw material. The new product will exhibit the same properties and qualities as the previous one, i.e., materials will remain in a continuous cycle of life. In general, a huge amount of this waste is composed of plastics whose versatility and wide range of properties makes it difficult to get a competitive alternative in terms of costs.  Table 1. Some of the main chitin sources and percentages [13].

Source
Percentage (%) Commercial chitosan ( Figure 1) is comp cosamine and is produced by the partial deace the change of acetamido groups into amino gr ymer depending on its molecular weight: low and oligochitosans [15].

General Features and Properties of Chito
The main properties which can contribute hibited by the chitosan are non-toxicity, biod theless, there are other interesting properties a tility which can be deduced from Table 2. Table 2. General properties of chitosan [16,17].

General Features and Properties of Chitosan
The main properties which can contribute to a sustainable development that are exhibited by the chitosan are non-toxicity, biodegradability, and biocompatibility. Nevertheless, there are other interesting properties and characteristics which explain its versatility which can be deduced from Table 2. Table 2. General properties of chitosan [16,17].

Property Conditions Use References
Solubility Dilute acids (pH < 6). Insoluble in organic solvents and water Water treatment [18,19] Activity Antibacterial, antifungal mucoadhesive analgesic, and hemostatic properties [20][21][22] Degradation Depends on molecular weight and deacetylation degree [18,23] Biocompatibility Physiological medium Biomedical applications [7,24] Chelating properties Capability to bind and adsorb diverse ions The removal of heavy metals and dyes from wastewater [25,26] Biodegradability Biodegradable to normal body constituents [24,27,28] Hemostatic Stop a hemorrhage [29,30] Catalyst Accelerates the formation of osteoblast [31] Fungicide Stopping the development of fungi [32,33] Spermicidal Reduce the mobility of spermatozoa [34] Anticholesteremic Reducing agent cholesterol [35,36] Anticancer Inhibiting the development of cancer cells [37] Conductivity Ionic conductivity [38,39] Flocculating agent Interactions with negatively charged molecules Water treatment [40] Thickener Increase the viscosity [41] Polyelectrolytes Acidic medium [42] Adsorption Separation and filtration [43][44][45] Clarifying agent Immobilization of enzymes [46] From the presentation of Table 2, it can be deduced that chitosan is a sustainable material as it is biodegradable and non-toxicity [47]. Another important reason for using chitosan is the presence of a large number of hydroxyl and amino groups in its structure which are suitable for chemical modifications [48]. This fact and the wide versatility of chitosan makes this material especially interesting for the preparation of suspensions, composites, functionalized materials, or (nano)hybrids for diverse eco-friendly purposes and applications. The interesting polymorphic behavior exhibited by the chitosan [49], together with the molar mass and degree of deacetylation, mainly defines its mechanical properties. The molar mass will also play an important role for other properties such as degradation degree or antibacterial activity as these are strongly affected by the changes in molar mass.
On the other hand, the degree of deacetylation is associated with the content of acetamide groups of polymeric chains. These groups will strongly affect the final features and properties of the chitosan, in particular its capacity to be biodegradable and its immunological activity. The deacetylation degree is defined between 50 and 99%, its content depends on the preparation methods. The deacetylation degree must be higher than 50% for the chitosan; below that value, it is considered chitin [18]. Some of the most important uses of chitosan are associated with biomedical applications. Nevertheless, new developments related to chitosan focus on agriculture, food packaging, textiles, or environmental applications [50]. The solubility of the chitosan depends on the medium being used to dissolve it; in acid mixtures with water, it is soluble, but it is insoluble in common organic solvents [51,52]. The reason for its solubility can be explained due to the presence of amino groups that transforms chitosan into a base, whose protonation produces a polyelectrolyte [53]. The presence of different functional groups is responsible for the reactivity and the flexibility of this polycationic polymer [54]. Chitosan biofilms show a semi-crystalline behavior, together with high hydrophobicity and little flexibility [55].

