Polyelectrolyte-Based Dental Materials

Polymers used as dental materials must meet several stringent requirements. Dental restorative materials must be nontoxic, have aesthetic appearance, and good adhesive and mechanical properties. In addition, these materials must exhibit long term stability in the presence of water, enzymes, and various oral fluids, and withstand thermal and load cycles. Finally, a desirable dental restorative materia] should be convenient to work with at the time of application. 

The properties exhibited by polyelectrolytes make them nearly-ideal candidates for dental material formulations. Dental polyelectrolytes are generally considered to be nontoxic and are able to adsorb chemically to the hydrophilic surface of tooth material through ionic interactions. Ionic cross-linking of the polyelectrolyte with multivalent cations (Zn2+, Mg2+, Al3+, Ca2+) results in the formation of a rigid and insoluble cement matrix. The stability and strength of the cement is attributed to the fact that, if a bond is broken, it can be reformed as long as the other bonds are maintained. Even today, polyelectrolytes are the only materials which are known with certainty to form a bond, which is stable with time, to tooth material [120]. In addition to long-term stability, many polyelectrolytes are translucent and possess cariostatic properties [121]. 

Polyelectrolyte-based dental cements or restorative materials include zinc polycarboxylates, glass ionomers, a variety of organic polyelectrolyte adhesives as well as alginate-based impression materials. Dental cements are primarily used as luting (cementing) agents for restorations or orthodontic bands, as thermal insulators under metallic restorations, and as sealents for root canals, pits and fissures. They are also sometimes used as temporary or permanent (anterior) restorations. For further introduction to dental materials the reader is referred to standard texts [122,123]. 

Zinc polycarboxylate, the first polyelectrolyte dental material, was developed and used as early as 1968 [124]. These materials are formed by the reaction of a zinc oxide powder with an aqueous solution of poly(acrylic acid). The zinc ions cross-link the polyacid chains and form a cement. A few years after the development of zinc polycarboxylate cements, Wilson and Kent introduced the first glass-ionomer cement (GIC) [125]. Glass-ionomer cements are formed 

A variety of organic adhesives which are capable of forming strong bonds between a polymeric (acrylate) restoration and the hydrophilic tooth material have recently been developed. A number of these monomers, which possess a pendent ionizable group, are polymerized in the mouth to form an adhesive layer. Alginates, which are used as impression materials, are formed by the reaction of the sodium salt of anhydro-beta-d-mannuronic acid with calcium sulfate. Calcium ions crosslink the linear polymer to form a gel. This reaction is carried out inside the mouth, and the gel formed retains the shape of the oral cavity. 

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