 | |
| Names | |
|---|---|
| Other names Alginic acid; E400; [D-ManA(β1→4)L-GulA(α1→4)]n | |
| Identifiers | |
| ChemSpider |
|
| ECHA InfoCard | 100.029.697 |
| EC Number |
|
| E number | E400(thickeners, ...) |
| UNII | |
| Properties | |
| (C6H8O6)n | |
| Molar mass | 10,000 – 600,000 |
| Appearance | White to yellow, fibrous powder |
| Density | 1.601 g/cm3 |
| Acidity (pKa) | 1.5–3.5 |
| Pharmacology | |
| A02BX13 (WHO) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Alginic acid, also calledalgin, is a naturally occurring, ediblepolysaccharide found inbrown algae. It ishydrophilic and forms a viscousgum when hydrated. When the alginic acid binds with sodium and calcium ions, the resulting salts are known asalginates. Its colour ranges from white to yellowish-brown. It is sold infilamentous, granular, or powdered forms.
It is a significant component of thebiofilms produced by the bacteriumPseudomonas aeruginosa, a major pathogen found in the lungs of some people who havecystic fibrosis.[1] The biofilm andP. aeruginosa have a high resistance toantibiotics,[2] but are susceptible to inhibition bymacrophages.[3]
Alginate was discovered by British chemical scientist E. C. C. Stanford in 1881, and he patented an extraction process for it in the same year.[4] The alginate was extracted, in the original patent, by first soaking the algae in water or diluted acid. The resulting insoluble alginic acid is then treated with an alkaline solution such assodium carbonate, creating sodium alginate, before finally precipitating and purifying the alginate from solution.[5][6]
Alginic acid is a linearcopolymer withhomopolymeric blocks of (1→4)-linked β-D-mannuronate (M) and α-L-guluronate (G) residues, respectively,covalently linked together in different sequences or blocks. Themonomers may appear in homopolymeric blocks of consecutiveG-residues (G-blocks), consecutiveM-residues (M-blocks) or alternating M and G-residues (MG-blocks). α-L-guluronate is the C-5epimer of β-D-mannuronate.[7]
Alginates are refined from brownseaweeds. Throughout the world, many of thePhaeophyceae class brown seaweeds are harvested to be processed and converted into sodium alginate. Sodium alginate is used in many industries including food, animal food, fertilisers, textile printing, and pharmaceuticals. Dental impression material uses alginate as its means of gelling. Food grade alginate is an approved ingredient in processed and manufactured foods.[8]
Brown seaweeds range in size from the giantkelpMacrocystis pyrifera which can be 20–40 meters long, to thick, leather-like seaweeds from 2–4 m long, to smaller species 30–60 cm long. Most brown seaweed used for alginates are gathered from the wild, with the exception ofLaminaria japonica, which is cultivated in China for food and its surplus material is diverted to the alginate industry in China.[9]
Alginates from different species of brown seaweed vary in their chemical structure, resulting in different physical properties of alginates. Some species yield an alginate that gives a stronggel, another a weaker gel, some may produce a cream or white alginate, while others are difficult to gel and are best used for technical applications where color does not matter.[10]
Commercial grade alginate is extracted from giantkelpMacrocystis pyrifera,Ascophyllum nodosum, and types ofLaminaria. Alginates are also produced by twobacterialgeneraPseudomonas andAzotobacter, which played a major role in the unravelling of itsbiosynthesispathway. Bacterial alginates are useful for the production of micro- or nanostructures suitable for medical applications.[11]
Sodium alginate (NaC6H7O6) is thesodium salt of alginic acid. Sodium alginate is a gum.
Potassium alginate (KC6H7O6) is the potassiumsalt of alginic acid.
Calcium alginate (CaC12H14O12) is the calcium salt of alginic acid. It is made by replacing the sodium ion in sodium alginate with a calcium ion (ion exchange).