Chitosan as an Ecofriendly Biopolymer and Its Applications
Chitosan is considered a natural biopolymer; it has received remarkable attention from the scientific community due to the fact that it can be easily biodegraded. Its residues are not toxic and can be easily eliminated and biodegraded by nature [7]. One of the most important problems associated with the raw materials is that these are limited, but chitosan is the most abundant biopolymer after cellulose. Furthermore, chitosan exhibits a great biocompatibility, limited by its low solubility which can be solved through chemical modifications and hydrolysis. Chitosan is a bioactive material which can be modulated and used in many applications [56]. Some of these applications are associated with biomedical purposes such as drug delivery systems, scaffolds, or membranes. Nevertheless, there are other important uses such as in the textile industry, wastewater treatments, agriculture, food, packaging, personal care, and biotechnology, among others. The adsorbent properties of chitosan are very useful for removing different heavy metal ions accumulated in water and derived from industrial processes such as Pb 2+ , Hg 2+ , and Cu 2+ , among others [57]. These can be accumulated inside the body and produce numerous diseases [58]. Chitosan can contribute to the agriculture by improving the harvest and productivity, being an ecofriendly material. It is used as a coating for seeds, enhancing the properties of the plants and the obtained products in terms of shelf life. This use as fertilizer is especially useful for plant protection as it can stimulate the plant defense, but it can also act as an antibacterial and antimicrobial agent [59]. Thus, chitosan acts as a plant growth-promoting agent and plant protector [60]. For that reason, it is considered a pesticide by several countries. The antioxidant properties of chitosan, together with its antimicrobial features, are suitable for the production of films for food packaging. The preparation of hybrid materials with chitosan allows modifying the permeability of those films depending on the requirements [2]. The chitosan can also be used as a food additive, dietary fiber, and functional ingredient [61,62].

Chitin Extraction
The extraction of chitin is necessary for the production of chitosan such as it was previously explained. A huge amount of chitin is obtained from crustaceans, but there are multiple advances in its production through insects or fungi and bacteria, thus avoiding the use of animal derivatives [63]. In general, the extraction requires several steps starting with the removal of mineral salts and proteins ( Figure 2). It is commonly carried out chemically, using acids and bases, which is not a sustainable process. These processes can destroy some properties of chitosan, reducing its versatility. Currently, there are multiple advances in natural deep eutectic solvents which could replace the hazardous solvents and preserve the features of chitin. There is another option based on the use of microorganisms for the extraction of chitin known as a biological method [64]. In general, these methods are especially indicated for the treatment of fungi and bacteria whilst chemical processes are related to the treatment of crustaceans. After removing the minerals and proteins, chitin requires a depigmentation process which is generally performed using oxidizing agents. The use of the enzymes could be a feasible way for removing the proteins, which can reduce the degree of depolymerization in comparison with traditional methods. That chitin also showed a better solubility in water probably due to a lower crystallinity of the product [65]. The specific use of the trypsin also induces the depigmentation, reducing the steps involved in the extraction of chitin [66]. There is a lot of ground to cover in terms of sustainability around processes for the extraction of chitin associated with environmental pollution, loss of chitin properties, and costs. One of the main consequences of this extraction is the polluted wastewater, which needs to be treated.

Chitosan Production
The production of chitosan requires the deacetylation of chitin; this process can be modulated through concentration, temperature, and time [7]. Scheme 1 shows the changes produced in chitin after being transformed into chitosan. The traditional method to obtain chitosan from chitin was reported in 1980, which promotes a high deacetylation due to rapid reaction rates at reduced temperatures [67]. There are different ways to carry out the deacetylation such as alkali treatment, the use of enzymes, or a steam explosion [16,68,69]. The degree of deacetylation will define the spectra of properties of the chitosan in terms of features such as solubility, viscosity, or biodegradability, etc. [70]. There are numerous alternatives where the energy consumption can be reduced, contributing to a green chemistry. Those methods explore the use of microwaves and ultrasonic waves in the deacetylation process. The use of ultrasonic waves leads to enhancing the reactivity of the deacetylation process [71]. Some of the new approaches are displayed in Table 3, showing some of the most interesting advances related to the sustainable production of chitosan.

Chitosan Production
The production of chitosan requires the deacetylation of chitin; this process can be modulated through concentration, temperature, and time [7]. Scheme 1 shows the changes produced in chitin after being transformed into chitosan.

