**1.1 Overview of hydrocolloids application in dentistry**

Hydrocolloids were the first elastic materials to be used in the Dentistry [2]. Elastic impression materials commonly used in the dental field include reversible hydrocolloids (agar-agar), irreversible hydrocolloids (alginate), and other synthetic and elastomeric materials such as polysulfide, polyether's, and silicone [2]. The properties and abilities of hydrocolloids materials enables the replication of the oral tissue with little or no deformity on withdrawal while abiding to both manufacturer and mechanical stipulations of its manipulation prior to being loaded on a tray to produce the gel or sol form [3].

Agar discovered by Sears in 1937 was the first hydrocolloids used in dentistry for making impressions to circumvent the cumbersome procedure and oral lacerations of using impression compound [9]. Agar is a vegetable colloid derived from seaweed found on the sea coast of Japan, a jelly-like substance softened when heated and solidifies when cooled [9]. However, the technique of using agar was complicated because of the need for special heaters and tempering Jars for heating and holding prior to use, syringes and water-cooled trays, even though it could be used severally without losing its chemical and physical properties before been discarded [10].

In 1947, alginate was introduced during the second world war as a result of the scarcity of agar from Japan by the extraction of alginic acid from marine seaweed [10]. Unlike agar which reaction is reversible; alginate reaction was chemical which resulted in irreversible hydrocolloids when the alginate gels are mixed with water [10]. Furthermore, the physical, mechanical, biocompatibility and fatigue properties and most importantly the hydrophilic nature that allows hydrocolloids to capture accurate impressions in the presence of some saliva or blood [10].

Equally significant, its low wetting angle makes it easy to capture full or partial arch impressions moderate ability to reproduce the detail and costs relatively little compared with other elastomeric impression materials [10]. Despite this, hydrocolloids materials are not accurate enough for fixed partial dentures but are used for partial framework impressions to the modelling materials, ability to adapt to the oral tissues and the formation of an elastic resilient film [3, 11].

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**Table 1.**

*Composition of agar gels.*

Potassium Sulphate

*Hydrocolloids in Dentistry: A Review*

*1.2.1 Agar*

from seaweed [3].

*1.2.1.1 Composition*

*DOI: http://dx.doi.org/10.5772/intechopen.94987*

**1.2 Types of hydrocolloids use in dentistry**

Generally, hydrocolloids used in dentistry can be typified as either reversible (agar) and or irreversible (alginate). This section, therefore, focuses on the proper-

Agar hydrocolloid has remained an excellent, cost-effective impression material since its discovery in 1937 from seaweed found on the coasts of Japan, and thus, has been used widely for the replication/duplication of models [12]. It is a reversible hydrocolloid which can repeatedly pass between highly viscous gel and low viscosity sol through heating and cooling [3]. In terms of its chemical composition and structure, agar is the sulfuric ester of a linear polymer of galactose extracted

The components of the agar gels are 12–15% agar, 1% potassium sulphate to ensure a proper set of the gypsum material poured in the impression, 0.2% borax as a strengthener for the gel, 0.1% alkyl benzoate as an antifungal during storage, and 85% water (**Table 1**). Borax and agar retard the set of gypsum products, so

The composition described in **Table 1** may differ slightly depending on the dispersing medium for the gel; which could be either loading on an impression tray or a syringe. When fine details of preparation are needed, a less concentrated gel type is used in a syringed. A more concentrated gel is used to in water-cooled tray to form the bulk of the impression. Agar possesses relatively good elastic recovery, reproduction of details, pleasant tasting and easy to clean up. But it cannot be used to produce electroplated pies due to its dissolution inside the electrolytic bath [14]. Agar is a technique sensitive impression material due to its low tear strength of 27.6 KPa. Agar is dimensionally unstable due to the loss of water from the agar gels even when stored at 100% humidity. The consequence of this is an inaccurate model if left for a while before the cast is poured [9]. Agar hydrocolloids are supplied as sticks or gel and require specific equipment for its manipulation before the impression making process. Thereby making the process cumbersome but can be reused once the setup is done. Although agar hydrocolloid is an inexpensive

potassium sulphate is added to cancel out their effect [13].

**Material Composition (Approximate** 

**percentage)**

Agar 12–15% Colloidal particle as basis of the gel

Water 85% Dispensing medium for the colloidal

Borax 0.2% Strengthens the gel Alkyl Benzonate 0.1% Antifungal agent

**Purpose**

suspension

1% Ensures set of gypsum material

ties, composition and application of these two hydrocolloids materials.
