Chitosan

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Chitosan
Chitosan2.jpg
Identifiers
CAS number 9012-76-4 YesY
ChemSpider 64870 YesY
Jmol-3D images Image 1
Related compounds
Related compounds D-glucosamine and N-Acetylglucosamine (monomers)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

Chitosan /ˈktɵsæn/ is a linear polysaccharide composed of randomly distributed β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It is made by treating shrimp and other crustacean shells with the alkali sodium hydroxide.

Chitosan has a number of commercial and possible biomedical uses. It can be used in agriculture as a seed treatment and biopesticide, helping plants to fight off fungal infections. In winemaking it can be used as a fining agent, also helping to prevent spoilage. In industry, it can be used in a self-healing polyurethane paint coating. In medicine, it may be useful in bandages to reduce bleeding and as an antibacterial agent; it can also be used to help deliver drugs through the skin.

More controversially, chitosan has been asserted to have use in limiting fat absorption, which would make it useful for dieting, but there is evidence against this.

Other uses of chitosan that have been researched include use as a soluble dietary fiber.

Manufacture and properties[edit]

Commercial chitosan is derived from the shells of shrimp and other sea crustaceans, including Pandalus borealis, pictured here.[1]

Chitosan is produced commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans (such as crabs and shrimp) and cell walls of fungi. The degree of deacetylation (%DD) can be determined by NMR spectroscopy, and the %DD in commercial chitosans ranges from 60 to 100%. On average, the molecular weight of commercially produced chitosan is between 3800 and 20,000 Daltons. A common method for the synthesis of chitosan is the deacetylation of chitin using sodium hydroxide in excess as a reagent and water as a solvent. This reaction pathway, when allowed to go to completion (complete deacetylation) yields up to 98% product.[2]

The amino group in chitosan has a pKa value of ~6.5, which leads to a protonation in acidic to neutral solution with a charge density dependent on pH and the %DA-value. This makes chitosan water soluble and a bioadhesive which readily binds to negatively charged surfaces such as mucosal membranes. Chitosan enhances the transport of polar drugs across epithelial surfaces, and is biocompatible and biodegradable. It is not approved by FDA for drug delivery though. Purified quantities of chitosans are available for biomedical applications.

Chitosan and its derivatives, such as trimethylchitosan (where the amino group has been trimethylated), have been used in nonviral gene delivery. Trimethylchitosan, or quaternised chitosan, has been shown to transfect breast cancer cells, with increased degree of trimethylation increasing the cytotoxicity; at approximately 50% trimethylation, the derivative is the most efficient at gene delivery. Oligomeric derivatives (3-6 kDa) are relatively nontoxic and have good gene delivery properties.[3]

Usage[edit]

Agricultural and horticultural use[edit]

The agricultural and horticultural uses for chitosan, primarily for plant defense and yield increase, are based on how this glucosamine polymer influences the biochemistry and molecular biology of the plant cell. The cellular targets are the plasma membrane and nuclear chromatin. Subsequent changes occur in cell membranes, chromatin, DNA, calcium, MAP Kinase, oxidative burst, reactive oxygen species, callose pathogenesis-related (PR) genes and phytoalexins.[4]

Natural biocontrol and elicitor[edit]

In agriculture, chitosan is used primarily as a natural seed treatment and plant growth enhancer, and as an ecologically friendly biopesticide substance that boosts the innate ability of plants to defend themselves against fungal infections.[5] The natural biocontrol active ingredients, chitin/chitosan, are found in the shells of crustaceans, such as lobsters, crabs, and shrimp, and many other organisms, including insects and fungi. It is one of the most abundant biodegradable materials in the world.

Degraded molecules of chitin/chitosan exist in soil and water. Chitosan applications for plants and crops are regulated by the EPA, and the USDA National Organic Program regulates its use on organic certified farms and crops.[6] EPA-approved, biodegradable chitosan products are allowed for use outdoors and indoors on plants and crops grown commercially and by consumers.[7]

The natural biocontrol ability of chitosan should not be confused with the effects of fertilizers or pesticides upon plants or the environment. Chitosan active biopesticides represent a new tier of cost-effective biological control of crops for agriculture and horticulture.[8] The biocontrol mode of action of chitosan elicits natural innate defense responses within plant to resist insects, pathogens, and soil-borne diseases when applied to foliage or the soil.[9] Chitosan increases photosynthesis, promotes and enhances plant growth, stimulates nutrient uptake, increases germination and sprouting, and boosts plant vigor. When used as seed treatment or seed coating on cotton, corn, seed potatoes, soybeans, sugar beets, tomatoes, wheat and many other seeds, it elicits an innate immunity response in developing roots which destroys parasitic cyst nematodes without harming beneficial nematodes and organisms.[10][11]