Chitosan Production
The production of chitosan requires the deacetylation of chitin; this process can be modulated through concentration, temperature, and time [7]. Scheme 1 shows the changes produced in chitin after being transformed into chitosan. The traditional method to obtain chitosan from chitin was reported in 1980, which promotes a high deacetylation due to rapid reaction rates at reduced temperatures [67]. There are different ways to carry out the deacetylation such as alkali treatment, the use of enzymes, or a steam explosion [16,68,69]. The degree of deacetylation will define the spectra of properties of the chitosan in terms of features such as solubility, viscosity, or biodegradability, etc. [70]. There are numerous alternatives where the energy consumption can be reduced, contributing to a green chemistry. Those methods explore the use of microwaves and ultrasonic waves in the deacetylation process. The use of ultrasonic waves leads to enhancing the reactivity of the deacetylation process [71]. Some of the new approaches are displayed in Table 3, showing some of the most interesting advances related to the sustainable production of chitosan. The traditional method to obtain chitosan from chitin was reported in 1980, which promotes a high deacetylation due to rapid reaction rates at reduced temperatures [67]. There are different ways to carry out the deacetylation such as alkali treatment, the use of enzymes, or a steam explosion [16,68,69]. The degree of deacetylation will define the spectra of properties of the chitosan in terms of features such as solubility, viscosity, or biodegradability, etc. [70]. There are numerous alternatives where the energy consumption can be reduced, contributing to a green chemistry. Those methods explore the use of microwaves and ultrasonic waves in the deacetylation process. The use of ultrasonic waves leads to enhancing the reactivity of the deacetylation process [71]. Some of the new approaches are displayed in Table 3, showing some of the most interesting advances related to the sustainable production of chitosan.

Circularity in the Chitosan Production
The traditional methods can also be adapted, at least partially, trying to get a sustainable production of chitosan. For that purpose, it is necessary to reduce the energy consumption by reusing the hazardous reagents. The recovery of sodium hydroxide used in the extraction of chitosan was reported in studies. The sodium hydroxide is part of wastewater and could be treated using ultrafiltration and nanofiltration membranes recovering the sodium hydroxide for a new cycle of life [74,75]. The reuse of sodium hydroxide can contribute to a decrease the environmental pollution and reducing the cost of the process, i.e., a lower amount of sodium hydroxide will be required. There were also reports for the preparation of chitosan at ambient temperature, following the general procedure of demineralization, deproteinization, and decolorization [76]. This fact could also be quite interesting, due to the reduced energy consumption. Thus, involving circularity in the production of chitosan can be very beneficial and economically better.

Applications of Chitosan for Sustainable Development
Chitosan can contribute to sustainable development through its applications and uses. This review tries to expose some of the most important applications related to the contribution of chitosan to a circular economy and sustainability. Figure 3 depicts the diversified application of chitosan. Deproteinization requires NaOH Optimized process [72] Rhizopus oryzae (fungi) Fermentation Cheap, low energy consumption, and soft conditions [73]

Circularity in the Chitosan Production
The traditional methods can also be adapted, at least partially, trying to get tainable production of chitosan. For that purpose, it is necessary to reduce the e consumption by reusing the hazardous reagents. The recovery of sodium hyd used in the extraction of chitosan was reported in studies. The sodium hydroxide of wastewater and could be treated using ultrafiltration and nanofiltration mem recovering the sodium hydroxide for a new cycle of life [74,75]. The reuse of s hydroxide can contribute to a decrease the environmental pollution and reduci cost of the process, i.e., a lower amount of sodium hydroxide will be required. were also reports for the preparation of chitosan at ambient temperature, followi general procedure of demineralization, deproteinization, and decolorization [76 fact could also be quite interesting, due to the reduced energy consumption. Th volving circularity in the production of chitosan can be very beneficial and econom better.

Applications of Chitosan for Sustainable Development
Chitosan can contribute to sustainable development through its application uses. This review tries to expose some of the most important applications related contribution of chitosan to a circular economy and sustainability. Figure 3 depic diversified application of chitosan.

Sustainable Use of Chitosan for Food Packaging and in Agriculture
Many biopolymers are being implemented in different coating materials d their excellent properties in terms of degradability and compatibility; these biopol include gums, starch, proteins cellulose, lipids, and their derivatives [77][78][79][80][81][82][83]. I sense, chitosan is a promising material for that purpose due to several reasons asso