The manufacturing process used to extract sodium alginates from brown seaweed fall into two categories: 1) calcium alginate method where the brown seaweed is first treated with calcium chloride to form a calcium alginate intermediate before washing with hydrochloric acid, and, 2) alginic acid method where there is no calcium alginate intermediate and the brown seaweed is treated only with the hydrochloric acid to extract sodium alginate.[9]
Chemically the process is simple, but difficulties arise from the physical separations required between the slimy residues from viscous solutions and the separation of gelatinous precipitates that hold large amounts of liquid within their structure, so they resistfiltration andcentrifugation.[9] The conventional process involves large amounts of reagents and solvents, as well as time-consuming steps.[4] Simpler and newer techniques, such as microwave-assisted extraction, ultrasound, high pressure, pressurized fluid extraction, and enzyme-assisted extraction, are the subject of research.[4]
The most common, conventional extraction process involves six steps: pre-treatment of the algal biomass, acid treatment, alkaline extraction, precipitation, bleaching, and drying.[4] Pre-treatments mainly aim at either breaking the cell wall to help extract the alginate, or removing other compounds and contaminants from the algae.[4] Drying is of the first kind, also helping to prevent bacterial growth; algae which is dried is also usually powdered to expose more surface area.[4] Common treatments to remove contaminants include treatments withethanol andformaldehyde, the latter of which is very common; ethanol solutions help remove compounds bonded to the alginate, and formaldehyde solutions help prevent enzymatic or microbial reactions.[4]
The algae is then treated with an acidic solution to help disrupt cell walls, which converts the alginate salts into insoluble alginic acid; a subsequently applied alkaline solution (pH 9-10), usuallysodium carbonate, converts it back into water-soluble sodium alginate, which is then precipitated.[4] It is also possible to extract the alginate directly with an alkaline treatment, but this is less common.[4]
Alginic acid is usually precipitated, through different techniques, with either an alcohol (usually ethanol),calcium chloride, orhydrochloric acid.[4] After the alginin is precipitated into a fine paste, it is dried, ground to the desired grain size, and finally purified through a variety of techniques.[4] Commercial alginate for biomedical and pharmaceutical use is extracted and purified through more rigorous techniques, but these are trade secrets.[4]
Various alginate-based materials can be produced, including porous scaffold material, alginate hydrogel, nonwoven fabric, and alginate membranes.[12] Techniques used to produce these include ion cross-linking, microfluidic spinning, freeze drying, wet spinning, and immersive centrifugal jet spinning.[12]
Calcium salts added to a sodium alginate solution to induce ionic cross-linking, which produces the hydrogel. Freeze-drying the hydrogel to eliminate water produces the porous scaffold material.[12]
Wet spinning consists of extruding an alginate solution from a spinneret into a calcium salt solution to induce ionic cross-linking (forming the gel), and thendrawing the fibers out of the bath with draft rollers. Microfluidic spinning, a simpler and more eco-friendly implementation of the process, involves introducing calcium salt flows flowing alongside and touching a central "core" flow of alginate. These flows form a "sheath". The fiber then emerges from the core flow. This technique can be used to produce shaped and grooved fibers.[12]
Alginate fiber, which is used in fabric, is usually produced through either microfluidic spinning, wet spinning, orelectrospinning to obtain thinner fibers.[12] Those fibers are used to produce alginate nonwoven fabric bycarding and needle punching. The resulting felts are used in wound dressings, facial masks, and tissue scaffolds due to its hygroscopic and water retention ability.[12]
As of 2022, alginate had become one of the most preferred materials as an abundant natural biopolymer.[12] It is particularly useful as abiomaterial because of its nontoxicity,hygroscopicity, andbiocompatibility, and can imitate local bioenvironments; its degradation product can be easily cleared by the kidneys.[12]
Alginate absorbs water quickly, which makes it useful as an additive indehydrated products such asslimming aids, and in the manufacture of paper and textiles.[13]
Alginate is also used forwaterproofing andfireproofing fabrics, in the food industry as athickening agent for drinks, ice cream, cosmetics, as agelling agent for jellies, known by thecode E401 and sausage casing.[14][15] Sodium alginate is mixed withsoybean protein to makemeat analogue.[16]
Alginate is used as an ingredient in variouspharmaceutical preparations, such asGaviscon, in which it combines withbicarbonate to inhibitgastroesophageal reflux.[17]
Sodium alginate is used as animpression-making material indentistry,prosthetics,lifecasting, and for creating positives for small-scalecasting.[18]
Sodium alginate is used inreactive dye printing and as a thickener forreactive dyes intextile screen-printing.[19] Alginates do not react with these dyes and wash out easily, unlikestarch-based thickeners. It also serves as a material formicro-encapsulation.[20]
Calcium alginate is used in different types of medical products, including skinwound dressings, in the form of hydrogels, to promote healing, since alginate can increase the fluid uptake capacity of dressings, reducing the amount of times the dressing needs to be changed.[21][22]
In research on bone reconstruction, alginatecomposites have favorable properties encouraging regeneration, such as improvedporosity,cell proliferation, andmechanical strength.[23] Alginate hydrogel is a common biomaterial for bio-fabrication of scaffolds and tissue regeneration.[24]
Covalent bonding of thiol groups to alginate improves in-situ gelling and mucoadhesive properties; the thiolated polymer (thiomer) forms disulfide bonds within its polymeric network and with cysteine-rich subdomains of the mucus layer.[25] Thiolated alginates are used as in situ gelling hydrogels,[26] and are under preliminary research as possible mucoadhesive drug delivery systems.[27] Alginate hydrogels may be used for drug delivery, exhibiting responses to pH changes, temperature changes, redox, and the presence of enzymes.[28]
