Alginates: a review of compositional aspects for dental applications.
Abstract: Alginate is an elastic irreversible hydrocolloid, widely used for recording the impressions of dentulous arches, impressions for making models for diagnostic purposes, and for duplication of casts. Today, their use far exceeds the use of all other impression materials in general dental practice because of their hydrophilicity, ability to record finer details and sufficient elastic recovery. Over the years, several modifications have been done to improve their clinical performance. Modified alginates in form of dust free, self-disinfectant and chromalginate have already been in the clinical practice and have shown their superiority over the conventional alginates. In addition to these modifications, other modifications have been introduced to further improve their clinical acceptance. This review is an overview of the evolution of dental alginate impression materials with an emphasis on the material aspects.
Article Type: Report
Subject: Dental materials (Chemical properties)
Dental matrices (Chemical properties)
Colloids in medicine (Chemical properties)
Authors: Srivastava, Akanksha
Aaisa, Jalise
Kumar T.A., Tarun
Ginjupalli, Kishore
Upadhya P., Nagaraja
Pub Date: 01/01/2012
Publication: Name: Trends in Biomaterials and Artificial Organs Publisher: Society for Biomaterials and Artificial Organs Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2012 Society for Biomaterials and Artificial Organs ISSN: 0971-1198
Issue: Date: Jan, 2012 Source Volume: 26 Source Issue: 1
Product: Product Code: 2899913 Alginate Chemicals NAICS Code: 325998 All Other Miscellaneous Chemical Product and Preparation Manufacturing SIC Code: 2869 Industrial organic chemicals, not elsewhere classified; 3843 Dental equipment and supplies
Geographic: Geographic Scope: India Geographic Code: 9INDI India
Accession Number: 304842702
Full Text: Introduction

Alginate is an elastic irreversible hydrocolloid widely used for recording the impressions of dentulous arches, impressions for making models for diagnostic purposes, and for duplication of casts. In addition to the dental applications, alginates are widely used in textile printing, food industry, pharmaceutical applications, paper industry, etc. Introduction of alginate as an alternative material to agar impression material was inevitable due to the scarcity of agar during World War II. E.C. Stanford, a British chemist, in 1881 carried out the experimental studies on the extraction of alginate from brown seaweeds. However, large-scale production of alginate was only introduced 40 years later [1]. S. William Wilding received a patent for the use of algin as a dental impression material. Alginate when dissolved in water forms a viscous solution, sol, which can be converted to a gel form using a calcium salt. It is hydrophilic and most pleasant impression material with an ability to record all the finer details of the undercuts with sufficient elastic recovery [2]. Ever since its introduction to dentistry, the composition of alginate has been dynamic with several modifications being incorporated from time to time to improve their properties and clinical performance. In the present review, an overview of the evolution of alginate impression material and its modifications are discussed with an emphasis on the material aspects. Table 1 shows the various applications of alginates.

Composition and chemistry of conventional alginate impression materials

Dental alginate impression materials contain a blend of several ingredients to impart desirable properties during its use in clinical conditions. Alginic acid is a colloidal polysaccharide extracted from various species of cell walls of brown seaweed (Phaeophyceae). Although alginic acid can be extracted from any seaweed, the chemical structure of alginic acid varies from one genus to other. Several brown seaweeds such as Laminaria, Ascophyllum, Durvillaea, Ecklonia, Lessonia, Macrocystis and Sargassum can be used as a source of raw alginate [4, 6]. Cell walls of brown seaweed are rich in calcium, magnesium and sodium salt of alginic acid. Seaweeds are crushed into pieces and stirred with an aqueous hot solution of sodium carbonate or other similar alkaline solution which causes the alginic acid to dissolve as sodium alginate to form a thick solution. This viscous solution is then diluted to facilitate its filtration to separate insoluble residue. From the filtered solution, desired form of alginate such as alginic acid or calcium alginate is precipitated. Addition of raw alginate with an acid converts it to alginic acid to form a soft gel. Alcohol is then added to remove water from the alginic acid. To this solution, sodium carbonate is added to convert the alginic acid to sodium alginate which is then separated from the mix, dried and milled into a desired particle size [4]. In another method, raw alginic acid solution is mixed with calcium salt to form calcium alginate as fibrous texture which is separated and re-suspended in aqueous acidic solution to form alginic acid. Alginic acid in fibrous form is separated and placed in a planetary mixer with alcohol. Sodium carbonate is then slowly added to this solution to convert alginic acid to sodium alginate which is then dried and milled to desired fineness.

