For some years now, biopolymers have attracted great interest from both academia and industry. Some of them have been investigated for a long time, such as rubbers , the interest in others, such as starch, cellulose  or PHA , is mainly being driven by ecology concerns. Chitin is somehow apart from this mainstreaming interest in biopolymers. Indeed, as the second major biopolymer worldwide after cellulose, it is mainly produced as a byproduct in shellfish industry. Therefore its production was less a concern for its valorization through different applications and subsequent purifications and derivatizations [4, 5].
Chitin is a polysaccharide, more precisely an aminoglucopyranan, composed of N-acetylated glucosamine (GlcNAc) and glucosamine (GlcN) units (Figure 1), linked by β(1,4) covalent bonds . Due to this specific linkage, chitin exhibits an extremely robust structure towards chemical and biological aggression, indeed the β(1,4) chitin bond is similar to the one found in cellulose, contrary to starch, that is much more easily digested by enzymes of several (micro)organisms than cellulose and presents an α(1,4) covalent bond between its monomeric units.
Moreover, the N-acetyl group attached to the major part of the glucosamine monomeric units of chitin confers to it extremely poor solubility properties, making chitin difficult to process and thus limiting its potential applications . To circumvent this issue, the hydrolysis of the acetyl group, also called deacetylation, can be applied (Figure 2). When GlcN units are predominant compared to GlcNAc units, the biopolymer is no longer designated as chitin, but as chitosan. Therefore the degree of acetylation (DA) (or the degree of deacetylation (DD)) is used to characterize the chitin/chitosan biopolymers as well as their degree of polymerization or source.
In this chapter the origin, different natural sources, subsequent issues related to the extraction and the purification of chitin, as well as its properties and applications are discussed.
2 Historical outline
Chitin was most probably discovered by an English scientist A. Hachett, who reported in 1799 “a material particularly resistant to usual chemicals”, but as he did not push his investigations any further, the discovery of chitin is usually attributed to a French naturalist from Nancy, Henri Braconnot , who identified this biopolymer in 1811 in extracts of mushrooms, and thus gave it the name of “fungine” . A few years later, in 1823, Auguste Odier found the same biopolymer in the insect’s exoskeleton, and named it “chitin” in reference to a Greek “chiton” meaning “tunic” .
The interrogation on the difference between cellulose, produced by plants, and chitin produced by arthropods was initiated by Payen in 1843 . In the same year Lassaigne found the presence of nitrogen in chitin, when working with the exoskeleton of silkworm butterfly, Bombix morii . Then, Ledderhose identified glucosamine and acetic acid as structural units of chitin in 1879, and Gilson confirmed glucosamine to be the repeated unit of chitin in 1894 . The final chemical nature of chitin was elucidated by Purchase and Braun in 1946 .
Chitosan was first obtained from chitin by C. Rouget , when boiling chitin in a concentrated alkali solution and noticing that the resulting compound was soluble in organic acids. Further, F. Hoppe-Seiler confirmed in 1894 that chitosan is the deacetylated form of chitin and thus gave it its actual name .
Another milestone in the discovery of chitin’s structure and arrangement was made by Bouligand. He discovered in 1965 that chitin adopts a stereotypic arrangement in arthropods . Thus, three main types were found, α-, β- and γ-types (Figure 3). The α-type exhibits an antiparallel disposal of chitin molecules, thus strengthening the intra- and inter-molecular hydrogen bonds. The α-type is the most robust type of chitin, it is the most resistant toward physical and chemical aggression and is also the one mainly found in nature. In contrast, the β-type chitin arrangement characterizes parallel chains’ aggregation, in this case N -acetyl groups play the role of spacers, allowing easier access to molecules of water, for hydration purposes and subsequent gel formations . Finally, the γ-type is mainly composed by 2 parallel and 1 antiparallel layers, its strength and resistance are closer to the β-type, the N -acetyl groups playing the same role in both those arrangements. In addition, non-crystalline, transient states have also been reported in a fungal system.