Sustainable Use of Chitosan for Food Packaging and in Agriculture
Many biopolymers are being implemented in different coating materials due to their excellent properties in terms of degradability and compatibility; these biopolymers include gums, starch, proteins cellulose, lipids, and their derivatives [77][78][79][80][81][82][83]. In this sense, chitosan is a promising material for that purpose due to several reasons associated with its biocompatibility and abundance [84,85]. The use of the chitosan in films can also provide other superiorities because of its antibacterial and antioxidant properties [86][87][88][89]. In general, chitosan is used in combination with other polymers due to some of its drawbacks associated with its low mechanical properties. Another important problem associated with chitosan is related to its water sensitivity [90]. The preparation of blends can diminish these problems, thus obtaining films with a wide range of properties. The miscibility problems between the mixtures of polymers can reduce the spectra of possibilities, but in general, the preparation of these films is easy and cheap. The preparation of these systems could be a good alternative regarding traditional films based on oil derivatives [91]. Table 3 displays some of the most promising blends of chitosan, based on the mixtures with other biopolymers. There are other mixtures with synthetic polymer of chitosan that are not included in this review, as those do not fit the sustainability criteria of the present review. Numerous composites of chitosan have been fabricated with graphene, carbon nanotubes, activated carbon, and metal nanoparticles [92][93][94][95]. One study suggests that poly(L-lactic acid)-ZnO multilayered with cationic chitosan and anionic β-cyclodextrin can be used as a promising material in applications for the active packaging of food [96]. A novel bilayer food packing film of Ag-Metal−organic framework loaded p-coumaric acid modified chitosan (P-CS/Ag@MOF) or chitosan nanoparticles (P-CSNPs/Ag@MOF) and polyvinyl alcohol/starch (PVA/ST) was fabricated. The bilayer composite film revealed a relatively smooth surface and higher tensile strength (27.67 MPa). The P-CS/Ag@MOF bilayer films displayed better oil resistance and oxidation resistance, and the bilayer film had good UVblocking properties and transparency [97]. The diverse blend composites of chitosan have been developed with various natural antimicrobial compounds and have been applied for antimicrobial food packaging; such antimicrobial compounds include thyme oil, spirulina, oregano essential oil, nisin, apple peel polyphenols, bamboo vinegar, cinnamon essential oil, custard apple leaves, plum peel extract, etc. [98][99][100][101][102][103][104]. The antibacterial nanofiber films were fabricated using gelatin, chitosan, and 3-phenyllactic acid (PLA) by electrospinning. Under acidic conditions, chitosan and PLA interacted and formed hydrogen bonds, which decreased the crystallinity of the nanofiber films. The nanofiber film had the best thermal stability, water stability, water vapor permeability, and more effective antibacterial effects against Salmonella enterica Enteritidis and Staphylococcus aureus, suggesting that the nanofiber film mat can be used as an active food packaging [105]. Similarly, Wang et al. discussed various chitosan and gelatin edible films, their synthesis strategies including casting, electrospinning, and thermoplastic method, and their properties in their review, thus highlighting importance of chitosan-based food packing films [106]. In Argentina, chitosan is produced from the waste of the shrimp industry; the synthesized chitosan has similar physicochemical properties to those of analytical grade chitosan. The chitosan coatings applied to processed lettuce at harvest increased nutritional quality and reduced microbiological contaminants in minimal processed lettuce [107]. Panda et al. fabricated ferulic acid-modified water-soluble chitosan and poly(γ-glutamic acid) polyelectrolyte multilayers films. These film surfaces possessed a reduced amount of protein adsorption; thus, these can be used as a potential good biomaterial for biomedical purposes to intensify the bio-active surface [108], thus prompting the concept of circularity and sustainability. Tables 4 and 5 show the effects of some films over the food due to the use of chitosan which could modify its properties. Table 4. Selection of blends of chitosan with other biopolymers for food packaging.
[118] Table 5. Effects of films based on chitosan over food.

Blend Food Effects References
Chitosan-glycerol film (Good mechanical and barrier properties. Stability)

Strawberry
Better preservation effect than the commercially available PE films. [119] Gelatin/chitosan film with nanocarriers (Fe III -HMOF-5) (Good results in mechanical properties and permeability)

Apple cubes
High content of nanocarriers allows the preservation of apple cubes during 5 days. [120] Chitosan films (modified with mango leaf extract) (Higher hydrophobicity and tensile strength) Cashew nuts High oxidation resistance. [121] Chitosan/gelatin film with silver nanoparticles (Better hydrophobicity and antibacterial properties) Red grapes Antimicrobial properties and high oxidation resistance. [122] Polyurethane/chitosan/nano ZnO composite film (Better mechanical properties, low permeability)

Carrot
Better shelf life than polyethylene film [19] Pullulan/chitosan film (good barrier to O 2 ) Papayas Maintained the physiological and nutritional attributes. High shelf life. [123] Chitosan-TiO 2 nanocomposite film (Better tensile strength and barrier properties)