Chemically, alginic acid is a block copolymer of anhydro [beta]-D mannuronic acid and anhydro [alpha]-D glucouronic acid. The properties of alginate depend on the degree of polymerization and the ratio of mannuronan (M) and Guluronan blocks (G). The mannuronan chains are stretched and flat which are more flexible compared to guluronan blocks. Guluronan blocks react with calcium and produces strong and brittle gel whereas mannuronan block produces weaker and elastic.

Alginic acid is not used as it is insoluble in water. Hence, for dental applications, salt of alginic acid (molecular weight of 32000-200,000) with monovalent ions such as sodium, potassium, ammonium or triethanolamine is generally employed. When the soluble alginates are mixed with water, they form a viscous sol quite readily. The viscosity of the sol depends on molecular weight (higher the molecular weight greater is the viscosity of the sol) [3, 4], concentration (higher the concentration of alginate, greater is the viscosity of the sol), temperature (viscosity decreases with temperature but at temperatures higher than 50[degrees]C, depolymerization of alginate may take place), pH (although no viscosity changes were observed between the pH range of 5 to 11, viscosity increases below 5 due to hydrogen bonding between the carboxylic groups of alginate where as viscosity decreases above 11 due to slow depolymerization of alginate).

Soluble alginate in the form of sol is converted to gel due to the cross linking the alginate chains by using a divalent ion such as calcium to form insoluble calcium alginate gel. Generally, hemihydrates or dihydrate form of calcium sulfate is used as source for the calcium ions although dihydrate form is more common. Other materials of dental interest are lead silicate and chromium sulfate. Set material consists of a brush heap structure of calcium alginate in which unreacted alginate powder, fillers, excess water, and reaction byproducts are enclosed.

[K.sub.2n]Alg + nCaS[O.sub.4] [right arrow] n[K.sub.2]S[O.sub.4] + [Ca.sub.n]Alg

Conversion of alginate sol to gel can occur by two mechanisms i.e. diffusion and internal gelation. Diffusion mechanism is generally used for the fabrication of alginate beads. This process involves the controlled addition of alginate sol into a reactor solution rich in calcium ions. The rate of formation of alginate gel is rapid and this process is generally used in pharmaceutical applications to encapsulate drug or other biomolecules into the alginate for the purpose of controlled release [4]. Internal gelation of alginate involves the use of calcium reactor which is available for the gelation throughout the alginate. Since the reactor is available throughout the alginate material in the form of a less soluble calcium salt, the rate of reaction is slow compared to diffusion method and hence is the preferred mode of mechanism used in dental alginate impression materials. Initially it was thought that the calcium forms ionic bridges with the alginic acid chains, later Rees in 1969 proposed Egg-box model which is now widely accepted. According to Egg-box model, a cooperative mechanism is involved wherein two alginate chains will form an egg-box with interstices occupied by calcium ions.

A rapid reaction between the alginate sol and calcium ions can result in the formation of alginate gel without adequate time for uniform mixing and carrying the mixed material to the oral tissues for recording the impression. Thus for dental applications, this reaction is delayed with help of a retarder which helps in providing adequate working time for the material [1]. Most commonly, retarder in the form of trisodium phosphate is used although disodium phosphate, tetrasodium pyro phosphate, potassium phosphate, oxalates, carbonates can also be used. In the presence of trisodium phosphate, the reaction between the calcium ions and alginate sol is deferred due to the preferential reaction of calcium sulfate with retarder than with alginate.