Further, the macroscopic arrangement of chitin layers and protein scaffolds surrounding them on a cholesteric helix was studied , a twisted plywood structure was thus found in the lobster Homarus americanus  and in the sheep crab Loxorhynchus grandis , it has also been reported to be responsible for the iridescence of the scarab beetle .
Chitin is present in the exoskeleton of arthropods , also in eukaryotic cells, such as those of fungi  and mushrooms . It is also found in the iridophores (reflective material) in the epidermis and the eyes of certain arthropods and cephalopods . One study  has even reported that the epidermal cuticle of a vertebrate, a fish named Paralipophrys trigloides, contains chitin; thus suggesting that chitin can also be produced by vertebrates.
As chitin is present in so many different species, it would be, of course, very tempting to use chitin for evolution and taxonomy of these different species, however to the best of our knowledge those studies have still to be done. Also, a comparison between different properties, contents, etc. of chitin from different sources would be of great interest, but for now scientists could not distinguish any specific characteristics for chitin extracted from different sources (α-chitin being dominating in any shellfish, insects, etc.), also the content comparison faces the issue of chitin extraction and characterization, detailed below.
Despite this very large diversity of chitin sources, until now, mainly chitin from the shellfish industry has been explored. It was thus shown that crustaceans mainly produce α-chitin, whereas cephalopods produce β-chitin, also the chitin content varies from 7 to 36% in crustaceans and from 20 to 40% in cephalopods, howeverwe have to acknowledge that in this study for 10 species of crustaceans explored, only two representative of cephalopods were considered, therefore the conclusions concerning this latter class may be less accurate .
A few studies performed on insect chitin were mainly concerned with butterflies . It was thus shown that the wings of the painted lady butterfly, Vanessa cardui Linnaeus, exhibits α-type chitin . The insects exhibit a complex hierarchical structure, where each epidermal scale represents one color. These scales are morphologically homogeneous and adherent to the wings in rows, which run parallel to the anterior-posterior axis of the wing .
The chitin content is usually in the range of 5 to 40% of dry mass of cuticle, thus honeybees were reported to have organic matrices with 23–32% of chitin , mushrooms 8–16%  and shellfish 5–40% , and even up to 49% were reported in squid pens . The chitin content within the same species during the lifetime of the organism seems to remain stable, it was found to be around 3% in Calanus helgolandicus .
Also, the determination of chitin content in its original source remains an issue. Indeed, no reliable analytic methods have been reported until now, infrared and diffraction analysis allowing only qualitative and non-quantitative approaches , therefore the only quantitative method mainly used upto now is the so called“alkaline extraction” method . However, it strongly depends on the source and extraction procedure used, which may explain severe differences observed in the literature concerning some species, namely cuttlefish was found to have only 5.8% of chitin by Hajji et al.  and up to 20% by Rhazi et al. , whereas shrimp was found to have up to 37.2% chitin by Hajji et al.  and only 22% by Rhazi et al. .
4 Extraction and purification
As we have seen from the previous paragraph, the extraction and purification of chitin is not only important for the recovery of the desired product but also to characterize the chitin content in different sources. Up to now two main approaches for chitin extraction have been studied: chemical and biological approaches.
4.1 Chemical extraction
Chemical extraction basically consists of two steps: acidic treatment for mineral elimination and basic treatment for protein elimination [27, 28]. The classical procedure can consist of an acidic treatment with HCl 1 M for 2 to 24 hours, followed with a basic treatment with NaOH 1 M for another 24 to 48 hours. The temperature usually is kept at 60–90 °C to avoid as much as possible chitin degradation and preserve its structure. The different parameters of these treatments can be tuned in order to adapt to a specific source of chitin or to combine different steps [17, 29].
Thus, for example, the mineral content is especially high within shellfish sources of chitin whereas lipid content is much higher in insects. Therefore, the acidic treatment can be adjusted, especially through the solid-state approach, to combine the elimination of both these sources of contamination and to obtain highly crystalline chitosan .