Mushrooms
Possessed effective antibacterial properties, non-cytotoxicity, and preservation performance [126] Active packaging films based on chitosan and sardinella protein isolate Shrimps Good antioxidant and antibacterial activities [127]  Chitosan-based biodegradable bags Palmer's mango Effective in delaying ripening and preserving the quality [133] Composite films based on chitosan and syringic acid Quail eggs Films exhibited higher density, water solubility, good preservation effect [134] Films based on quaternary ammonium chitosan, polyvinyl alcohol, and betalains-rich cactus pears (Opuntia ficus-indica) extract

Shrimp
Enhanced the UV-vis light barrier, elongation-at-break, and antioxidant, antimicrobial and ammonia-sensitive properties [135] Chitosan coating with vacuum packaging Beef Extend the shelf life of beef Inhibited S. aureus [136] Chitosan coatings Lettuce Improve quality and extend shelf-life of minimally processed lettuce [107] Chitosan films incorporating litchi peel extract and titanium dioxide nanoparticles Watercored apple Coating treatment significantly inhibited respiration rate, weight loss, and softening [137] Polylactic acid/chitosan films Indian white prawn Antimicrobial properties [138] Chitosan-Gelatin (CHI-Gel) based edible coating incorporated with longkong pericarp extract (LPE)

Shrimp
Edible coating as a natural antioxidant, antimicrobial activity and inhibiting melanosis, retain the quality and extend the shelf-life [139] Pink pepper residue extracts incorporated in a chitosan film Salmon fillets Shelf-life of the skinless salmon fillet could be extended by 28 days [140] Chitosan film incorporated with citric acid and glycerol Green chilies Improved mechanical, thermal, and antioxidant properties of the film were and increased shelf life [141] The chitosan can act as protector, coating material, stimulator of the growth, nutrient, fertilizer, or pesticide in agriculture. It was also observed that the use of chitosan can increase productivity. Furthermore, the use of chitosan could replace some dangerous chemicals used as compounds of fertilizers in agriculture, protecting soil, aquifers, and ecosystems [142]. It was reported that excellent antimicrobial activity was observed in chitosan against many viruses, bacteria, and fungi. Nevertheless, its activity is higher against fungi than bacteria. In general, the chitosan seems to inactivate the replication of viruses [143]. Moreover, it is considered a potent elicitor which can induce plant defense against diseases [144]. Table 6 shows some of the effects observed of chitosan over some fruits and vegetables. Table 6. Effects of chitosan and derivatives over some products.

Material/Use Plant Effects Reference
Chitosan with copper Tomato Plant defense (Enzymatic and anatomical changes). [145] Seed-priming with chitosan Cucumber Disease protection and enhanced plant growth. [146] Foliar application of chitosan Sweet pepper Enhancement of the adverse effects of salinity and improved the growth and yield. [147] Chitosan solution (using a hand sprayer) Dracocephalum kotschyi Increase of antioxidant enzyme. [148] Chitosan (foliar spray or pre-sowing seed treatments in Cd-stressed plants) Pea Improvement in growth, photosynthetic pigments, and reduction in oxidative damage. [149] Chitosan (protective spray) Mango (Amrapali and Dashehari) Reduced malformation of mango. [150] Chitosan nanoparticles Durum wheat Increase the leaf antioxidant pool. [151] Chitosan oligosaccharide (COS) Tea plant (Camellia sinensis) Improved the antioxidant enzyme activities and the content of chlorophyll and soluble sugar. [152] Chitosan nanoemulsion containing allspice essential oil Maize Preserved maize samples from aflatoxin B1 and lipid peroxidation. [153] Chitosan nanoparticles loaded with garlic essential oil Wheat, oat, and barley As a seed dressing agent found to have antifungal activity against Aspergillus versicolor, A. niger, and Fusarium oxysporum. [154]

1.5% chitosan solution treatment Berry
Inhibit postharvest berry abscission of the 'Kyoho' table grapes. [155] Preharvest chitosan sprays Muskmelons Induced suberin polyphenolic deposition at wound sites during healing thus promoted wound healing and reduced disease development. [156] Chitosan film containing Akebia trifoliata (Thunb.) Koidz. peel extract/montmorillonite A. trifoliata fruits Significant effect on the delaying crack and mature of the fruits. [157] Chitosan-based nanoencapsulated Foeniculum vulgare Mill. essential oil Sorghum bicolor Significantly preserved the nutritional and sensory characteristics of S. bicolor seeds. [158] Encapsulated peppermint essential oil in chitosan nanoparticles -Biological efficacy against stored-grain pest control. [159] 3.