Agricultural applications of chitosan can reduce environmental stress due to drought and soil deficiencies, strengthen seed vitality, improve stand quality, increase yields, and reduce fruit decay of vegetables, fruits and citrus crops (see photo right).[12] Horticultural application of chitosan increases blooms and extends the life of cut flowers and Christmas trees.[13] The US Forest Service has conducted research on chitosan to control pathogens in pine trees[14][15] and increase resin pitch outflow which resists pine beetle infestation.[16]

NASA life support GAP technology with untreated beans (left tube) and ODC chitosan biocontrol-treated beans (right tube) returned from the Mir space station aboard the space shuttle – September 1997

Chitosan has a rich history of being researched for applications in agriculture and horticulture dating back to the 1980s.[17] By 1989, chitosan salt solutions were applied to crops for improved freeze protection or to crop seed for seed priming.[18] Shortly thereafter, chitosan salt received the first ever biopesticide label from the EPA, then followed by other intellectual property applications.

Chitosan has been used to protect plants in space, as well, exemplified by NASA's experiment to protect adzuki beans grown aboard the space shuttle and Mir space station in 1997 (see photo left).[19] NASA results revealed chitosan induces increased growth (biomass) and pathogen resistance due to elevated levels of beta 1-3 glucanase enzymes within plant cells. NASA confirmed chitosan elicits the same effect in plants on earth.[20]

Nontoxic, low molecular weight chitosan polymer solutions appear to be safe enough for broad-spectrum agricultural and horticultural uses.[21][22] In 2008, the EPA approved natural broad-spectrum elicitor status for an ultralow molecular active ingredient of 0.25% chitosan.[23]

A natural chitosan elicitor solution for agriculture and horticultural uses was granted an amended label for foliar and irrigation applications by the EPA in 2009.[12] Given its low potential for toxicity and abundance in the natural environment, chitosan does not harm people, pets, wildlife, or the environment when used according to label directions.[24][25][26] The US Forest Service tested chitosan as an ecofriendly biopesticide to prearm pine trees to defend themselves against mountain pine beetles.

Water filtration[edit]

Chitosan can also be used in water processing engineering as a part of a filtration process. Chitosan causes the fine sediment particles to bind together, and is subsequently removed with the sediment during sand filtration. It also removes phosphorus, heavy minerals, and oils from the water. Chitosan is an important additive in the filtration process. Sand filtration apparently can remove up to 50% of the turbidity alone, while the chitosan with sand filtration removes up to 99% turbidity.[27] Chitosan has been used to precipitate caseins from bovine milk and cheese making.[28][29]

Chitosan is also useful in other filtration situations, where one may need to remove suspended particles from a liquid. In combination with bentonite, gelatin, silica gel, isinglass, or other fining agents, it is used to clarify wine, mead, and beer. Added late in the brewing process, chitosan improves flocculation, and removes yeast cells, fruit particles, and other detritus that cause hazy wine. Chitosan combined with colloidal silica is becoming a popular fining agent for white wines, because chitosan does not require acidic tannins (found primarily in red wines) with which to flocculate.[30]

Manufacture[edit]

Large three-dimensional functional objects made from chitosan.[31]

Bioinspired materials, a manufacturing concept inspired by natural nacre, shrimp carapace or insect cuticles,[32][33][34] has led to development of methods to manufacture large scale consumer objects using chitosan.[35][36] This method is based on replicating the molecular arrangement of chitosan from natural materials into fabrication methods, such as injection molding or mold casting.[37] Once discarded, chitosan-constructed objects are biodegradable and non-toxic.[38] The method is being explored to engineer human organs or tissues using three-dimensional bioprinting.[39][40]

Pigmented chitosan objects can be recycled,[41] with the option of reintroducing or discarding the dye at each recycling step, enabling reuse of the polymer independently of colorants.[42][43] Unlike other plant-based bioplastics (e.g. cellulose, starch), the main natural sources of chitosan are from marine environments and do not compete for land or other human resources.[44][31]

The fabrication of large objects with chitosan applies concepts from Shrilk, a chitosan material that replicates insect cuticle and a fibroin protein from silk for uses in bioengineering.[45][46]

Winemaking and fungal source chitosan[edit]

Chitosan has a long history for use as a fining agent in winemaking.[47][48] Fungal source chitosan has shown an increase in settling activity, reduction of oxidized polyphenolics in juice and wine, chelation and removal of copper (post-racking) and control of the spoilage yeast Brettanomyces. These products and uses are approved for European use by the EU and OIV standards.[49]

Potential industrial uses[edit]

Scientists have developed a polyurethane coating that heals its own scratches when exposed to sunlight.[50][51] The self-healing coating uses chitosan incorporated into traditional polymer materials, such as those used in coatings on cars to protect paint. When a scratch damages the chemical structure, the chitosan responds to sunshine by forming chemical chains that bond with other materials in the substance, eventually smoothing the scratch. The process can take less than an hour.[52]