Also the alkali concentration for the deproteination step is of specific interest, as at high temperature and concentration the reaction continues up to deacetylation and the obtained product is no longer chitin, but chitosan .
The utilization of acidic and basic conditions at high temperature may also damage the integrity of the chitin polymer, and oligosaccharides can be thus obtained. It can present advantages for some applications, shorter chains inducing better solubility, but also drawbacks of chain alteration .
Another issue consists of a very poor ability to analyze the obtained product, several attempts through infrared , diffraction , fluorescence  or NMR [21, 26] techniques were made, however they did not give sufficiently satisfying results [25, 28, 29], and only a combination of several of these approaches allows the confirmation of the structure and purity of the chitin.
Another aspect of the purification of chitin is its color. Indeed, melanins and other sclerotins [12, 17] attached to the exoskeleton of arthropods or other chitin sources make it difficult to obtain a pure white chitin at the end of the process, therefore bleaching agents, such as hydrogen peroxide, are often used to finalize the chitin purification . Other techniques, such as γ-radiation, were also tested to improve the extraction of chitin .
The main drawback of the utilization of chemical extraction remains however its energetic and environmental impact due to the extensive utilization of acidic and basic solutions requiring further neutralization and elimination .
4.2 Biological extraction
Although more environment friendly, these methods remain less efficient, one of the main reasons being the up to now poor understanding of the main covalent links between chitin and the surrounding proteins and melanins [12, 17]. Indeed, the main enzymes used for the purification of chitins are proteases, they cut protein bonds to give peptides and amino acids, however they cannot liberate chitin from catechols or even amino acids directly attached to it.
In this case, it becomes even more difficult to avoid bleaching steps, thus reducing the scope of pure biological extraction and making it much more biochemical .
As neither chemical nor biological methods are completely satisfying for now, several academic teams and industrial companies are still working on their improvements. This extraction stage is, indeed, essential for chitin and chitosan to fulfill the huge potential those molecules can present.
Chitin and chitosan have recently been reported to present several properties such as biocompatibility , biodegradation , scavenging of heavy metal  and of cholesterol or other fats , antimicrobial and antioxidative behaviors [43, 44], etc. However, we have to acknowledge that even if those biopolymers seem very promising, until now, the actual applications remain rather limited.
This can be explained by different aspects: the difficulty of extraction and purification of the original chitin, the necessary transformation of chitin to chitosan followed by the derivatization of the latter for most of applications [45– 47], etc.
Also, some of the previously mentioned assertions appeared less obvious that they seemed at first glance, for example, it would be difficult for the same molecule to present antimicrobial and biodegradation properties, actually if the antimicrobial aspect of chitin and chitosan seems to be proven, their biodegradation appears much more doubtful . Similarly, the fact that chitin is difficult to purify, namely from fats, does not necessarily mean that it makes it good candidate for lowering cholesterol in blood.
For all the reasons mentioned above, we will focus our attention in this chapter mainly on biomedical, agricultural, materials and water purification applications of chitin and chitosan.
5.1 Biomedical applications
The main biomedical applications of chitosan are in wound healing. This application combines two of the most interesting properties of chitosan: antimicrobial behavior and biocompatibility . These products have been on the market since the early 1990s, mainly in Asia and North America, but also in Europe, however to a lesser extent.
These products are commercialized by companies such as BioSyntech (Canada), Hemcon (USA), Medovent (Germany), Marine Polymers Technologies (USA), Cytosial (France).
Some other biomedical applications of chitosan were also studied, such as bone substitutes [4, 49], blood interactions [4, 50], drugs vectorization , implants , or anti-inflammatory [52, 53], antihypertensive  and anticancer  drugs; however most of these applications have not yet reached the actual market of biomedical products.