2[Na.sub.3]P[O.sub.4] + 3CaS[O.sub.4] [right arrow] [Ca.sub.3] [(PO4).sub.2] + 3[Na.sub.2]S[O.sub.4]

In clinical practice, alginate is generally mixed with cold water to extend the time available for the manipulation of alginates. A formulation containing only soluble alginate and calcium salt would result in the formation of calcium alginate gel that is sticky without adequate strength and is not desirable for dental impression recording. To improve the consistency and to reduce the tackiness, inert filler in the form of diatomaceous earth is added. This material, in fact, makes up the bulk of the alginate impression material composition and may be present in more than 50%. It helps in the formation of a non tacky gel with improved strength and stiffness [1]. Silicic anhydride, talc, calcium carbonate, perlite or other similar materials can also be used as fillers. In addition, zinc oxide in small concentrations is also used to modify the properties of resultant gel. In order to counter the retardation effect of alginate on the setting of gypsum product, an accelerator of gypsum setting in the form of potassium titanium fluoride is added to ensure hard and dense dental cast surface [1]. Potassium zinc fluoride and potassium sulfate can also be used as accelerator of gypsum setting. Finally, coloring pigments and flavoring agents will be added to improve the patient acceptance.

Compositional modifications of alginate impression materials

Need for the modification

Ever since its introduction to dentistry, alginate impression material remained the material of choice in routine dental practice to record the impressions of dentulous and partially dentulous mouth. Good elastic recovery coupled with sufficient flexibility, ability to record the fine details and low cost makes the material attractive. However, the material is far from being ideal. These materials suffer from low tear strength, dimensional instability during disinfection and storage, technique sensitivity and biocompatibility problems with the dust evolved during manipulation [2]. Several modifications in the composition have been attempted to address these problems which led to a variety of alginate impression materials.

Dust free alginates

Conventional alginates contain diatomaceous earth in the form of fine particles as filler. However, during storage of the material, these filler particle were found to settle due to high density leading to inhomogeneous distribution of the filler. In order to achieve uniform distribution, the container is tumbled before mixing [1]. On opening the container, about 5 to 10% of the filler particles in the form of dust are evolved. Dust may also be evolved from the material during its initial mixing with water. It was reported that the size of these particles is similar to that of asbestos fibers and their prolonged inhalation was reported to cause silicosis, pulmonary hypersensitivity, and carcinogenesis or fibrogenesis [1, 7]. In an attempt to avoid or minimize the dust during manipulation, alginate powder is coated with de-dusting agent to agglomerate the powder to a more dense form. In general, glycerin, glycol, polyethylene glycol and/or polypropylene glycol are used as dedusting agents[8]. Surface-active substances and hydrocarbons such as squalene, decane, dudecane or specific isoparaffin have also been tried [9, 10]. Attempts have also been made to partially substitute diatomaceous earth as fillers in the alginate. Sepiolite, a natural mineral fiber containing magnesium silicate with a mean particle size of 1 to 40 |xm in about 20% was added to alginate as a substitute. Sepiolite, when added traps the alginate particles thus reducing dust generation [9, 10]. Tetrafluoroethylene, on the other hand, traps the alginate particles by forming cow-web like structure due to the stresses applied during manipulation [10, 11].

Alginate with polyacrylamide incorporation

Conventional alginates when mixed with water may form a grainy mass with lumps of unmixed material as the water does not wet the powder easily. In order to improve the mixing characteristics, the conventional alginates were added with thickening and stabilizing agents such as 0.01-0.25wt% polyacrylamide (molecular weight-200,000 to 6,000,000) to help in the formation of smooth mix of alginate with water [12, 13].

Storage solution for alginates

Conventional alginate, being hydrocolloid, undergo syneresis and imbibition which is responsible for their dimensional instability. Hence, alginate impressions cannot be stored and have to be poured up with gypsum product soon after the impression recording [14]. A storage solution is now commercially available to store the alginate impressions without any dimensional changes. A recent investigation reported that storage of alginate up to 100hr did not result in significant dimensional changes [15].

Tray adhesive for alginate

Conventional alginates do not exhibit retention to the impression trays and hence perforated trays are generally employed to achieve retention [7]. Molten sticky wax on the tray, securing the wisps of cotton wool on the tray with sticky wax, coating the surface of the tray with varnish before alginate loading seems to improve the retention of the alginate to the tray. Recently, a tray adhesive containing polyamide, ester gum and rosin in isopropyl alcohol or a combination of isopropyl alcohol with ethyl acetate has been introduced [16]. Application of such adhesives prior to the loading of alginate was found to improve the bond strength of alginate to the metal and plastic trays [17].