The polymer can only repair itself in the same spot once, and would not work after repeated scratches.[52] Whether this technology can be applied to industrial materials, however, depends on a number of factors, such as long-term persistence of the repair, stiffness and heat resistance of the coating.[53]

Biomedical uses[edit]

Chitosan's properties allow it to rapidly clot blood, and has recently gained approval in the United States and Europe for use in bandages and other hemostatic agents. Chitosan hemostatic products have been shown in testing by the U.S. Marine Corps to quickly stop bleeding and to reduce blood loss, and result in 100% survival of otherwise lethal arterial wounds in swine.[54] Chitosan hemostatic products reduce blood loss in comparison to gauze dressings and increase patient survival.[55] Chitosan hemostatic products have been sold to the U.S. Army and are currently used by the UK military. Both the US and UK have already used the bandages on the battlefields of Iraq and Afghanistan.[56] Chitosan is hypoallergenic and has natural antibacterial properties, which further support its use in field bandages.[57]

Chitosan hemostatic agents are often chitosan salts made from mixing chitosan with an organic acid (such as succinic or lactic acid).[58] The hemostatic agent works by an interaction between the cell membrane of erythrocytes (negative charge) and the protonated chitosan (positive charge) leading to involvement of platelets and rapid thrombus formation.[59] The chitosan salts can be mixed with other materials to make them more absorbent (such as mixing with alginate),[60] or to vary the rate of solubility and bioabsorbability of the chitosan salt.[58] The chitosan salts are biocompatible and biodegradable making them useful as absorbable haemostats. The protonated chitosan is broken down by lysozyme in the body to glucosamine [59] and the conjugate base of the acid (such as lactate or succinate) are substances naturally found in the body. The chitosan salt may be placed on an absorbable backing.[61] The absorbable backing may be synthetic (for instance made from existing absorbable suture materials e.g. Tephaflex polymer) or natural (e.g. cellulose or gelled/solidified honey).

Chitosan's properties also allow it to be used in transdermal drug delivery; it is mucoadhesive in nature, reactive (so it can be produced in many different forms), and most importantly, has a positive charge under acidic conditions. This positive charge comes from protonation of its free amino groups. Lack of a positive charge means chitosan is insoluble in neutral and basic environments. However, in acidic environments, protonation of the amino groups leads to an increase in solubility. The implications of this are very important to biomedical applications. This molecule will maintain its structure in a neutral environment, but will solubilize and degrade in an acidic environment. This means chitosan can be used to transport a drug to an acidic environment, where the chitosan packaging will then degrade, releasing the drug to the desired environment.[62] One example of this drug delivery has been the transport of insulin.[63]

Preliminary research and FDA review[edit]

Supposed to limit fat absorption in the body, chitosan may be sold in tablet form as a "fat binder". A Cochrane meta-analysis,[64] which evaluated clinical trials performed with dietary chitosan over a minimum of four weeks, body weight, blood pressure and parameters related to cholesterol were changed only in some low-quality trials, indicating a minor effect on body weight. Other higher quality trials indicated no significant effect of chitosan and no clinical justification for advising overweight patients to take chitosan supplements.[64]

In an experimental model of the digestive tract, chitosan was shown to interact with oil, which inhibited duodenal absorption and enhanced lipid excretion.[65] However, the mechanism of interaction between chitosan and fat is not well understood and has not been proved clinically.[66] In mice, dietary ingestion of chitosan did not depress the level of iron, zinc or copper.[67]

The U.S. Food and Drug Administration (FDA) has issued warning letters to supplement retailers who made inappropriate claims about the supposed health benefits of using chitosan.[68]

Chitosan is under research for several potential dietary or clinical applications:[69]

  • As a soluble dietary fiber, it may increase gastrointestinal lumen viscosity and retard emptying of the stomach, creating a sense of satiety.[70][71]
  • It alters bile acid composition, increasing the excretion of sterols and reducing the digestibility of ileal fats.[72][73][74] It is unclear how chitosan does this, but the currently favored hypotheses involve the increase of intestinal viscosity or bile acid-binding capacity.[75]
  • Chitosan is relatively insoluble in water, but can be dissolved by dilute acids, which would make it a highly-viscous dietary fiber.[75] Such fibers might inhibit the uptake of dietary lipids by increasing the thickness of the boundary layer of the intestinal lumen, which has been observed in animal experiments.[76]
  • Having very few acetyl groups, chitosan contains cationic groups. This may cause chitosan to have bile acid-binding capacity, which causes mixed micelles to be entrapped or disintegrated in the duodenum and ileum.[75] This would interrupt bile acid circulation, causing reduced lipid absorption and increased sterol excretion, which has also been observed in animal experiments.[74][75][76]

See also[edit]

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