Finally, chitosan was also shown to be active against cryptosporidiosis in goats, significantly reducing the excretion of oocysts of C. parvum and diarrhea, and enhancing the weight gain of young animals .
5.2 Agricultural applications
Chitin and chitosan were reported to contribute to the protection of plants against pathogens. However, in this application, chitin and chitosan were described to play very different roles.
Chitin was reported to contribute to the protection of seeds, mainly by allowing the growth of microbial pesticides, such as Trichoderma harzianum P1, which was reported to be active against foliar disease. The addition of chitin to the medium specifically contributed to the growth of T. harzianum and partially inhibited the growth of certain pathogens such as R. solani, thus playing a double role: enhancing the production of suitable microorganisms and lowering the invasion of unsuitable ones through actual inhibition and further demographic control .
Chitosan was described to act as an elicitor, indeed, several plants possess chitinolytic enzymes, which help them to defend themselves against pathogen aggressors, such as fungi. The introduction of chitosan in the growth medium stimulates the production of chitinolytic enzymes in plants, thus making them more resistant towards their natural aggressors [58, 59].
5.3 Materials applications
For a long time chitosan as a material was used as a plastic for the production of antimicrobial films for the food industry, i.e. packaging protecting fresh vegetables, fruits or meat .
More recently, some novel applications received greater interest, among them the utilization of chitosan as a catalyst support is of particular interest. Indeed, this application combines several up-to-date techniques, such as freeze drying  or utilization of supercritical CO2 (scCO2)  to increase the surface exchange capabilities and/ or utilization of ionic liquids [66, 67]. It thus contributes to the implementation of green chemistry principles by minimizing the amount of required product – catalytic and not stoichiometric proportions, and utilization of renewable raw materials – second most abundant biopolymer worldwide.
5.4 Water purification
The quality of water remains one of the huge issues humanity is currently facing. Access to pure water in some parts of the world remains difficult, thus generating malnutrition, diseases, and even military conflicts. Therefore the purification of water that was initially unsuitable for use or polluted is of crucial importance. To contribute to the wellbeing of a larger population the chosen water treatment techniques have to be technically and economically efficient.
Different techniques are usually applied to water purification, such as scavenging, adsorption, flocculation or biological treatments. Chitosan can be applied for adsorption [68, 69] and flocculation  purposes, the fact that it is an environment friendly compound, very abundant and rather cheap makes it a solution of choice in certain cases. Thus chitosan was shown to be particularly efficient for the flocculation of cardboard-mill secondary biological wastewater , unfortunately the actual applications in industry remain rather rare, as concurrent flocculating agents are cheaper. Indeed, even if chitosan shows better properties, the traditional cheaper products are sufficient to fulfill current regulatory frameworks.
Chitin is a second most abundant biopolymer on Earth after cellulose. Present in several species, it is, until now, mainly obtained from the discards of the shellfish industry. Its extraction and purification as well as the reliability of available sources remain an issue and unfortunately impacts its widespread applications. Also, as chitin has a rather poor range of applications that can be exploited, mainly focused on the agricultural area, it has to be transformed by deacetylation to chitosan, and sometimes even further by derivatization of the latter.
Despite extremely interesting properties, such as biocompatibility or antimicrobial behavior, the applications of chitosan in the biomedical area remain limited, mainly due to the extreme difficulty to access sufficient purity and source reliability of the biopolymer. The materials applications until now are rather limited, mainly due to the cost of the product, which remains higher than that of petroleum based polymers with similar properties. The applications in the agricultural fields seem very promising but require more research and development to achieve significant results, therefore the main utilization of chitosan is now the purification of water, achieved by scavenging of heavy metal residues.
Finally, for the widespread utilization of these truly very promising biopolymers, beside strong scientific developments, intellectual rigor is also mandatory, to promote only actual properties and to avoid attributing fashionable but not proven ones.
This article is also available in: Luque/Xu, Biomaterials. De Gruyter (2016), isbn 978–3–11–034230–7.
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