*1.2.1 Agar*

*Colloids - Types, Preparation and Applications*

results and application.

produce the gel or sol form [3].

discarded [10].

blood [10].

**1.1 Overview of hydrocolloids application in dentistry**

Recently, hydrocolloid (alginate) gels have also been actively investigated for their ability to mediate the regeneration and tissue engineering of different tissues and organs, including skeletal muscle, nerves, the pancreas, and liver. Current strategies for skeletal muscle regeneration include cell transplantation, growth factor delivery, or a combination of both approaches [6, 7]. Also, Studies within the pharmacological field have demonstrated how alginate-antacid formulations can decrease post-prandial symptoms by neutralizing the acidity of gastric contents by forming a gel-like barrier to displace the "acid pocket" from the oesophagogastric junction and protect the oesophageal and gastric mucosa with controlled released drug products used as model system for mammalian cell culture in biomedical studies [6]. In Dentistry, for example, hydrocolloids are widely used in the fabrication of dental and maxillofacial prostheses impression due to their biocompatibility with the tissues, ease of use, physical properties and hydrophilicity with the oral tissues [8]. Other areas of hydrocolloids applications include orthopaedic structures and stone models in surgical cases [8]. This chapter aims to discuss the different hydrocolloids used in dentistry, their preparations, uses and storage for optimal

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

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

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

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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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 from seaweed [3].