Alginate containing biocidal agents

Disinfection of the dental impressions is the most important barrier system in infection control [18]. Although, sterilization is more effective than disinfection, dental impressions cannot be subjected to sterilization as it is associated with significant dimensional changes. The importance of disinfection of dental impressions is well recognized and is the routine practice in the dental office. Several investigations have proved that disinfection indeed helps prevention of transfer of diseases from the clinics to the laboratory [19, 20]. However, it was observed that disinfection of the impression is not performed systematically in routine dental practice [21]. Disinfection of alginates either by immersion or spray technique was found to cause dimensional inaccuracies, although reports are available supporting the disinfection of alginates without clinically significant dimensional changes [22]. In a study it was observed that immersion of alginate impressions in hypochlorous acid did not result in significant dimensional change [23]. In addition spraying of 0.5% sodium hypochlorite and 2% Glutaraldehyde did not cause any dimensional changes in alginate impression [14, 24].

To avoid dimensional inaccuracies associated with disinfection process, manufacturers have incorporated disinfectant materials into the alginate. A disinfectant material that is added to the alginate must be efficient enough without affecting the clinically important properties and the castability of the recorded impression [25]. Antimicrobial compounds which are water soluble and easily dispersible materials such as quaternary ammonium compounds, bisquanidine compounds, dialkyl quaternary compounds, quinoline compounds, substituted phenols, chlorhexidine, didecyldimethyl ammonium chloride, and a mixture of these materials are generally employed [26].

Commercially, combinations of disinfectants are added to alginate compositions. For example, Virkon contains Potassium monopersulphate, potassium hydrogen sulphate, potassium sulphate, sulphamic acid, maleic acid, sodium hexameta phosphate, sodium dodecyl, benzene sulphonate, and sodium chloride and C & J Algisept contains p-chloro-m-cresol, tetra-bromo-cresol, N, N, BIS (3-aminopropyl) dodecylamine [27]. Disinfectants are either physically blended or they are coated onto the alginate powder. Alternatively, attempts have also been made to incorporate disinfectants in the form of microcapsules which will release the disinfectant on mixing with the liquid. Similarly, disinfectants can also be added to the mixing liquid. In this regard, several investigations have evaluated the efficiency of chlorhexidine, sodium hypochlorite and other similar disinfectant solutions with varying success. Among these, chlorhexidine was most widely used and was proved to be efficient without affecting handling and physical characteristics of the alginates [28].

Chromalginate

Conventional alginate impression materials undergo gelation with an increase in the viscosity quite rapidly. During the gelation process, pH of the alginate varies from being above 8 immediately after mixing and gradually decreases as the gelation progresses. In order to indicate the progression of the gelation to help the operator in timely manipulation, manufacturers have incorporated pH indicators that change color during the gelation process [7]. Initially, conventional alginates were simply modified with the addition of pH indicators. However, most pH indicators shows a color change at an early stage before actual gelation time This is because most pH indicators show a color change slightly above pH 8 which is the pH immediately after mixing. However, during the gelation pH of the mix decreases below 8 and the pH indicator does not show any color change at this pH. Further, the pH indicators are poorly soluble in water during mixing. To overcome this problem, a combination of inorganic and/or organic pigment along with pH indicator has been incorporated [29]. In such cases, the initial color of the alginate soon after mixing is a combination of pigment and pH indicator where as the color of alginate after setting is the color due to pigment alone. Various pH indicators such as Cresol Red, c naphtholphthalein, Tropaeolin OOO, Thymol blue, and phenolphthalein can be used. In addition, water soluble additives such as glycols may be added along with pH indicators which will prevent pH indicator from changing the color tone due to the presence of other salts in the alginate mix before gelation.