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The chitosan also showed good results associated with ions, as it can be observed in Table 8. These are only some examples of the good results that can be achieved.  Fluoride Zirconium (IV)-impregnated magnetic chitosan graphene oxide Adsorption capacity was 8.84 mg/g [220] U(VI) Chitosan-based aerogel U(VI) adsorption capacity of 160 mg/g [221] Au(III) Chitosan functionalized with N,N-(2aminoethyl)pyridinedicarboxamide Maximum adsorption capacity of 659.02 mg/g [222] Cr(IV) Chitosan composite Adsorption capacity was 18 mg/g [223] Cu(II) Benzothiazole functionalized chitosan Maximum copper adsorption capacity of 1439.7 mg/g [224] Cr Chitosan can be used for paper manufacture due to its mechanical properties which can provide better resistance to recycled paper, reducing the consumption of chemical additives [234]. Table 9 displays the various roles of chitosan in paper production. Table 9. Effects of chitosan in paper production.

Material/Use Paper Application Effects Reference
Nanoparticles with chitosan and starch Old corrugated containerboard (OCC)

Increase tensile and burst strength
Decrease tear resistance [235] Chitosan and cellulose nanofibers Paper recycling (decolorization) Remove water-based inks [236] Microparticules with chitosan and bentonite Paper reinforcement Chitosan is a good dry strength additive [237] Chitosan as additive Papermaking (aging stability of paper) Increase tensile strength. Decrease the hydrophilicity of paper [238] Chitosan with zeolite as filler Papermaking Improve the mechanical properties of paper Chitosan as additive Paper reinforcement (Kenaf paper (Hibiscus cannabinus)) Give a good mechanical and dry strength properties [239] Graphene ink from the exfoliation of graphite in pullulan, chitosan, and alginate For strain-sensitive paper Paper-based strain sensor, the chitosan-graphene has the best resistivity value and demonstrates the highest sensitivity towards strain [240] The chitosan can also be used as amino-functionalized structures for CO 2 capture. Many industrial processes could reduce their emissions using these systems. Furthermore, there are many other options where chitosan can be used to reduce the greenhouse gas emissions [241]. Table 10 displays the chitosan-based materials used for gas capture. Table 10. Chitosan-based materials used for gas capture.

Future Perspectives
It is expected that chitosan uses will increase replacing other traditional materials due to its interesting properties and functionalities, but also due to it being abundant, it can be extracted using green chemistry and easily treated as waste. For these reasons, chitosan is considered a rich renewable resource where some of its shortcomings associated with solubility, mechanical properties, and porosity are being addressed due to the potential of this source.
This article shows some of the most prominent fields where chitosan is an interesting alternative to other conventional materials, but its properties will be reflected soon in other many fields due to its versatility and properties. Some of the most promising applications could be associated with specific areas such as medicine, food packaging, or biotechnology, among others.
There is a lot of room to grow in terms of the production of chitosan, the current goal of which is clearly focused on the removal of hazardous solvents and reducing the energy consumption. On the other hand, chitosan can contribute to sustainability in terms of recycling and waste management due to its degradability.

Conclusions
Chitosan shows an interesting range of properties which make it very useful for sustainable development due to it being abundant, biodegradable, biocompatible, and versatile. The production of chitosan is improving in terms of green chemistry, due to the hazardous chemicals being replaced by eutectic solvents, lower energy consumption has been achieved, and circularity can be applied to secondary processes. The use of chitosan in films for food packaging shows better properties than traditional films composed of polyethylene. The edible food packing with enhanced antimicrobial activity can be developed using chitosan. Numerous blends of chitosan have been developed with various essential oils and extracts which are excellent antibacterial and antifungal agents. On the other hand, the chitosan provides interesting and multiple features for a sustainable agriculture, such as a protection for the plant and increasing the production. Finally, the chitosan can contribute to green chemistry in multiple processes such as the paper industry or the treatment of wastewater, reducing the impact and contributing to the circularity of industrial processes. The chitosan-based composites, hydrogels, and membranes can be used for the remediation of diversified pollutants including dyes, antibiotics, phenols, metal ions, etc. Thus, being a second abundant biopolymer in nature, chitosan can be a potential sustainable future material. Funding: Author wants to thank the Erasmus+ KA107 scholarship.

Conflicts of Interest:
The authors declare no conflict of interest.