Alginate in two paste form

Traditionally, alginate is supplied as powder to be mixed with water. Proportioning of the powder alginate is done using the plastic scoop provided by the manufacturer. Tendency to evolve dust during manipulation [1], inconsistency in dispensing accurate amount of powder, separation of ingredients and contamination of the powder during storage are some of the drawbacks of using powder dispensing system [7]. To counter these disadvantages, alginate in the form of two paste system has been introduced [30]. Base paste contains a mix of soluble alginate, water and fillers mixed with a paste forming material. Polysaccharides such as carrageenan, pullulan, xanthene gellan gum, guar gum and gum arabic can be used to form a paste without the separation of components of alginate paste [30, 31]. The reactor or catalyst paste contains calcium salt mixed with a viscous liquid that is non-reactive towards calcium salt such as liquid paraffin, fatty acids or aliphatic alcohol in the form a paste. Polybutene can also be used as paste forming material to ensure uniform mixing of calcium salt and stabilize the reactor paste. A pH stabilizing agent such as magnesium hydroxide is also added [31, 32]. In order to improve the surface of the model or cast, a fluoride containing salt may also be added to the reactor paste. By varying the concentration of fillers, this material may be supplied as syringe and tray consistencies [33]. Alginate in two paste form is quite convenient to mix and it can also be used with mechanical mixing units. The best surface quality can be obtained with the paste-type material. Studies suggest that a paste-type material would better meet the requirements of an alginate impression material [34].

Permeation speed enhancer

Conventional alginate powder requires longer time to form a wet mass with water during mixing due to the low permeation of the water. Hence, formation of smooth and uniform mix requires longer mixing time with resultant decrease in the working time. Also, during storage, alginate powder is deteriorated, probably due to the absorption of water with a resultant mix that is too fluid. To overcome these drawbacks, conventional alginates are incorporated with hydrophobic material to prevent the deterioration of alginate during storage. In order to enhance the permeation of alginate, a surfactant such as polyoxyethylene alkyl phenyl ether,

Polyoxyethylene/polyoxypropylene alkyl ether, polyoxyethylene alkyl ether, are also added along with hydrophobic material [35]. Such materials when mixed with water, rapidly forms sol with ease and exhibits sufficient viscosity. Polysaccharide materials such as carrageenan, pullulan, curdlan, xanthan gum, gellan gum, pectin, konjak, glucomannan, xyloglucan, guar gum, gum Arabic and locust bean gum are used to maintain the viscosity of the material.

Alginate in different flavors

Conventional alginate impression material without flavoring agents is unpleasant and may result in gagging in of some patients [1]. In order to improve the patient acceptance and to make the material more pleasant during use, alginate is now available in variety of flavors. Flavoring agents such as cinnamon, strawberry, watermelon, bubblegum, cherry, orange, peppermint, and spearmint are used to provide flavors. Initially, these flavors were supplied as liquids which were to be mixed with the water before mixing the material. But, liquid flavoring agents were not popular because of the need for separate liquid and leakage problem during supply. Now the flavorings agents are incorporated in the powder itself [36] and thus the use of separate flavoring liquid is not necessary.

Conclusion

Alginate impression materials are routinely used to record the preliminary impressions of dentulous and partially dentulous mouths. The composition of alginate has been changing from time to time to improve the clinical performance. Modified alginates are much superior with respect to handling and clinical performance compared to conventional alginates. Such modifications are helpful in improving their clinical performance and extending the use of the material in various dental applications.

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Akanksha Srivastava, Jalise Aaisa, Tarun Kumar TA, Kishore Ginjupalli *, Nagaraja Upadhya P

Department of Dental Materials, Manipal College of Dental Sciences, Manipal University, Manipal--576104, Karnataka, India

* Corresponding author (email: kishore.ginjupalli@gmail.com)

Received 28 March 2011; Accepted 26 September 2011; Available online 8 February 2012
Table 1: Various applications of alginates

Specialty              Description

Textile printing       Used a thickening agent for the paste
                       containing dye (3)

Food                   Used in sauces, syrups and toppings of ice
                       creams (3,4)

Pharmaceutical         Used as excipient in drug delivery system and
applications           is also used in the form of micro and
                       nanoparticles as a carrier of the drug (3,4)

Medical applications   Used as wound dressings (5)

Paper                  Used as surface sizing material to achieve
                       smooth surface. (4,5)

Fish industry          Used as a binder for fish feed (4)
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